Journal of Hydrology, 107 (1989) 133-145
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
133
[1]
AN EVALUATION OF THE FACTORS DETERMINING THE
EFFECTIVENESS OF WATER QUALITY BUFFER ZONES
JONATHAN D. PHILLIPS
Department of Geography and Planning, East Carolina University, Greenville, NC 27858 (U.S.A.)
(Received April 22, 1988; accepted after revision July 15, 1988)
ABSTRACT
Phillips, J.D., 1989. An evaluation of the factors determining the effectiveness of water quality
buffer zones. J. Hydrol., 107: 133-145.
This study examine s the relative role of slope length, slope gradient, surface roughness, and soil
hydrologic properties ~,n determining the pollution control effectiveness of vegetated buffer zones.
Two models describing buffer conveyance capacity are introduced. The first assumes that pollutant
transport through the buffer depends on the energy of overland flow and is based on Bagnold's
stream power concept. The second assumes that buffer effectiveness is a function of total contact
time of both surface runoff and throughflow and is based on Darcy's law and the Manning equation.
The hydraulic and detention models, respectively, are applied to the problem of estuarine shoreline
buffer zone delineation in Carteret County, North Carolina. Results show that where solid-phase
pollutants transported as suspended or bedload in overland flow are the major concern, slope
gradient is t~-:e most critical factor, followed by soil hydraulic conductivity. Where dissolved
pollutants that are transported by both surface and subsurface flow are of concern, buffer width
is by far the most important factor, with soil moisture storage capacity also playing a role. Methods
developed here may be applied to any water quality buffer delineation problem to determine the
relative influence of soil properties, geomorphology, and surface conditions.
INTRODUCTION
One of the most effective tools for coping with nonpoint source pollution is
the maintenance of buffer zones - - vegetated strips of land separating runoff
and pollutant contributing areas from surface waters. A buffer ~one allows
runoff and associated pollutants to be attenuated before reac~Jng surface
waters, via infiltration, adsorption, uptake, decay, filtering, and deposition
(Tollner et al., 1976; Karr and Schlosser, 1978; Vanderholm .?t al., 1979;
Lowrance et al., 1984, 1985, 1986). General evidence of the ability of vegetated
areas to remove pollutants is the increased use of overland flow methods for
wastewater treatment (Smith, 1982; Abernathy et al., 1985) and use of wetlands
for secondary and tertiary treatment (Carter, 1985; EPA, 1986).
Existing guidelines for buffer delineation are site- and situation-specific and
typically hold several factors which affect buffer effectiveness constant. These
factors include the size and slope of the buffer, resistance to flow, infiltration
0022-1694/89/$03.50
© 1989 Elsevier Science Publishers B.V.
134
capacity, and the ability of the soil to hold moisture. Since several of these
factors are usually assumed constant by existing guidelines, it is difficult in
developing buffer delineation procedures for new areas and situations to
determine which factors are most critical. The purpose of this research is to
develop a method for evaluating water quality buffers in terms of the relative
influence of the factors mentioned above.
For example, it is difficult to apply design standards developed for overland
flow wastewater treatment systems to design of natural buffers because the
former are always constructed such that infiltration capacity is negligible.
With this property held constant, soil moisture storage capacity has no
influence. Further, overland flow treatment systems are all grass-covered and
constructed over a fairly narrow range of slope gradients. Therefore systems
can be designed solely on the basis of slope length and application rates (Smith,
1982; Abernathy et al., 1985), and design criteria provide little insight into the
roles of soil properties, surface roughness or slope steepness.
In other cases, buffer delineation guidelines implicitly or explicitly assume
certain characteristics (or narrow ranges of characteristics) of various
properties. Palfrey and Bradley's (1982) recommendations for estuarine
shoreline buffers in Maryland assume natural vegetation and permeable soils.
Recommendations for vegetative filters for livestock feedlot runoff by
Vanderholm et al. (1979) assume a constant surface roughness and a narrow
range of soil hydraulic properties. Roman and Good's (1983) proposed buffer
delineation model for wetlands in the New Jersey Pinelands is based on
conditions unique to that region and on criteria unique to that situation. In
other cases the nature of the problem dictates that certain buffer characteristics will not vary widely and/or that a single pollutant is of importance.
