ICSE6 Paris - August 27-31, 2012
- Michael Heibaum: Natural and artificial material for scour protection
ICSE6-125
Combining Plants, Natural and Artificial Building Material for the
Optimal Protection of Banks and Coasts
Michael HEIBAUM1
1
BAW – Federal Waterways Engineering and Research Institute
Kussmaulst.17 – D.76187 Karlsruhe, Germany - e-mail : [email protected]
Banks and coasts often need protection due to the dense population of our world. The hydraulic loads and interactions
at the coast and at inland waterways and the resulting need of protective structures are often contradictory to the
natural environment, arising the question if nature or safety has to be focused on. The elements of revetments are
discussed in how far technical solution can be replaced or complemented by biological elements. Since weight is a
major factor, pure biological solutions are hardly possible, but combining natural and technical elements can be
successful.
Key words
Erosion protection, biological-technical measures, revetment stability.
I
INTRODUCTION
Wherever banks or shores may not develop naturally, the desired (anthropogenic) geometry needs to be
protected. This requires a revetment or other stabilizing structures which can be built using various
construction methods, depending on local conditions and strategy. The range reaches from large breakwaters
for coastal protection, which are to absorb the highest loads without damage, to all kind of bank protection
systems for inland waterways from sheet piles to technical-biological structures. Loads and strains that affect
a bank protection, and according to which it has to be designed, include waves, drawdown (the quick drop of
the water level) and – unfortunately way too often – vandalism. A fact in advance: Weight is crucial – for all
load types and all application cases.
II LOADS AND INTERACTION AT THE COAST
Tides and seasonal influences change the morphology of coasts. Currents parallel to the coast, wave runup, storm surge, and other sources of hydraulic strain are the causes for the constantly changing geometry of
coasts. These processes can be very quick but may also take several years.
At a beach, erosion and depositing processes may offset each other over the year in some places.
Substantial loss of land occurs only in case of significant storm surge. Other places are subject to permanent
erosion and thus require measures for coastal protection. However, while these measures are useful in one
place, they may be detrimental in other places (e.g. like groin fields that lead to land gain upstream but to
erosion downstream). If protection or repair measures have to be executed repetitively (e.g. beach
nourishment) because of being damaged by every more severe action, they might cause high costs. In case of
cliffs or undercut cliffs, erosion causes even more damage. The result is often a sudden and dramatic loss of
land, which cannot be restored.
As every sandy beach reveals clearly, shores undergo permanent flattening due to dynamic hydraulic loads,
i.e. loads developing from currents, waves, and the interaction between pore water and surface water. A cliff
may be artificially created through digging or developed as a natural escarpment; however, it will not last
long until it will be flattened again. The final inclination of the shore depends on local conditions and the
soil, particularly its granularity and grading. The steepest slopes, which can be created through hydraulic
495
ICSE6 Paris - August 27-31, 2012
- Michael Heibaum: Natural and artificial material for scour protection
filling in the sea, are in the range of 1:40 to 1:15 (CUR 1992). But the geometry of such a hydraulic fill
cannot be considered as permanent and needs a cover to remain stable under various hydraulic conditions.
Maritime slope protection is not limited to the shores but can apply also to sub-sea slopes. At the Eider
barrage (Germany), for example, scour has developed with a gradient of up to 1:1 in the upper section, prone
to progress toward the barrier. This steep gradient developed due to clay layers in the overall sandy soil,
these acting as a kind of reinforcement. Clay can even produce vertical slopes for a limited period of time.
Clay erodes less quickly than sand; however, protection measures are nevertheless needed. To prevent
retrograde erosion and thus damage to the barrage, the scour slope towards the structure had to be protected
by a revetment with corresponding dimensions.
