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Appendix B - a review of the literature on the strategic placement and design of buffering
features for sediment and P in the landscape
Appendix B - Literature review .............................................................................................................................. 1
Introduction ........................................................................................................................................................ 1
Pollution of surface waters by sediment and sediment associated chemicals ......................................................... 1
Buffer types and their role in sediment trapping ................................................................................................ 2
Grass buffers ...................................................................................................................................................... 2
Hedgerows.......................................................................................................................................................... 4
Trees and woodland ........................................................................................................................................... 4
Wetland features................................................................................................................................................. 5
Infrastructure ...................................................................................................................................................... 7
Management practices........................................................................................................................................ 7
Implementing buffer features in the UK .......................................................................................................... 12
Implementing buffer features elsewhere .......................................................................................................... 13
Buffer design, placement, management and performance................................................................................ 13
Summary, conclusions and gaps in literature ................................................................................................... 23
References ........................................................................................................................................................ 25
Introduction
Diffuse pollution is high on the political agenda because of the need to meet water quality targets set under the
EU Water Framework Directive (2000/60/EC). Diffuse pollution is caused by a suit of chemicals and substances,
some of which can be mitigated by controlling the rate or timing of application into the landscape while others,
such as sediment and sediment associated chemicals, require additional controls in order to reduce or prevent
transfer from the landscape into aquatic systems where they can cause ecological damage. This review considers
why sediment and sediment associated chemicals, specifically P, are considered a pollution problem; what
features within the landscape can act to buffer sediment transfer; subsurface buffering function; UK policy for
establishing buffers in agricultural environments; buffer design, location and maintenance.
All references to P in this document relate to sediment associated P unless clearly stated otherwise. This
literature review forms part of Julia Duzant’s PhD thesis (submission due January 2008).
Pollution of surface waters by sediment and sediment associated chemicals
Sediment is an integral and dynamic part of surface water systems and it plays a major role in the hydrological,
geomorphological and ecological functioning of river basins (Owens et al., 2005). In natural and agricultural
systems, sediment originates from the weathering of rocks, the mobilisation and erosion of soils and river banks,
and mass movements such as landslides and debris flows (Owens, 2005; Morgan, 2006). In most river basins
there are also important contributions to the sediment load of organic-rich material from a range of sources such
as riparian trees, macrophytes and fish (McConnachie and Petticrew, 2006). This material is susceptible to
transportation downstream by flowing water, from headwaters and other source areas towards the outlet of the
river basin. While a certain level of sediment is essential to maintain system functioning, too much sediment or
too little sediment can be detrimental, and as such sediment can be considered a pollutant in its own right
(Owens et al., 2005). For example, the excessive delivery of fine-grained sediment to surface waters due to soil
and/or bank erosion can smother sensitive aquatic habitats, such as salmonid spawning gravels, thereby causing
reductions is egg and larval survival rates. On the other hand, too little sediment, due to river impoundments
trapping sediment, can result in downstream channel degradation and the loss of important habitats such as salt
marshes and tidal flats. As well as sediment quantity, sediment quality is also important for the functioning and
quality of surface waters (Förstner and Owens, 2007). This is because many pollutants, such as certain metals,
radionuclides, organic pollutants etc are associated with sediment (Horowitz, 1991; Van der Perk, 2006). Thus,
sediment sources, delivery and transport dynamics control the fluxes and storage of sediment-associated
pollutants and contaminants, including particulate phosphorus, pesticides, metals and PCBs, within surface water
systems (Owens et al., 2001; Owens and Walling, 2002; Meharg et al., 2003; Carter et al., 2006).
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Chemical elements are preferentially associated with either water or sediment, although some elements can be
associated with both water and sediment phases in approximately equal amounts (Horowitz, 1991). Thus, for
sediment-associated elements, such as certain metals and nutrients etc, it is well known that there is a
relationship between particle size and the concentration of the chemical elements. There is a fairly large body of
literature on this relationship for heavy and trace metals, radionuclides etc (for reviews see Horowitz, 1991; Van
der Perk, 2006) but few studies have specifically examined this relationship for nutrients, and specifically
phosphorus. An exception is the work by Owens and Walling (2002) for suspended sediment collected from the
River Aire in England. They separated a bulk suspended sediment sample into fractions of various particle size
classes and examined the P content (total, inorganic and organic) content of the various particle size fractions. In
all cases there was a very strong relationship (r2>0.85) between P content and particle size as measured by
specific surface area. This shows that there is a strong affinity of P for particulate material. This works supports
direct measurements or inferred statements made by others (Pionke and Kuniski, 1992; Pacini and Gachter,
1999) on the likely relationship between P and particle size. Other papers discuss the specific relationships
between P and certain sediment composition such as the oxy-hydroxides (Golterman, 2004; Van der Perk, 2006).
Features within the landscape that buffer sediment transfer
“A buffer acts to protect from impact or “cushion the blow” (Dabney et al., 2006). The impact or “blow” that
this study is concerned with is the pollution of surface waters by sediment. In this context the purpose of a buffer
feature is to intercept field runoff and to trap pollutants before they can enter a watercourse (Dosskey et al.,
2002).
Buffer types and their role in sediment trapping
Water quality may be managed at source by reducing inputs or preventing the dislodgement of soil particles, at
transition by intercepting the transfer of detached soil particles or, once the pollutant has reached the water,
through in-stream amelioration techniques. Buffering is provided at the sediment transport stage by performing
one or more of the following roles:
- Reducing runoff water speeds by reducing the angle or length of slope.
- Delaying the flow time and reducing the peak flow of runoff through the system.
- Trapping pollutants carried in runoff before it reaches the river.
The term buffer feature usually describes a vegetated strip between a river, stream or creek and an adjacent
upland land-use activity (Hickey and Doran, 2004). The following section however, includes infrastructure,
water retention features and management practices that have the potential to buffer the impact of sediment laden
runoff on agricultural land. It includes features located adjacent to a water course as well as further up in the
catchment either within-field or at the field edge. It includes features intended as a water pollution measure as
well as those which are incidental, having been established for an alternative purpose such as creating habitats or
maintaining biodiversity. The features listed below have been selected from the literature because they share the
potential to perform one or more of the three roles listed above. They have been categorised according to feature
type.
1.
2.
3.
4.
5.
6.
Grass features - riparian buffer zones, grass strips (field margins, vegetated filter strips, beetle banks,
conservation headlands, set-aside) grass hedges or vegetative barriers.
Hedgerow features - ditches, banks.
Trees and woodland – riparian buffer zones, in field trees, woodland barriers.
Infrastructure - fences, stone walls.
Water retention - wetlands, ponds, floodplains, grass waterways.
Management practice - strip cropping, contour cropping, terracing (or contour bunds), contour
cultivations, soil berms.
Grass buffers
Riparian buffer zones
“A buffer feature may be considered as a permanently vegetated area of land most likely, but not exclusively,
adjacent to a watercourse and managed separately from the rest of a field or catchment” (Muscutt et al., 1993).
Indeed the most widely reported buffer feature in the literature is the riparian buffer zone. Occurring along the
edges of objects to be protected, like rivers and reservoirs, these zones aim to protect water quality and to
function as a nutrient filter (Nieswand et al., 1990). They do not necessarily consist of only grass but stands of
native vegetation including mature woody vegetation (Correll, 2005) and may be 20 to over 100 m long in the
direction of flow.
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Muscutt (1993) outlined the major water quality abatement functions of riparian buffers as:
• Surface runoff reduction
• Surface runoff filtration
• Groundwater filtration
• Bank erosion reduction
• Stream filtration
Additional benefits from a riparian zone include stabilising channels, preventing stock access to waterways,
filtering sediment and other particulates, removing soluble nutrients, providing terrestrial and aquatic habitat,
provide corridors for the movement of native fauna and flora between geographically separate areas, maintaining
invertebrate communities (Parkyn, 2004), moderating stream temperature and light, inputting organic debris (Lee
et al., 2004).
A number of reviews on riparian buffer zones exist (Barling and Moore, 1994; Muscutt et al., 1993; Wenger,
1999; Hickey and Doran, 2004; Parkyn, 2004; Correll, 2005) and the literature of Correll (2005) may be found in
an annotated and indexed bibliography at http://www.riparian.net/. It should be noted that the majority of the
literature cited has been carried out in the US and only one of the reviews referenced above (Muscutt et al., 1993)
was carried out in the UK.
Grass strips
The filter processes of a riparian buffer are similar to those in grass strips (Van Dijk et al., 1995) but grass strips
are usually much shorter than riparian buffers, varying between 1 and 25 m. Grass strips can consist of
permanent vegetation, but may also be part of the crop rotation cycle and may exist at the bottom or middle of a
field. They are applied for both the filtration of sediment and the removal of nutrients from runoff. The strips
change the hydraulic characteristics of the runoff (Dillaha et al., 1987) so that, (a) infiltration is enhanced, (b)
sedimentation increases while the flow velocity and transport capacity decrease, (c) filtration of suspended
material by vegetation increases, (d) adsorption of solutes to plant and the soil surface increases and (e)
absorption of solutes by vegetation increases.
Grass strips exist in UK agriculture in a number of forms, not necessarily intended for the control of runoff and
sediment transfer including (Defra, 2005):
a) Beetle banks, which are tussocky grass ridges, generally about 2 m wide, that run from one side of a
field to the other whilst still allowing the field to be farmed. They provide habitat for ground nesting
birds, small mammals and insects (including those which feed on crop pests). When carefully placed
across the slope such banks can help reduce run-off and erosion;
b) Buffer strips, which may be 2 m, 4 m or 6 m, require maintaining a grassy strip by controlled cutting,
not applying fertilisers or manure and not using the strip for access. They are intended to protect
sensitive features such as wetlands and woodlands from fertilisers and pesticides as well as providing
wildlife habitat;
c) Field corner management refers to the provision of a grassy area less than 1 ha, e.g. one that is awkward
to reach with machinery or of lower productivity, in order to provide habitat for insects and birds;
d) Conservation headlands are 6 to 24 m strips along the edge of cereal crops where the careful use of
sprays allows populations of broadleaved weeds and their associated insects to develop.
