Environmental Impact :: Flood Risk Assessments

Proposals for Flood Defence

6.1 Current Defences

The subject site is currently defended from tidal and fluvial flooding by existing flood defences along the Seaton Tramway embankment to the east and the Esplanade to the south. These defences are not considered adequate to defend the site to the current required standard. (Their top level is 3.0m AOD and the 1-in-200 peak surge tidal level will be 3.96m AOD in AD 2082).

The Environment Agency considers the risk from tidal flooding to residential development is of high category and will require flood defences to the current national guidance levels.

6.2 Site Levels

Note on Survey Data

The site and its environs have been subject to various surveys over the last few years. The earlier surveys were based on the Ordnance Survey bench mark (OBM) levels and covers most of the site.

Subsequent surveys for the developer in 2006 are based on the Ordnance Survey GPS active network which has given a datum approximately 180mm below the land based OBM datum.

Asset surveys in 2006 carried out for the Environment Agency (EA) give a level difference between the GPS-based systems and the OBM land-based system of approximately 45 – 50mm. Differences between the developers’ surveyors’ GPS datum and the EA’s asset surveyors’ GPS datum vary between approximately 90 and 130mm.

It is accepted that the EA’s asset survey baseline is the most reliable, and levels shown in Appendix B3 have been adjusted to conform to that survey on the assumption of an average 90mm correction requirement between the two GPS surveys, and adjusting the land based OBM surveys accordingly. (This is a conservative assumption, and before actual works are designed a thorough check will be required to verify or adjust correction).

However, for the purposes of assessing flood risk the calculations in Appendix D and the sea wall level survey in Appendix E1 are based on the worst possible case – that which assumes that the developers’ GPS surveys is correct, setting the Regeneration Site 180mm below land based OBM surveys. (It is likely that this level is actually between 90mm and 130mm higher).

Existing and predicted sea levels are based on the EA’s GPS based survey datum.

Summarising –

* drawings in Appendix B3 show the most likely existing levels for The Underfleet and Harbour Road, adjusted to match the EA’s Asset survey baseline;

* calculations in Appendix D and the drawing in Appendix E1 use / show the potential flood risk worst existing case levels, taking them to be 180mm below land based OBM levels and 130mm below the EA Asset survey levels;

* this methodology is conservative (safe) for flood risk assessment and design purposes but will require to be adjusted in line with current EA surveys before design and construction is implemented;

* sea levels are based on the EA’s GPs-based datum.

Required Changes in Level

At present, the site is defended from flooding to a 1-in-10 year standard along the eastern boundary. The site is low lying behind this level at around 1-2m AOD and is considered at risk from overtopping from the sea.

The Environment Agency require that the development ground level to the proposed residential site should be targeted at the 1-in-200 years return period extreme tidal level allowing for combined surge effects plus climatic change risk in sea level in 75 years’ time. External paved areas will be generally required to be at 3.96m AOD (to provide escape to residential property).

Residential building ground floor levels will require to be set at 3.96m AOD plus 0.15m freeboard = 4.11m AOD;

The average existing ground level in the areas behind the flood defence embankment is around 1-2m AOD. Due to the site’s sensitive proposed end use (residential) it will be required to raise levels in this area to tie in with the 4.11m AOD domestic floor level and 3.96m AOD flood escape route levels.

Commercial & retail buildings’ ground floor levels may be designed (for a 50 year design life) to be at 3.96m AOD.

Should PPS require residential properties to be designed to meet minimum standards of flood defence for 100 years (rather than the 75 years currently proposed), then these levels will be required to be raised by 150mm (25 years’ sea level rise at approximately 6mm per year) to 4.26 and 4.11m AOD respectively.

6.3 Wave Over-Topping

Application Site

Historic flooding has been known to occur by the sea escaping inland from the River Axe estuary over the Yacht Club slipway (lowest point) and out through the Yacht Club access onto Harbour Road.

