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Traditionally revetments are designed to provide a safety. This leads to large scale monotonous application of materials and shapes that are not optimized for habitat diversity. The Rich Revetment concept aims to create highly variable habitats in the intertidal and subtidal zone of dikes and foreshores while maintaining safety levels. This approach utilizes a variety of different materials, gradients and shapes to create differences in height and hiding places. This leads to a variation of environments with different exposure levels to currents and waves. Tidal pools, on the picture right, is an example of a rich revetment solution to provide shelter and attachment opportunities for species of animals and plants. These solutions are suitable for engineers involved in dike (re)construction and maintenance project or other water related infrastructure such as harbours, quays and piers.
General Building Block Description
Structural works like dikes are mostly designed from a safety point of view. Optimization for enhancing ecological functions is not often considered in the design phase. The objective of rich revetments is (besides coastal protection) to contribute to the ecological value of a dike. The aim is to create a nett positive contribution to natural value of the dike section. Other objectives are to create educational opportunities and to raise awareness of designers, constructors, students and visitors.
Advantages compared with 'traditional solution'
When the physical conditions or design constraints require hard solutions, the rich revetment concept still provides opportunities for added services. A rich revetments design can be a robust design that does not require additional maintenance. The following advantages are recognized:
- Provision of valuable hard substrate habitats to the surrounding ecosystem
- Provision of connectivity along the shore, in the form of a continous ribbon of diverse hard substrate habitats (migration hubs). Potential to link previously disconnected areas of high natural value.
- Potential for collection of edible plants and animals, for instance to support of fish population for fisheries
- Contribution to an attractive landscape for recreation such as visitors, recreational fishing, and divers (Barbier et al. 2011).
- Construction of a rich revetment solution could provide mitigation and compensation measures with respect to requirements of environmental legislation such as water framework directive and natura 2000.
- No loss of existing foreshore, because the footprint is equal to the classic revetment solution. This will reduce conflicts with environmental protection regulations, especially Natura 2000.
Disadvantages compared with 'traditional solution'
- Construction of rich revetments could add to the costs of the project.
- Added attractiveness for recreation could add additional safety risk for the public or hinderance for the surrounding.
- Provision of connectivity along the shore, could provide habitat for invasive species as stepping stones (see for instance DELOS project, http://www.delos.unibo.it/).
- In some countries, creating pools in revetments is not allowed due to danger of breeding mosquito's.
How to Use
To determine the feasibility and design of a Rich Revetment project guidance is presented below. As a first step of the design process an analysis of the planned works and its bordering ecosystem is needed. On the basis of conceptual design principles a technical design can be realized and budgeted.
- Discussion with client on details of civil engineering design and specification of ecological and recreational potential
- Analysis of local ecosystem biodiversity of hard substrate species.
- Analysis of physical forcing factors and limiting design criteria as parameters and boundary criteria for ecologically enhanced design. In this case wave exposure and siltation risk are critical design factors.
- Analysis of surrounding ecosystem and identification of possible interactions and reinforcements. In the Netherlands Eastern Scheldt case the value of the diverse hard substrate community is already established. Maintaining this values is a requirement for dike reconstruction in the Eastern Scheldt.
- Discussion with client on selection of suitable dike sections, depending on planning
- Pre-design, valuation and costing of ecologically enhanced structures and presentation to the client
- Analysis of the physical aspects of the water system and the dike section (foreshore and intertidal morphology, wave conditions, shape of the dike and the toe)
- Consideration of the possible interests involved in the area (recreational activities such as scuba diving, commercial fishing etc.)
- Selection of most promising eco-structure. In the Eastern Scheldt case focus was put on keeping the water at the dike toe and provide suitable substrate for attachment of macro-algae.
- Technical design of most promising eco-structure within overall civil engineering planning and requirements
- Slopes and orientations
- Choice of materials
- Size distribution, porosity
- Application method
- Seeding method (if required)
- Detailed costing and phasing within major project planning
- Small scale field trial
- Setup of monitoring and maintenance plans (if required, this is a necessity in the case of pilots)
- Technical design should include interaction with biologist to ensure optimum design. It is noted that the proposed approach is mainly empirical in nature. Field trials are an important way to establish the feasibility, site constraints and availability of biological recruits to colonize newly established hard substrates for any local situation. Some field trials concerning effectivity of maintaining wet conditions, material type and sorting are executed by University of Applied Sciences Hogeschool Zeeland in cooperation with project Zeeweringen, Imares and Deltares.