Grass hedges
Like grass strips grass hedges, or vegetative barriers, are set out along contour lines and separated by strips of
arable land. They consist of 0.3 to 1 m wide permanent strips of stiff erect grass which spread concentrated
runoff, retaining sediment and, in the longer term, develop terraces (Dabney et al., 1993). Essential features of
suitable grass species are a stiff stalk, which does not bend under the pressure of the water flow, and a high stalk
density in order to reduce the flow velocity and cause ponding (Van Dijk et al., 1995). Runoff that accumulates
against the barrier is released gradually to the field downslope. Sedimentation mainly takes place in the ponds
upslope of the barriers. The runoff itself is affected in three ways by the barriers (Van Dijk et al., 1995):
1) The catchment discharge is delayed as a result of water storage in the ponds along the barriers and the
subsequent delivery of this water. Peak discharge will also be reduced.
2) If barriers follow the slope contours accurately, the water will spread out against the barriers thus reducing the
risk of gully erosion.
3) The combination of these two effects will lead to an increase in infiltration.
The main advantage of barriers is their small spatial extent (Van Dijk et al., 1995). However, the narrow width
makes them vulnerable to breaching by water breaking through the sods; often as a result of animal holes that
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facilitate subsurface flow and tunnelling which strongly increases the erosion hazard (Kemper et al., 1992). The
risk of breakthrough can be reduced by a careful layout of the strips (parallel to slope contours and a wellfounded distance between the barriers) and by conscientious maintenance (Dabney et al., 1993).
The function of hedges as buffering features has been recognized by a number of researchers. Dabney et al.
(1995), Dewald et al. (1996) and Rafaelle et al. (1997) reported on the use, as a conservation measure, of narrow
grass hedges planted on the contour. Gilley et al. (2000) described substantial reductions in runoff and soil loss,
under both tilled and no-till conditions, with grass hedges covering only 7% of the total experimental plot area.
Hedgerows
In a UK context a hedge can be defined as “a more or less continuous line of woody vegetation that has been
subject to a regime of cutting in order to maintain a linear shape” (The Countryside Survey, 1993). Commonly
hawthorn or blackthorn, they are likely to offer a different structure and a lower stem density to the grass hedges
described above. The term hedgerow is used to include the vegetation within, above and alongside the hedge
itself including hedgerow bottom flora, hedgerow trees and any adjacent field margins (Barr and Gillespie, 2000).
When hedge management is abandoned and the overall natural shape is regained, or when the bottom 2 m (or
less) of the feature is not more or less continuous, then the feature must be described by another term, for
example, a scattered line of shrubs (Countryside Survey, 1993). Outside the UK hedgerows are frequently much
bigger and less subject to management regimes (for example, fencerows in the US and Canada and roadside
vegetation in Australia and South Africa).
It is not always clear within the literature on buffers which type of hedge is being referred to. Generally though,
UK hedgerows do not appear within the literature on buffer features and it is not clear how much of the
published information specifically on grass hedges in other climates can be directly transferred. However, a well
maintained hedgerow planted along the contour does constitute a ‘porous barrier crossing the paths of
concentrated flow channels’ which Carter (1985), Dabney et al. (1995) and Gilley et al. (2000)(Gilley J.E. et al.,
2000) suggests will have a buffering effect in slowing runoff, causing temporary ponding upslope of the barrier
and allowing time for the settling of suspended sediment. Herzog (2000) is one of the few authors to use the term
hedgerows suggesting that linear landscape elements such as this are powerful tools in soil conservation and
water quality improvement because they have the potential to control the fluxes of matter and energy in
landscapes while requiring only relatively limited surface. Ryszkowski (1992) suggested that hedgerows and
shelterbelts root deeper than annual crops and have higher evapotranspiration which enables them to function as
“ecological water pumps”. At the same time they intercept nutrients contained in lateral flows of water in the
subsoil (Herzog, 2000).
Ditches
Hedges in UK agricultural environments are often sited adjacent to a ditch. Agricultural drainage ditches are
constructed primarily to facilitate removal of excess surface and shallow ground water (Dabney et al., 2006).
They range in size from small intermittently flooded ditches draining individual fields to higher order,
permanently flooded channels with nearly riverine capacities (Bouldein et al., 2004). Moore et al. (2001)
demonstrated that when these ditches are vegetated, they may significantly attenuate the movement of nutrients
and pesticides through them. The mechanism of the contaminant processing in vegetated ditches is not clear but
is likely associated with leaves, stems and roots providing additional surfaces for deposition, adsorption,
absorption and the activity of associated micro-organisms. Properly managed vegetated ditches can function as a
special class of constructed wetlands, and both of these landscape features can function as buffers (Dabney et al.,
2006).
Trees and woodland
Trees and woodland may exist as a riparian buffer zone or as in-field trees, woodland fences or woodland edges
protected in UK agriculture as important landscape features as well as for their role in habitat maintenance and
soil erosion control. Forested or planted native trees will provide a buffering function in providing water quality
benefits as well as returning ecological function to the stream (Parkyn, 2004). In forestry systems buffer zones
generally consist of production trees left beside the stream when the surrounding area is harvested (Parkyn,
2004). Plants such as flax for weaving, fruit and nut trees or high value native tree species that can be selectively
harvested may provide ecological function and a mechanism to remove nutrients such as P from the riparian
zone. Forest vegetation in particular can shade streams and lower stream temperatures (Parkyn, 2004) and trees
provide organic matter inputs in the form of leaves and woody debris, creating a diversity of food resources and
habitats for in-stream fauna.
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Riparian buffers are often remnants of former river plain forests with willows (Salix sp.), alder (Alnus glutinosa)
and a variety of hardwood trees (Fraxinus excelsior, Ulmus sp., Acer sp., Quercus robus) (Herzog F., 2000).
McKergow et al. (2004) compared riparian rainforest buffers with grass buffers and found that the former
performed poorly due to low vegetation density and a lack of under-storey. Although sediment was deposited
during storm events the material was not permanently trapped and was resuspended during subsequent events
creating a sediment source area.
McKergrow et al. (2006) observed higher SS concentrations in a tree buffer compared with a grass buffer and
attributed this to the lack of understorey vegetation and hence surface cover. Tree riparian buffers have been
sediment source areas in other locations, with more sediment leaving the riparian buffer than entering (Smith,
1992; Jordan et al., 1993; Daniels and Gilliam, 1996; McKergrow et al., 2004).
Verchot et al. (1997) found that on North Carolina Piedmont sites, forested buffers might be either sources or
sinks of nutrients in surface runoff depending on season. Forest buffers were ineffective during the winter and
spring when water-filled pore space exceeded 25 to 35% and infiltration was low. Conversely Daniels and
Gilliam (1996) found that during the dry season forested ephemeral channels had little vegetation and were
effective sediment sinks but were ineffective during large storm events because there was little resistance to flow.
Forested buffer strips that are sufficiently dense may also improve water quality by restricting the access of
livestock to streams, thereby both reducing inputs of nutrients and bacteria associated with livestock faeces and
reducing erosion resulting from streambank trampling (Barling and Moore, 1994; Muscutt et al., 1993).
Generally however, woody buffer vegetation appears to be more important for nitrogen removal than sediment
control. For example Correll (1997) recommended that woody vegetation, especially forest, may be more
effective than grass at removing nitrate from groundwater and more effective at providing organic matter in the
deeper subsoils, where it is needed for effective denitrification in groundwater.
Wetland features
“Water diversions reduce the speed of runoff and therefore erosion by intercepting water flow across the slope
and diverting it to existing water structures or proposed structures such as retention/water basins. It is generally
applied to soils subject to severe erosion due to soil type or slope or climate in order to reduce rill and gully
formation” (Hilton et al., 2003).
Grass waterways
Grass waterways (GWW) are broad shallow channels often located within large fields, with the primary function
of draining surface runoff from farmland and preventing gullying along natural drainage ways (Atkins and Coyle,
1977). Their purpose is to reduce runoff volumes from agricultural watersheds due to their comparably high
infiltration rates and the reduction in runoff velocity that prolongs the potential infiltration time (Fiener and
Auerswald, 2003). Vegetation within the channel may act as a filter in removing some of the sediment (Hilton et
al., 2003). They are commonly used on roadside margins in sustainable urban systems where they can handle
large flow rates, but are also used in large sloping ranges in the US.
Runoff volume is reduced when adjacent fields produce runoff while the rain intensity does not exceed the
infiltration rate in the GWW itself. The amount of runoff volume reduction depends on:
a) The size of the area where runoff from the adjacent fields overflows the GWW (effective area);
b) The difference between rain volume and infiltration volume plus surface storage capacity in the GWW;
and
c) The infiltration volume after the rain caused by the runoff time lag between the inflow and outflow.
Sedimentation is mainly controlled by:
a) A decrease in transport capacity caused by reduced runoff velocity;
b) The sieving of particles by dense vegetation and litter; and
c) The infiltration of sediment-laden runoff.
There is usually a selection of fast growing grass sown in the GWW and it is mowed frequently to prevent
sward-damaging sedimentation (Fiener and Auerswald, 2003). The cross section of waterway depends on slope,
soil texture and area to be drained but should be no deeper than 0.1 to 1 m (Manyatsi, 1998). Urbonas (1994)
claims that they are effective on slopes less than 0.03 m/m. The performance of a GWW to reduce runoff volume
will depend on the length of the side-slopes, the shape of its cross-section in the area of concentrated flow and on
the sediment settling due to decreased runoff velocity and the infiltration of sediment-laden runoff. Sediment
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settling takes place primarily during sheet flow on the side slopes, where Reynolds numbers are small (<200).
Most of the settling is expected to occur in the first few metres of the grass filter (Fiener and Auerswald, 2003).
Retention ponds
Ponds have been installed in many countries in Europe; their main purpose is water supply to agriculture but
they aid hydropower, irrigation and flood control and some are constructed specifically for capturing sediment to
prevent the pollution of rivers downstream (Verstraeten and Poesen, 2000). In this case ponds reduce the
velocity of runoff and therefore help to limit downstream sediment losses (Hawkins and Scholefield, 2000) as
well as allowing some treatment of dissolved pollutants (Hilton et al., 2003). Since they require either a reliable
base-flow or a surface catchment of at least 5 ha to stop them drying out, they are generally, but not universally,
located lower down in the catchment, such as the low point of a drainage slope, ditch or surface runoff area such
as a farmyard or car park (Hilton et al., 2003). Their effectiveness may be increased by adding inlet and outlet
sumps (Hilton et al., 2003), by forming a sequence of ponds and/or by aeration of the water and application of
oxidants. However, their performance in nutrient retention is varied, especially in the case of P, and their
capacity for storage of nutrients and organic matter finite (Hawkins and Scholefield, 2000). The useful life of
ponds is therefore very short unless they are frequently dredged (Verstraeten and Poesen, 2000).
