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Harbours are often seen as abiotic environments that are optimized for economic activities. While this is sometimes true, harbours also provide a habitat for many species. Simple measures can do a lot to provide additional ecological value.

Building with Nature Design Traditional Design

Floating and hanging artificial surfaces can enhance habitat diversity and filter feeder biomass, if port water quality is good enough (like in Rotterdam). Constructions made of standard nylon ropes, strategically strung between the piles of a jetty, are a cheap way to do this. The photo shows such ropes four months after installation.


Smooth steel and concrete structures, like sheet-pile walls or jetty piers, provide little grip for mussels and sea anemones. Further, compared to natural rocky habitats, artificial structures lack cracks and crevices, and profile variation.

    General Project Description


    Title: Harbour and Port Opportunities
    Location: Rotterdam Harbour (Scheurhaven & Pistoolhaven)
    Date: March 2009 - September 2010
    Companies: Deltares, Ecoconsult, Port of Rotterdam
    Costs: Not yet known for application purpose
    Abstract: Enhancing biodiversity and productivity in harbours and ports by increasing habitat complexity. By increasing filter feeder biomass water quality may be improved.
    Topics: Water quality

    Project objective

    Project objective
    Port areas consist mainly of man-made structures, 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. Too smooth a substrate, however, will hamper organism attachment and provide little hiding place for larger animals such as fish, lobsters and crabs. Traditional harbour structures create a smooth underwater environment. The present pilot experiment concerns simple but effective measures to make this environment more suitable for attaching organisms and to create more habitat complexity for fish.

    Project solution

    In order to promote the settlement of mussels and consequently contribute to water quality in the harbour, this pilot experiment aims to create a less smooth underwater environment via free-floating artificial substrates. This would enlarge the substrate suitable for settlement, increase the biomass of filter-feeders (e.g. mussels) and enhance habitat diversity in the port area. To enrich the habitats underneath pontoons and jetties so-called 'pontoonhulas' were developed. To enrich habitat around poles so-called 'polehulas' were used (‘hula’ is the name of the skirts of Polynesian hula dancers).

    Governance context

    The Port of Rotterdam has the ambition to become a sustainable and energy-neutral port. In this context this authority is investigating various alternatives to make the port ‘greener’. When the Rijkswaterstaat-funded WINN project Levende Waterbouw approached port representatives, they found an open attitude towards developing a number of pilot experiments.

    Costs and benefits

    The present pilot investigates simple and low-cost options to enhance filter-feeder biomass in harbour basins. Given their filter capacity, mussels are known to improve water quality by decreasing fine sediment concentrations, thus increasing light penetration into the water (Dolmer, 2000). Light stimulates algae growth and living algae produce oxygen. Since all organisms living in the water column use oxygen to grow, mussels can be claimed to promote biodiversity in port areas. Moreover, mussels are able to improve water quality by retaining contaminants (Strogyloudi et al., 2006). Also, generating more habitat variety and complexity by creating hanging underwater forests is thought to provide hiding places for fish.

    Planning and Design


    As part of the Rijkswaterstaat Innovation Program (WINN), Deltares sought cooperation with Port Centre Rotterdam. Ideas developed by Deltares and Ecoconsult as part of the Rich Revetment study and new ideas on the hanging ‘hula’ structures were proposed for implementation in a number of pilot experiments in the Rotterdam harbour.


    In the Rich Revetment philosophy eco-dynamic design alternatives were considered for four habitat types:

    1. high-dynamic shallow habitats,
    2. high-dynamic deep habitats,
    3. low-dynamic shallow habitats and,
    4. low-dynamic deep habitats.
      The fourth habitat type is found in harbours and several pilot alternatives for this habitat type were evaluated. The large amount of hard substrate present in harbours was considered to offer opportunities for epifauna that needs hard substrate to attach to. In this respect artificial hard substrates can function as a reef structure. Yet, artificial hard substrates usually lack the variety of habitat characteristics of, for example, natural rocky coasts. Therefore, the optimal harbour wall was designed to resemble a natural reef by adding cracks and crevices of distinct sizes, making the wall suitable for colonization by small organisms such as algae, mussels and sponges, as well as by larger organisms such as fish and lobsters. Implementing such a reef wall, however, would require the construction of new walls. Therefore, the present project focuses on simpler solutions that can be applied in existing harbour basins.

