Southeast Regional Coastal Monitoring Programme

Temporal interval for topographic surveys

The temporal interval between surveys has been determined in a similar manner to the spatial interval. The risk framework indicates the following broad categories of temporal coverage. Categories indicate the annual frequency of survey.

High risk beach management plan sites

Hold line, managed retreat

High/medium exposure

Low- exposure

Do nothing accessible beach site

Do nothing inaccessible cliff site

Temporal interval

Full surveys will typically be repeated every five years at all sites. More detailed surveys may occasionally be required at high-risk beach management plan sites, where a complex bed shape is identified on the baseline bathymetric survey.

The key factors to be considered when determining temporal interval are:

  • Tidal range
  • Bedrock type
  • Exposure to wave attack
  • Bed composition, below low water.
  • Proximity to inlet channels and managed navigation channels
  • Presence of nearshore bars and mobile sediments
  • Presence of nearshore sand and shingle banks

    There are sound arguments for a variable approach to the frequency of bathymetric surveys within the region; these are considered below. There is limited value in conducting regular bathymetric surveys at sites where the bed surface is composed of hard bedrock, without surface sediments. Surveys conducted more frequently than once every five years are unlikely to provide significant benefits at many of these sites. Sites at which the nearshore sub-tidal zone is active will show significant changes on a regular basis i.e. where active surfical sediments and nearshore bars occur. These sites are (regionally) most common in areas of low tidal range and sandy foreshore.

    The tidal range of the region varies significantly from west to east, with the lowest tidal range occurring within Swanage, Poole and Christchurch Bays. The bedrock platform is never exposed in this area and the nearshore zone is highly dynamic, usually to a water-depth of about 10m. The rapid and large-scale changes, which occur within this zone, cannot be detected using either aerial photography, LIDAR, or land survey methods. Bathymetric surveys are conducted at least annually within Poole and Christchurch Bays.

    The temporal interval for subsequent monitoring of the nearshore zone needs to be evaluated on a site-by-site basis. Some areas will need monitoring at least annually, e.g. the whole coast between Swanage and Hurst Spit, whilst others could be monitored more cost effectively at an interval of approx. five years. A degree of flexibility is needed here to optimise value for money. Surveys across areas with a rocky bed are less likely to produce accurate, but more importantly, repeatable results. Areas with either a soft substrate or mobile bed need to be monitored more frequently, but can be monitored more accurately. Examples are accurately given below.

    Bathymetric surveys are conducted regularly for navigation purposes, within many of the harbour areas, but data sets rarely extend to the shoreline. Dynamic estuarine areas with mudflats may change rapidly and there are often opportunities to combine harbour navigation surveys with those for shoreline management. Bathymetric surveys are conducted annually adjacent to the Western Solent marshes; this approach provides useful information across a rapidly eroding mudflat area that cannot be monitored safely using land survey methods. The mouths of Chichester, Langstone and Poole Harbours are also surveyed annually, to combine management of sediment transport and navigation.

    The temporal interval for each of the generic categories is shown below.

    5 years

  • Low exposure tidal inlets eg. Portsmouth Harbour, Pagham Harbour.
  • High exposure beach frontages where the beach toe is exposed on a bed platform at low water e.g. Worthing
  • Do nothing sites
  • Hard cliff sites with no beach e.g. Portland

    2 years

  • Medium exposure eroding saltmarsh/mudflat frontages at narrow tidal range sites (2m) e.g. W Solent

    1 year

  • Inlet entrances to tidal harbours eg East Head, Hayling Island
  • Hold the line sites with narrow tidal range where beach toe does not dry on bed rock.

    6 months

  • BMP sites in areas of low tidal range with active nearshore bar or mobile bed
  • Tidal inlets associated with regular beach recycling e.g. Chichester approach channel

    A baseline bathymetric survey of the open coast is in progress within 1km of the shoreline. GIS layers are available of the baseline.