III LOADS AND INTERACTIONS AT INLAND WATERWAYS
Banks at inland waterways, canal embankments in particular, are subject to predominatly hydrodynamic
processes generated through navigation. The main loads result from drawdown and return flow, in sensitive
places also from the dropping of anchors. Other strains result from the weather, animals and people. These
cannot be quantified. Yet, they still need to be considered when designing construction measures.
The maximum load revetments at inland waterways can tolerate is known from test results and can be
calculated rather precisely (GBB 2012). Economic efficiency and risk limit the load to a value less than the
maximum possible, but often the load is higher than expected e.g. by vessels passing a speed limit.
Theoretical calculations and experience can be contradictory. Often you hear: “we have never observed any
damage” but sometimes you know that additional riprap has been dumped every year. On the other hand, the
real causes of damage are very difficult to identify precisely. Real prove is rare and does not apply for all
construction methods.
IV INTERACTIONS OF PROTECTIVE STRUCTURES AND NATURAL ENVIRONMENT
Structures for bank protection need to take aspects of environmental protection more and more into
account. From an ecological perspective, the waterway itself is the best transport route. Thus, when it comes
to structures installed for protection purposes on a waterway, traditional construction methods should also be
revised in an ecological perspective and options should be considered which are closer to nature. This is
particularly important as structures which have existed for decades have meanwhile been ‘integrated’ into
the natural environment. A new construction replacing such old ones bears the risk of a painful intervention.
If new construction measures are inevitable, the impact should at least be as small as possible in order not to
damage the delicate balance which has developed over time and which will more or less be disturbed by any
human interference.
This is not to say that every measure should be prohibited or that pseudo-ideal environments should be
installed elsewhere as a substitute, since on the one hand, technical structures are a prerequisite for living in
an industrial society. And, on the other hand, nature itself also is subject to change, and thus it hardly makes
sense to reject any change of the environment. Nature is always dynamic and undergoes alterations. It can
change so dramatically that if these natural events were man-made, we would speak of destruction:
avalanches, landslides, eruptions, and floods. But in such cases natural processes and their consequences are
taken for granted and are often considered as an opportunity for a new beginning. Responsible anthropogenic
processes should be judged more often in a similar way.
Nature needs time to adapt to altered conditions. This process can be shortened and the change can be
supported by using certain appropriate construction measures and building materials. As a first step,
individual elements which are close to nature can be installed in a technical construction if there is not
enough experience or if regulations do not allow implementing an entire concept closer to nature.
Certain traditional bank protection measures have proven helpful: fascines, live brush mattresses,
vegetation mats, wattle fences, and others. If planned right, constructing with living material and
496
ICSE6 Paris - August 27-31, 2012
- Michael Heibaum: Natural and artificial material for scour protection
bioengineering measures can meet both the technical and the ecological demands, maybe in combination
with technical measures. However, it needs to be considered that, in most cases, bioengineering measures
need more room than technical alternatives. Examples for bioengineering measures can be seen at many
standing and flowing waters. At waterways and at the coasts, only few approaches have so far been made.
V SAFETY VS. CLOSE TO NATURE?
Embankment dams and dikes show the contradiction between technical and ecological demands
particularly well. From a geomorphologic point of view, dikes and embankments are foreign bodies to the
environment, especially if they contain culverts or other structures in and through the embankment. Such a
construction system requires great effort and substantial means if to be integrated into the environment in
such a way that it cannot be recognized as an artificial construction. Apart from the question of aesthetics,
the building of embankments or dikes needs people’s approval. The necessity of the majority of our
embankments and dikes has never been questioned. But there is a lot of discussion what kind of flora is
allowed – and if there are already trees and large bushes, deforestation is opposed even though regulations in
many countries ask for nothing but a grass cover.