Detention basins
Like retention ponds detention basins retain storm runoff to spread out and reduce the runoff peak. However,
whilst retention ponds are designed to retain some water at all times detention ponds retain runoff for only short
periods and remain dry during dry weather. They perform this purpose following a storm in order to reduce the
flow rate, allow settlement of solids and retain/dilute the first flush of pollutants, usually draining into a
watercourse through a flow reducer. Drainage basins are located close to the runoff source and can be combined
with other practices, such as water diversions, ditch management, grass waterways, grass hedges, (roof and
farmyard runoff intersection and porous pavements), which aim to direct run-off and erosion away from water
bodies (Hilton et al., 2003).
Wetlands
Riparian wetlands and floodplains have proven to be important depositional zones for sediment and nutrients
(Barling and Moore, 1994). Constructed wetlands, like buffer zones and ponds, have been established to slow
down runoff water from agriculture, enhance infiltration and sorb P to soil and vegetation, thereby trapping
sediment and nutrients (Uusi-Kämppä et al., 2000).
Wetlands consist of permanently or semi-permanently flooded land, generally, but not necessarily, adjacent to a
river, which can slow down the rate of runoff through temporary storage and develop microbial communities in
the soil which help break down pollutants (Hilton et al., 2003). An approximate ratio of wetland to catchment
area of 0.1 to 1% is required (Hilton et al., 2003).
Uusi-Kämppä et al. (2000) observed that several processes, in addition to assimilation of nutrients, may be active
in constructed wetlands:
1. A local reduction of turbulence may be important, reducing water velocity.
2. Vegetation increases retention time under high water velocities as compared with basins without
vegetation (Jadhav and Buchberger, 1995). The P load usually increases with runoff, and a high
retention time is important for the settling of PP.
3. Vegetation mitigates resuspension of sediments in shallow waters. This is clearly seen in the
constructed wetlands as vegetation cover increases (Braskerud, 1995).
4. Leaves and stems create local deposition surfaces.
Floodplains
Floodplains may act either as a conduit or a barrier to water movement and associated sediment and solute
transport from hillslopes to the river channel (Burt, 1997). In small to medium-sized (less than 10000 km2)
basins floodplains are usually less than 1 km2 (Burt, 1997). Additionally, Walling and Owens (2003) reported
that overbank sedimentation on river floodplains can result in significant reduction of the suspended sediment
load transported by a river and can thus represent an important component of the catchment sediment budget.
The ability of a floodplain to act as a pollution buffer between farmland and the river depends fundamentally on
the hydrological properties of the floodplain sediments providing opportunity for deposition of suspended
sediment load from surface runoff, as well as processes such as denitrification (Burt, 1997).
Cooper et al. (1987) used 137Cs to map the aerial extent and thickness of sediment to determine the amount of
deposited sediment in riparian areas during the previous 20 years. Although only a thin layer of sediment had
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been deposited in the floodplain swamp, the large area available made it an important depositional zone.
Approximately 80% of the 137Cs sediment was deposited above the floodplain swamp and this was high in silt
and clay. Approximately 85-90% of the sediment removed from cultivated areas remained in the catchment. In a
similar study by Lowrance et al. (1986) the sediment deposition rate was estimated to be 35-52 Mg/ha-1/yr-1
(Barling and Moore, 1994).
Infrastructure
No reference is made within the literature to the buffering value of stone walls and fences as buffer feature.
However, some suggestions are given below on their potential buffering roles.
Stone walls
The UK Defra encourages the protection and maintenance of stone walls on agricultural land for stock
management and as landscape and historic features (Defra, 2005). Since they fit Herzog’s (2002) definition of a
linear landscape feature it does not seem unreasonable to assume that they might perform a role in preventing the
transfer of runoff and sediment. However, they might encourage the flow of water along the wall edge until the
flow meets a gate or opening. Flow through this is likely to be deeper and more concentrated perhaps causing
more damage than it might have done as a shallow dispersed flow. This will depend on the porosity of the stone
wall.
Fences
Fences may have a similar role to stone walls but may be of a more open structure. This might allow coarser
material to flow through the fence until a bank of transported soil and debris builds up against the fence. The
bank might then promote a buffering role in the reduction in velocity and volume of runoff.
Management practices
Brief definitions are provided for field management practices which aim to reduce run-off water speeds by
reducing the angle or length of slope. Whilst they perform a buffering function little detail is provided here
because the focus of the review is the establishment of vegetated features. More detailed descriptions are
provided by Hilton et al. (2003), from which the following information is adapted.
Strip cropping
Strip cropping alternates strips of row crops with closely growing crops (e.g. grass, clover), planted on the
contour (Manyatsi, 1998). Erosion is largely limited to the row-crop strips and soil removed from these is
trapped in the next strip down slope which is generally planted with a leguminous or grass strip. In Swaziland
strip cropping is encouraged where maize is grown. The grass strips are about 2-4 m wide and the cropped area
about 15-45 m wide depending on the slope. The size of strip is a function of the machinery to be used and the
slope angle. Tippett and Dodd (1995) suggested that their effectiveness was similar to vegetated field strips but
reported no efficiency data for either field borders or strip-cropping.
Contour cropping
Contour cropping involves extending seed lines or sowing crops across the slope in line with contours. This
reduces both erosion and runoff by removing natural drainage channels down slope which occur with
conventional cropping. The crops provide a perpendicular barrier to runoff due to stem density. Reducing runoff
improves infiltration and reduces soil erosion. It can be combined with contour cultivations and strip cropping.
Terracing
Contrary to the normal UK usage of this term, this management practice does not reduce the length of a slope by
creating a series of steps down the contour. In this context it means a series of regularly spaced embankments
across a slope that form a channel on the up-slope side of the embankment to retain water behind the bank
improving infiltration and sedimentation. It is normally accompanied by contour and/or strip planting and acts as
a check on any contour row failure (Tippett and Dodd, 1995). In its original US form it requires extensive
earthworks and therefore significant potential changes to existing land-use practices. However, a single plough
pass along the contour could be viable on bare land following maize. Manyatsi (1998) suggests that it is
applicable on slopes from 2-14%. It may well result in significant loss of useable area and would, therefore, tend
to be focused on areas where erosion is severe.
Contour cultivation
By focusing cultivations along contours any surface runoff and erosion will be directed across the slope rather
than down existing seedbeds/tramlines. As a consequence speed of run-off will be reduced improving infiltration
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and sedimentation. The success of this best management practice is dependent on very careful attention to field
slope, shape and soil types. It can be combined with contour cropping.
Soil berms/contour bunds
Redirect runoff or erosion materials away from watercourses thereby reducing direct pollution. It can involve
deep ploughing to a riparian edge to create a raised berm or more engineered operations such as strengthened
soil structures around a retaining pond or sediment basin or across a field slope to break it up onto segments to
prevent runoff.
Summary
Table 1 summarises the buffering functions of the various buffer types discussed. Figure 1 summarises the
processes displayed by buffers whilst Figure 2 illustrates the most likely location of each feature in a catchment.
Based on the literature reviewed the buffer types which appear to offer the greatest number of benefits consist of,
either solely or in combination, a vegetated strip. This may be a beetle bank, field margin or grass barrier, part of
a large, mixed species riparian area, floodplain or wetland or the ground cover to a forested area. In all cases the
vegetation, which is usually grass, is repeatedly reported to provide the potential to slow and spread runoff, to
increase infiltration and to encourage filtration, deposition and retention of sediment and associated nutrients.
Aptly, vegetated strips are the most common buffering features in UK agriculture and the most dominant
buffering feature of the most recent UK agri-environment schemes.
Table 1 Buffer function
Grass buffers
Hedgerows
Trees
Wetland features
Built
Management practice
The strategic placement and design of buffers for trapping sediment and phosphorus
8
Contour cultivation
Terracing
Contour cropping
Strip cropping
Stone walls
Fences
Grassed waterways
Floodplains
Detention basins
Retention ponds
* * *
** ** ** ** * * * * * * * *
* * *
** ** ** ** * * * * * * * *
*
** **
* * * * *
*
*
* *
In-channel wetlands
**
**
**
*
In-field trees
**
**
*
*
Woodland barriers
Hedgerows
*
**
**
**
Ditches
Grass hedges
Promote adsorption and absorption of
nutrients
Vegetated filter strips
Increase infiltration capacity
Riparian buffer zones
Buffer function
Spread out and reduce velocity and
turbulence of runoff
Encourage filtration, deposition and retention
of sediment and nutrients
Soil berms/contour bunds
Class
Buffer type
Defra Project PE0205
Slow incoming surface runoff
Reduce risk of gully erosion
Spread incoming surface runoff
Increase surface roughness
Reduce turbulence of incoming surface runoff
Increase infiltration
Dilute runoff by rainfall
Reduce flow volume
Reduce transport capacity
Increase infiltration of
soluble pollutants into
soil
Encourage sediment deposition
Deposition in ponded area
Ponding of surface runoff
Adsorption to plant and soil surface
Absorption of solutes by vegetation
Filter out sediment
Reduce soil loss
Reduce transport of sediment associated nutrients
Figure 1 Buffer processes
Close to source
Within field
Bottom of field
Adjacent to watercourse
DETENTION
BASIN
GRASS
WATERWAY
MANAGEMENT
PRACTICE
GRASS
BUFFER
IN-FIELD
TREES
GRASS
BUFFER
RIPARIAN
BUFFER:
GRASS AND
TREES
RETENTION
BASIN
FLOODPLAIN
WETLAND
Figure 2 Buffer types and likely locations
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Defra Project PE0205
Subsurface buffering function
The effectiveness of a buffer feature in preventing phosphorus and sediment transfer out of a field system is
dependent on both above and below ground vegetative components. Subsurface components such as root density,
root architecture, soil properties and soil structure are often overlooked in favour of the more visible surface
features. This section reviews these subsurface components and their role in the buffering systems.
The impact of roots on soil erosion
The role of roots in reducing soil erosion is often overlooked (De Baets et al., 2006). Most studies that consider
the role of vegetation in reducing soil loss have focused on the above-ground biomass only. However, roots can
play an important role in both preventing soil erosion by binding soil particles and reducing the erosive force of
surface water through increased surface roughness. It can also be assumed that roots are more effective in
reducing concentrated flow erosion rates than surface vegetation, as the decrease in rate of soil detachment with
increasing root area ratio has been shown to be greater than that observed with increasing plant cover (De Baets
et al., 2006).
