Remote sensing approaches for assessing wave attenuation by saltmarsh vegetation : benefits for monitoring managed realignment schemes

Coastal ecosystems around the world are commonly affected by frequent flood events, in particular during winter. This impact is expected to be exacerbated because of climate change. To address this issue, coastal defences are usually built to reduce or mitigate flooding using hard engineering techniques such as sea walls but they are significantly expensive to be created and maintained over time. Alternatively, nature-based strategies, known as soft engineering, are being adopted to recreate or restore natural features. In Europe, including the UK, there is an increasing trend of developing these strategies in coastal management such as managed realignments schemes to restore saltmarshes which are recognised to mainly provide wave attenuation due to their vegetation structure. However, permanent monitoring of this function is challenging due to the extensive data collection needed over time. This thesis takes into account the use of satellite data to retrieve leaf area index (LAI) as an alternative structural parameter of saltmarsh vegetation instead of the traditional frontal surface area (FSA) for wave attenuation purposes. The aim of this research is to assess how optical remote sensing can be used to assess vegetation structure in saltmarshes for wave attenuation. To accomplish this aim, the following objectives are presented: (1) to evaluate the potential of Sentinel-2 MSI imagery for characterising intra-seasonal variation in saltmarsh LAI, (2) to determine whether remotely-sensed LAI may be an alternative for field-based FSA as structural vegetation parameter for wave attenuation, and (3) to assess whether remote sensing-based LAI can effectively be used as an alternative for FSA in wave attenuation modelling. The Brancaster managed realignment scheme was selected as the main study area. The results of this thesis have shown the importance of seasonality to accurately characterise vegetation structure using remote sensing approaches. The remotely-sensed LAI is found to be a suitable descriptor of both saltmarsh vegetation structure and wave attenuation by vegetation. This thesis mainly contributes to improving monitoring of saltmarsh vegetation of managed realignment scheme to assess wave attenuation by developing a brown LAI model for the winter season when storm events are more frequent, highlighting the relationship between satellite-derived LAI and wave attenuation estimations, and incorporating LAI into wave numerical modelling. Remote sensing approaches potentially provide a rapid evaluation of coastal protection service by managed realignment schemes and monitor their performance over time.