Riparian buffers for forestry operations may safely assume a forested buffer
area and that sediment is of paramount importance. Design of secondary
wetland wastewater treatment systems may assume low slopes, dense
vegetation, and a certain range of hydrologic properties.
The effectiveness of vegetated filters for nonpoint source pollution control
is widely appreciated. Guidelines for delineating riparian buffers or for quantitatively and objectively evaluating buffer effectiveness have been lacking,
however. Existing guidelines are largely site-specific, and are primarily rulesof-thumb or "best guesses" (see Roman and Good, 1983; McCullough, 1985;
Budd et al., 1987). There is a body of literature dealing with pollutant removal
in vegetated filters and riparian areas, and with the geomorphic, hydrologic,
and ecological structure and function of riparian areas (for reviews, see
Wharton et al., 1982; Carter, 1985; Lowrance ~t al., 1985; EPA, 1986; Mitsch and
Gosselink, 1986). This literature does not address the buffer delineation
problem, however.
In short, there are no generally applicable guidelines for determining which
factors are most important in establishing criteria for water quality buffer
zones. The purpose of this paper is to present a method, for determining the
relative importance of topography, soil hydrologic prcperties, and surface
135
roughness in determining buffer effectiveness. The method is applied to the
problem of estuarine shoreline buffer establishment in Carteret County, North
Carolina.
Models describing the conveyance capacity of buffers are developed below
for two separate cases. In the firstcase conveyance capacity (or buffer effectiveness) is considered to be a function of overland flow energy, and subsurface flow
is ignored. In the second case buffer effectiveness is considered to be contingent
on detention time of runoff within the buffer, and subsurface flow is included.
After the models are developed, they are applied to determine the relative
importance of factors influencing shoreline buffer zones in Carteret County.
M O D E L S OF B U F F E R EFFECTIVENESS
The effectiveness of a water quality buffer zone is a function of its ability to
assimilate, cleanse, or delay runoff-borne pollutants passing through it.
Potential effectiveness of a given buffer may differ according to water quality
and management goals and the pollutants of major concern. Some pollutants,
such as sediment, and heavy metals or pesticides adsorbed to sediment
particles, are transported solely or primarily in particulate form in surface
runoff. Subsurface flow is unimportant in transporting these pollutants, whose
movement depends on the energy or transport capacity of overland flow. Other
pollutants may be transported in dissolved form (or as very small suspended
solids) in either surihce or subsurface flow, and any significant flow is sufficient
for their transport. Likewise, some pollutants are conservative and may cause
problems even if their delivery is delayed, while delaying nonconservative
pollutants may result in their degradation.
Two separate models have been developed to analyze buffer criteria to
reflect two genera~ categories of pollutants: Those whose transport depends on
the energy of overland flow; and those whose likelihood of reaching water
bodies is a function of detention time within the buffers. The models are called,
respectively, the hydraulic and detention models.
Hydraulic model
The major assumption of the l~ydraulic model is that transl~rt of runoffborne pollutants from upslope contributing areas through a buP~br is directly
proportional to the energy of overland flow. This assumption is presumably
applicable to sediment, other large particulates, a~d adsorbed pollutants (such
as heavy metals, phosphorus, and some pesticides).
Some water which infiltrates into the soil also reaches surface waters, and
dJsso!ved substances can be transported by this pathway. In a water quality
l~aanagement context it is necessary to justify neglecting the subsurface
component. Subsurface flow is much slower than surface runoff. This lag time
allows for decay and degradation of many pollutants. The soil matrix itself may
also filter many contaminants, and water in the root zone is available for plant
136
use, which may further remove pollutants. Since the subsurface component of
runoff is delayed, it is less likely to contribute to acute impacts associated with
high concentrations during or immediately after storms. Temporally and
spatially localized impacts (high bacterial counts in shellfish areas, fish kills,
algae blooms) are often the critical water quality concern, so subsurface flow
is relatively unimportant (though the serious impact of subsurface flow in some
other contexts is recognized). These factors may justify the assumption that
runoff pollutant delivery is proportional to the energy of surface flow. In any
case this approach is consistent with historical approaches to urban
stormwater and nonpoint source pollution control which have focused on
reducing surface runoff (Office of Water, 1983; Field, 1985).