For the purposes for the calculation produced in Appendix D this route will be treated as the primary vulnerability, the spit area being generally

higher, as well as subject to recent earthworks operations (making levels higher, but uncertain).

The Yacht Club is at present estimated to be at risk of flooding of about 10% annual probability, and this will rise to 63% by 2082.

Whilst the subject site will be protected from flooding, Seaton will remain vulnerable until the Yacht Club levels and the Tramway river embankment (set at similar levels) are raised or a flood defence is provided.

Breaking waves and sea spray water over topping the Esplanade is estimated to produce a peak discharge of 0.73 m3/s during extreme wave conditions (with the beach scoured away so that no significant energy is lost during wave run-up). The water produced by this event will be routed through the site along the Primary Flood Escape route (where it will run at an approximate depth of 245mm in the worst case) to discharge into the marshes north of the site.

Axe Riverside

In addition to raising site levels, this area will require a sea-wall to prevent overtopping by waves in the Axe Estuary.

Outline Calculations (sheets AR-47 to AR-61, pages (97) to (111) in Appendix D) indicate that a riverside wave wall of approximately 1.1m to 1.2m in height (i.e. having a crest level of approximately 5.32m AOD) will limit peak wave overtopping to acceptable levels.

Water overtopping the new riverside wall will drain through the surface water and flood escape route systems to the marshes north of the Regeneration Area. At the sea level which causes overtopping of this wall the marshes will already be submerged below sea water flooding in over the Tramway north of the site (threshold levels approximately 3.0m AOD), so impact from wave overtopping on drainage of the marshes will be negligible.

6.4 Surface Water Drainage

Application Site

It is proposed to generally provide conventional gravity drainage discharging into infiltration systems (soakaways) placed in the fill required to raise site levels. (Natural ground below the existing site has limited soakaway potential due to high water table). This will depend for effectiveness on the quality of fill (its porosity) and its depth. Since this is currently unknown, attenuation has been calculated assuming little can be provided by the soakaways. The attenuation will be provided by a narrow widening of the Rhyne to form a 9m wide access berm for 360m with an operational depth of 500mm. Discharge to the Rhyne will be by means of gravity overflow pipes from soakaways and conventional drains.

Because of the slight intensification of land use as a result of the proposed development (principally the foodstore and its car parking), there will be an increase in surface water peak run-off rates in extreme events. In order to ensure that there is no increased rate of discharge to the Marshes, the existing receiving Rhyne (at the northern boundary of the subject site) will be supplied with a low level access 9m wide strip along its southern side to accommodate the increased discharge anticipated. See Appendix D, calculation pages (73)-(89).

The proposed development will not increase the depth of flooding in the marshes.

Axe Riverside Area

This land will also be drained by gravity systems. These will discharge either directly to the River Axe (as present) through flap-valved drains, or, where practicable, into infiltration systems (soakaways) placed in the fill required to raise levels, and where the underlying natural ground is suitable. Infiltration system overflows and any gravity systems which cannot drain to the river or to soakaways will discharge to the Rhyne systems north of the Tramway depot via a retained and extended length of the existing culvert on the boundary of the Application Site.

No attenuation will be required because –

* site redevelopment will lead to a less intensive use of impermeable areas, and

* the current proposals include infiltration systems which will reduce peak surface water run-off from the site.

6.5 Flood Escape Routes

Application Site

Flood escape routes are to be provided to cater for surface water extreme discharges (as shown in Appendix B2). Escape Route 2 (and an existing one) generally follow existing Rhyne systems. A new flood escape route (Escape Route 1) is proposed through the development site. This will comprise a 4.5m wide cycleway set at approximately existing ground level between Harbour Road and the Marshes.

The sewer in Harbour Road is a combined egg-shaped culvert which receives surface water from a substantial area of Seaton west of the subject site. Should it become surcharged and overflow, the discharge will drain to the marshes through Flood Escape Route 1 which has ample capacity for such a flow.