Questions regarding safety, including aspects such as stability and impermeability, need to be assessed
primarily from a technical point of view. The following demands must be met:
x The protection measure must be sufficiently resistant against hydraulic strain (waves, current,
drawdown, return flow).
x All possible failure mechanisms must be excluded with sufficient certainty. The failure of an
embankment dam or dike may have disastrous effects.
x An impervious lining must not leak in any case. What is crucial here is not the possible loss of water,
but the flow forces developing from percolation which threaten the inner stability of the construction
and promote its outer erosion.
x Banks of waterways must be sufficiently protected from boat collision.
x Vandalism must not be underestimated as it may restrain or disable important functions of a protection
measure, even if it is not done on purpose.
These demands are crucial for any engineering method and cannot be ignored: Flood protection is
indispensible for protecting communities and habitats, and for ensuring a high quality of life and waterways
are unrivalled, especially in the transportation of bulk goods. So all structures and structural elements that
contribute to their safety have to be installed with great care and with all the technical knowledge. Only then
it can be discussed, to what extent we can build closer to nature. In the following possibilities and limitations
concerning revetments are discussed.
VI REVETMENTS FOR BANK PROTECTION
VI.1
6.1 General
Revetments ensure the protection of an (inclined) bank. Vertical banks protected by sheet-piles or
reinforced soil structures are not considered in this paper. Layers of a revetment – of which some are not
always needed – are top-down:
x Top layer
x Cushioning layer
x Filter
x Lining
x Levelling layer/separation layer
x Subsoil.
497
ICSE6 Paris - August 27-31, 2012
- Michael Heibaum: Natural and artificial material for scour protection
The individual revetment may vary, since local conditions as well as traditions determine the construction
method to a great extent. At rivers, geological conditions often require specific revetments. But even at
canals, very different construction methods may be applied despite identical conditions.
As debates on the construction of revetments showed that objective dimensioning methods would be
desirable, several attempts have been made to find universal rules. This requires considering individual
components as well as the revetment as a whole. Geotechnical and hydraulic experts need to work close
together because the dimensioning of the revetment concerns both fields:
x The top layer is primarily determined by hydraulic criteria.
x The cushioning layer results from material aspects.
x The filter is to be dimensioned according to geohydraulic aspects (which are determined by pore water
flow and its interaction with the grain skeleton).
x The impervious lining must meet certain geotechnical material demands.
x The levelling layer is most often selected according to design constraints. However, it needs to be
adapted to local geotechnical and/or geohydraulic conditions.
x The subsoil must be stable (in geotechnical terms).
The interactions of subsoil, revetment, surface water and pore water have been compiled by de Groot et al.
(1988):
x Surface currents during high and low tide and pressure fluctuations due to waves lead to the grain
relocation and removal according to direction and intensity as well as according to the geometry of the
slope ("transfer function I").
x These hydraulic loads also influence the pore water ("transfer function II"), which, like the surface
discharge, flows seawards, yet much more slowly.
x Which effects the pressure fluctuations have on the surface or a revetment depends on the gradient
when the pore water exits from the slope ("transfer function III").
The interaction between surface water and pore water in the grain skeleton is often not sufficiently
considered. Wave movements on the surface of the water lead to pressure fluctuations on the surface of the
sand. The pore water cannot follow simultaneously, due to the air enclosed in the water even at greater
depths. Compared to the surface water, the pore water thus undergoes permanent change between negative
and positive excess pressure. The positive excess pressure in the pore water, however, results in an
increasing outward flow and a reduction of the effective stresses. Depending on the strength of the subsoil,
these effects can lead very quickly to the liquefaction of the soil and thus to downward relocation and to the
flattening of the slope.
In the following, the mentioned components are described separately.
VI.2
Top (armour) layer
Dimensioning the top layer is a primarily hydraulic problem: Currents and waves are not to damage the top
layer; i.e. the relocation of construction components is to be prevented, wave impact must be tolerated
without damage, and energy is at the same time to be dissipated. Furthermore, loose components must be
sufficiently interlocked and the used elements must meet certain quality criteria. Due to the high demands on
such protection structures, primarily technical solutions are chosen. But combinations with bioengineering
measures are possible.