Given a certain quantity of runoff and associated contaminants from shore
development, the total energy of overland flow available to transport
pollutants is a function of: (a) the proportion of runoff which moves as overland
flow; (b) slope length (i.e., width of the buffer zone); (c) slope gradient; and (d)
surface roughness.
The proportion of.runoff moving on the surface can be determined from
saturated hydraulic conductivity. Infiltration capacity at a given site varies
according to soil moisture status, and is at a m a x i m u m when the soil is dry. The
saturated hydraulic conductivity, however, represents the steady-state infiltration rate under saturated conditions (Skaggs and Khaleel, 1982), and is
strongly related to the cumulative infiltration associated with a given event.
Most models of ponded infiltration capacity (making the assumption here that
infiltration of a slug of runoff passing through a buffer is closely approximated
by ponded infiltration rates) have a general form consisting of a constant term
(hydraulic conductivity) added to or multiplied by a second term related to
time-specific conditions such as initial moisture conditions, distance from the
surface to the wetting front, and factors such as vegetation which m a y be
included in empirical formulations (Skaggs and Khaleel, 1982). The widelyused Green-Ampt equation, based on Darcy's law, is one example:
fp = Ks(Ho + Sf + Lf)/Lf
(1)
where fp is infiltration capacity, Ks is hydraulic conductivity of the transmission zone, H0 is depth of water at the surface, Sf is the effective suction of
the wetting fron~, and Lf the effective depth to the wetting front. In comparing
the ability of buffers to assimilate a given volume of runoff via infiltration, H0
is in effect a constant. Ks~ SE, and the variation of Lf through an infiltration
event can all be described as functions of K (Skaggs and Khaleel, 1982, pp.
143-144). Thus relative saturated hydraulic conductivity provides an accurate
reflection of relative ability to infiltrate a given imposed mass of surface water.
Saturated hydraulic conductivity is also readily available in soil surveys and
databases (sometimes referred to as permeability).
For a given mass of water, Bagnold's (1966, 1977) stream power, originally
designed to predict bedload sediment transport, provides a physically-based
measure of the energy available to transport material. The time rate of energy
137
expenditure per unit weight of water, for water flowing downslope (unit stream
power) is:
Pu = ( p g A L Vs)/(pgAL)
=
Vs
(2)
where p is density of water, g is the gravity constant, A is cross-sectional area,
L is the length of the reach, V is mean flow velocity, and s = sin 0, where 0 is
the angle of the slope relative to the horizontal.
For overland runoff through a vegetated buffer it is reasonable to assume
uniform, turbulent, kinematic flow. It also is assumed that flow is steady-~tate,
and p and g are constant. Uniform, steady-state flow with constant specific
gravity is unlikely to obtain in any particular event, but these are reasonable
assumptions for a range of conditions and events. With these assumptions, it
can be shown from stream power theory and the Manning equation that:
Pu =
L°'4(81"3/n°'6)
(3)
for a unit of runoff or precipitation excess, where n is the Manning roughness
coefficient. Moore and Burch (1986) derived this using Yang's stream power
concept, but it can also be derived using Bagnold's stream power (Phillips,
1987).
Then, denoting saturated hydraulic conductivity with K, a general index of
buffer effectiveness (B) is obtained, reflecting both stream power and the
infiltration assimilative capacity of the soil. For any given buffer (subscript b)
and a reference buffer (subscript r) the relative ability of the buffers to reduce
total overland flow energy of a given runoff flow into the buffer is given by:
Sb/Br--
(Kb/Kr)(Lb/Lr)°'4(Sb/Sr)-l"3(nb/nr) °'s
(4)
Detention model
Transport of some pollutants -- especially dissolved substances -- does not
depend on overland flow energy. The hydraulic model (and the hydraulic
approach in general) emphasizes kinetic energy of a moving water mass and/or
force exerted at the flow margins. The subsurface flow is neglected, and more
permeable media lead to greater buffer effectiveness in the hydraulic model.
