Environmental Impact :: Flood Risk Assessments

Potential Impacts of Flooding

7.1 Tidal Flooding

Published tide data indicates that a high tide might rise at a peak rate of about 17mm per minute during a highest astronomical tide (HAT) condition; this extrapolates to possibly 26mm per minute during the once-in-200 year extreme event tide condition.

At these rates the tide will rise 1.0m in about 60 minutes during HAT conditions and 1.0m in about 40 minutes during the once-in-200 years extreme event tide condition.

7.2 Wave Action

Application Site

The subject site in its current location is not at direct risk of flooding from wave action, nor is the site a risk from high waves. The site is however at risk form the over spill / spray from waves over topping the Esplanade to the south of the subject site.

Water from high waves hitting and overtopping the sea defence wall will run to Harbour road as the topography of the area dictates this. In order for the water to be disposed of it is proposed to allow two main flow routes through the site to the marshes north of the subject site. These are clearly highlighted on the proposed surface water drainage strategy plan in Appendix B2.

Axe Riverside

The site requires a wave defence wall to control overtopping. Some water will still come over, but will drain via the surface water system to the marshes. A wall height of 1.1m to 1.2m is recommended. This will require a detailed assessment and design (which might permit a reduced height), since assessment has been based on extremely conservative assumptions.

7.3 Fluvial Flooding

The site, adjoining land and the marshes to the north are defended from fluvial flooding by the tramway embankment alongside the River Axe. Additional defences are provided by the Staffords Brook flood bank (the bank which crosses the marshes upstream of the site and precludes flood flows to the west of the tramway embankment passing down to the site).

Should these defences fail (e.g. by a breach in the embankment or inoperation of flap valves to the Rhyne outfalls), then the marshes would become vulnerable to fluvial flooding from the River Axe.

At its present topographical levels, the subject site could also be at potential risk of fluvial flooding in such circumstances, although the degree of flooding would be marginal, given its elevation above the level of the marshes.

Proposals for the subject site include raising its levels to defend against extreme tidal surge sea levels. At such an elevation the marginal fluvial flood risk to the subject site is eliminated.

The site is not considered by the Environment Agency to be “functional floodplain” of the River Axe given the presence of the tramway riverside embankment and the Stafford Brook flood bank.

Thus, by raising site levels in the estuarial tidal flood plain there is no loss of river flood plain storage or conveyance. That is, the proposal to raise ground levels within the defended area will not cause an adverse impact upon flood flows or storage capacity of the River Axe. Thus neither the subject site nor adjoining or downstream land is put at increased risk of fluvial flooding by this scheme.

7.4 Surface Water Flooding

Surface water will be collected via soakaway systems (where practicable) with overflows and a conventional gravity drainage system to discharge directly into a proposed shallow attenuation widening of the Rhyne between the site and the marshes to the north, ultimately to drain into the River Axe between tides.

The proposed development will not increase the depth of flooding in the marshes (catchment areas and characteristics remain unchanged).