- Habitat enhancement measures such as those proposed will be designed not to interfere with the prime objectives of coastal defence.
- Habitat enhancement measures should not lead to unwanted proliferation of nuisance or invasive species.
- Assessment of project implementation by biological expertise
- Execution of the project
- Evaluation of ecological development through monitoring during 3 consecutive years by biological expertise; especially for pilot like enterprises
Costs of rich revetment solutions widely differ and there is no standard guideline that indicates costs.
Below, a number of different field cases are presented providing applications of the concept of Rich Revetment. Design or application can greatly differ amongst sites, depending on goals (coastal protection, nature development, retention) and specific local conditions. Experiences from these cases can be used as inspiration for future designs. For more information on the Building with Nature cases we refer to the case description pages.
The pilot project in Yerseke is the first location in the Netherlands that uses tidal pools, called ecobasins, in the "toe" of the dike. The "toe" is the fortified link between the slope and the front shore. The dike toe consists of stones, protecting the dike against erosion and to support the dike cover. As a pilot, ecobasins are designed on a dike section of 1 km. 10 short pools (basin type 1) and 2 long pools (basin type 2) are installed on this dike section. The basins are waterproof and water remains in the ecobasins during low-tide.
Within the basins various stone types and sortings provide fixation for algae and shelter for juvenile fish and macrofauna. The basins at the toe of the dike are sealed with sheet asphalt and then filled up with lava stone. The dike toe is located near the low tide line, so that water remains in the basins after high tide.
In all basins the total number of species have increased. Basins that fall dry contain less species than basins in which water remains during low tide.
For several locations along the Eastern and Western Scheldt the dike foreshore needed to be strengthened in 2009. The requirements for this contract to ensure the dike stability was originally focussed on economical aspects only. However, the special ecological value of two locations in the Eastern Scheldt were recognized.
The approach to enriching the foreshore involved creating as many different habitats as possible. The engineering design called for a variety of different materials, gradients and shapes. The design aims to create differences in height, hiding places, and variations in the exposure to and shelter from the currents. To ensure flexibility, the design consists of a modular system of blocks consisting of round, criss-crossed and atoll-shaped piles of stones and linear elements, all in varying sizes. Combining these blocks made it possible to achieve more variety at a larger scale.
The development of the underwater landscape is monitored by divers. A fast recovery was stated as well as a high biodiversity. Also rare organisms settle within the landscape. The site has developed as a popular diving spot, this enlarges the supportive group for these types of alternatives.
The breakwaters of the entrance of the North Sea Channel at IJmuiden (The Netherlands) protect the port against wave attack. The breakwaters consist of concrete blocks. The surface of the blocks, the cracks, and spaces between these blocks are habitats for a diversity of marine flora and fauna like algae, insects, crabs and shellfish, fish and birds (including a red list species). Because of this, it is important that during and after renovation of breakwaters, the affected hard substrate habitats will recover quickly.
In order to stimulate the growth of marine species on the breakwaters, the aim of this pilot study was to test slabs with various textures and geometric shapes attached to the concrete blocks for algal and macro-faunal colonisation (see picture).
In conclusion, small adaptations of both texture and structure of concrete constructions within the intertidal zone of the marine environment lead to better settlement and growth conditions for algae and macrobenthos to settle and grow. Thus primary and secondary production is enhanced, without decreasing the safety level of the port.
Port areas consist mainly of man-made constructions, such as seawalls, piles and pontoons. These hard structures are favoured as a settling substrate by different organisms, such as algae, mussels, sponges and oysters. However, substrates in harbours are often smooth hampering establishment of organisms, and provide little hiding place for larger animals such as fish, lobsters and crabs. Traditional harbour design results in a smooth underwater profile. Possibilities to create simple but effective profiled structures in harbour areas are investigated in pilot study.
