Foreshores in freshwater environments
Foreshores in freshwater environments can consist of freshwater wetlands or floodplains. A shallow foreshore or floodplain can contribute in different ways to the strength, the stability and the flood protection capacity of a dike. The foreshore can reinforce the macro stability of the dike and can strengthen the dike by increasing the seepage length. Under certain conditions the dike’s crest can even be designed lower than in a traditional design due to the wave-attenuating effect of the foreshore. This may be enhanced by introducing vegetation such as reeds, shrubs or trees. In the absence of a natural foreshore, a gradually sloping foreshore can be constructed in front of the dike.
Main characteristics of this foreshore dike are the gradual slope, in comparison with the steep slopes of traditional dikes, and the different vegetation zones on this slope that play a role in reducing flood risk. The habitats that can develop on the slope of this dike provide services to the adjacent ecosystem and opportunities for recreation and landscaping. If the adjacent water contains enough sediment to be trapped by the vegetation, the combination of sediment trapping and vegetation-induced soil formation enables the foreshore to adapt to a slowly increasing water level (relative sea level rise).
A freshwater foreshore in combination with a green dike
Dikes bordering freshwater systems such as rivers or lakes are usually built as relatively steep structures consisting of a sand core with a clay cover and a grass mat on top of it. Synergy of these dikes with the surrounding landscape is generally not included in the design. Depending on the situation, the area around the water line is protected by a revetment of rock or concrete elements, or by a vegetation zone, often a band of reed. These levees are designed to withstand design water levels and overtopping of locally generated wind waves. Especially the overtopping restriction can lead to a significant extra design height and the need to stabilize the inner slope (usually with a berm). Adding a berm often requires considerable space and consequently leads to substantial additional costs.
Some older levees still have a core of material that was locally available by the time, such as peat or seaweed. Apart from this, many older dikes don’t meet present-day stability requirements, because they have been built too steep. Also, dikes in areas with a sandy subsoil may be susceptible to piping (the formation of sediment transporting seepage channels), which can undermine the dike (for instance see IVW 2011). In order to maintain the prevailing flood safety level, dikes which are not up to the prevailing quality standards have to be re-engineered, or other measures need to be taken. If there is enough space available at the water side, constructing or preserving a shallow foreshore may be an option to increase safety, provided that these foreshores are included in the safety assessment procedure (which is not the case in the Netherlands, so far, also see V&W 2007).
Natural foreshore on the Frisian coast of Lake IJssel, the Netherlands
In summary, shallow foreshores may help solving the above problems without socially and environmentally intrusive dike strengthening via:
- reduction of wave impact/overtopping by attenuating waves; incoming waves lose their energy through breaking in shallow water and through the resistance they meet in vegetated areas;
- stabilisation of the outer side of the dike: the soil mass resting on the dike acts as a stabilising berm;
- reduction of piping by increasing the seepage length and - if necessary - by including low-permeability elements like a clay layer.
Additionally, foreshores, whether naturally present or constructed as part of a levee system, generate other functions and services, such as recreation, natural value and carbon sequestration. Furthermore, they form a stepping stone or a corridor that connects existing ecosystems. Other ecosystem services they can potentially provide are water quality improvement and resting, foraging and breeding grounds for migratory and residential birds. Sometimes this type of composite flood defence system can be built entirely without hard elements. In other cases limited use of hard elements, e.g. around the waterline, may be necessary. Also, nourishment schemes can be considered for maintenance.
Lakeside foreshores can be constructed from the water side, using well-known dredging and deposition techniques. This reduces the disturbance of local communities behind the dike (inconvenience, noise, sometimes also removal of property and houses), as well as the potential damage to roads and buildings due to heavy traffic. From a life-cycle point of view, the cost can be less than those of repeated dike strengthening, especially if the foreshore is able to form enough new soil and trap enough sediment.