But if both surface and underground flows are considered, permeability is a
double-edged sword m high infiltration to reduce surface runoff is offset by
rapid subsurface runoff movement.
Using the same assumptions about the overiand flow component as the
hydraulic model, detention time (T) of surface flow on a slope is given by:
7"=
n°'6Ls-°'3q~°'4
(5)
where qs is surface discharge and the other tvrms are as previously defined.
Darcy's law for velocity of flow through a porous medium is:
v
=
(6)
138
where K is a conductivity coefficient. For purposes of this discussion, this is
equivalent to the saturated hydraulic conductivity as already defined.
Downslope subsurface flow will occur almost entirely in the saturated state. In
the unsaturated state both gravitational and tensional gradients are greatest
in the direction of downward percolation, and tend to be insufficient to drive
significant downslope flow (Kirkby, 1985). When saturation is reached,
however, downslope flow occurs, almost entirely under gravitational forces
(Kirkby, 1985). Under these circumstances detention time for subsurface
saturated flow is given by the following, with K as saturated hydraulic conductivity:
T
(7)
= KsL
Considering the entire downslope flow, an index of detention time (T*) for
a given imposed flow is:
T*
(8)
= [n°6Ls-°3(qs/q)-°4][gsL(qg/q)]
where q is total downslope discharge and qg is the subsurface component.
Since the goal is the ability so compare relative effectiveness of buffers over
a range of runoff inputs (q), it ~u not necessary to know the absolute values of
q, qs, and qg to construct the model. The proportion of the discharge which
travels overland or subsurface is a function of the infiltration capacity, which
(as before) is assumed to be a function of K. For the overland component,
detention time varies as the - 0.4 power of qs. For a given imposed stormwater
mass, since K is an index or surrogate for qs, then T = f ( K - ° 4 ) . Since the
proportion of flow travelling on the surface is an inverse function of hydraulic
conductivity, we reverse the sign to obtain T = f(K°4).
The proportion of a given input flow which moves underground in the
saturated zone is a direct function of K. Substituting K °4 for (q~/q)-O.4 and K for
(qg/q) in eqn. (8) we obtain:
T*
=
n°'6L2s°'7 K K
(9)
TM
Now we can express eqn. (9) as a buffer effectiveness ratio, rearranging and
simplifying so that any term will be greater than unity if the proposed buffer
is more effective t h a n the reference in delaying runoff and less than unity if the
converse is true:
T~b /T~r
=
(nb/nr)°'6(Lb/Lr)2(Kb/Kr)°'4(Sb/Sr)
-°'7
~10)
At this point the detention model contains the same basic elements as the
hydraulic model. It is worthwhile to compare the two. The roughness term in
the two models is identical, but the others vary significantly. Slope length (or
buffer width) assumes far greater relative importance in the detention model.
This result is reasonable, since in the hydraulic model the positive benefits of
slope length on buffer effectiveness are partially offset by an increase in stream
power with slope length. Slope gradient is less important in the detention t h a n
in the hydraulic model. This, too, is reasonable, since the hydraulic model is
139
based on force (P, = Vs) and slope gradient exercises an independent influence
in addition to its influence on velocity. In the detention model, slope gradient
is important only for influencing velocity. Finally, saturated hydraulic conductivity assumes a less important role in the d~tention model as compared to the
hydraulic model. While K is more important in developing the detention model,
its ultimate impacts on the buffer effectiveness ratio are reduced because the
positive impacts on surface flow (reducing surface volume) are partially offset
by the negative impacts on saturated throughflow (higher throughflow
velocity).
Still to be accounted for in the detention model is the proportion of infiltrated runoff which moves downhill as saturated throughflow as opposed to the
proportion percolated in unsaturated conditions. There is no generally-available parameter which can represent this partition in absolute terms. However,
the relative tendency for the soil to become saturated can be determined by
comparing the available water or moisture Capacity. The latter is defined as the
difference between the moisture available at field capacity and wilting point.
