Project Description: Flood Consequences Assessment for Planning Application.

Client: Saint Malo Valley Ltd.

Introduction

Any proposed development scheme located within a floodplain requires the submission of a Flood Consequences Assessment in accordance with TAN 15 (PPG 25 in England).

Project Brief

Egniol were commissioned by Saint Malo Ltd to prepare and submit a Flood Consequences Assessment for a site in Valley, Anglesey. The site is located in an area of tidal flood risk, although it is afforded some protection from tidal flaps on the Cymran Strait. The report examined the effect of a successive failure of the tidal flaps and the resulting inundation pattern that would occur between breach and repair.

Project Delivery

Inundation modeling was produced for a range of tidal events up to the 1 in 1000-year return period (0.1%) and for the routing of flood water towards the development site.

A topographical survey of the Valley Mart site and immediate surroundings, including the flood bund levels at the Cymran Strait was also commissioned. This resulting data was then integrated into Ordnance Survey digital mapping for the area to show the predicted flood routing and likely point(s) of overtopping against the tide readings, allowing for a sea level rise of 5mm/year due to climate change.

Environment Agency Wales (EAW) requested design levels of 4.38m and 4.63m AOD for the 1 in 200 and 1000 year events respectively.Tide curves for these levels were subsequently developed for the site using the Holyhead tide curve data and plotted graphically to estimate the time / tidal level relationship over the tidal cycle.

In order to estimate the progressive inundation pattern, Digital Terrain Mapping (DTM) was employed to provide ground level data and the surface area of water bodies for storage volume below bank level. Calibration of the DTM data was undertaken by comparison to actual ground levels.

From the produced tide curve data, ordinates at 30minute intervals were produced and successive flood volumes predicted.
The flood inundation areas have been predicted using LSS 3D modelling software which uses the DTM map data to create successive filling. The image is then further enhanced by importing OS mapping to show ground features in relation to the site.