There is a huge range of possible construction elements. Excluding special constructions for breakwaters,
there is still a large choice of alternatives. At German inland waterways, riprap is most common, sometimes
partially grouted. Above the zone of fluctuating water levels, riprap can be covered and filled with topsoil
and then planted. In the zone of fluctuating water levels, however, plants cannot grow due to loads generated
by the current or waves. Depending on their habitat, animals seek refuge in the cavities of the riprap.
498
ICSE6 Paris - August 27-31, 2012
- Michael Heibaum: Natural and artificial material for scour protection
To allow plants on fully grouted revetments, often plant pockets are arranged. But such effort is pointless,
as the individual root balls are too small and can hardly tolerate strain. Additionally, vandalism is moreover
frequent.
Due to the insufficient supply of natural stones, concrete blocks are used in many cases. If placed
regularly, they are less permeable than riprap and offer no cavities for habitat. As the concrete surface is not
a particularly pretty sight, the revetments protecting the Danish North Sea coast for example are installed
further inland with a flat sandy beach filled hydraulically in front. So there is only the risk that the beach
erodes during winter storm surges and has to be restored in the spring.
At Dutch canals, concrete blocks are used with holes to increase the top layer’s permeability and to allow
planting. However, the stems of plants must be prevented from being scraped at the edges because damaged
stems hinder a permanent settlement of vegetation. The same applies to loose armour stones, which, if too
small or not heavy enough, may be relocated due to drawdown and waves and then may also damage the
plants.
Revetment systems incorporating plants and riprap have been examined for inland waterways with
considerable success. These gabion-like revetments include riprap and precultivated plants. Thus, hydraulic
loads are excluded from the critical first growing phase, and the mats can provide sufficient protection to the
bank right after their installation. The mats are filled with stones to ensure the necessary stability, and with
lava to provide water storage and sufficient humidity for the plants also when being temporarily above the
water level.
VI.3
Cushion layer
A cushioning layer is recommended if the components of the top layer, at their installation or during
service, represent an overload to the layer below, e.g. an asphalt lining or a geotextile filter. This can occur in
case of impact due to dumping or in case of abrasion due to current- or wave-induced rocking of the stones.
A cushion layer may also serve to distribute the top layer’s weight evenly, which is necessary, for example,
for impervious linings wit geosynthetic clay liners (GCL, bentonite mats). The specific construction method
is chosen according to general design considerations. However, it must be ensured that the material of the
cushioning layer cannot erode through the cavities of the top layer, i.e. filter criteria have to be obeyed.
Usually sandy material is chosen for cushion layers which means no negative effects on roots of plants.
VI.4
Filter
The filter is one of the revetment’s most important components (Heibaum 2004). Either granular or
geotextile filters can be applied. The use of granular filter is well known since long and geotextile filters are
installed increasingly over the past 30 years. Filters must fulfil two tasks: On the one hand, they are to
prevent the removal of soil – usually called "retention criterion". On the other hand, and this aspect is often
neglected, they are to ensure sufficient water discharge under the given conditions for their entire operating
life – the so called "permeability criterion". These demands are in many aspects contradictory. That is why it
is particularly important to know the local conditions in order to find the optimal solution.
Finally, the difficulties which go along with the installation are to be taken into account. Often we are
"fishing in murky waters", i.e. in sediment laden water with no sight to depth. Building contractors have
gained certain experience; however, the possibilities of monitoring still need to be improved. Checks and
monitoring are not a question of distrust, but of ensuring optimal implementation. There is still a lot to do in
this area.
Like cushion layers, sandy filters will not restrain root growth. Only coarse gravel might hinder root
penetration. Geotextile filters are penetrated by roots and do not influence adversely the plant development.
499
ICSE6 Paris - August 27-31, 2012
VI.5
- Michael Heibaum: Natural and artificial material for scour protection
Impervious lining
An impervious lining is required if water loss from the waterway is too high or if interaction with the
groundwater is to be prevented. The latter is becoming increasingly important. However, it makes the
dimensioning of the revetment even more difficult, if the groundwater table is high.