This information is readily available ir~.soil surveys. Therefore the relative
ability of a proposed and reference buffer to assimilate infiltrated water (as
opposed to allowing downslope saturated throughflow),is given by the ratio
Cb/Cr. C is the soil moisture storage capacity, obtained by multiplying the
available water or moisture capacity [L L- ~] by the thicknes~ of ~he soil profile
t~
"
above a confining layer. For purposes of water quality buffer del|neatlon,
on 1 y
that portion of the profile lying above a seasonal high water t~ble should be
used to compute C. Depths to water tables are routinely inc~!uded in soil
surveys. Adding this term to eqn. (10), we obtain:
~
Bb/Br
= T~b/T~r = (Tlb/nr)°'e(Lb/Lr)2(Kb/Kr)°'4(Sb/Sr)-°'7 (Cb/Cr)
(11)
Influence of individual factors on buffer effectiveness
Given a specific range of conditions (soil types, topography, surface
roughness) the models developed above can be used to estimate the relative
importance of any of the model factors in determining buffer effectiveness.
Using the subscript x to denote the maximum and 1the minimum value for any
parameter found in any situation (such as a particular geographic area, or in
data from a specific study), the maximum possible variability (M) in buffer
effectiveness is given by:
M~l =: (K,,/Kl)(Lx/Ll)°'4(sx/sl)-l3(nx/nl) °'6
(12)
for t~e: hydraulic model and by:
MD := (nx/ne)°e(Lx/Lt)2(Kx/KI)°4(Sx/St)-°7 (Cx/Ct)
(13)
for the detention model. Every term on the right of either equation is of the
form (Zx/Zt) y~ where z is any of the parameters and Yz the derived exponent for
that factor. The relative contribution of any factor zi to the maximum expected
variation in buffer effectiveness is then given by:
140
(14)
Mz,[M = [(zx[zt)Yz][Mz
ESTUARINE SHORELINE BUFFERS IN CARTERET COUNTY, NORTH CAROLINA
Study area and data collection
Carteret County, North Carolina (Fig. 1) has experienced tremendous
growth in recent years, especially along its estuari•e shorelines. Stormwater
runoff from shoreline developments is a major concern, and is known or
thought to contribute significantly to a number of problems, including closure
of shellfish beds, damage to estuarine nursery areas, estuarine eutrophication,
violation of water quality standards, and a general decline in fisheries
(McCullough, 1985; NCDEM, 1985a, b; NCCRAC, 1986).
Estuarine shoreline land use, geomorphology, vegetation, surface
conditions, and soil properties were surveyed throughout the county. Information was obtained by field visits to shoreline sites, aerial photographs, the
county soil survey (Goodwin, 1978), and interviews with the county planning
,J'"-/ ....................................
i
CAROLINA
. . . . . . . . .
-~"....." " ' N O R T H .
[
......... 0
80 MI.
4'0 8'OKM
40
~ . . ~
~..~~ t~-
,.
",
, ,,,,,
Carteret
~,~Vt{")fl~W"
County
Fig. I. Location map for Carteret County.
TABLE 1
General estuarine shoreline types of Carteret County
Extensive marsh
Low sediment bank
High sediment bank
Fringe marsh
Backbarrier
Broad, nearly level wetlands. Soils poorly drained, mucky and sandy, with
high water table and frequent flooding
Nearly level to gently sloping, often with steep erosional scarp (relief 0.10.7 m) at water's edge. Soils of the well-drained, sandy Wando-SeabrookKureb association or the poorly-drained Leon-Murville-Mandarin association, which is generally sandy with a low.permeability organic subsoil
As above, with with a steep bluff or cliff (relief 1.5-7.0 m) at water's edge
Low or high sediment bank with a narrow strip of fringing salt marsh at
the water's edge
Nearly level to moderately steep, well-drained, sandy soils of the NewhanCorolla-Beaches association. Often includes fringe marsh
141
PERCENTAGE OF SHORELINE
0
20
40
!