Digging a clay layer into a foreshore in order to increase the seepage length
A point of attention concerning soft-sediment foreshores, especially in wave-exposed areas, is that they will have a more dynamic interaction with the principal forcing factors (wind, waves and currents) than a dike covered with a healthy grass mat or stone revetments. This can be dealt with by including a zone in which natural processes, such as erosion and sedimentation can take place. In that case, a setback line behind which no erosion is allowed needs to be defined (similar to the case of dune erosion on the coast). If erosion proceeds until or beyond this line, maintenance and adaptation measures are necessary, but these will be relatively cheap as they will mainly boil down to earthwork.
Moreover, the foreshore vegetation is a live element in the system, which means that it will go through a number of succession stages and will vary through the seasons in biomass and flow resistance properties, hence in effectiveness to flood protection. Also, the possibility of degradation due to pests and diseases has to be taken into account (like with grass-covered dikes). Regular monitoring, assessment and maintenance are required to make sure that the system is in place when needed. Finally, there may be legal aspects, as the existing foreshore may be protected habitat (e.g. under Natura2000) or may come under a different jurisdiction than the levee.
How to use
Shallow foreshores or floodplains take space, so they can only be applied where enough space is available. In the case of lakes this is usually not a problem, but in rivers this point requires special attention. Depriving a river from its storage space has implications for the height and propagation speed of flood waves. Since flood storage in a river is determined by the width at the water surface, this means that the foreshore level should preferably stay below the design water level. On the other hand, the level should be high enough to be effective in attenuating waves under design conditions.
Water retention area and natural foreshore in the river Scheldt, Belgium
Note that the effect of storage on flood wave propagation only counts at some distance from the mouth. In medium-size rivers, like the Rhine, the fetch for local wave generation is not very large there, but in larger rivers like the Mississippi or the Yangtze locally generated waves can be higher and their attenuation by shallow foreshores can be an issue, also further upstream.
Freshwater wetland vegetation generally requires suitable hydrodynamic conditions and not to long submergence times. Rivers wetlands can be threatened by unnatural flow conditions, caused for instance by upstream dams, locks and sluices. In lakes wetland development is often hindered by fixed or unnatural water levels, such as a highstand during summer for water storage, and a lowstand during winter to reduce flood risk. Even if the lake level is constant or low, however, inundation of low-lying areas can occur due to storm-induced water level set-up. In Lake IJssel (IJsselmeer, the Netherlands), for instance, wind-induced downwind set-up during extreme events can amount to 1 m above the mean lake level (RWS-RIZA, 2007).
The profile of a foreshore levee is determined by inundation depth, inundation frequency, mean water depth (lower parts) and groundwater level (higher parts). In the lacustrine case, this profile often consists of a soil retaining ridge of relatively coarse sand, if necessary with a hard revetment at the outer side around the mean water level. It also contains a low-lying area supporting a wetland habitat, an upward sloping part to attenuate the incoming waves, a sandy buffer in front of the dike and a shallow ditch in front of the dike in order to control the groundwater level. Reeds can grow in shallow water up to 0.5 m depth with respect to the mean water level. Wetland vegetation will be limited to a zone up to approximately 0.5 m above mean water level. At higher levels grass and shrubs will tend to dominate. Note that such foreshores may also occur naturally. One example is the wetlands on the Frisian coast of Lake IJssel, which evolved from the salt marsh present there before the lake was closed off from the sea in 1932.
Cross-shore profile of a lake-bordering shallow foreshore, from the Houtrib dike pilot
As far as the material used is concerned, much will depend on what is available in the vicinity. In any case, the vegetation-supporting part must contain enough fines and nutrients to be sufficiently fertile. Wave attenuation is often less of an issue in the narrower parts of a river, i.e. upstream of the mouth area. There the main function of a forelshore in front of the dike is to increase the stability of the outer slope and to prevent seepage-related phenomena such as piping, heaving and bursting. The latter is usually achieved by digging in a clay layer and covering it with the original topsoil, to increase the seepage length and thus to reduce the probability of piping. This method also stimulates the re-establishment of vegetation. In the case of lacustrine foreshores, the sand ridge at the offshore end must either provide for a sufficient buffer to accommodate erosion during an extreme event, or be adequately protected against erosion. An important material property is the soil permeability after placement. On the one hand, permeability should be small enough to prevent piping and drying out of the wetland part, on the other it should allow for sufficient groundwater supply to the vegetation.
