The effect of seagrass meadows on sediment dynamics and shoreline stability through modifications of waves and tidal flows

Significance

Seagrasses are of significant importance ecologically, but may also serve an important natural coastal protection function by extracting energy from waves and currents, and thus promoting sedimentation processes. As a result, seagrasses possess significant ‘natural capital’, that is, are of value to our society through the various services they provide. However, how exactly moving seagrass fronds effect wave, current and sediment transport processes and to what extent they promote sedimentation and provide natural coastal protection is largely unknown. These are important questions as there is a lot of interest focussed on seagrass maintenance, but also restoration and creation of seagrass meadows as a nature-based solution to coastal erosion. Better understanding of the interactions between seagrass and wave/current hydrodynamics is expected to significantly increase the success rate of seagrass restoration projects (currently estimated at 30%).

Approach

An extensive research programme was executed:

1. Carry out air-based and water-based surveys of the larger study area, encompassing bare sand, seagrass meadows and kelp beds.

2. Collect sediment samples from within and beyond a seagrass meadow to infer sediment pathways and hydrodynamic sorting processes.

3. Measure spatial variability in wave and current conditions within a seagrass meadow to quantify wave/current attenuation due to seagrass. 

4. Conduct field measurements of wave/current hydrodynamics, turbulence characteristics, sediment dynamics within a seagrass meadow.

Mapping

The study area was extensively mapped from the air and from the sea surface using autonomous vehicles (‘drones’ and uncrewed surface vessels). This provided not only very accurate maps of seagrass presence, but also provided information on abundance and length of the seagrass fronds. Moreover, these data also suggested that regions of dense seagrass are associated with local areas of shallower water, pointing towards the sediment trapping ability of the seagrass.

Sediment sampling

To compliment the mapping of seagrass extent and bathymetry, more than 100 sediment samples were collected within and outside seagrass meadows, and the beach. These samples will be analysed for sediment texture and carbonate content. The hypothesis here is that if regions of seagrass represent relatively calm areas and areas of deposition, the sediment should be finer inside than outside seagrass meadows.

Self-logging instrumentation

To directly assess whether seagrass are filtering wave and currents, an extensive array of self-logging wave and current sensors were deployed around the larger seagrass meadow north off Great Ganinick. The pressure sensors were deployed for a 2-month period. By comparing waves and currents entering the meadow with those exiting, the ability of seagrass to extract energetic from the waves and currents can be evaluated. The clarity of the water meant it was relatively easy to deploy and retrieve the instrumentation.

Main instrument rig

Very detailed hydro- and sediment-dynamic measurements of the processes within the seagrass canopy were made using a range of acoustic, electro-magnetic and optical instruments. The data was collected remotely via a 240-m long cable that ran from the beach into a mobile field laboratory parked in one of the fields at the back of the dunes. Amongst others, the instruments measured detailed flow characteristics right at the sea bed, as well as the amount of sediment in the water column. The instrument rig was carefully prepared on Par Beach and then carried into the water and installed within a small seagrass patch 100 m from the low tide line.