Shallow interlocking root networks not only reduce erosion because of increased surface roughness but also act
to anchor the soil. Root systems stabilise the surrounding soil in two ways. Firstly, roots physically bind the soil
particles forming a mechanical barrier to soil and water movement (Kutschera-Mitter, 1991; Morgan and
Rickson, 1995; Gyssels and Poesen, 2003): plant roots effectively increase the shear strength of a soil making it
less vulnerable to erosion (De Baets et al., 2006). Tengbeh (1993) found that for two contrasting soils in which
Loretta grass (Lolium perenne) was grown soils had greater shear strength at moisture contents ranging between
saturated and the plastic limit than the soil with no root system, and that shear strength also increased with
increasing density: ranging from 1.0 kPa in a sample with a root density of 0.20 g cm-3 to 5 kPa in samples with
a root density of 1.80 g cm-3, for a sandy clay loam. Tengbeh (1993) also found the amount by which shear
strength was increased by the presence of a root system was dependent on soil texture. In a sandy clay loam
shear strength increased by 500% in comparison to root free soil while a clay soil showed an 850% (root density
1.80 g cm-3) increase over root free soil. In general the difference in shear strength was 1.7 times greater in the
clay than in the sandy clay loam soil. The second method by which roots stabilise the soil is by excreting binding
agents and attracting other micro-organisms that in turn produce other organic bindings (Gyssels and Poesen,
2003). Tengbeh (1989) also found that in some soil textures (sandy soils) but not all, grass roots increased the
angle of internal friction which would also increased soil strength. The stabilisation of the soil by the root system
reduces the susceptibility of the soil to rill and gully erosion by reducing scouring and sediment transport (De
Baets et al., 2006; Gyssels and Poesen, 2003). Grasses are particularly good for erosion control because they
germinate quickly, provide a complete ground cover and a dense root network, in particular in shallow surface
soil that reinforces the soil by adding extra cohesion (Brindle, 2003; De Baets et al., 2006).
The presence of roots near the soil surface increases the surface roughness. An increase in surface roughness
reduces overland flow velocity, which in turn reduces both the erosive force and sediment transport capacity of
the moving water. A review by (Gyssells et al., 2005) has suggested that water erosion rates decreases
exponentially with increasing root mass in a similar way as would be expected with changes in surface
vegetation cover.
Presently it is impossible to say which plant element has the greatest impact in reducing soil loss, only that it is a
combination of effects of roots and canopy cover. It is possible that the relative importance of different parts of
the plant changes through time. In the early plant stage plant roots are crucial in preventing soil loss because the
above ground cover is fairly limited. As the plant matures the relative importance of roots over surface cover
becomes less apparent and requires further investigation.
Plant uptake of P
Phosphorus may be quickly taken up by the clay fraction of soil but this is a finite process dependent on cation
exchange capacity of the local soil. However, this is not the only P sink within the buffer system, P will also be
removed by the buffer vegetation. The efficiency of a plant to take up P from the surrounding soil depends on the
capability of the root system to absorb P, the amount of actively absorbing roots per unit of shoot, age of root
segment, size of root system, root architecture and season (Föhse et al., 1988) adopted by plants to increase
efficiency of uptake differ among species, for example ryegrass (Lolium perenne) produce large root systems
while other species have higher uptake per unit of root (Raghothama, 1999; (Föhse et al., 1988). Phosphorus is
directly available to plants only in a dissolved form but this can represent only a minor component of the total
quantity of P in the soil (Walk et al., 2006). Soil bound P can be solubalised by plant secretion of organic acids
The strategic placement and design of buffers for trapping sediment and phosphorus
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Defra Project PE0205
or by bacteria. Phosphorus uptake is then limited by soil moisture, root surface area, root hair density and length,
contact of root or mycorrhizal surfaces with the soil solution, and kinetics of uptake across the root surfaces
(Walk et al., 2006). Root architecture plays an important role in plant uptake of P because of the chemicals low
mobility in soil which leads to greater accumulation in the soils surface than subsurface (Sánchez and Boll, 2005;
Lynch and Brown, 2001). Computer simulations and observations of root architecture suggests that root systems
with enhanced topsoil foraging acquire P more effectively especially in a stratified soil with more P in the top
soil (Lynch and Brown, 2001; Ge et al., 2000).
Root architecture determines the exploitation and exploration of localized P resources. An optimal root
architecture is one that enhances P acquisition at minimum carbon cost or resources required for root growth
(Lynch and Brown, 2001), but facilitates acquisition of water, water-soluble nutrients and physical support for
the plant (Fitter, 1991). Different species use different strategies for example comparison of three grass spices
showed them to have very different strategies on exploiting vacant soil patches of varying concentrations.
Meadow-grass (Poa angustifolia) had a high growth rate into the vacant space with minimal change to root
morphological properties irrespective of the concentration of available nutrients, Field Woodrush (Luzula
campestris) entered the vacant space slowly and showed little growth response to increase in nutrients, however,
they appeared to increase nutrient uptake per root length (Caldwell, 1994 ), and Ribwort plantain (Plantago
lanceolate) responded to enriched soil patches by producing more densely branched root systems suggesting a
targeting of local resources (Šmilauerová and Šmilauer, 2002).
A plants’ root system depletes P from the rhizosphere (Tinker and Nye (2000)). Ryegrass has been shown to
reduce total P by 1.7-12.6% (Chen et al., 2003). However, plants can also return P to the soil through leaf fall
and decomposition. With no other mechanism for removal except erosion, in time the buffer zone could
potentially become saturated and unable to trap P (Barling and Moore, 1994). To overcome this problem either
the soil must be removed or the vegetation harvested/grazed. Experiments in which grass was continuously
cropped and no additional P added caused a rapid decrease in the P concentration in soil solution (Koopmans et
al., 2004). Methods to remove P could include selective harvesting for wood or fruits, light grazing of grass with
sheep and harvesting of grasses for haymaking (Parkyn, 2004). However, if the above ground vegetation is to be
managed to maintain its capacity to take up P from the soil, care must be taken to avoid grazing or mowing
reducing the effective cover below that at which erosion starts to increase.
Soil hydrology and root interaction
The effectiveness of a buffer feature in reducing soluble chemical transfer and small sized sediment transfer is
often linked to the infiltration capacity of the soil below the buffer feature and not deposition (Muscutt et al.,
1993; Gharabaghi et al., 2002; Parkyn, 2004; Abu-Zreig et al., 2003). Infiltration capacity is a function of water
flux and a soils hydraulic conductivity, which is dependent on the size and abundance of connected pore space
through the soil. It has been shown that the width of a grass buffer strip influences the infiltration: a longer strip
tending to increase infiltration (Schmitt et al., 1999a; Seobi et al., 2005) primarily because of increased surface
roughness reducing overland flow velocity leading to greater infiltration. The porosity of a buffer feature is a
dynamic characteristic of the soil system being created by natural packing of the primary soil particles, frost
heavage, shrinkage and the burrowing activity of soil forna and flora.
Infiltration rate is important because it determines the proportion of overland flow and potential for chemical
interaction between solution and soil particles. Delayed infiltration can result in increase P transport in runoff.
While preferential flow along cracks and larger macro-pores allows rapid transport of P and sediment bound P
from the soil surface to depth, either to the water table or to a connected watercourse, with reduced opportunity
for P adsorption (Simard et al., 2000, Shipitalo et al., 2004). Even if macro-pores make up only a small
proportion of the total pore space they can exert considerable control on vertical flow rates. Although clay soils
may be associated with higher absorption rates of P to clay particles, finer textured soils tend to have a greater
extent of preferential pathway than course textured soils because of the increased likelihood of cracking (Simard
et al., 2000). Phosphorus adsorption to clay surfaces is limited by the cation exchange capacity of the soil
particles and once a pathway becomes saturated more P will remain in solution. The effectiveness of a buffer
feature in reducing soluble P and fine sediment transfer therefore partly depends on the pore size distribution of
the soil system. The longer the residence time in the soil the greater the potential for sediment to settle out,
soluble P to be absorbed onto soil particles and plants to absorb nutrients. Therefore, the greater the proportion
of flow occurring through smaller meso- and micro-pore networks the greater the potential for reducing the
transfer of P from the field system.
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Plant roots play an important part in the dynamics of soil hydrology. There is evidence to suggest that a
perennial root system has a limited life span of a year or less (Langer, 1979). Thus to sustain productivity there is
a continuous cycle of new formation, aging, death and decay. Plant roots can have opposing effects on soil
hydraulic conductivity. At one extreme root systems and old decaying roots can act as macro-pore conduits
increasing infiltration capacity (Muscutt et al., 1993; Collier et al., 1995; Archer et al., 2002; Seobi et al., 2005).
At the other extreme dense fibrous mats of roots at shallow soil depths can reduce hydraulic conductivity
(Morgan et al., 1995; Archer et al., 2002). A grass buffer strip may consist of many different plant species that
have different affects on the hydraulic conductivity, for example red clover (Trifolium practense) have stout
taproots that increase hydraulic conductivity, whilst other grasses such as Agrostis tenuis and Lolium perenne
have been found to reduce hydraulic conductivity because of a dense network of fine roots near the soil surface
(Morgan et al., 1995; Archer et al., 2002). Shrubs and trees that tend to have coarser roots have a slower
turnover rate and grow to deeper soil depths. When these roots die back they leave large channels through which
water can infiltrate rapidly to depth in the soil. However, it is generally agreed that in most cases buffer strips
have a greater infiltration capacity than an adjacent field under crop because of a more stable and continuous
network of pores (Seobi et al., 2005; Muscutt et al., 1993). Initially the infiltration capacity of a newly
established buffer strip can be low, there are two main reasons given for this, firstly the network of pore spaces
takes time to develop, possibly several years (Van Der Kamp et al., 2003). Secondly, during the initial
establishment phase roots preferentially move into larger pore spaces which reduces the hydraulic conductivity
but after time roots begin to die back and larger macro- and meso-pores increase in number thus leading to an
increase in hydraulic conductivity (Pommier, 1996; Quinton, 1996; Archer et al., 2002).