If the vegetation is an essential part of the flood defence system because it has to provide for a significant part of the wave attenuation vegetation development needs to be considered. Several options are possible to account for this, such as a more robust design of the soil part of the foreshore or possible planting of more developed tree or shrub vegetation. However, if conditions are suitable for vegetation development and there is sufficient seed availability natural vegetation establishment with considerable biomass can be extremely rapid. Vegetation may need some form of maintenance, such as grazing or mowing. In the very least, it needs to be adequately monitored.
Willow vegetation for wave attenuation
Construction costs of foreshores are largely determined by the cost of the required volume of sediment. Maintenance costs involve maintenance of the offshore sand ridge and the vegetation cover. After an extreme event it may be necessary to restore parts of the profile. Construction costs will be comparable to or even less than those of traditional dike strengthening. Monitoring and assessment may involve extra costs as compared to a traditional dike (Table 1, see Haskoning 2011).
Cost estimates for two foreshore project designs described further below (after Haskoning, 2011).
From a life-cycle costing perspective, construction of a levee with a shallow foreshore is likely to be cheaper than a traditional design, as cost savings on construction are relatively high compared to the extra costs for maintenance. Especially if the foreshore designs are able to trap sufficient sediment to keep up with gradually changing hydrodynamic conditions (lake level, river flood levels and sea level rise) future costs for maintenance and levee upgrading may be considerably less.
The foreshore ecosystem can provide a variety of ecosystem services:
- regulating services, such as wave attenuation, erosion prevention, sediment trapping, soil formation, nutrient cycling, greenhouse gas sequestration, water purification;
- provisioning services, such as groundwater storage, food production for herbivores, resting/foraging/breeding place for migratory and residential birds, biomass production, corridor between separated existing ecosystems, nursery for fish, shellfish and crustaceans;
- supporting services, such as the availability of freshwater wetland habitat, room for a variety of recreation functions;
- cultural services, such as the aesthetic value of a ‘natural’ landscape.
Foreshores may be used for a variety of other functions, such as recreation (beaches, small marinas) where there is room for it, eco-tourism, extensive herbivore grazing and meat production, biomass production (in relation to vegetation maintenance), etc. Some existing foreshores have been built up to a level well above flood level and are being used as industrial zones. In that case, there are no nature functions, but the dike stabilisation and piping reduction effects remain.
In order to make room for the "Nieuwe Merwede", a river branch in the downstream Rhine-Meuse complex, an area of the polder Noordwaard (about 2000 ha), will be used for flood conveyance. To that end, part of the existing dike, designed for water levels with a probability of exceedance of 1/2000 per year, will be lowered to a crest level which is exceeded by floods with a p.o.e. of 1/100 per year.
The polder Noordwaard after implementing Room for the River measures. Fort Steurgat can be found in the top right corner.
In the northeast corner of the area Fort Steurgat is located, an old fortress that has been turned into apartment buildings. The dike around the fortress now becomes part of the primary flood defence system, which in present practice would mean that it has to be raised and strengthened to resist a flood with a p.o.e. of 1/2000 per year while bordering a large open water area (the inundated Noordwaard) with wave heights up to 1 m. A first 'traditional' dike design around Fort Steurgat resulted in a dike height of NAP + 5.5 m (NAP = datum and approximately the ground level here), with an armour layer of concrete blocks. This led to opposition among the local population and an alternative was sought with a visually acceptable wave-attenuating element in front of the existing dike.