In order to create a more profiled underwater environment this pilot aimed to come up with designs for suspended artificial substrates. The goals was to
- promote the settlement of mussels and consequently contribute to water quality in harbours.
- enlarge available substrate for settlement,
- increase biomass of filter-feeders (e.g. mussels)
- enhance habitat diversity in port areas.
For the Rotterdam harbour two specific structures were selected for further elaboration: polehulas and pontoonhulas. The hulas resemble Hawaiian skirts and consist of bands with ropes that could be wrapped around poles or attached to pontoons.
Use of artificial substrates, such as hulas, can increase the amount of biomass considerable.
Wave reducing poles - Nieuwe Waterweg (Netherlands)
One of the first experiments carried out in the framework of Rich Revetment are the wave reducing poles. The poles serve as attachment site for all kinds of plants and animals and simultaneously reduce the height of incoming waves. This project experiments with different types of poles (wood and concrete) and ropes (nylon and sisal) to determine the best design.
The poles provide a hard substrate for the establishment of all kinds of plants and animals. E.g. mussels can filter the water and add to the water quality. These mussels are an important food source for birds. Behind the poles, the birds can find a sheltered area for foraging.
A wave reducing pole forest reduces the wave load. It can be an alternative in case of a shortage or inadequate crown height trim, but does not solve an unstable embankment or dike with probability of piping. Calculations with the SWAN wave model have been performed for the wave reducing willow forest in for the Noordwaard. A similar broad pole forest is a (very) expensive construction but the wave reducing poles can be an alternative for wave reducing reef. The poles should be sturdy enough for storm conditions, sufficiently deep rooted and sufficient high above the waterline.
In order to make room for the river "Nieuwe Merwede", a section of the polder Noordwaard (about 2000 hectare), presently protected by the river dike will change into land on the outside of the dike, exposed to regular flooding. The area has several functions, mainly agriculture, tourism, nature, living and some industry. A new primary river dike is required in the North Eastern corner of the Noordwaard to protect the inhabitants at Fort Steurgat. During a 1/2000 year discharge event the average water depth in the polder will be 3 meter whereby, in combination with a severe storm, waves up to 1 meter high are expected near Fort Steurgat. A first 'traditional' dike design around Fort Steurgat resulted in a dike height of 5.5 meter above NAP, with concrete blocks as armouring layer, leading to protests from the local population.
To create an ecodynamic design that provides safety, that provides additional values for nature and recreation and that is practical from the viewpoint of costs and durability. The construction of the wave reducing eco-dike has a number of impacts on the dike design. The objective of the design is to produce wave reduction in order to reduce wave overtopping on the dike. In the final design a continuous willow tree plantation in front of the dike provides at 80% reduction of incoming wave height at 1:2000 storm conditions. This allows the design of a dike with a 70cm reduced crest height, without violation of maximum overtopping limitations. Furthermore, reduced wave attack allows for the design of a clay clad dike in stead of a dike with hard armour layer. The willow plantation is inspired by a centuries old traditional culturing of willow trees for use as brushwood in swamp areas
The construction of the wave reducing eco-dike has a number of impacts on the dike design. Firstly, the new dike has a wider footprint than the 'traditional' dike. The design includes the implementation of a willow tree plantation. This living element is outside of the expertise of traditional dike designers and necessitates expert input of biologists. Some uncertainty remains on sensitivity of tree plantation to disease, ice-winters, forest fire, stability under extreme wave forces. Therefore some contingencies against failure are implemented in the design.
Ecological dike reinforcement - Ellewoutsdijk (Netherlands)
The seawall of the village of Ellewoutsdijk was in need of repairs. However, raising the dikes is not a viable option as that would mean doing away with an ancient fort. Innovative solutions are required in order to preserve safety (ComCoast). It was investigated whether it would be possible to reinforce the dike coverings instead of raising them. That way, in extreme situations the highest waves could crash over the seaside dike without the inward dikes failing as a result. A water retaining top layer has been added to of the armour layer. Additionally, a small (coffee cup size) hole has been made in some stones. At low tide, small puddles remain in these holes, further stimulating algae growth.