I
60
I
EXTENSIVE MARSH
FRINGE MARSH ~
21.0
LOW SEOIMENT BANK ~ 1 5 . 6
HIGH SEDIMENT BANK IIm 1. 9
Fig. 2. Proportion of mainland shoreline of Carteret County composed of various general shoreline
types. Shoreline types, including the backbarrier type found on the landward side of the county's
barrier islands, are described in Table 1.
department. A general description of estuarine shorelines in the county is
given in Table 1 and Fig. 2.
A range of likely shoreline buffer widths was established by observing
existing shore zone land use, examining proposals for new developments, and
by interviews with the county planning director. It was determined that a
minimum 15 m setback from mean high water could be expected for any future
developments, and that 300 m is the maximum setback which could be expected
for any land use likely to have significant direct impacts on estuarine waters.
Maximum and minimum saturated hydraulic conductivities and soil
moisture storage capacities for shore zone soils were determined from the soil
survey. Both K and available water capacity (in in -~) are published as a range
of values. T[,e lower end of the range was used in all cases. Soil moisture
storage capacity was determined by multiplying the available water capacity
for each horizon by its average thickness as given in the soil survey. Only the
portion of the profile above a confining layer or seasonal high water table was
considered. Conductivity values range from 1.0 to 50.0cm h-~. Soil moisture
~torage capacities ranged from 1~5 to 101.6cm.
Roughness values were determined by comparing the range of surfaces
which occur along the shoreline to the table of n values for overland flow given
by Engman (1986, p. 51). The range of values is 0.01 to 0.45.
The range of slope gradients in the shore zone was determined from air
photos, topographic maps, and site visits. Only the portion of the shore zone
landward of the wetland/upland interface or the top of a cliff or bluff shoreline
was considered. Slopes range from 0.0025 to 0.1.
Results
Table 2 shows the range of values for each parameter~ the appropriate
exponent (Yz) for each term (zx/zt) yz in the hydraulic and detention models, and
the maximum expected variation for each term or factor. These relative contri,.
butions to the maximum variation are given in percentage form in Table 3.
In the hydraulic model it is seen that slope gradient and saturated hydraulic
conductivity are the most important factors, together accounting for 93% of
142
TABLE 2
Variation of topographic, soil, and surface roughness factors for estuarine shoreline areas of
Carteret County, N.C.
Parameter
Buffer width
Conductivity e
Manning's n
Gradient
Soil moisture
capacity
Range of
.~ariation
15-300 m
1-50 cm h-1
0.01-0.45
0.0025-0.10
1.524-101.600 cm
Exponents a
Max variation b
H
D
H
D
0.4
1.0
0.6
1.3
NA
2.0
0.4
0.6
0.7
1.0
3.314
50.000
9.816
120.970
NA
400.000
4.782
9.816
13.266
66.667
a Absolute value of the exponent of appropriate term in eqns. (13) and (14) for the hydraulic (H) and
detention (D) models.
bMaximum value divided by minimum value and raised to the indicated exponen~ in the hydraulic
(H) and detention (D) models.
e Saturated hydraulic conductivity.
TABLE 3
Relative importance of factors influencing buffer effectiveness, Carteret County, N.C.
Parameter
Contribution to maximum
hydraulic model
Expected variation (%)
detention model
Buffer width
Saturated hydraulic
conductivity
Manning's n
Slope gradient
Soil moisture
capacity
2
27
81
1
5
66
NA
2
3
13
the maximum variation. Length was least important, accounting for only 2%
of the variation. The predominance of slope gradient is not unexpected, since
slope is included as a term in ~he unit stream power equation (Pu = Vs) and
also influences power indirectly du~ to its effects on flow velocity. The role of
conductivity, as a surrogate for infiltration capacity, is also important because
the hydraulic model does not consider subsurface flow.
Conversely, length or buffer width is the predominant factor in the detention
model, accounting for 81% of the maximum variation. Soil moisture storage
capacity accounts for 13%, while hydraulic conductivity, roughness, and slope
gradient have only minor influences. While saturated hydraulic conductivity
is a critical factor in deriving the detention model, its ultimate contribution is
minor because of offsetting effects. High conductivity tends to enhance buffer
effectiveness in that it allows surface water to be infiltrated. But higher
143
conductivity tends to reduce buffer effectiveness in that it allows for rapid
throughflow in the saturated zone. The net effect is a slightly positive influence
on buffer effectiveness.