As an alternative, a building- with-nature design was made that is cost-effective and provides not only safety, but also sustainability and nature value: a wave-attenuating willow forest in front of the dike. In the past, willow forests were ubiquitous in the Dutch river floodplains. They were used for the production of brushwood, among others for application in hydraulic engineering projects.
Design of the willow plantation in front of Fort Steurgat (Dekker en De Vries, 2009)
The final design includes a continuous willow tree plantation in front of the dike, which is expected to yield 80% reduction of the incoming design wave height. Although the willows are planted on a low embankment, it is mainly the vegetation that does the job here. The reduced discharges due to wave overtopping allow to dike crest to be 70 cm lower than in the traditional design. Moreover, a clay-clad dike is able to resist the remaining wave attack, so there is no need for a concrete block revetment. The overall ‘footprint’ of this alternative design is wider, which is not a problem in this case.
Cross profile of design for willow plantation and dike at Fort Steurgat (Dekker en De Vries, 2009)
Having a living element as part of a flood defence system is outside of the present expertise of dike engineers and inspectors. This means not only that expert input from biologists is needed, but also that new methods of assessing the flood defence system have to be developed. Moreover, this living element involves a certain degree of uncertainty whether the forest will be in good shape at the moment it is needed. In order to reduce the system’s sensitivity to pests, diseases, ice winters, forest fires, etcetera, special measures have been taken, such as the use of a mix of willow species, and zonation of the forest with fire corridors in between. An alternate mowing an cutting scheme assure the continuous presence of zone with a dense forest of 2-year branches.
Kop van ‘t Land - Island of Dordrecht
Near the mouth of a river, especially in areas where the influence of tides is felt, the width increases significantly and locally generated waves become a point of attention. In that case, their effects can no longer be ‘hidden’ in a safety margin for the crest height, but have to be taken explicitly into account in the design of the flood defence system.
One location where this is the case is the Kop van ‘t Land, the easternmost tip of the embanked part of the Island of Dordrecht. Here the Nieuwe Merwede, one of the main branches of the Rhine-Meuse complex, is already wide, with the Noordwaard area at the other side. This means that during high floods, when the Noordwaard is inundated, there is a considerable fetch for winds from south-westerly and south-easterly directions.
The dike at Kop van ‘t Land has a key function in protecting the urbanised area of the Island of Dordrecht from flooding. From a risk management perspective, it is therefore wise to make it very strong and safe. This will indeed be done in a forthcoming strengthening project. So far, however, possibilities to reduce wave attack have not been considered. There is ample space for vegetation-covered foreshores at either side, and also across the river vegetation can help reducing the wave exposure. In a forthcoming pilot project, the possibilities to better use vegetation and shallowness to reduce wave attack on this dike will be further investigated.
The Island of Dordrecht, with Kop van ‘t Land in the red circle
Wieldrechtse Sea Dike - Island of Dordrecht
At the Island of Dordrecht (a most interesting place from a flood risk management point of view), there is also the Wieldrechtse Sea Dike, which now serves as a secondary flood defence at the south side of the urbanised part of the island. South of it is a zone of agricultural land, protected by a primary flood defence which presently satisfies the safety standards. The west part of the area includes some vital infrastructure, such as the north-south motorway A16, the railway between Rotterdam and Brussels and the high-speed railway Amsterdam-Rotterdam-Brussels-Paris.
Location Wieldrechtse Sea Dike
As the urbanised area north of the Wieldrechtse Sea Dike is located in a small dike ring that will rapidly flood if anywhere the flood protection system fails, timely evacuation will be difficult. Hence the individual risk is above the norm of 10-5 per year that will be part of the new flood safety norm system to be introduced in the Netherlands. Therefore, the flood protection of this area will have to be enhanced. As there are no plans to urbanise the southern part of the island, the present safety norm can be maintained there. This has led to the idea not to raise and strengthen the existing primary flood defence, but to use the southern part of the island as a shield against flooding of the urbanised area. South of the eastern part of the Wieldrechtse Sea Dike a park with wave-attenuating vegetation is planned (the light green area in the figure above), which in combination with the existing dike must provide for the required safety. The present primary dike will stay in place. This spatially extended option of a flood defence system (as opposed to the usual line approach), which is probably cheaper than strengthening the existing primary defence, will be further explored in a forthcoming pilot study.