Summary
Subsurface removal of P and fine sediments depends on the infiltration capacity of the soil, pore size distribution,
the storage capacity of the soil, soil moisture content, plant root architecture and the ability of the vegetation to
absorb available P from the rhizosphere.
The subsurface component of a buffer strip can potentially play an equally important roll in the prevention of P
and sediment transfer as the surface component. However, it is less easy to examine and therefore its exact
influence remains poorly understood. In particular few studies have examined the effects of a natural vegetation
root system on erosion or the impact of grass roots on concentrated flow erosion rates.
Implementing buffer features in the UK
Established in the UK in 1986 Agri-environment schemes provide the government’s main mechanism for
compensating farmers for income lost when establishing or improving environmentally beneficial practices on
farmland. Table 2 provides a summary of Agri-environment schemes and their aims. The current protocol for the
implementation of buffer features is described by the Defra’s Environmental Stewardship which is the latest UK
Agri-environment scheme, launched in 2005.
The primary objectives of the scheme are to
• Conserve wildlife (biodiversity
• Maintain and enhance landscape quality and character
• Protect the historic environment and natural resources
• Promote public access and understanding of the countryside
• Natural resource protection
Within these objectives it has the secondary objectives of
• Genetic conservation
• Flood management
Entry Level Stewardship (ELS) is open to all farmers and landowners and promotes simple and effective land
management. Higher Level Stewardship is another element of the scheme which encourages targeted
environmental management. All of the options available can be accessed in the scheme handbooks (Defra, 2005)
but those options likely to perform a buffering role are listed in Table 2.
The strategic placement and design of buffers for trapping sediment and phosphorus
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Defra Project PE0205
Table 2 Summary of Agri-environment schemes and their aims
Scheme
Date
implemented
Aims
Environmentally
Sensitive Areas (ESA)
1986
To protect threatened landscape and ecology and create
improvements in public areas
Nitrate Sensitive Areas
(NSA)
1989
To improve water quality on waters under threat of nitrate pollution
by reducing fertilizer applications
Countryside stewardship
1991
To include conservation as part of farming practice and land
management for the protection of targeted landscapes, wildlife
habitats and historic features. To improve opportunities for public
access
Habitat
1994
To create, protect and enhance wildlife habitats through
environmentally beneficial land management
Countryside incentives
1994
To increase the benefits from land entered in the Arable Areas
Payment Scheme by providing incentives for farmers to improve
public access
Organic Aid
1994
To assist farmers who wish to convert to organic production
Moorland
1995
To protect and improve moorland by encouraging extensive grazing
practices
Entry Level Stewardship
2005
To continue
management
or
introduce
beneficial
environmental
land
Implementing buffer features elsewhere
In the USA ‘vegetated filter strips’ are an approved BMP. Installation is now part-funded by the US Department
for Agriculture under the Conservation Reserve Program and in certain cases blanket installation along all
perennial streams has been undertaken (Dillaha et al., 1989). In New Zealand a policy of ‘retirement’ of riparian
zones (i.e. removal from active use) has been implemented along all perennial streams to protect riparian and
aquatic habitats.
Buffer design, placement, management and performance
Buffer design
“Riparian buffers are the most important control of non-point source pollution but you can’t just establish a
riparian strip and call it a buffer! A good job has been done in selling buffers but a poor job has been done of
educating about how they work” (Gilliam, 2004 – conf.).
Height
Prosser and Karssies (2001) proposed that the height of the vegetation in a buffer is important. Whilst heights of
10 to 15 cm have been recommended (Dillaha et al., 1986) longer grass should not pose a problem as long as the
stem density near to ground level is still high and does not suffer from lack of light (Prosser and Karssies, 2001).
However, experiments by Hook (2003) do not support the use of height as a predictor of buffer capacity and
Pearce et al. (1997) reported that, as long as submergence does not occur, vegetation height is not a significant
variable in VFS performance.
Pearce et al. (1998 a, b) applied simulated rainfall, run-on and sediment to 10 and 2 m long plots that were
clipped to the ground surface, clipped to 10 cm or left unclipped. Stubble height generally did not affect runoff
(Frasier et al., 1998) or sediment retention except over distances less than 2 m (Pearce et al., 1998a, b). In deeper
The strategic placement and design of buffers for trapping sediment and phosphorus
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Defra Project PE0205
flows representative of concentrated runoff or overbank flooding short stubble enhanced sediment deposition
most but taller vegetation favoured subsequent retention (Abt et al., 1994; Clary et al., 1996).
The limited research available on sediment retention in rangeland riparian areas shows that where herbaceous
vegetation is dense commonly recommended stubble height criteria in the 5 to 15 cm range (Clary, 1995; Clary
and Leininger, 2000) will probably maintain vegetation capable of trapping sediment in either shallow overland
runoff (Pierce et al., 1998 a, b; Hook, 2003) or overbank flow (Abt et al., 1994; Clary et al., 1996).
Height may be a factor in sediment trapping and remobilisation. 10 to 15 cm most commonly recommended.
Density
A number of studies have found an increase in buffer effectiveness with an increase in vegetation density.
Results from Hook (2003) indicated that the dense vegetation of moist and wet riparian sites generally retained
sediment effectively, whereas lower sediment retention was associated with sparse vegetation. The difference in
sediment retention was attributed to gross differences between flow paths.
Pearce et al., (1998a) found the plant density, cover, growth form and bare soil area influenced sediment
retention. In both simulated and real filter strip vegetation sediment retention has been found to increase with
stem density (Tollner et al., 1976; Munoz-Carpena et al., 1999; Ghadiri et al., 2000). Abu-Zreig et al., (2004)
observed a steady increase in sediment trapping efficiency with an increase in percentage cover. Upland range
studies show that erosion and sediment transport are sensitive to surface cover, especially when very sparse
(Hook, 2003). In a study of grassed filter strip performance in south-central Montana cropland, presence of
vegetation was more important than the species planted (Fasching and Bauder, 2001).
Agreement that buffer efficiency will increase with vegetation density. No quantification of this or guidelines.
Several observations suggest that practical field indicators of a site’s potential for sediment retention should
emphasize major differences in vegetation that can be evaluated visually rather than accurate, quantitative
measurements (Hook, 2003).
Species
Wilson (1967) suggested that the requirements for a suitable filter grass species should be (a) a deep root system,
(b) a high stalk density, (c) insensitivity to submergence and droughts and (d) ability to grow through sediment
coverage (van Dijk et al., 1996).
Taddesse and Morgan 1996 found that a dense uniform grass strip can effectively reduce erosion on an erodible
sandy loam soil on slopes up to 13° whereas a barrier formed from a more open grass with a less dense rooting
system is effective only up to 9°. They recommend a perennial species with an erect, dense and uniform habit
rather than tussocky growth, a wide range of stem and leaf angles with respect to ground surface providing a
barrier to flow and a system of roots to bind the soil and reinforce its strength. The species of grass tested was
Festuca ovina which Hayes et al. (1978) recommends for forming a dense rigid and permanent barrier.
There is general agreement with the properties proposed by Wilson (1967) although very few studies provide
recommendations on vegetation properties or species. This may reflect the difficulty in isolating the influence of
vegetative factors on buffer performance. Abu-Zreig et al. (2004) found that experimental evaluation of the
influence of vegetation type on filter performance was very difficult because of the difficulty in constructing
identical filters varying only in vegetation type.
Age
Vuurmans and Gelok (1993) identified the age of the grass as an important factor because it determines its
stiffness. Young grass is more flexible and bends more easily than older grass. Old grass is slightly more
effective in reducing erosion and more effective in retarding the water flow. As well as greater stem stiffness this
has been attributed to higher grass density and lower frequency of mowing activities (Van Dijk et al., 1995).
Width
The meaning of buffer width in this document refers to the distance between front edge of buffer at the field
edge to the back of buffer in the direction parallel to flow. Trapping efficiency generally is improved when the
width of the buffer is increased (Magette et al., 1989; Chaubey et al., 1994; Barfield et al., 1998; Borin et al.,
2005) with longer buffers increasing the percentage of mass reduction of both nutrients and sediment (Lee et al.,
2000). When tested against other factors filter width has been stated as having the greatest influence on the
The strategic placement and design of buffers for trapping sediment and phosphorus
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Defra Project PE0205
sediment trapping efficiency of vegetative filter strips (Abu-Zreig, 2001, 2003, 2004; Hook, 2003). For a given
soil type filter width also has a great effect on the amount of infiltration and consequently on the volume of
water outflow and peak flow rate (Abu-Zreig, 2001).
Phillips (1989) employed mathematical models to estimate the relative importance of soil hydrologic properties,
topography and surface roughness in determining the effectiveness of water quality buffers in North Carolina,
USA. He found that although slope was the most critical factor for effective removal of sediment or particulate
pollutants transported in surface runoff, buffer width was by far the most important factor for effective removal
of dissolved pollutants in surface or subsurface flow.
Many examples have been cited in the literature. Lalonde (1998) found that trapping efficiency varied between
68 to 98% as filter widths increased from 2 to 10 m (Abu-Zreig, 2001). Abu Zreig (2001) observed that the
average trapping efficiency for a 15 m filter was three times higher than a filter 1 m long (95 and 30%).
Dillaha et al., (1989) tested six filter strips with widths of 0, 4.6 and 9.1 m. Sediment trapping efficiency (STE)
varied from 53 to 86% on the 4.6 m strips and from 70 to 98% on the 9.1 m filter strips. Similar results were
obtained by Maggette et al., (1989) who tested filter strips vegetated with Kentucky-31 fescue established on a
silty loam soil. Removal efficiencies of 66% and 82% were obtained for 4.6 and 9.2 m strips respectively. This is
in agreement with Coyne et al. (1995) who reported 99% of sediment removal efficiency in two 9 m long filter
strips vegetated with tall fescue and Kentucky bluegrass at an even higher slope of 9%.
According to Hook (2003), averaged across other factors, decreasing buffer width from 6 to 1 m reduced average
sediment retention from 99 to 83 % which corresponded to 13 times more sediment in runoff. In contrast,
moving from wetlands to uplands or increasing slope from 2 to 20 % each reduced average sediment retention
from 96 to 91 % which corresponded to just 2.5 times more sediment.
A number of studies, however, do not show such an obvious relationship. Neibling and Alberts (1979) found
very little difference between the amount of sediment retained in strips of width 0.6 and 4.9 m and most of the
sediment was deposited at the upslope side of the strip width (Van Dijk et al., 1995). Line (1991) observed no
significant difference in trapping efficiency between 3 m and 6.1 m long VFS of a mixture of ryegrass and
fescue.