    At first a study of requirements and boundary conditions was executed by Ecoconsult (Paalvast 2007a). Several other pilot projects connected to the Rich Revetment concept concept started out in a similar way, by conducting feasibility studies that would define general questions or problems and offer potential solutions. Subsequently, the envisaged pilot project was investigated in more detail, making a selection of techniques and locations (Paalvast 2007b). Suitable harbour basins were selected based on expert knowledge of ecologists familiar with the system. Locations were selected based on salinity levels. Large fluctuations in salinity level are a restricting factor for many organisms. As mussels were one of the target species, locations and timing of the experiments were adapted to optimize the potential for mussel establishment and survival. Some alternatives that were generated in the pre-feasibility phase are in the figures accompanying this text.


    For the Rotterdam harbour two specific structures were selected for further elaboration: polehulas and pontoonhulas. 
    Polehulas consist of ropes suspended from a band that can be wrapped around a pole. The polehulas that were used in this experiment were composed of 301 ropes with a rope length of 55 cm and a rope diameter of 6 mm. The total rope length per polehula is 165 m and the total surface area is 3.1 m 2 of artificial substrate. 
    Pontoonhulas are floating elements from which ropes are suspended. The floating element used in this experiment consists of a rectangular frame of PVC-tubes (diameter 125 mm) with on the inside a stretched nylon net with a mesh size of 12.5 x 12.5 cm. At the crosses of the nylon net, ropes with a diameter of 12 mm are connected. The area of the pontoonhula, the length of the ropes and the rope density may vary per pontoonhula. Two types of pontoonhulas were used in this pilot.

    Pontoonhula Type I has a rope length of 150 cm, whereas the rope length of Type II decreases from 150 cm at the outside to 30 cm in the middle (bell shape, in Dutch "klokmodel"). The largest pontoonhula measures 160 x 200 cm with a rope density of 208 ropes per pontoonhula and a rope length of 150 cm. The total rope length is 312 m and the maximum total surface is 11.8 m 2 of artificial substrate.
    An uncertainty that provided a potential bottleneck for the Port was whether or not the Port would be allowed to remove the temporary structures. There was fear that once the floating structures would be colonized by algae and mussels they would attract protected fish species, which would restrict the Port’s possibilities to (temporarily) remove these structures if so needed for maintenance or other common harbour operations.

    Request for Proposal

    The time from initiation to contract took about one year of regular, albeit low intensity, interaction. Before the project could actually start Port of Rotterdam insisted on solving the perceived risk of valuable nature developing in harbour basins limiting options for maintenance and normal harbour operations. To tackle this problem the Dutch Ministry of Agriculture, Nature conservation and Fisheries was approached with the request for a written and signed permit to remove the structures at any time. When this permit was acquired the project could start and after reaching agreement on the proposed pilot structures and locations, Port of Rotterdam asked Deltares to submit a proposal for enriching underwater habitat diversity in their harbour areas. Deltares, Port of Rotterdam and Ecoconsult developed the plan together. Ideas on how to enrich habitat diversity were further developed by both Deltares and Ecoconsult (Paalvast 2007a and Paalvast 2007b). The full program, including financing and performing of all construction and monitoring tasks, was agreed and undertaken by the various parties in a constructive and effective cooperation.