Other examples of floodplain restoration and engineering
In various river basins around the world large areas of floodplain are being reconnected to the river, for instance by removing or laying back levees. Thus flood levels are brought down, the water quality and spatial and environmental quality are enhanced and new opportunities are created for economic activities such as recreation. Examples are: the Make Room for the River program for the Dutch Rhine in the Netherlands, various floodplain restoration schemes along the Danube, and floodplain projects in the Mississippi basin. Another example is the unembanked Meuse River in the Netherlands, where accreted floodplains are reconnected to the river by lowering them.
Floodplains along tidal rivers are important to accommodate and attenuate tides and storm surges penetrating into the river. Encroachment on these areas in the past has led to an increase of flood levels, tidal range and, in combination with channel deepening for navigation, to a net import of fine sediment and very high levels of turbidity. Examples are the Scheldt in Belgium (increased flood levels and tidal range; e.g. Dam et al., 2013), and the Loire in France and the Ems in Germany (increased tidal range and turbidity; Winterwerp and Wang, 2013; Winterwerp, 2013). Examples of floodplain reconnection to a tidal river can be found along the Belgian Scheldt, for instance (Temmerman et al., 2013).
Oeverdijk - Lake Marken
Lake Marken (Markermeer) used to be part of the former Zuiderzee, later Lake IJssel, and was separated in 1976 from Lake IJssel by the Houtrib Dike. The original intention to reclaim the land was given up and the area remained a shallow lake. In the 2006 assessment of the Dutch flood defence system the dikes at the west side of this lake were found insufficiently stable and susceptible to piping. Raising them, however, might be problematic, as they are resting on a thick layer of peat. This has led to two responses: a study on how much weight the peat subsoil actually can carry and the development of an alternative to dike strengthening, the so-called Oeverdijk. It is basically series of shallow wave-attenuating, piping-preventing and stabilising foreshores at the lakeside of the existing dike.
Location of Oeverdijk along Lake Marken, between the cities of Hoorn and Amsterdam, the Netherlands
The design of the foreshore varies alongshore, depending on the local bathymetry and possibilities for ecological optimization. At many locations, the designed foreshore is higher than the design water level, which would render the existing dike redundant. Yet, this is maintained for security and because removing it will be more expensive than leaving it in place.
One aspect that needs to be tested is the chance that the foreshore is eroded away or that its top breaches during an extreme storm. In order to have a worst-case estimate, a dynamic dune erosion model has been applied to the foreshore profile as if it were bare loose sand. Conclusion: the profile tested provides for a sufficient buffer for the top part not to be eroded or breached. Clearly, this a test is case-specific and has to be repeated for every new profile.
Houtrib Dike - Netherlands
Although the Houtrib Dike separates two bodies of water, it is part of the primary flood defence system. In the 2006 assessment the dike’s revetments (among other elements) at the side of Lake Marken were found to be insufficiently stable. A wave-attenuating (and ice-resistant) foreshore would be an obvious option here. As strengthening of this dike is considered less urgent and is actually subject to debate, the authorities responsible decided to first carry out a pilot project with a shallow foreshore along part of the dike.
Floating ice at Lake Marken piling up against the Houtrib Dike (source: www.nufoto.nl)
Along a short stretch of this dike Rijkswaterstaat and EcoShape are about to carry out a combination of three pilot experiments, with foreshores of different width (i.e. the distance between the intersections of the foreshore profile with the existing dike profile and the lake water level). The foreshores mainly consist of sand, whence their capacity to cope with the design wave attack has been tested with a standard dune erosion model (XBeach). The sand bodies are to be planted with vegetation to attenuate incoming waves and to prevent wind erosion. Moreover, the sand body is supposed to keep floating ice away from the dike and the road.
