Guidance Note 1 - Appendix 1
As part of the requirements for the granting of a Dredging Licence (and some River Works Licences), the licensee is required to undertake a pre- and post-dredge hydrographic survey of the dredging area and any adjacent areas of the river bed likely to be affected by the dredging operations.
Both the pre- and post- dredge surveys must be undertaken by a professional hydrographic surveying contractor. A copy of the survey specification, including the craft to be used, surveying methodology, equipment calibration and tidal reduction, must be submitted in conjunction with a survey report and hydrographic survey results plotted at a scale commensurate with the area to be sounded – generally 1:1000 for berth surveys. A 1:1000 scaled survey requires a sounding to be plotted at a minimum interval of 5m. All survey data is to be provided in digital ASCII XYZ format in conjunction with hardcopy survey plot sheets. All data is to be referenced to the local Port of London Chart Datum in the vertical and OSGB 36 National Grid in the horizontal (see Note 1).
The pre-dredge survey report must clearly outline the area to be dredged (including any side slopes) and the proposed dredging depth. A volumetric calculation of the amount of material to be removed based on the pre-dredge sounding dataset, the target depth and proposed dredge area should be included in the report.
The post-dredge survey must be undertaken within a week of the completion of dredging operations and any deviation from this period must be agreed in writing with the PLA River Engineering and Environment department. The report submission is to include the calculated volume of material removed based on the difference between the pre- and post-dredge bathymetric datasets.
With certain licences it may be necessary for the survey to determine that the works area and berth approaches are free of any obstructions. In these instances the hydrographic survey will need to attain 100% coverage of the associated river bed thus requiring either a Multibeam Echosounder [MBES] or Side Scan Sonar [SSS] survey to be undertaken.
The area that may be affected by dredging operations and/or associated works will be determined by the method of dredging proposed, the location on the river and other environmental factors influencing the river regime at a particular location. There may be a requirement to undertake bespoke monitoring of sediment types, sediment chemical analysis and transport rates in the vicinity. This data may be readily available via existing models or previous licence application submissions. Applicants should liaise with the Port of London’s River Engineering and Environment Department in order to assess the extent and types of data required.
Note 1: Clarification of survey specifications may be obtained from the PLA Hydrographic Department (Tel 01474 562207). Further information is also available here.