NATURAL RESOURCES WALES

FLOOD RISK ANALYSIS

NORTH WALES AREA

FLOOD RISK MAPPING & RIVER MODELLING CONSULTATION

REFERENCE: A160043

TITLE:Proposed Residential Development, Talybont

DATE RECEIVED: 20th April 2016

DATE REPLIED:4th May2016

  1. NRW (Natural Resources Wales) FRA (Flood Risk Analysis) have received an FCA (Flood Consequences Assessment) from Francis Sant (FS) for the construction 22 residential dwellings on greenfield land adjacent to the Afon Ceulan, in Talybont, to the North of Aberystwyth.
  1. The modelled hydrology does not match the peak 1% and 0.1% APE flows shown in table A.7 within the supplied report. The modelled flows appear to match flows within table A.5, which were derived from ReFH software. The consultant does not appear to have addressed comments made in March by NRW Hydrology Dept and as such hydrological inputs have not been agreed. This review therefore focuses upon the model build and does not consider model results as these are likely to change with updated hydrological inputs.
  1. The supplied model is an ‘update’ to an NRW catchment wide ISIS TUFLOW model constructed in 2009. The consultant has removed unrequired river reaches, leaving only the Afon Ceulan, which flows adjacent to the proposed development site. The consultant has changed the 1D modelling software from ISIS to Estry.

The cut down model extent is considered suitable for assessing flood risk to the proposed development site.

  1. The NRW model used in channel survey data collected in 2009. The consultant has used this survey without ensuring its applicability and representation of the present day. This is particularly relevant following the significant floods in June 2012 which could have resulted in significant channel changes. The 2D domain is also represented by a DTM derived from LIDAR flown prior to 2009 – no checks have been conducted to ensure its suitability for use.
  1. The supplied model has retained a 5 metre 2D grid resolution as used within the NRW model. The focus of the NRW model was catchment wide, and as such, a 5 metre grid was considered relevant. Use of a 5 metre grid is not considered to offer sufficient detail for an FCA study where the channel width is in the region of 5 metres and the overall model extent measures ~750 by 150 metres. No justification has been provided as to why a 5 metre grid has been retained and why it is applicable to the study area. The coarse nature of the grid, and lack of detail, is highlighted by the fact that the overall model run time is 49 seconds.
  1. Modelled cross sections have been ‘exported’ directly from ISIS into .csv format to be read into Estry software. However, the consultant has failed to acknowledge that ISIS uses ‘panels’ to identify the in channel area of the cross section. As such, the modelled cross sections include significant areas of floodplain and are significantly wider than the deactivated 1D domain. This must be corrected to ensure correct 1D and 2D conveyance.
  1. In total, three bridge structures have been modelled, all with a form loss of 1. No LC tables have been used to define structure losses. The supplied model report does not detail how structure losses in ISIS (which automatically calculates losses) have been transferred to, and considered within, Estry software.
  1. TMF file appears to be incorrectly labelled with regards to the roughness value for grass, which should form the default underlying roughness.
  1. The proposal shows all residential dwellings to be located outside the extreme flood outline. However, this outline could change, based upon model amendments detailed in the above points.
  1. Overall, the submitted model has fundamental deficiencies that could result in changes to model results. As such, this review has not considered model health or results. The deficiencies exhibited by the model affect the two most important aspects of the model, namely, the input hydrology and the topographical representation (both 1D and 2D). The input hydrology should be reviewed, and agreed through consultation with the NRW Hydrology Dept. The topographical representation must be addressed through 1) ensuring that all topographical data has been included within the model correctly and 2) that topographical data is representative of the present day and is of sufficient detail to provide confidence in model results.

The supplied model has been modified from a catchment wide NRW model. The consultants must ensure that the NRW model utilised within the supplied FCA model is appropriate, and is representative of the present day, to be used as part of the FCA assessment.