Robinson et al. (1996) and Mickelson and Baker (1993) even reported lower trapping efficiency for longer strips
and higher trapping efficiency for shorter strips. They found that more than 70% of the sediment load was
removed by strips of 4.6 m long or less whereas sediment removal in 9.1 m long strips was less than 85%.
Daniels and Gilliam (1993) also obtained lower trapping efficiencies of 45 to 58% for the 3 m long vegetated
filter strip and of 57 to 62% for the 6 m long vegetated filter strip, under natural rainfall conditions.
Pollutant type appears to make a difference to the buffer width required. For example for trapping very fine
particles a longer filter strip may be required (Jin and Romkens, 2001). Wilson’s (1967) results indicate that
smaller particles require longer filters for maximum deposition than do larger particles. Neibling and Alberts
(1979) observed that sod strips as short as 0.61 m reduced sediment discharge rates by 90% but sediment
discharge of particles less than 0.002 m through the same width strip was reduced by just 37%. Gharabaghi et al.,
2002 suggest that much of the larger particles of sediment may be removed in 5 m of grass buffer, but finer
particles may require 10 m. Abu-Zreig (2001) advocates a poorly vegetated 3 m long filter as sufficient for silty
sediments whilst, for clay type sediments, the width of the filter should be no more than 15 m.
Abu-Zreig et al., (2001) states that short filters (2 and 5 m) which are somewhat effective in sediment removal
are much less effective in P removal. Increasing the filter width beyond 15 m is ineffective in enhancing
sediment removal but is expected to further enhance P removal.
Factors other than particle size are likely to influence the relationship between buffer width and trapping
efficiency. The optimum width of a buffer strip has been associated with the number of successive rainfall events,
whether the flow meeting the buffer strip is uniform or concentrated, stream order, soil, landscape, geology and
the type and load of pollutant. Pearsons et al., (1994, 1995) noted that increasing the width of a herbaceous filter
from 4.3 to 8.6 m reduced runoff and sediments passing through the buffer but a longer buffer was less effective
during very intense rainfall, probably due to increased runoff speed. Manning’s roughness coefficient has a
moderate influence on filter performance compared with width and slope. The increase in TE with n is much
higher at shorter filter widths compared with longer filter widths (Abu-Zreig, 2001).
The strategic placement and design of buffers for trapping sediment and phosphorus
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Defra Project PE0205
Dabney et al. (2006) suggest that it is wrong to assume that narrow buffers do not improve water quality; rather,
the presence of a continuous buffer edge is critical because the first increment of buffer has a much larger impact
than any subsequent increment. It is generally agreed that the majority of sediment retention takes place in the
first 1 to 3 m of filter strips (Tollner et al., 1976; Daniels and Gilliam, 1996; Robinson et al., 1996; Pearce et al.,
1998b, Jin and Romkens, 2001). Dillaha et al. (1989) and Simmons et al. (1992) have demonstrated an
acceptable effect (50 to 80%) with 3 to 5 m wide buffers.
Based on study results, Hook (2003) suggests 6 m as a starting point for designing rangeland buffers for
sediment retention, consistent with cropland filter strip guidelines (USDA Natural Resource Conservation
Service, 2000).) Under the conditions tested, even the least favourable sites studied by Hook (2003) (10 to 20%
slopes and 90 to 265g/m2 biomass) achieved 95 % sediment retention at a 6 m width, suggesting that this width
may be effective across diverse rangeland riparian sites.
Whilst some authors suggest 10 m as the minimum width for satisfactory pollution control (for example
Haycock and Pinay, 1993; Castelle et al., 1994), Neibling and Alberts, 1979, Mickelson and Baker, 1993 and
Robinson et al., 1996 (Abu-Zreig M. et al., 2004)observed no appreciable difference between the sediment
trapping efficiency of 10 m and 15 m long filters and Schmitt et al. (1999b) found that incremental
improvements in sediment retention are relatively small beyond 7.5 m widths.
Even where recommendations exist there appear to be exceptions suggested. Buffers of a given width are less
effective for the retention of relatively mobile contaminants e.g. suspended clay, bacteria, dissolved nutrients
than for coarse sediment (Schmitt et al., 1999b). Although increasing the filter width beyond 15 m is ineffective
in enhancing sediment removal it is expected to further enhance P removal (Abu-Zreig et al., 2003). Widths
narrower than 6 m may be adequate on level or gently sloping areas with dense vegetation, such as moist
floodplain sites (Hook, 2003).
Figure 3 illustrates the complexity of deriving appropriate buffer widths based on previous studies. Based on the
graph a 10 m buffer strip could provide anything in the range of 0 to 100% pollutant removal. The graph does
not, however, distinguish between the natural variability in results and that introduced by differences in
experimental methods, scale and site characteristics.
Parkyn (2004) observed that the width required to optimise nutrient removal has been debated with little
systematic study of the issue and that one problem in assessing minimum widths is that many studies have had to
use existing buffer widths, rather than deriving it experimentally. It is difficult to pick out an optimum width
from the literature. In any case Hickey and Doran (2004) observed that in many cases the width of buffer strips
are set based on what is politically acceptable or what landowners can reasonably be expected to “give up”.
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Figure 3 Buffer zone pollutant removal efficiency variation with width (Ducros 1999)
Flow rate
“The ratio of the buffer area to the upslope source area captures one source of variability in buffer loading but
does not capture the large variations caused by differences among individual storm events or due to flow
concentration prior to runoff entering the buffer. The specific flow rate (i.e. the volume rate of flow per unit
width of buffer perpendicular to the direction of flow) is a more fundamental way to describe flow rate though a
buffer. Buffer hydrologic contact time with surface runoff is determined by specific flow rate, buffer width in the
direction of flow and buffer hydrologic roughness” (Dabney, 2003).
Shape
Riparian forest buffers are designed with three zones. Zone 1, a 5 m forested strip adjacent to the waterbody,
receives little disturbance. Zone 2 is a managed forest area beyond Zone 1. Zone 3 of a riparian forest buffer is a
filter strip; it is optional and used mainly adjacent to agricultural land to trap sediment (USDA NRCS, 1999;
Dabney et al., 2006).
Buffer placement
General
Once the perfect buffer design has been recognized it must be established in the landscape. It has already been
implied from Figure 2 Buffer types and likely locations that different buffer types will have an optimum location
in the landscape and this may not always be adjacent to a watercourse. The following section aims to derive from
the literature whether there is an ideal place to locate buffer features and if so what determines the ‘right’ and
‘wrong’ place. At the catchment scale questions asked of the literature will include ‘how high in the catchment
should features be sited?’, ‘should target areas be selected based on sediment yield, on connectivity to a
watercourse or on the value of the watercourse to be protected?’, ‘if to be placed adjacent to streams then which
size of streams should be protected and should the entire stream be protected or just specific sections?’; at both
the catchment and the field scale is there a maximum slope angle or an ideal soil type for effective buffer
performance or are there more important factors than these?
Is placement important?
“There is a requirement to understand more about catchment level hydrology so that buffers aren’t established in
the wrong place” (Wigington, 2004 – conf.). Verstraeten et al. (2002) suggested that many soil conservation and
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sediment control techniques are known and widely studied and yet their impact at a catchment scale generally
remains unclear. They warn that this information is crucial if one needs to evaluate the effect of a technique, for
example a buffer strip, on the reduction in sediment delivery to a river. Soil erosion and sediment delivery
processes vary in space with different kinds of processes operating at various locations (Verstraeten et al., 2002).
Therefore, the implementation of a conservation measure will be most effective when applied to those locations
where it is most suited (Verstraeten et al., 2002).
“Daniel and Gilliam (1996) noticed that the position of conservation buffers in landscapes significantly affects
their effectiveness in dispersing runoff and removing sediment, nutrients and chemicals from runoff” (Qiu, 2003).
Vennix and Northcot (2004, in (Tim U.S. et al., 1995)) suggested that the effectiveness of a buffer strip is a
complex function of the characteristics of the delivery area as well as the streamside area where the buffer is to
be installed. Similarly Vandervalk and Jolly (1992) observed that the water quality impact of vegetated buffer
strips depends on their location and interaction with other watershed elements as well as on their physical
characteristics. When the vegetated buffer strips were placed along certain segments of a perennial stream in a
watershed in southern Iowa Tim et al. (1995) found that sediment yields were disproportionately reduced and
advised that in order to obtain the maximum benefits of vegetated buffers researchers and resource managers
must understand the impact of the physical characteristics of vegetated buffers (e.g. width, placement or location
along the major stream) on watershed quality.
In the UK Blackwell et al. (1999) maintain that riparian buffer zones are often bypassed where natural
hydrological flows are intercepted by ditches or drains which are common features in the UK landscape. In such
cases a riparian buffer zone can be rendered ineffective and it may prove more effective and cost efficient to
establish buffer zones in association with ditches or areas of discharge (Blackwell and Maltby, in press). Similar
conclusions have been drawn by Haycock and Muscutt (1995) who reported that while 85% of some subcatchments of the River Avon in Hampshire in the UK were served by effective riparian buffer strips, 60% of
polluting material in the river was delivered by roads and drains that effectively bypassed them. Observing the
same pattern in Ontario et al. (2004) suggested that buffers zones may be most effective in preventing the
deterioration of water quality in areas where the natural drainage patterns are intact.
Type of buffer / field position
At the field scale Dabney et al. (2006) categorise a number of buffer features according to the position that they
might best perform and describe the benefits of each location.
a) In-field buffers: grassed waterway, contour buffer strip, alley cropping, vegetative barrier.
b) Edge-of-field buffers: field border, filter strip, riparian forest buffer, vegetative barrier.
c) After-field buffers: constructed wetlands, vegetated ditches, channel bank vegetation.
In-field buffers are recommended close to the source because they offer the best opportunities to encounter sheet
flows and therefore can most effectively reduce runoff and control erosion and pollutant transport. These buffers
are complementary to edge-of-field and after-field buffers. On flat lands, buffers that impede drainage within
agricultural fields are impractical so edge-of-field buffers are advocated. After runoff has left the field and
passed through edge-of-field buffers, water quality can still be improved by after-field buffers.