There are three fundamental types of lining: asphalt surface, top layer grouted with impervious mortar, and
clay lining. The first two options combine armour layer and impervious lining. Thus, the revetment needs
fewer layers and is less high. However, from an ecological point of view, these methods are not preferable,
since they don't allow for vegetation.
As is the case with all rigid construction methods, fully grouted revetments or pavements are inflexible and
susceptible to fractures. Rhizomes and roots may damage the asphalt lining, as asphalt is a highly viscous
liquid that cannot bear the pressure of growing roots or rhizomes. Asphalt and mortar are difficult to install
on slopes with a gradient steeper than 1:3 because certain flow properties need to be ensured to allow for
filling all voids, but, at the same time, movement down the slope has to be prevented.
As to clay lining, based on experience, undrained shear strength values of cu=15 to 25 kPa have been
deemed optimal for deformability but sufficient stability. Exerting a certain pressure on the clay lining
during its installation provides further advantages. Any clay lining requires more revetment thickness
because, apart from the clay lining itself, a filter (as long as it is a granular filter) and a top layer contribute to
the total thickness. This is due to the fact that clay is susceptible to erosion and thus needs to be protected.
Usually riprap or partial grouted riprap is used for armour. Similar to natural clay, geosynthetic clay liners
can be used to provide an imperious lining. Installed in the dry, these systems have proved their reliability.
Installation under water is rather delicate and needs special care. Both natural clay liner and geosynthetic
clay liner are susceptible to root penetration. So near such lining, only grass should grow on the top soil.
Applying geosynthetic membranes as lining has so far been considered inappropriate for waterways even
though they hinder root penetration. The material is very sensitive and cannot be self-healing in case of
leakage. Such membranes have nevertheless already been installed at inland flood protection dikes (as well
as for landfills), where it can be installed in the dry.
VI.6
Levelling layer/separation layer
Levelling layers are installed to ensure an even subgrade for the revetment, if this cannot be provided by
digging. If the soil is not very stable, the levelling layer (which then has to be thicker) functions as soil
replacement, e.g. if the slope cannot be shaped properly because the soil in-situ tends to fluidize. When
dimensioning levelling layers, filter rules have to be obeyed, since the goal is to prevent the soil in-situ from
penetrating the layer and to prevent the layer material from eroding. If a levelling layer is installed below a
clay lining, interface mechanisms like degradation of the clay surface must be prevented. This means in
particular that fill material should not be too coarse.
A separation layer is required below impervious top layers. It is mostly implemented in form of a
geotextile. The separation layer promotes the self-healing of cracks with respect to percolation of water.
VI.7
Subsoil
No revetment will perform properly if the stability of the subsoil is not ensured. The stability is determined
by the interaction of grain skeleton, surface water and pore water.
Slope failure, i.e. the sliding of the soil body on a curved sliding surface, is the most prominent example of
insufficient stability. Pore water pressure and pore water flow need to be particularly considered in the
corresponding calculations: An allegedly stable slope may slide, if, e.g. in case of flooding, the soil is
saturated and the water level drops relatively quickly. By this, the induced flow of the pore water produces
500
ICSE6 Paris - August 27-31, 2012
- Michael Heibaum: Natural and artificial material for scour protection
additional and often substantial forces! Furthermore, a rapidly falling water level (drawdown) causes excess
pore water pressure in the soil, affecting the (geotechnical) stability of the bank.
To evaluate the stability it is important to establish whether the pore water in the underlying soil is able to
follow the changes in the water level of the river or canal without significant excess pressures being
generated. A comparison of the drawdown rate of the water level and the hydraulic conductivity of the soil
can provide a conservative estimate of whether excess pore water pressure is being generated. Drawdown
rates less than the hydraulic conductivity result in only small gradients of the pore water flow, so the
associated flow force can be neglected with respect to the bank stability.
