Data Collection and Modelling
One of the outcomes of the Making Space for Sand project is to build on existing data about the coastline to more accurately understand what future coastal change may look like. To help us predict how beaches and dunes will change over time, it is important that we collect data about sand availability and sand movement. In principle this appears quite simple, but the data required to better understand this considers a significant amount of research and subsequent modelling.
The process of data collection delivered through the Making Space for Sand Project will enable project partners at the University of Plymouth to run a range of models. These models can be looked at in isolation, but when considered collectively, will help us understand, with greater accuracy, how the coastline will change and how we can adapt to these changes.
So to understand how a dune may respond to coastal change there is a need to understand where sediments stores are and how they are connected to one another. These sinks or stores are very dynamic and increased storminess and sea level rise will result in dunes becoming an increasingly important source of sand for beaches.
What data is being collected?
Making Space for Sand project partners from South West Coastal Monitoring and the University of Plymouth will be collecting a wide range of data, on the land and in the sea, to help feed information into models that will more accurately predict what change could look like. It is important for us to understand what data is being collected and understand why it is being collected so that we can appreciate the complexities associated with the modelling process.
LiDAR Surveys
LiDAR stands for Light Detection and Ranging. It is a remote sensing technology used to collect information on the elevation of things without making physical contact with them. LiDAR uses lasers, which are pulsed to the ground, where they are reflected back to the LiDAR sensor. By measuring the time it takes for each light ‘pulse’ to return to its source, it is possible to accurately measure the distance from the LIDAR sensor to the ground. This process all happens very quickly as typical LiDAR sensor are capable of emitting thousands of pulses per second.
Where large areas are being surveyed LiDAR is deployed on aircraft that will fly over a location. As LiDAR technology improves, sensors have become cheaper and more lightweight, which has enabled them to be mounted to drones, which are sometimes referred to as Unmanned Aerial Vehicles (UaV’s). This has enabled more detailed surveys to take place in difficult or dangerous environments, more quickly and more cheaply than before.
Why are we collecting LiDAR data? LiDAR can show us how the surface of beaches and dunes have changed overtime. If you know how the surface is changing you can understand where sand is eroding or building on a site.
Image of aircraft carrying out LiDAR surveys
Bathymetric Surveys
Bathymetric surveying is the study of the surface of the seabed. These surveys are very important to help ensure safe shipping but can also help tell us how the seabed is changing. Traditionally this was achieved by lowering a pre-measured weighted line over the side of a vessel to get a depth measurement, but nowadays uses sonar. An echo-sounder on a survey vessel transmits sound ‘pulses’ which reflect off the seafloor, or other objects below the surface, and then ‘return’ the pulse back to the sensor. The seafloor depth is computed using the time it takes for the sound to leave the array, hit the seafloor, and return to the array.
Why are we collecting bathymetric data? Bathymetry surveys can show us where the surface of seabed is, and with multiple surveys, how its changed over time. The ‘return’ pulse can also help identify the type of sediment that is on the seabed. Collectively this information helps us understand if there are stores of sediment further offshore, how this sediment moves and whether it is likely to be a source of sand to nearby beaches and dunes.
Image of boat carrying out bathymetric survey close to the shore
Weather Stations
At a number of locations, the Making Space for Sand project is installing weather stations. A weather station is a cluster of instruments that measure information on atmospheric conditions. These weather stations are collecting information relating to wind speed, wind direction and rainfall. These conditions are not the same on all the beaches on Cornwall so the weather stations will be located on a number of project sites, so that we can understand how conditions vary along the length and breadth of the Cornish coastline.
Why are we collecting data on wind and rain? When conditions are dry and windy it is more likely that sand on a beach or dunes will move. During periods of high rain or low wind sand is less likely to move. By monitoring the wind speed and direction, along with how wet the environment is, we can better understand whether conditions are enabling sand on a beach to migrate further inland. This is an important consideration when building models on coastal change.
Image of weather station installed at Crantock
Passive Seismic Surveys (Tromino)
When visiting a sandy beach or a dune system you may see lots of sand, but you can only see the surface of this layer of sand. Exactly how deep is the sand? Is there a thin veneer of sand covering the bedrock or tens of metres of it? It’s intrusive and difficult to dig lots and lots of holes to find this out, so is there a passive way to find out how deep the sand is?
To help ‘see’ through the ground we will use a measurement device called ‘Tromino’. This uses broadband three-component sensitive seismometers to measure extremely small vibrations in the ground caused by background environmental vibrations. These vibrations called ‘microtremors’ are often generated by wind and waves impacting the earth surface. Tromino measurements will be taken from the back of the dunes all the way to the low tide level. We can then compute the full bedrock profile beneath the beach and dune sand.
Why are we collecting sediment depth data? The Tromino surveys will help us understand how large the sediment store on the beach and in the dunes is. When combined with the other data collected it will help us understand whether other stores of sand further offshore could replenish sand levels on the shore and help us understand if a beach or dune is more or less resilient to change as a result of stormy conditions and sea level rise.
Sediment Analysis
Coastal sediment comes a diversity of shapes and sizes. Some sediment is rounded, some angular and others plate like in shape. Some sediment is resilient whilst some is more easily broken down. Some sediment is course and gravelly whilst others is fine and silty. The characteristics of sediment will influence how it moves when under the sea and on the land. Sediment samples will be collected, both on the beach and from the seabed and after it is cleaned and dried it can be sieved and sorted to understand its characteristics.
Why are we collecting sediment data? By understanding what type of sediments there are on the beach and out at sea, we can understand how mobile it may be and whether it can easily move from store to store. It will also help give insight into how long it takes for the sediments to break down and how likely the stores will replenish.
Coastal Change Modelling
All of the data collected will feed into a range of different models that can be used to predict how the coastline, beaches and dunes will look in the future. These models can estimate the retreat or erosion of a shore in response to sea level rise. The better the quality and the greater the breadth of the data that is fed into these models, the better the quality of the prediction that they will ultimately make.
The Bruun Rule
A model that will be used at the early stages of the project is the Bruun rule. The Bruun rule is a mathematical model developed in the 1960s by the Danish coastal engineer Per Bruun and can help us predict how our beaches will change with sea-level rise. It is based on the theory that beaches will shift upwards and landwards, in order to ‘keep up’ with sea level rise.
While the Bruun rule is widely applied, it is also accepted that it is not perfect. For example, the Bruun rule assumes there is unlimited space behind the beach, whereas in reality there are often cliffs, seawalls or development.
ShoreTrans Modelling
In recent years, more complex modelling tools have been developed to better predict future beach and dune evolution. Whereas the Bruun rule doesn’t consider space behind the beach, the impact of storm events or the influence rivers and deeper water currents on sediment availability, there are other models, such as ShoreTrans, that do.
Using the ShoreTrans model will improve the accuracy of coastal change predictions and will enable the project to better consider the impacts of coastal squeeze, where the built environment limits the coastline from naturally rolling back. However, like all models, ShoreTrans has its limitations, but it is hoped that, as data is collected, some of these can be considered to help improve its prediction accuracy.
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