Rich structures are small-scale structures that provide habitat for organisms. There are different types of rich structures (see image below). Examples of rich structures are floating marshes (type 1), pontoon hulas (type 2) en pile hulas (type 3/4). Rich structures are relatively easy to apply and offer opportunities for added services even when physical conditions or design constraints require hard solutions and/or provide little space for Building with Nature. Application of rich structures increases the natural value of the area, dampens waves, reduces maintenance needs of quay walls and contributes to attractive landscapes.
Advantages compared with a 'traditional solution'
Advantages of rich structures compared to ‘traditional solutions’ include:
- Ecosystems and physical conditions:
- Provision of ecologically valuable habitats for organisms. Floating marshes are appealing to birds and insects, fish can find shelter beneath them. Pile/pontoon hulas provide valuable substrate for many species including mussels. Furthermore, they serve as a source of food for fish and as breeding grounds.
- Decreased hydraulic loads on the foreshore / embankments. Floating structures can reduce hydraulic loads caused by currents and waves.
- Reduced turbidity. More substrate for filter feeders increases the amount of filtering of the water, resulting in reduced turbidity.
- Contribution to an attractive landscape for recreation. Floating marshes increase spatial quality and provide opportunities for recreational fishing.
- Potential for collection of edible (shell)fish and plants.
- Rich structures may extent lifetimes of structures (e.g. shellfish frames against quay walls may slow down corrosion).
- Construction of a rich structure can contribute to meet the requirements of environmental legislation such as water framework directive and Natura 2000.
- Floating marshes can be used as climate mitigation measure as they can capture CO2.
Disadvantages compared with a 'traditional solution'
Disadvantages of rich structures compared to ‘traditional solutions’ include:
- Construction of rich structures could add to the costs of the project in case application does not reduce or other costs (e.g. reduction maintenance costs of quay walls);
- Added attractiveness for recreation could add additional safety risk for the public or hinderance to the surrounding;
- Rich structures could provide habitat for invasive species.
- Maintenance required for the structures.
How to use
A guideline for determining the feasibility and implementation of rich structures is presented below. In the pre-feasibility phase it is important to get insight in the physical boundary conditions and social-economic interests to determine the possibilities. In the feasibility phase the design is created in close collaboration between technicians and ecological experts. Field trials can be used to improve the design for area specific conditions. After this, the design can be constructed and monitored. Costs depend on the design, construction implications, maintenance and operational cost and benefits.
Activities in the pre-feasibility phase are:
Discussion with client on ecological and recreational potential in area of interest;
Consideration of the possible interests involved in the area (creating new recreation areas, improving spatial quality, contributing to nature and climate goals, improving ecological value)
Analysis of physical forcing factors and limiting design criteria as boundary criteria for application of rich structures;
Analysis of the physical aspects of the water system (flow and wave conditions, water depth, sediment characteristics, water quality (turbidity, light attenuation, dissolved oxygen, salinity, temperature), morphology)
Analysis of local ecosystem biodiversity;
Analysis of surrounding ecosystem and identification of possible interactions and reinforcements;
Pre-design, valuation and costing of rich structures and presentation to the client;
Selection of most promising rich structure.
In the feasibility phase the following steps need to be taken:
Technical design of most promising rich structure including planning and requirements:
Determining location(s) for structures;
Size of structures;
Choice of materials;
Seeding method (if required);
Detailed costing and phasing within major project planning.
Small-scale field trial to improve the design for the specific area
Setup of monitoring and maintenance plans (if required, monitoring especially required in the case of pilots)
Technical design should be created in interaction between technicians and ecologic experts to ensure optimum design for improving ecological value without increasing the number of invasive species.
In this phase the technical design is made. Field trials can be used to improve the design for the specific area.
In the project phase the following steps need to be taken:
Assessment of project implementation using technical and biological expertise;
Execution of the project;
Evaluation of ecological development over a period of several consecutive years (especially in case of a pilot).
Project costs differ for different rich structures and locations.
Cost indication for a floating structure (2012) (buildingwithnatureinthecity.com):
Willow mattress measuring 20 x 5 metres tested for wave attenuation.
Delivery and construction: EUR 6,500 including VAT;
Planting with reed shoots: EUR 1,000 including VAT;
Anchoring to the bed/embankment: EUR 11,000 including VAT;
Total: EUR 18,500 = EUR 185/m 2).
This cost estimate does not include measures that need to be taken to ensure public safety.
For hula structures no cost indication is available. The material used is relatively cheap, but the hulas must be made by hand (buildingwithnatureinthecity.com).
On the right side, links are provided to different projects with rich structures. A short description of three projects where rich structures were applied is given below. In these projects different types (see image above) of rich structures were applied.
Harbour opportunities - Rotterdam (Netherlands)
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 they provide little hiding place for larger animals such as fish, lobsters and crabs.
The goal of the project harbour opportunities was to invent designs for suspended artificial substrates to (e.g. Paalvast, 2007):
- promote the settlement of mussels and consequently improve water quality;
- enlarge available substrate for settlement of organisms;
- 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: pole hulas and pontoon hulas (type 2+3 from overview image). 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 substantially.
Floating marsh as shoreline protection - Houtribsluices, NL
Dikes often border the water directly with relatively steep slopes along the coast of large Dutch freshwater lakes. 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 re-create gradual land-water transitions brushwood mattresses were constructed in front of the dike (type 1 from overview image). These mattresses might facilitate development of floating reed marsh in the shallow zone in front of a dike. The floating marshes reduce wave impact on the dike, enhance sedimentation and create a clear shallow water zone with (submerged) vegetation. On top of this, the initial substrate of the mattress could be suitable for establishment of filter feeders, such as zebra mussels (Dreissena polymorpha) and other species.
Widening Amazonehaven basin – Rotterdam, NL
Rich structures were applied at the new quay wall constructed when the Amazonhaven basin was widened. The Amazonehaven basin was constructed between 1988 and 2000 (Broos and De Gijt, 2014). To make the basin suitable for larger vessels (Ultra Large Container Ships, ULCS) the basin was widened. The present quay wall had to be demolished and a new quay wall had to be built.
Rich structures were attached to this new quay wall (see figure below). They consist of fixed frames with wires (type 4 from overview image). Oysters can attach to these structures. The frames can be easily removed for maintenance.
- Besix-Van Oord aannemerscombinatie (2013). Verbreding Mondin Amazonehaven (ppt).
- Broos, E. J., & de Gijt, J. G. (2014). The Demolishing of the EMO Quay Wall in the Amazonehaven. In Port of Rotterdam, HTG Congress.
- Paalvast, P., 2007. "Pakket van eisen voor hangende substraten en ecoplaten in het Rotterdamse havengebied". Ecoconsult report to Port of Rotterdam (In Dutch).
- Paalvast, P. (2015, January). Application of string and rope structures, pole and pontoon hulas, to increase productivity and biodiversity in man-made hard substrate aquatic environments. In Congress on artificial reefs: from materials to ecosystems–ESITC Caen (Vol. 27, pp. 28-29).
- Project pages ‘Harbour opportunities - Rotterdam (Netherlands)’ and ‘’ Floating marsh as shoreline protection - Houtribsluices, NL’
- Van Geest, G., Geerling, G. and de Vries, M.B. 2010. Pilot drijvend rietmoeras NMIJ. Deltares report.
- Wesenbeeck, B. (2009). Mosselen in de haven [online]. Available at: http://www.sciencepalooza.nl/?p=937 [date of search: 28-02-2019].
- www.buildingwithnatureinthecity.com (factsheets ‘Floating wicker mats’ and ‘Pile and pontoon hulas’)
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