Critical areas
Identifying critical areas provides a means of directly linking buffers with problem areas rather than promoting
widespread implementation of buffer strips throughout a catchment. Few attempts have been made to establish
criteria for selecting critical areas for implementing best management practices such as buffers (Maas et al.,
1985). Maas et al. (1985) describe two distinct critical-area perspectives: the land resource perspective and the
water resource perspective. Critical areas, from the land resource perspective, are those lands on which soil loss
exceeds the soil loss tolerance which is the rate at which soil can be replaced by natural processes. Although
areas of severe soil loss often are the most critical areas for treatment of agricultural nonpoint-source pollution,
this need not be the case. From the water resource perspective, critical areas are those areas where the greatest
improvements to an impaired water resource can be obtained for the least investment in best management
practices.
Determining critical areas
Determination of critical areas requires consideration of such factors as the type of water quality impairment, the
dimensions and dynamics of the impaired water resource, the hydrology of the watershed, the magnitude of
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pollution source areas, and the investment in BMPs. Implied in this approach is the concept of treatability of the
resource (Maas et al., 1985). Out of a number of projects investigated by Maars et al. (1985) erosion rate,
distance to nearest watercourse, on-site evaluation and present conservation status were the most commonly used
factors for critical area selection. Other factors included type of water resource impairment, manure sources,
fertilizer rates and timing, pathogen source magnitude, distance to nearest watercourse, planning timeframe and
designated high priority sub-basin.
Variable Source Area (VSA) method
Qui (2003) proposed a strategy for placing conservation buffers in agricultural landscapes that incorporates a
concept of variable source area. “According to Hewlett (1982) a VSA is an area which varies from less than 1%
of a watershed’s area during small storms to more than 50% during large storms, depending on spatial variability
in the watershed (Qui, 2003)”. “A VSA-based approach for placing conservation buffers is a process of
incorporating the VSA hydrology and strategically placing conservation buffers in agricultural landscapes based
on site-specific natural resource conditions. This approach distinguishes landscapes in terms of their roles in
runoff generation and proposes to place conservation buffers within VSAs” (Tim et al., 1995). “Within-VSA
conservation buffers generate less surface runoff and more subsurface and return flows than edge-of-field buffers.
In addition, the within-VSA buffers better intercept the flow path, prevent the generation of concentrated flow
and increase retention time for runoff. The within-VSA buffers outperform edge-of-field buffers for almost every
water quality parameter” (Tim U.S. et al., 1995). “The results show that placing conservation buffers within
VSAs is more effective and cost-effective than placing them along the edge of the field, which is a typical
practice when conservation buffers are applied at the field scale” (Tim U.S. et al., 1995).
Tim et al., 1995 tested various segments along a stream in a watershed in southern Iowa. They a large difference
in sediment yield reduction by simulated buffer strips with land use and land-management practise. The greatest
reduction in sediment yield was predicted for those segments where farmers and land operators had yet to
implement the required conservation practices that reduce soil erosion and enhance water quality. Advantage of
being applicable for implementing conservation buffers from field to regional scales.
Ratio of buffer to contributing area
(Hayes J.C. and Dillaha T.A., ) suggested that for maximum effectiveness of vegetated buffer strips the ratio of
the buffers strip area to the area of runoff contributing area should be 1:50 or less (Tim et al., 1995).
Soil type
Soil drainage properties influence buffer performance with free draining soils minimising the generation of
surface runoff, both on the hillside and within a buffer (Parkyn, 2004). Runoff is generated soonest, and seepage
forces that encourage erosion are greatest, on the shallowest soils in a watershed. These are therefore the zones
that need the greatest protection and are also amongst the least productive areas from an agricultural point of
view (Dabney et al., 2006). Shallow soils ensure that ground water plumes have hydrological contact with (and
do not pass beneath) the riparian root zone, increasing potential for buffering nutrient fluxes through
denitrification and for nutrient uptake (Tabacchi et al., 1998).
For maximum removal efficiency the buffer soil should have a high porosity to enable infiltration of a large
amount of runoff (Lee et al., 2000). Infiltration is important because the finer particles and soluble nutrients
enter the soil profile along with infiltrating water and because it decreases runoff thus reducing sediment
transport capacity of the runoff (Phillips, 1989; Dillaha and Inamdar, 1997).
Within the literature soil type is discussed more in the context of N pollution control than sediment control.
Those factors important in N control may conflict with those beneficial to the mitigation of pollution by
sediment and other associated nutrients. For example Verchot et al. (1997) observed that infiltration was the key
factor controlling N pollutant removal from surface runoff and suggested that buffers in the clayey soils of the
Piedmont may, therefore, not be as effective as sandy coastal plain soils.
Slope angle
Jin and Romkens (2001) proposed that as slope increases, the trapping efficiency of a buffer will decrease as
deposition moves downslope through the buffer feature. De Ploey et al. (1976) found, in their experiments, that
when slope exceeds 18% grass covered slopes could result in more erosion than bare soil slopes because of
concentrations of fast flowing run-off between individual clumps of vegetation. Similarly, Ligdi and Morgan
(1995) observed that contour barriers (simulated with metal rods) provided good protection on slopes up to 11%
but resulted in twice as much erosion as bare soil on slopes of 19%. Furthermore, this confirms
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recommendations by the FAO (1976) and the experiments of Wenner (1981) with respect to the suggested
maximum slope for which the technique is an effective soil conservation measure (see
Table 3 for values).
The slope angle at which buffers may perform effectively can be increased where dense vegetation exists and
this has been attributed to the greater reduction in flow velocity in comparison with a more uniform vegetation
structure. Jin and Romkens (2001) simulated a vegetated filter strip using bunches of polypropylene bristles
inserted into a flume. They found that whilst a vegetated filter strip with a density of 2500 bunches/m2 was
effective in trapping sediment on slopes up to 4%, a VFS with a density of 1000 bunches/m2 was effective on
slopes up to 6%. Lakew and Morgan (1996) suggested that barriers with a dense root system and an interwoven
mat of rigid leaves were more effective on slopes of up to 23% whereas more open tussocky species were
effective only up to 16%. Boubakari and Morgan (1999) confirmed this proposing that perennial grasses with
dense leaf, stem and root systems will control erosion on steep slopes largely by ponding water upslope of the
barrier, encouraging infiltration and promoting deposition of sediment within the ponded area. Conversely, grass
species with low stem densities and low rigidity will concentrate flow within the strip and provide passages
whereby rill channels can extend through the barrier on to the slope below (Taddess and Morgan, 1996).
Tadesse and Morgan (1996) deduced that on the steeper slopes the main effect of the barrier is to filter sediment,
whereas on the gentler slopes the main effect is to pond the runoff and thereby increase infiltration and reduce
runoff volume. On steep slopes the ponded area is small and rills developed in the non-cohesive material in the
deposition zone are able to extend their channels through the barrier and on to the slope below.
Abu-Zreig (2001) observed that with a vegetative cover slope angle of the vegetative filter strip showed a minor
effect on trapping efficiency as well as on infiltration rate whilst at higher filter widths slope effect is smaller in
small filter widths.
At a larger scale Dillaha et al. (1989) observed through a farm survey that vegetated filter strips were ineffective
for removing sediment and nutrients from surface runoff in hilly areas because drainage usually concentrated in
natural drainage ways within the fields before reaching the vegetated filter strips. Flow across these vegetated
filter strips during the larger runoff producing storms (the most significant in terms of water quality) was
therefore primarily concentrated and the filters were locally inundated and ineffective. This assessment was
confirmed by the fact that little sediment was observed to have accumulated in the majority of the filters
observed. In flatter areas vegetated filter strips were more effective; slopes were uniform and significant
sediment accumulations observed. Several one to three year old vegetated filter strips were observed that had
trapped so much sediment that they were higher than the fields they were protecting. In these cases runoff tended
to flow parallel to the vegetated filter strips until a low point was reached where it flowed across as concentrated
flow. Therefore the vegetated filter strips acted more like a terrace. Flow parallel to the vegetated filter strips was
also observed on several farms where mouldboard ploughing was practical. When soil was turn ploughed away
from the vegetated filter strips a shallow ditch was formed parallel to the vegetated filter strips. If this ditch was
not removed by careful disking later runoff once again concentrated and flowed parallel to the vegetated filter
strips until it reached a low point and crossed as channel flow. Dabney (2006) recognised the importance of flow
redirection caused by edge-of field berms. Standard filter strips specifies that the gradient along the edge of the
filter strip must be less than 0.5% and calls for the field upslope of the filter strip to have a slope steepness of
between 1 and 10% (Dabney, 2006).
Summary: The literature reviewed appears to present concurring recommendations on slope angle (Table 3).
Based on those studies the mean maximum slope angle for effective sediment trapping by vegetated buffers can
be calculated as 12.5% and, with a dense uniform grass cover, this might be increased to 23% (based on
Taddesse and Morgan, 1996). Differences in experimental approach have not been factored into this calculation.
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Table 3 Recommended maximum slope angles taken from the literature
Recommended maximum
slope (%)
Slope angle
(degrees)
Reference
10
5.7
Hayes & Dillaha (1992)
10
5.7
Herson-Jones et al. (1995)
11
6.3
Ligdi & Morgan (1995)
15
8.5
Neswand et al. (1990)
15
8.5
FAO (1965)
16
9.1
Lakew & Morgan (1996)
18
10
DePloey (1976)
Mean = 13.6
7.7
Economic cost
Another factor in determining best placement of buffer features is the economic cost. Nakao and Sohngen (2000)
suggested that the cost of a ton of soil erosion reduction varies across site characteristics in a watershed. They
claim that very few studies have assessed how cost effective either riparian buffers or filter strips are for
reducing environmental impacts. Their research into the effect of site quality on the costs of reducing soil
erosion with riparian buffers (which can be applied to other types of filter strips) revealed that:
1.
2.
3.
The costs of riparian buffers themselves depend heavily on soil type and land opportunity costs. However,
when costs are estimated in terms of the costs per ton of reduced soil erosion, the costs are likely to vary
across a watershed, depending on a range of other factors including field shape, tillage methods, and field
size plus the effectiveness of the riparian buffers and soil type.
Of the factors investigated tillage practices have the most dramatic influence on the cost per ton. Buffers at
the edge of fields cultivated with no-tillage systems may be three times more expensive than those installed
along fields cultivated with conventional tillage systems.
