Groyne Groynes are barriers or walls perpendicular to the coastline, often made of greenharts, concrete, rock or wood. Material builds up on the downdrift side, where littoral drift is predominantly in one direction, creating a wider and a more plentiful beach, thereby protecting the coast because the sand material filters and absorbs wave energy. However, there is a corresponding loss of beach material on the updrift side, requiring another groyne there. Groynes do not protect the beach against storm-driven waves and if placed too close together create currents that carry material offshore.
Coastal erosion and the availability of drinking water are two major coastal management issues that necessitate precise monitoring of the morphological changes to the shoreline. UAV Lidar is being used in this region to produce comprehensive topographic surveys with unprecedented productivity and level of detail, and several Lidar surveys are planned over the next two years.
Read on for details of the context, technical setup and results of the very first survey which took place in January A regional consortium involving several private companies and public laboratories, led by the French Geological Survey BRGMhas been set up to thoroughly investigate the availability and possible evolution of groundwater.
As a consequence, its estuary experiences dramatic morphological changes, both seasonal and linked with extreme meteorological events such as flooding and storms.
Technical Choices BRGM already monitor sections of the shore using photogrammetry, but the benefits of additional Lidar surveying include: The m study area can be surveyed by UAV Lidar within minutes 3 Lidar is expected to produce better reliability than photogrammetry over weakly textured surfaces although a thorough data comparison still has to be performed to confirm this.
The UAV equipped with the Lidar system. Survey and Data Pre-processing The survey area encompasses a m-long section of the shoreline, from the sea to m inland.
The swath obtained at 30m height was in excess of m. The trajectory resulting from the PPK processing provides an estimated position accuracy of 2cm and an angular accuracy of 0. The classified point cloud.
Sea surface echoes are only present underneath the UAV track and in the swash zone. The remaining water surface has been coloured in plain blue. Point Classification and Quality Assessment Five classes were defined: Separating land and sea echoes in the swash zone was a challenging task; due to breaking waves on the shore, the altitude of the water surface varies, covering and uncovering the beach.
In some places, the instantaneous water surface is far above the mean water surface, so that a crude screening based on altitude is not suitable. In the point could, the waves are seen as patterns similar to low vegetation, and some manual reclassification was needed to extract all the information from the data.
The transition from land to sea is characterised by two criteria Figures 4 and 5: In some cases it was possible to follow the terrain below the mean sea surface, when the ground was uncovered by a wave trough during the scan.
A land and bathymetric survey was also performed on the same day as the Lidar survey. Vertical cross-section through the point cloud through the swash zone 50cm thick.
A straight comparison between the points collected during the land survey and the Lidar DTM displays an average error bias of 4.
The main reason for this relatively high figure has been identified as the specific sampling strategy of the land survey; it was focused on the land-sea interface, where ground point classification is the most difficult, as well as on the sharp break in the slope of the beach visible in Figure 1where interpolation from the point cloud to a DTM is expected to produce the worst results.
In view of the fact that the soft sand on the beach is far from a hard reference surface — a mere footstep will create a 5cm indentation — the resulting accuracy can be considered as a good outcome in very demanding test conditions. Conclusions and Next Steps This first survey has confirmed the suitability of UAV Lidar for the purpose of comprehensively surveying the coastal topography in a context of morphological changes.
The point density in this survey is actually excessive and can be reduced significantly without any loss of information. With the current setup and present regulations, a team can reasonably survey up to 10km of coastline per day in favourable conditions, outside urban or restricted areas.
With a fixed-wing UAV and the relevant permits from the civil aviation authority, provided the safety of the flight and reliability of the UAV can be ensured, more than km would be technically achievable.
YellowScan Lidar systems have already been tested successfully in several fixed-wing UAVs and will soon be implemented in operational surveys.
This flexibility of mobilisation is one of the major benefits of UAV operations.
A series of surveys over a two-year period are expected to bring new insights into the dynamics of the river estuary. Corresponding sketch showing the mean sea surface blue linethe sea surface position at different instants blue dashed line and the Lidar measurement points red dots.
Acknowledgements Thanks are due to the development team of the open-source software CloudCompare used for data visualisation and analysis in this project. Sediment dispersal from a typical Mediterranean flood: Continental Shelf Research, 28 Origins and processes of salinization of a Plio-Quaternary coastal Mediterranean multilayer aquifer:Waves erode the base of cliffs, undermining them and making them collapse.
This can create coastal features such as the Twelve Apostles in Victoria, Australia. There are two common definitions of coastal attheheels.com, coastal erosion is often defined as the loss or displacement of land along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms.
This landward retreat of the shoreline is measured to a given datum over a temporal scale of tides, seasons, and other short-term cyclic. Western Australia is facing some of the worst flooding and coastal erosion it has ever seen in the next decade, as the tidal cycle adds to .
Poole and Christchurch Bays SMP2 - the first revision of the original Shoreline Management Plan (SMP1) produced in The review took place between October - November , and the full Draft SMP2 was made available for public examination on 25th November There are two common definitions of coastal attheheels.com, coastal erosion is often defined as the loss or displacement of land along the coastline due to the action of waves, currents, tides.
wind-driven water, waterborne ice, or other impacts of storms. This landward retreat of the shoreline is measured to a given datum over a temporal scale . Assessing the impacts of climatic and, in particular, land use changes on rates of soil erosion by water is the objective of many national and international research projects.