Planview of the Houtribdijk foreshore pilot experiments
Sand engines - IJsselmeer (NL)
At the time Lake IJssel was formed by the construction of a 30 km dam that closed off the sea arm Zuiderzee, the Frisian coast was bordered by extensive saltmarshes. As the water of the lake gradually turned fresh, the saltmarsh turned into freshwater wetland. In the meantime, this has become a most valuable ecosystem supporting rare plant species and large numbers of resident and migratory birds.
Situation at Workumerwaard, Frisian coast, Netherlands
Although this foreshore was not man-made, but only influenced by man-made works (the dam that closed off the sea arm), it functions as a wave-attenuating element in the flood defence system. Now that the vegetation has reached its climax stage and is gradually losing its vitality, and the shoreline is slowly eroding, a pilot experiment with an offshore nourishment is being run, with the intention to gradually bring sediment onshore, stop erosion and create space for new pioneer vegetation without destroying the existing one. For more details and figures, see Sand Engine Workumerwaard.
- Depending on the situation, dike raising and strengthening can be avoided by creating a shallow wave-attenuating foreshore.
- Such foreshores also create room for multi-functional use, thus to enhance the system’s benefits.
- There is no generally applicable design, the solution to be chosen depends on the local physical, ecological and societal situation.
- Until this has become proven technology, many decision makers are reluctant to adopt this type of solution, because of the perceived risks involved (project delay, uncertainty of the nature component). For the time being, implementation of these concepts in practice depends on ‘champions’ at high political and administrative levels. They can help to get a number of pilot projects off the ground in order to show that the concept works in practice and can be cost-effective.
- Cost-benefit analyses should take a long-term perspective, with a time horizon exceeding the lifetime of the present project. In that case, the adaptation capacity of shallow foreshores becomes part of the equation.
- Discounting, as usual in cost-benefit analyses, does not apply to nature components in hydraulic engineering projects, as these cannot be purchased at an arbitrary point in time, but need time to develop.
- Dam, G., S.E. Poortman, A.J. Bliek & Y. Plancke, 2013. Long-term modeling od the impact of dredging strategies on morpho- and hydrodynamic developments in the Western Scheldt. Proc. WODCON XX: the Art of Dredging, 14 pp.
- Dekker, F and M.B. de Vries, 2009. Rapportage Ontwerp Groene Golfremmende Dijk FortSteurgat definitief
- Deltares, 2012. Synergy of safety and ecology. Report no. 1205256-000, 80 pp. (in Dutch)
- Haskoning 2011 (details to follow)
- IVW 2011. Third assessment primary flood defences. Netherlands Ministry of Infrastructure and the Environment, Inspectie Verkeer en Waterstaat, Report IVW/WB/2011/000002, November 2011, 31 pp. (in Dutch).
- RWS-RIZA 2007. Hydraulic Design Conditions 2006 for the assessment of flood defences. Ministerie van Verkeer en Waterstaat, September 2007, 295 pp. (in Dutch). ISBN 978-90-369-5761-8.
- Temmerman, S., P. Meire, T. Bouma, P. Herman, T. Ysebaert & H. de Vriend, 2013. Ecosystem-based coastal defence in the face of global change. Nature, 504: 79-83, doi: 10.1038/nature12859
- V&W 2007. Directions for safety assessment of the primary flood defences (VTV2006). Ministerie van Verkeer en Waterstaat, August 2007, 447 pp (in Dutch). ISBN 978-90-369-5762.
- Winterwerp. J.C. & Z.B. Wang, 2013. Man-induced regime shifts in small estuaries. Part I - theory. Ocean Dynamics 63: 1279-1292, doi 10.1007/s10236-013-0662-9
- Winterwerp, J.C., 2013. On the response of tidal rivers to deepening and narrowing. Deltares, March 2013, 83 pp.
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