Restoration of gravel beds

Gravel beds as biodiversity hotspots

Gravel beds were widespread in our North Sea in the past. Today, some of these gravel beds still exist, but unfortunately in a highly degraded state (see figure). Gravel beds are biodiversity hotspots and have an important function as breeding and nursery grounds for many species. Indeed, they are essential for species that need a hard surface to deposit their eggs (e.g. whelk, Atlantic bobtail, dog whelks). Other species, such as the cuttlefish, dogfish and other shark species, attach their clutches to attached sponges, moss animals and colonies of hydrozoa that occur within healthy gravel-bed ecosystems.

• Disturbance of gravel beds

• Restoration of gravel beds

            Objectives

            Preparation

            Implementation

            Monitoring

            Maintenance

Disturbance of gravel beds

The systematic removal of large boulders by fishermen in the past and the effects of silting by other human activities have greatly reduced the size and structural quality of gravel beds in our North Sea. The species characteristic of healthy gravel beds are mostly long-lived, slow-growing species that are very sensitive to disturbance.  The species composition of the perennial gravel beds has therefore changed significantly as a result of continued disturbance: here too, long-lived species that reproduce slowly largely gave way to short-lived, opportunistic species. A number of small gravel beds have been partially spared from human disturbance due to their hard-to-reach location. These relict gravel beds appear to provide a refuge where certain vulnerable species associated with gravel beds, such as whelks, gyratory hornets, spider and harvestman crabs, sea sprite and dead man's thimble, can still develop relatively well. Moreover, gravel bed was recently discovered  was recently discovered under one of the main navigation channels in the Belgian part of the North Sea that is still in remarkably good condition. The existence of these relict gravel beds shows that restoring gravel beds to the biodiversity hotspots they once were is still possible

Restoration of gravel beds

Situation in our North Sea and surrounding waters
A preliminary study was already carried out in Belgium on the restoration of a gravel bed (see bibliography). However, due to the lack of fisheries restrictive measures, the necessary preconditions to carry this out successfully could not be met so far. Indeed, the introduction of measures that exclude bottom-feeding activities is a very important requirement for the successful recovery of gravel beds.

Even among our neighbouring countries, not many projects have so far been carried out around restoration of gravel beds. However, two examples show that gravel bed restoration is indeed possible and can provide the desired results: gravel-seeding in the UK and blue reefs in Denmark (see bibliography). Currently, there is still a gap in knowledge on long-term effects of gravel-bed dumping, as the transition from pioneer community to a stable, mature community with more long-lived species can take several years.

Objectives of restoring gravel beds

Main objectives:

Sustainable restoration of healthy gravel beds and associated long-living fauna: The ecosystem at the level of the gravel beds reaches a mature state, replacing pioneer communities with a community of long-living, slow-growing species and habitat-structuring species. Restore the function of gravel beds as spawning and nursery grounds for various fish species

Intermediate objectives: 

1. Substantially reduce pressure from bottom-feeding activities on gravel beds > passive restoration

• Reduce/eliminate bottom disturbance by taking measures to protect bottom integrity in the areas where valuable gravel beds occur and/or allowing nature restoration valuable gravel beds.

• Ensure effective implementation of soil integrity protection measures by providing adequate enforcement.

2. Increase our knowledge of residual gravel beds and potential restoration measures

• Determine location(s) where active restoration of a gravel bed is necessary and feasible based on already available data and possible additional research.

• Draw up a specific monitoring plan and start monitoring in function of the target species. Thorough monitoring of the T0 situation (the situation at time 0, i.e. before the measures take effect) is crucial here.

• Implementation of a pilot project in which at least 1 ha of gravel bed is restored by dumping gravel (see MSFD programme of measures). The aim of this project is to gain a better understanding of the technical aspects of the physical restoration of a gravel bed (such as the suitable grain size of the gravel to be dumped, distribution of different grain sizes, use of accelerators, etc.) and the added value that the dumping of gravel can offer compared to passive restoration of gravel beds.

• Investigate other environmentally beneficial measures, such as vacuuming gravel beds to remove overlying fine sediment. If appropriate, a pilot project will also be initiated for this purpose.

3. Restoring gravel beds and associated long-lived fauna > active restoration

• Larger-scale creation of gravel beds, if appropriate based on the results of the pilot project of dumping a minimum of 1 ha of gravel.