Along coasts of large Dutch freshwater lakes, dikes often border the water directly, with relatively steep slopes. Shallow zones and the gradual slope from land to water are lacking. Consequently, species that inhabit these zones are decreasing. In addition, constant lake-water levels cause erosion of shores. To dampen waves and recreate gradual land-water transitions brushwood mattresses were constructed in front of the dike. These mattresses might facilitate development of floating reed marsh in the shallow zone in front of a dike. This marsh reduces wave impact on the dike, enhances sedimentation and creates a clear shallow water zone with (submerged) vegetation.
The innovative application of braided brushwood mattresses aims to create floating foundations for emergence of reed vegetation. The floating mattresses locally reduce currents and waves thereby decreasing hydraulic loads on the dikes and creating valuable habitats above and below water. More benign conditions stimulate settling of suspended solids and promote the stabilisation of silty soils.
Tiles at seawalls - Singapore (Netherlands)
Increasing urbanisation worldwide has resulted in extensive replacement of natural habitats with man-made habitats (Chapman et al., 2009). A good example is the artificial seawall, that has becomes an ubiquitous feature of the coastline with the increased need for protection (Moschella et al., 2005). This has resulted in habitat fragmentation and a global loss of various coastal ecosystems that occur at the transition from sea to land (Chapman et al., 2003).
The physical conditions along the gradient from lower to the upper shore are extremely variable. The subtidal is essentially fully marine and not exposed to the same stressors as the intertital The intertidal is a much more stressful environment for marine organisms, primarily due to the time spent ‘emersed’, i.e. exposed to the air. On seawalls the high littoral zone tends to host very low in diversity as the conditions are too harsh to support marine life. Natural rocky shores tend to have more refugia and can therefore support more life at the higher zones.
To aim for a greater biodiversity, concrete (complexity) tiles are retrofitted to existing seawalls. Concrete complexity tiles can be seen in the picture. There is a five-step design plan:
Select maximum hydrodynamic exposure where still useful to try to enhance diversity.
Choose maximum seawall slope where still useful to try to enhance diversity.
Select maximum height (= shortest inundation period) where useful to try to enhance diversity by adding complexity tiles.
Select optimal structure (i.e., complexity tiles) to enhance diversity.
Select optimal placement of structures to enhance diversity
Retrofitting of tiles has a strong positive effect on the seawall diversity. This result offers promising opportunities to incorporate the best-tile designs within concrete blocks as typically used for seawall construction.
- Barbier, Edward B., Hacker, Sally D., Kennedy, Chris J., Koch, Evamaria W., Stier, Adrian C. and Silliman, Brian R. (2011), The Value of Estuarine and Coastal Ecosystem Services . Ecological Monographs, Vol. 81, No. 2, pp. 169-193, 2011
- Chapman, M. G. & F. Bulleri, 2003. Intertidal seawalls–new features of landscape in intertidal environments. Landscape and Urban Planning, 62: 159-172
- Chapman, M. G. & D. J. Blockley, 2009. Engineering novel habitats on urban infrastructure to increase intertidal biodiversity. Oecologia, 161: 625-635.
- Dekker, F and M.B. de Vries, (2009). rapportage Ontwerp Groene Golfremmende Dijk FortSteurgat definitief
- Moschella, P. S., M. Abbiati, P. Åberg, L. Airoldi, J. M. Anderson, F. Bacchiocchi, F. Bulleri, G. E. Dinesen, M.
Frost, E. Gacia, L. Granhag, P. R. Jonsson, M. P. Satta, A. Sundelöf, R. C. Thompson & S. J. Hawkins, 2005. Low-crested coastal defence structures as artificial habitats for marine life: Using ecological criteria in design. Coastal Engineering, 52: 1053-1071
- Paalvast, P. 2008. "Rijke Berm Oosterschelde Tussenrapportage 2008". Monitoringsrapport Ecoconsult
- Paalvast, P. 2009. "Rijke Berm Oosterschelde Tussenrapportage 2009". Monitoringsrapport Ecoconsult