As field size increases, riparian buffers can become more or less cost effective depending on whether the
size of the buffer increases. In this case it will depend on how the buffer is managed and used. Riparian
buffers may become less expensive if the percentage change in acres is less than the absolute value of the
percentage change in effectiveness.
Buffer management
“Longer-term performance of filter strips can decline if not managed properly” (Dosskey, 2001).
Time
Buffer performance is expected to vary over time and to depend on management criteria (Borin, 2005). A better
understanding of long-term accumulation of pollutants has important implications for management of buffers to
maintain filtering capability (Dosskey, 2002).
For example, according to Vellidis et al. (1994) the efficiency of new or restored buffers initially increases with
time (Borin et al., 2005). This effectiveness later decreases as sediments accumulate and encourage concentrated
flow across the buffer (Dillaha and Inamdar, 1997). Conversely Parkyn (2004) suggested that dissolved nutrient
uptake by plants may be greatest during early growth phases and decline as vegetation matures. During
experiments by Dillaha et al. (1989) the effectiveness of vegetated filter strips for sediment removal decreased
with time as sediment accumulated in the vegetated filter strips. They proposed, however, that this may not be a
problem in “real world” vegetated filter strips because filter strips vegetation should be able to grow through
most sediment accumulations. The success of vegetated filter strips in surviving burial by sediment will be a
function of random variables associated with rainfall, runoff, vegetative growth rate, depth of sediment
accumulation and other factors (Dillaha et al., 1989).
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Rodriguez (1997) claimed that as roots and other plant residues lodge against a grass hedge hydraulic resistance
progressively increases, causing deeper backwaters, longer settling lengths and increased sediment trapping so
long as the grass hedge is strong enough to remain erect. Over 6 years it was observed that grass hedges caused
substantial sediment deposition as evidenced by the appearance of berms above the hedges.
Whether sediment removal is effective over the long term appears to be a matter of debate. Cooper et al. (1987)
found that the riparian zone was a sediment sink over the 20 year period they studied. Lowrance et al. (1986)
reached a similar conclusion (using different methods) examining sediment deposition over a 200 year period.
Both of these studies were conducted in watersheds that were characterized by >50% forest cover. Whether
narrow buffers are able to retain sediments over the long term is not clear as most studies have been too short in
duration to detect remobilisation of sediments during infrequent intense storms (Hickey and Doran, 2004).
Flow
Several reports suggest that sediment retention decreases substantially when the grass becomes submerged
(Beibling and Albers, 1979; Magette et al., 1987; Vuurmans and Gelok, 1993). This may be due to surface
morphology forcing the water to concentrate and build up to a greater depth before entering the grass strip thus
submerging the grass. According to Vuurmans and Gelok (1993), the resistance encountered by the flow
suddenly drops when the concentrated water flow forces the grass to bend. As a result, the flow velocity will
further increase and sedimentation will decrease. Concentrated runoff often develops on fields with long slopes.
Water becomes concentrated into flowpaths and reaches the filter strip at a few points (Dillaha et al., 1988). In
such a situation sedimentation will be less than expected on the basis of plot measurements. This also explains
why results of on-farm measurements with filter strips are, in many cases, disappointing. Dillaha et al. (1989)
state that ‘unless vegetated filter strips can be installed so that concentrated flow is minimized, it is unlikely that
they will be very effective for agricultural non-point source pollution control’ (which includes sediment
retention). Therefore, grass strips should follow the slope contours precisely. Effects of filter strips on runoff
amounts are not as evident in the literature. Edwards et al. (1983) found that runoff sometimes increased after
passing a filter strip. This is also reported by Magette et al. (1987). This is attributed to higher soil moisture
contents in the filter strip compared with the upslope field, which limits infiltration (Van Dijk et al., 1995) and
thus maintains sediment load in surface runoff.
Sediment accumulation at the field edge of buffers has been observed to create dikes that divert field runoff to
low points along buffers where it flows across as concentrated flow (Dillaha et al., 1989). In order to maintain
sheet flow it may be necessary to remove sediment deposits or other modification of surface topography
(Dosskey, 2001).
A possible approach would be to disperse the runoff from the drainageway or ephemeral channel at the footslope
so the grass and riparian system could function without being overloaded (Daniels and Gilliam, 1996).
Within the field orientation of crop row direction could be adjusted so that it would discourage, or at least not
contribute to, flow into swales before reaching the field margins. On relatively hilly topography this could be
accomplished by locating the riparian field margin on the contour rather than a constant distance from the stream
(Dosskey et al., 2002)
For grass buffers, or the grass strip component of Correll’s three zone riparian buffer, he recommends the zone
be carefully contoured to create sheet surface flows during storms, and must be intensively maintained by
mowing. When a berm of trapped sediment develops, it should be re-contoured and replanted (Correll, 2005)
Unless vegetated filter striips can be installed so that concentrated flow is minimized it is unlikely that they will
be very effective for agricultural non-point control (Dillaha et al., 1989).
Other management features
Management practice that might improve buffer performance:
• In field practices that reduce total sediment load to buffers such as implementing appropriate tillage, land
shaping and in-field buffers can improve the trapping efficiency of buffers so long as other runoff
characteristics, such as size distribution of sediment particles, remain relatively unaltered (Dosskey et al.,
2002).
•
The formation of berms at the edge of buffers by tillage operating parallel to contour buffers and
perpendicular to waterways act as linear elements that interact with topography and soil properties and may
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alter runoff patterns (Dabney et al., 2006). To avoid these berms, contour buffer strips must be periodically
renovated (USDA-NRCS, 1999).
•
In contrast, vegetative barriers can be designed to control runoff by using these berms as miniature gradient
terraces to redirect runoff to a stabilised concentrated flow outlet (USDA-NRCS, 1999).
•
To promote plant growth and the trapping of suspended solids, herbaceous buffers should be mown and the
residues removed two to three times per year (Dillaha et al., 1989).
Management practice that might impede buffer performance:
• Traffic on the field lanes during the wet seasons exposed bare soil and served as a secondary field edge
or sediment source (Daniels and Gilliam, 1996).
•
Uphill-downhill farming is not recommended because it would increase erosion rates and counteract
improvement in the trapping efficiency of the buffer resulting from improved runoff distribution
(Dosskey et al., 2002).
Buffer performance
The efficiency of a buffer can be defined in terms of sediment trapping and reduction in surface runoff. Table 4
provides a summary of buffer efficiency as defined by other authors.
Table 4 Summary of grass buffer efficiency
Author
Thomas (1988)
McGregor and Dabney (1993)
Neibling and Alberts (1979)
Dillaha et al. (1987)
Parsons et al. (1990)
Van Dijik (1995)
Van Dijik (1995)
Van Dijik (1995)
McKergrow et al. (2004)
Barfield and Albrecht (1982)
Hayes and Hairston (1983)
Hayes et al. (1984)
Barfield et al. (1998)
Meyer et al. (1995)
Abu-Zreig et al. (2004)
Abu-Zreig et al. (2004)
Lee et al. (1990)
Gilley et al. (2000)
Abu-Zreig et al. (2001)
Lalonde (1998)
Cyne et al. (1995)
Le Bissonnais (2004)
Borin et al. (2005)
Daniels and Gilliam (1996)
Dosskey et al (2002)
AVERAGE
Reduction in soil loss (%)
70
40 (1st season), 75 (2nd season)
>80
>80
>80
50 to 60 (1 m)
60 to 90 (4-5 m)
90 to 99 (10 m)
40 to 95
>90
>90
>90
>90
>90
68 (2 m)
98 (15 m)
62 to 69
6?
45 to 99 (3 to 9.1 m)
85
99
30 to 100 (mostly >90)
Reduction in runoff (%)
50
78
60 to 90
Potential 66.5, actual 28.8
76.8
64
Summary, conclusions and gaps in literature
•
Buffer studies have been conducted at a range of scales and it is not clear how accurately results can be
scaled up from the plot to the field to the catchment.
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•
Most research on buffers has been conducted in laboratory or manipulated field experimental conditions and
few quantitative data exist on buffer performance under natural field conditions (Hickey and Doran, 2004).
Also there are still very few studies that measure the effectiveness of either vegetated filter strips or riparian
forest buffers under natural rainfall conditions at a scale appropriate to represent management units
realistically (Lowrance and Sheridan., 2005).
•
Detailed monitoring after buffer zones are established is often lacking (Briggs et al., 1994). Not only does
this have implications for sink source relationships during extreme events, but also for seasonal differences
in buffer performance, and whether sediment and associated nutrients stay within the buffer or move
through it.
•
The majority of the literature focuses on riparian zones and there is a lack of information on mid-field buffer
features. If buffer features are at risk of becoming sources of pollutants over time then harvesting may be
required to remove P permanently. Therefore mid field buffers might be ideal as they would keep the
destabilization and erosion associated with harvesting further away from streams.
•
A number of different terms are used to describe the features performing a buffering function including
vegetative filter strip, buffer strip, grass contour strip, grass hedge, filter strip, riparian buffer, grass barrier,
vegetative barrier strip. Literature describing the features is spread over a wide range of scientific disciplines.
This may impede the transfer of information to those that are involved with the practical establishment of
buffer features.
•
Buffer studies have been conducted at a range of site conditions with different plot sizes, rainfall
characteristics, soil types and slope angles so direct comparison is difficult. Furthermore there are no
established methods for measuring and monitoring existing buffer features in the field.
•
There is little information on the impacts of breaching points within buffer features e.g. channels, poaching,
farm gates etc.
•
There may be conflict in optimal buffer characteristics where, for example, saturated conditions will
encourage denitrification but will not be beneficial for the removal of P. Buffer features should ideally be
targeted to the specific pollution problem that they are trying to address.
•
The lack of design guidance has been flagged by a number of researchers but there appear to be even fewer
guidelines on the placement of buffer features both at the field and the catchment scale. The strategic
placement of buffer features may be critical on relatively small farms, typical of UK agriculture, where
farmers might be reluctant to take large areas of land out of production and therefore must target buffer
zones effectively.
•
Little evidence exists to illustrate the implications for incorrect buffer placement and design and what
happens when buffer features are located in the wrong place. Yet, it is likely that the ‘wrong’ location may
enhance erosion beyond that which might occur with no intervention. Little information is available on
whether the effectiveness of the barriers alters if placed on a convex or concave break of slope. The interrelational effects of buffer placement must also be considered e.g. the stripping of sediment from a
hydrological pathway may increase its erosivity or transport capacity.
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