• Apply other environmental enhancement measures to the gravel beds on a wider scale, if this proves appropriate based on the results of any pilot project on this.

4. Raise awareness on the importance of healthy gravel beds with stakeholders and the general public .

Flowchart restoration on gravel bed

For the restoration of gravel beds, we focus in the short term on establishing a test site of at least 1 ha in size. The results of this test can later be used to possibly restore gravel beds on a larger scale. The flow chart presented below focuses on the construction of the test gravel bed. 

Preparation

To avoid sedimentation of the constructed gravel bed, it is important to look for a location where the soil shear stress is high enough and/or where little sand is present. A study by IMDC (see bibliography) examined the suitability of the then existing zones within which fishing measures were proposed to deposit gravel in terms of bottom shear stress and from an ecological point of view. Once the location of zones to protect soil integrity is determined, we will consider whether IMDC's study can still be considered up to date.

Besides choosing the appropriate zone for restoring a gravel bed, the choice of the type of gravel and its origin is also important (see table). Using natural marine gravel is cheap if extracted in the vicinity and gives the greatest chance of developing a community typical of marine gravel beds. A major disadvantage here is that this gravel has to be taken away from another marine area. A possible alternative is to use material sourced from on land; this can be either natural stones or artificial materials. However, we expect that the community that will develop on these substrates will differ from those found on natural marine hard substrates. The preferred substrate is therefore recycled substrate of marine origin. For example, from gravel extraction for cable construction, or from seawalls degraded for port expansion.

Gravel grain size distribution is also an important factor. Based on extant gravel beds in England and the Netherlands, it is assumed that there is a heterogeneous distribution of gravel and sand in natural gravel beds. A plan should be drawn up to determine the optimal distribution of gravel versus sand, possibly testing different scenarios.

Implementation

The pouring of gravel can be carried out by a professional contractor and, in principle, takes only a few days. If a gravel sub-base is used, it is best to pour it first, followed by the larger stones and finally the rocks. The gravel can be dumped using either a 'fall pipe vessel' or a 'side stone installation vessel', with the first option being more precise but more expensive.

Monitoring the gravel bed

The monitoring of the gravel bed can be divided into 3 facets: ecological monitoring, geophysical monitoring and hydromorphological monitoring. It is crucial to start a strong monitoring programme for the construction of the gravel bed, so that the necessary knowledge from this project can be taken into account when determining the required monitoring of the pilot project and its concrete elaboration. 

Ecological monitoring of the gravel bed

The installation of the gravel bed will most likely take place in summer, when weather conditions are most suitable for such engineering operations. Since most larvae of epibenthic species (species living on the surface of the soil) settle in spring, little growth is expected during the first year. Colonisation of a hard substrate is a slow process that has already been extensively documented at other sites. As we aim to develop a mature community, an initial monitoring campaign can therefore take place at least 2 years after installation of the gravel bed. In the long term, monitoring campaigns can be repeated every 2-3 years. This is a minimum monitoring proposal, a more extensive monitoring programme can be drawn up depending on the predetermined needs for outreach, among other things. An additional interesting option to explore here is the use of Autonomous Reef Monitoring Structures: 3D structures placed on the seabed for a certain period of time after which the organisms present on the structures are studied.

Geophysical monitoring of the gravel bed

The purpose of geophysical monitoring is to map the gravel bed, determine changes in morphology in and around the project site and determine sedimentation based on bottom shear stress.

The final position of the test site will be determined based on a 'multibeam echosounder survey' (MBES) bathymetric survey for the installation. This survey uses sonars to determine the depth of the seabed and map its relief. The installation of a gravel bed disturbs the natural morphological balance of the site. As the strongest disturbance of the seabed is expected immediately after the installation of the gravel bed, a high survey frequency in the short term is desirable (interval of about 3 weeks, can be done in combination with hydro and morphodynamic monitoring). In the longer term, the system will evolve to a new equilibrium and the frequency of geophysical monitoring can be greatly reduced. To assess the impact of storms on gravel and to get a picture of nearby seabed mobility, monitoring before and after the winter season is recommended.

Maintenance: enforcement

Proper enforcement is essential to safeguard the newly created gravel beds from disturbance and seabed disturbance. A study to determine which forms of enforcement are appropriate for this purpose is recommended. This study is best conducted before the gravel bed is laid, so that enforcement can be guaranteed immediately after implementation.