At present when the drainage system in Harbour Road exceeds its capacity, surface water ponds up to the thresholds of the former caravan park and Lyme Bay Holiday Village, as described in section 5.3 (existing drainage systems) above. The provision of the new Flood Escape Route 1 will enable more efficient draining of Harbour Road (by setting its thresholds lower) so that such ponding is reduced.

On The Underfleet there are at present two low points: one on the bend to the north west of the Regeneration Area, and one on the east side of Underfleet Roundabout. If The Underfleet highway drainage capacity is currently exceeded, surface water either overspills on the inside of the bend in the road, draining into the Underfleet Brook, or it overspills into the Underfleet car park entrance (which is to become the access to the new “town square”).

The proposed development will cut-off the Underfleet Roundabout flood escape route, diverting flows back up to the low point on the inside of the road bend some 170m north of the roundabout. Here excess surface water will discharge into the adjacent Underfleet Brook and new Flood Escape Route 2. If the eastern lane of the Underfleet roundabout (i.e. that area within the Application Site red line) is left at present levels, then, in the event of blocked offsite highway drainage, the water at the channel (gutter) line will rise to 300mm in depth before escaping north to Flood Escape Route 2. If the lane is rebuilt up to existing channel levels on The Underfleet, then the maximum depth of any ponding before the highway run-off drains back to Flood Escape Route 2 will be no more than 50mm.

In summary, the flood escape routes from Harbour Road will improve, and those on The Underfleet rearranged.

Axe Riverside

Should the combined sewerage Pumping Station in Riverside Way become inoperable or overwhelmed by surface water inflows, or should the surface water drainage system in Riverside Way become overwhelmed, then excess surface water will escape north to the marshes via a proposed new flood escape route (Flood Escape Route 3) adjacent to the proposed realignment and extension to Riverside Way, entering the Marshes through the existing Rhyne behind (west of) the Tramway Depot.

6.6 Downstream Flood Risk and Sustainable Drainage (SUDS)

The subject site will discharge water to the river estuary dominated by tidal conditions via the existing Rhyne system which incorporates a widened berm in order to attenuate water during peak surface water discharge conditions. No additional offsite flood risk then results from surface water discharge from the site.

The proposed use of the infiltration systems will assist in reducing offsite flood risk due to discharges from the Regeneration Area.

6.7 Warning Systems

It is proposed that the site be linked to the Environment Agency's automated early warning systems.

A site evacuation management plan is proposed (details to be agreed with the Environment Agency) to be implemented at the site.

Because the land surrounding the development site is all below the 3.96m AOD level, in extreme conditions the site would become an island, with surrounding water in excess of 960mm deep. Accordingly refuges are proposed with emergency medical attendance in two publicly accessible buildings on the site (e.g. the foodstore and the visitor centre).

6.8 Site Evacuation

The site itself will become a self contained island in the event of extreme flooding. In addition to the provision of refuges, therefore, consideration will be given to the supply of one or two special access vehicles (e.g. deep-wheeled transport, or shallow draft boat) to be stationed at one of the refuges.

6.9 Temporary Flood Defences (During Construction)

It is proposed to develop the site in phases progressing from west to east.

The Rhyne widening proposed for the permanent works will be carried out at the beginning of the first phase to ensure that the surface water attenuation required is always suitable.

Alongside the eastern boundary of the early phases of the development of the Application site there will be formed a flood escape route to be retained until the phase in which the cycleway is to be constructed so that that there will always be a controlled route for flood water from Harbour Road to the Marshes.

It is envisaged that the Application Site redevelopment will proceed earlier than the Axe Riverside redevelopment (although it could conceivably be carried out simultaneously). In order to ensure that flood escape is adequately catered for in either eventuality, the existing flood escape route from the ditch behind the Riverside Workshop (Riverside Way) and the marshes north of the site will be maintained until a new flood escape route is provided (“Flood Escape Route 3”) as part of the Axe Riverside redevelopment.