The extent of the bathymetric survey programme has been determined by a preliminary estimate based upon most recent surveys; this is reliant on Admiralty Charts in many areas. Areal extent of surveys has been considered in context with: (a) the presence of surficial sediments, determined in conjunction with offshore bed sampling (b) the complexity of bedforms, such as offshore sand and shingle banks (c) the offshore wave climate and (d) existing wave transformation models. The resultant extent will necessarily be different for each area. The risk-based programme design process provides a spatial interval with either one of two alternative survey line spacings; this will be reviewed following the baseline survey, depending on the regularity of the nearshore bathymetry, and presence of nearshore surfical deposits. The spatial interval is recommended on the basis that it is suitable for generation of surface models and is adequate for most numerical modelling applications. Line spacings may need to be closer to monitor changes at the mouth of tidal inlets. Surveys will extend offshore typically between 0.5-1km, depending on anticipated beach profile closure depth. As a general guide, changes to the bathymetry in water depths greater than about 10m-15m have little impact on wave conditions, in this region.

A high quality baseline survey is needed for the whole of the region. Some areas are already well covered in this respect, but most areas are lacking in suitably detailed data. The programme will seek to identify changes to the plan position of a series of contours, at a regional scale; these might typically include: MLW, 5mCD, and 10mCD. Surveys will be conducted to the same datum (ODN) as topographic surveys across the whole region. Baseline surveys will typically be generated at a spatial interval of 50m, although this too will vary according to local bathymetry. When used in combination with the MHW line survey from the aerial survey this data could be used to examine (a) large scale changes in nearshore steepening and (b) recession or advance of the contours.

Hydrographic survey profile spacing

Data will typically be analysed using digital terrain models and grid surfaces produced for use in GIS

Example bathymetric surveys of the nearshore sub-tidal zone

Copyright New Forest District Council

On some occasions, for instance at tidal inlet entrances, data is combined with topographic data to produce combined surface models of the beach and submerged bathymetry.

Ground model of a combined bathymetric and topographic survey, showing the mean high water contour and track plots of survey lines

Copyright New Forest District Council