DISCUSSION
With respect to Carteret County, North Carolina, the implications of the
results are clear. If the chief nonpoint source pollutants of concern are
sediment or sediment.associated contaminants, then buffer delineation criteria
should focus on slope gradient and soil hydraulic conductivity. Roughnecks is
of only minor importance, and buffer width is of little concern as long as a
minimum 15 m shoreline setback is observed. Thus, sediment-control buffers
around construction sites, for example, should be designed with special
attention to gradients and soil hydraulics.
With the exception of construction sites and other high-erosion areas, the
major concern with respect to runoff from shore developments in Carteret
County is bacteria, which result in shellfish bed closures and make water
unsafe fer swimming and water skiing. Nutrients, especially nitrogen, are also
important due to a general concern with algae blooms and accelerated eutrophication in North Carolina estuaries. With respect to these pollutants the
detention model is more appropriate, since delays allow for natural processes
to break Q~n'-- *~'~,~contaminants. Where these pollutants are concerned buffer
delineations should focus on adequate buffer widths, with a secondary concern
for the capacity of soils to store moisture. Recall that storage capacity is
considered only for the portion of the profile above a seasonal high water table,
implying that this factor is significant, in addition to the available water
capacity of the soil matrix
It is important to note that the relative importance of factors suggested by
the model depends not only ~n whether the hydraulic or detention model is
applied, but vlso on the range of conditions encountered. In the example above,
in the detention model, length accounted for 81% of the maximum variation
and slope gradient for 3%. But suppose buffer widths under consideration had
varied only from 15 to 20 m and slope gradient had varied from 0.0025 to 0.3. In
that case (with other factors the same as in the example above) buffer width
would account for only 1.6% of the maximum variation and slope for about
26%. Soil moisture storage capacity would become the most important
variable, accounting for about 60% of the maximum variation.
With regard to ~:he general issue of water quality buffer delineation, the
hydraulic and detenticn models d~veloped here provide a tool for determining
which factors may be most critical. Once the pollutant(s) of primary
management concern are identified, either the hydraulic or detention model
can be chosen. Then the expected range of conditions can be determined, and
either model used as above to determine which factors are most important.
Both models also have the potential to be used ~o provide quantitative
evaluations of buffer effectiveness for any given or proposed buffer with
144
reference to an established ideal buffer. The development and application of
eqns. (4) and (11) for this purpose is the subject of ongoing investigation.
CONCLUSIONS
In summary, this study presents a method for estimating the relative
importance of soil hydrologic properties, topography, and surface roughness in
determining the effectiveness of water quality buffers. The general method is
based on the concept of relative conveyance capacities. There are two versions
of the method: a hydraulic model, for pollutants whose transport is a function
of overland flow energy; and a detention model for pollutants whose t r a n s p o r t
is a function of detention time within the buffer. The models determine the
relative influence of buffer width, slope gradient, surface roughness, soil
hydraulic conductivity, and soil moisture storage capacity in terms of the
proportional contribution to the maximum expected variation. In Carteret
County, North Carolina, the analysis shows that slope gradient and s a t u r a t e d
hydraulic conductivity are the most important determinants of estuarine
shoreline buffer effectiveness with regard to sediment and other c o n t a m i n a n t s
for which the hydraulic model is most appropriate. The detention model, which
i~ more appropriate for the major pollutants associated with runoff from
shoreline developments in the area, shows t h a t buffer width is the most
important variable.
ACKNOWLEDGEMENTS
The assistance of Lynn R. Phillips (Carteret County Planning Director) in
data collection is greatly appreciated.
REPERENCES
Abernathy, A.R., Zirschy, J. and Borup, M.B., 1985.Overland flow wastewater treatment at Easley,
S.C.J. Water Pollut. Contr. Fed., 57: 291-299.
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