The reason is that natural surface water and pore water in the subsoil are not an ideal, incompressible fluid.
Small microscopic air (more generally: gas) bubbles are dispersed in the water, so the fluid shows a certain
compressibility. Compressible pore water causes a delayed reaction of the pore water pressure on any
pressure change at the boundaries if the hydraulic conductivity of the subsoil is lower than the velocity of
that pressure change. Due to this phenomenon, bank stability is affected by the interaction of surface water
and pore water. Figure 1 shows the development of excess pore water pressure for a river bank subjected to a
sudden drawdown. Excess pore water pressure will reduce the effective stresses in the soil, in the limit state
to such extent that the shear resistance is too low to avoid sliding. To regain stability, the effective stresses
have to be increased which can be done by an appropriate surcharge.
Figure 1: Pore water pressure distribution due to sudden draw down.
Figs. 2 and 3 show clearly that a strong but light erosion protection is not sufficient: The soil below the
erosion blanket was relocated downslope due to the waves (i.e. the drawdown of the wave trough) during a
moderately higher water level. Such soil transport will lead finally to failure of the erosion protection. So
weight is needed that cannot be provided by plant systems (e.g. willow brush mattresses). There is the
possibility to fix a (light) erosion protection (with sufficient tensile strength) by prestressed anchors that
cause a membrane tension in the cover. Only then soil movement would be prevented. Without prestressing
the soil would be relocated below the anchored cover. The associated effort is rather high, so it would be
easier to add weight, e.g. by the aforementioned gabions combining rock and pregrown plants.
VI.8
Installation
A part from the differences described in this paper, any revetment will only function properly if installed
properly. Experience gained at inland waterways and at the coasts shows that even well-proven construction
methods may undergo damage if installed wrong. Thus, possibilities and limits of the installation must be
considered at the design stage. Detailed requirements to the tender are useless if these cannot be realized or
monitored. Regarding the revetment as a whole is required not only for the design but also for the
anticipation of installation problems:
– The contractor should not be confronted with requirements if problems are known beforehand (e.g.
under the rough conditions at coasts, requirements in a 5 cm range are illusory).
– Limit values are only effective if monitoring is possible (e.g. considering the evenness of the
subgrade).
501
ICSE6 Paris - August 27-31, 2012
–
- Michael Heibaum: Natural and artificial material for scour protection
On the other hand, monitoring should be performed as often as possible. However, there are many
areas where monitoring is hardly possible or not possible at all.
Figure 2: Coir erosion protection blanket immediately after installation.
Figure 3: Protection blanket deformed by downslope relocated soil after moderate flood waves.
VII CONCLUSIONS
Due to the hydraulic loads and interactions at the coast and at inland waterways protective structures are
often inevitable. Such (technical) structures are often contradictory to the natural environment. There is no
real question if nature or safety has to be focused on: safety comes first! But discussing the elements of
revetments reveals certain possibilities in how far technical solution can be complemented by biological
elements. Since weight is a major factor, pure biological solutions are hardly possible, but combining natural
and technical elements offers ecological and strong solutions.
VIII
REFERENCES
GBB (2012): Principles for the design of bank and bottom protection for inland waterways. Mitteilungen
der Bundesanstalt für Wasserbau. Karlsruhe: Eigenverlag 2012
Groot, M.de, Bezuijen, A., Burger, A.M., Konter, J.L.M. (1988): Te interactions between soil, water and
bed or slope protection. In: Kolkman et al. (eds.): Modelling Soil-Water-Structure Interactions. Rotterdam:
Balkema 1988.
Heibaum, M. (2004): Geotechnical filters - the important link in scour protection. Keynote paper in:
Proceedings ICSE-2 (Second International Conference on Scour and Erosion), Singapur, 14.-17. Nov. 2004.
502