    Status 2012

    The hula structures will be applied in another project for the Port of Rotterdam: the widening of the Amazonehaven at Maasvlakte 1. The goal is to increase habitat diversity and the idea is proposed that corrosion of the sheetpiled harbor wall may be delayed by having the mussels filter oxygen out of the water before it reaches the steel wall.



    The project and its scope were defined by the parties collaboratively.

    Detailed design

    Before the actual construction, the conceptual designs from the previous phase were further detailed to enable actual construction. 
    In the Scheurhaven polehulas were placed around 5 wooden and 2 steel poles and in the Pistoolhaven the polehulas were placed around 3 steel poles. At the wooden pole one polehula was placed above mean low water (MLW). Below MLW three polehulas were added with an overlap of 10 cm. The aim of this variation is to determine the optimum position of the polehulas with respect to the water level. At the steel poles only one polehula is placed below MLW around each pole. 
    Pontoonhulas type I, with a size of 85 x 230 cm, and a varying rope density of 40, 80 and 168 ropes were used in the Pistoolhaven. In the Scheurhaven two pontoonhulas type I, with a size of 160 x 200 cm and a fixed number of ropes per pontoonhula of 208, were used. Moreover, two pontoonhulas type II were used there, with an area of 160 x 200 cm and a fixed rope density of 208 ropes per pontoonhula.

    Project delivery

    The bare polehulas and pontoonhulas were placed in the Scheurhaven and the Pistoolhaven in March 2009.

    Operation and Maintenance

    Delivered project

    The project comprised the construction of polehulas and pontoonhulas, to be placed in the Scheurhaven and the Pistoolhaven (harbour numbers 5390 en 6360, respectively) of the Port of Rotterdam. The main aim of the pilot experiments in the Scheurhaven and the Pistoolhaven was to investigate the success of the proposed structures in attracting biomass. It was decided that monitoring would be limited to two monitoring moments when species on the structures would be determined and biomass on the ropes would be weighed. Furthermore, it was decided that structures should be maintained on a monthly basis to check whether they were still in place and whether there was no damage to the structures.


    Given the nature of the pilot a monitoring program was set op to monitor the ecological development on the structures. Twice a year biomass and species diversity on the structures was determined. Status of the structures was checked each month for safety purposes. These monthly maintenance check-ups were also used to measure present biomass on the strings.


    The polehulas in the Scheurhaven were monitored 15 and 34 weeks after construction. After 15 weeks, barnacles (Balanus improvisus) and mussels (Mytilus edulis) were predominant below MLW. Above MLW algae species Porphyra umbilicalis and Ulva linza, were predominant.

    After 34 weeks a significant shift to mostly mussel abundance was observed. Given biomass of a reference pole (9,925 g/m 2), the biomass of the polehulas is maximum 11.4 times larger (112,996 g/m 2) and 8.5 times higher when averaged over all polehulas (84,031 g/m 2).

    As compared with the polehulas, dominance of mussels was even stronger at the pontoonhulas. No algae were observed there, possibly due to the limited light penetration underneath the pontoonhula. After 19 weeks average biomass was about 11 g/cm rope. Biomass was largest at the edges of the pontoonhulas and decreased towards the centre, which may be explained by competition for food. At the Scheurhaven the biomass per cm rope was significanty larger than in the Pistoolhaven, possibly due to the lower and more dynamic salinity in the Pistoolhaven, a factor that negatively influences the growth of mussels (Almada-Villela, 1984; Kautsky, 1982). Moreover, higher turbulence levels in the Scheurhaven may have facilitated a larger supply of oxygen and food to the mussels.

    Given a maximum biomass of 348 kg at one pontoonhula in the Scheurhaven, the total time to filter the entire volume of water in the Scheurhaven can be calculated. Based on a study on the colonization of an artificial hard substrate by the same mussel (Joschko et al., 2008), a biomass of 348 kg corresponds with some 247,000 individuals, One mussel is assumed to filter around 35 cm3/min (Dolmer, 2000). Given the total volume of water in the Scheurhaven of about 195,000 m 3, one pontoonhula is able to filter this entire volume in 16 days. Upscaling this to the entire Rotterdam harbour, around 35 pontoonhulas per harbour basin are able to filter the entire volume of water in the harbour in one month.

