Creating hanging and floating structures
Hanging and floating structures are small-scale structures that provide habitat for organisms, can make a landscape more attractive and can be used to dampen wave energy which can contribute to lower maintenance costs of quay walls. There are different types of hanging and floating structures (see image below). Examples are floating marshes (type 1), pontoon hulas (type 2) en pile hulas (type 3/4). The 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.
Advantages compared with a 'traditional solution'
Advantages of hanging and floating 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.
- Hanging and floating 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 hanging and floating structures compared to ‘traditional solutions’ include:
- Construction of a hanging or floating structure could add to the costs of the project in case application does not reduce 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;
- Hanging and floating structures could provide habitat for invasive species.
- The structures require maintenance.
How to use
A guideline for determining the feasibility and implementation of hanging and floating 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 hanginig and floating 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 hanging and floating structures and presentation to the client;
Selection of the most promising hanging/floating structure.
In the feasibility phase the following steps need to be taken:
Technical design of most promising hanging/floating 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
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 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).
A short description of three projects where hanging/floating structures were applied is given below. In these projects, different types of structures were applied.
Harbour opportunities - Rotterdam, NL
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 space 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. 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
Along large Dutch freshwater lakes, dikes are often located directly along the lake and shallow zones and a 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
Hanging 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.
Hanging structures were attached to this new quay wall (see figure below). They consist of fixed frames with wires. Oysters can attach to these structures. The frames can be easily removed for maintenance.
Wave reducing poles - Nieuwe Waterweg - NL
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.
- 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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