    The monthly check-ups revealed that after a couple of months pontoonhulas increased significantly in weight due to increasing biomass. Consequently, initial buoyancy did not suffice anymore and the structures started sinking. Fortunately, they were still attached to existing pontoons by several hawsers. To prevent the pontoonhulas from breaking loose, the number of hawsers attaching them to the pontoons was doubled to keep the pvc-construction of the hulas afloat.

    End of life

    Pontoonhulas can also help reducing wave heights as the ropes with the mussels attached to them can absorb wave energy. The ropes of the pontoonhulas were included in SWAN-VEG (40.81) which is a numerical model to calculate wave properties influenced by thin stems of vegetation. In this model wave height, wave period, length of section with ropes and the biomass were varied. Depth was 4 m and it was assumed that the ropes were 4m as well. Structures achieved wave damping between 14%, with 10 m structure, a low biomass and a wave height of 0.1 m, and 80%, with 30 m structure, a high biomass and a wave height of 0.5 m. By varying variables such as biomass and length, a conservative estimate can be determined for the functionality of the structure if applied in the field. 
    The hula-structures were removed from the field by the end of 2010. The pontoonhulas where transported to the Deltaflume of Deltares. There they were put in the water and were exposed to both regular and irregular waves to determine their ability to reduce waves. It was found that for floating structures wave transmission closely resembles wave transmission by floating breakwaters with the addition that mussel structures penetrate deeper in the water column and are porous and thus, both these parameters influence wave attenuation capacity of the structure (van Steeg & van Wesenbeeck 2011). 
    Ideally, the mathematical model exercise should be expanded and the flume study should be used as a validation case for SWAN-VEG. However, this is not yet done due to budget constraints.

    Lessons Learned

    • Artificial substrates, such as polehulas and pontoonhulas, can increase the biomass in a harbour basin considerably. Thirty-four weeks after construction one polehula yields on average 8.5 times as much biomass as a pole with no hula around it. A pontoonhula is able to gather a total biomass of some 350 kg after 19 weeks. On both artificial substrates the mussel Mytilus edulis is the dominant species. On polehulas also different algae species were found.
    • The amount of mussels on a single pontoonhula is able to filter the entire water volume of the Scheurhaven basin in 16 days. This will contribute to light penetration into the water, contaminant sequestration and a better overall ecological condition of the basin. This is in line with the main objective of the European Water Framework Directive.
    • Biomass development and species composition have only been monitored in the first year after construction. This implies that so far there is no clear indication of their long-term development.
    • After mussel settlement the weight of the pontoonhulas increased with almost 300 kg in less than four months. Consequently, the original buoyancy of pontoonhulas was no longer sufficient to keep the structure afloat. For future designs buoyancy should be adapted to the increasing weight.
    • The design of pontoonhulas can be optimized by making use of design elements of MZI's (Mussel Seed Capture Installations), which are developed to capture mussel seed at open sea. These structures are anchored and consist of steel cables and a large number of float levers.
    • Polehulas can be improved by using ropes with a larger diameter. In the current design the rope diameter in the polehulas was 0.5 cm. This was rather small, as individual mussels ended up settling on several ropes.
    • Floating mussel structures attenuate waves and can be used for this purpose in harbour basins. The amount of wave attenuation depends on the length of the structure, the rope density,
    • To realize this pilot a step by step approach was followed: 1) scope the opportunities and costs for potential eco-design solutions, 2) implement a small-scale pilot to obtain a proof-of-concept, 3) modify the design according to the lessons learned from the pilot and upscale to full scale. This gradual approach has proven helpful in the efforts to achieve an innovative approach that was actually implemented in the field.




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