Environmental Impact :: Flood Risk Assessments
Flood Risk, Sources and Potential Extent of Flooding
5.1 Flood Risk
The Environment Agency’s indicative flood plain map (reproduced in Appendix C) indicates that the Regeneration Area site lies wholly within the tidal flood risk area. The flood risk at the site is therefore dominated by tidal water rather than fluvial or other sources (i.e. flood risk arises from high tides, surge effects and waves, not river conveyance discharge overspill).
The site is also at potential risk from groundwater under a possible artesian effect. This means that the ground water could rise up through the ground. This effect has been investigated with the use of boreholes and groundwater monitoring and it has been determined that groundwater does not rise at sufficient a rate to contribute perceptibility to flooding events. High groundwater levels are nevertheless possible prior to flood events, resulting in increased levels of flooding of the Marshes in tide-locked conditions. (These adverse antecedent conditions have been assumed to be present when calculating depths of possible flood water in the marshes due to tide-locked surface water; in the event of flooding from the sea all antecedent conditions and surface water flooding will, of course, become irrelevant since flood levels will be determined by sea levels).
5.2 Sources & Potential Extent of Flooding
Sources of Flooding and Methods of Flood Estimation
The level of flood risk to currently consider (and the minimum level of flood protection to provide, where development is permitted) is that corresponding to an annual probability of flood occurrence of 0.5%, (i.e. once-in-200 years), during a 75 year period.
The current frequency of local flooding which may impact the site is, however, significantly more frequent than once-in-200 years, based upon local knowledge and observations over several years, and calculated predictions of sea levels in relation to the Tramway embankment level (which provides the marshes and the site with its defences from coastal flooding).
There are five potential sources of flooding: -
* tidal (the primary source);
* rising groundwater;
* trapped surface water run-off, due to rain fall, when unable to drain under tide locked conditions;
* sea wave overtopping and run-off drainage as surface water and trapped by tide-locked conditions;
* fluvial conveyance (low risk).
(In addition, consideration needs to be given to failure or incapacity of the existing surface water drainage system in Harbour Road, and / or of the pumping station in Riverside Way).
The primary source, tidal flooding, has a number of components: -
(1) high ‘still water’ tide levels;
(2) tidal surge caused by low atmosphere pressure;
(3) wind set-up, causing the surface level of tidal water to be raised by the effects of the friction drag of shoreward winds;
(4) wave set-up, caused by the effects of waves running up the beach;
(5) waves overtopping sea defences;
(6) the effects of climate change (“global warming”).
These components are each increased for rarer, more extreme events. Some of the extreme events tend to occur together also, due to common atmospheric factors, particularly (2), (3) and (5) which may also occur closely in time with high rainfall and surface water flood risk.
For the purposes of this Flood Risk Assessment, components (1), (2) and (3) have been evaluated from Posford Haskoning Ltd’s “Environment Agency South West Region Report on Regional Extreme Tide Levels” (February 2003).
Component (4), wave set-up, has been assessed in the calculations using an iterative approach based on the wave-breaking heights of waves running up the fore-shore and beach toward the sea defences along the Esplanade.
Component (5), wave overtopping flood risk, has been assessed from the EA’s R&D Technical Report W178 “Overtopping of Sea walls – Design and Assessment Manual”, taking the significant wave height to be the highest wave-breaking height on the foreshore (in accordance with the guidance given in Posford Haskoning’s report above), using the sea level due to wave set-up at the wave breaking line as the “still water level”, and retaining the wave periods and frequencies found for the full height waves out in Seaton Bay. For the purposes of wave over-topping calculations, the highest still water level (prior to the addition of wave set-up) has been taken as 3.0m AOD, the height of the top of the Tramway which provides the sea defence to the existing site; were it higher, then the quantity of flood water due to wave overtopping would be completely masked by inundation of the marshes and the site (at its current level) by the sea. (Also, for calculation purposes, the extreme wave conditions and highest tide level have been taken to be constant throughout the tide-lock events – a very conservative assumption).
Three conditions of the overtopping have been considered: -
i) with the existing shingle beach substantially intact in its most adverse current topography,
ii) with the beach scoured away to the front of the existing sea wall, and
iii) with the shingle beach banked up to the crest of the sea wave return wall.
In the first condition (i), negligible quantities of sea water overtop the existing sea defences; in the second condition (ii), however, very substantial qualities can overtop. In the third condition (iii), considered here because local reports indicate it to be a relatively common occurrence, supplementary calculations indicate that a peak discharge intermediate between conditions (i) and (ii) is possible.
It is therefore the second extreme condition (ii) which is considered when assessing the level of surface water flooding in the marshes under tide-locked conditions.
Component (6), climate change, has been assessed using current EA recommended criteria guided by draft PPS 25 and UK CIP data, as described above, section 3.9.
Combined high tide – surge – high wind – rainfall event effects have been treated as partially interdependent joint events in accordance with the guidance given in the DEFRA / Environment Agency Flood and Coastal Defence R&D Programme – “Joint Probability: Dependence Mapping and Best Practice” – R&D Interim Technical Report FD2303/TR1 (July 2003). Thus for an annual probability of 0.5%, the combined return period required is 200 years. With a correlation co-efficient of 0.7 (taken from FD2303/TR1, interpolating between Newlyn and Weymouth), the worst combination of marginal return periods which together give the 1-in-200 overall event probability (Ts, Tw and Tr) was found to be Ts= 200 years for surge tide levels, Tw= 1.37 years for the worst wind conditions and Tr= 1.37 years for rainfall event.
Table 1 of the calculations reproduced in Appendix D below [see section P(3.2), page (4) of the calculations], presents the effective “marginal return periods” which, taken together for joint surge and wind events, correspond to a 1-in-200 year event scenario in each case.
Extreme tide levels under surge conditions are tabulated (together with climate change effects) in Table 2, section C (1.1) of the calculations reproduced in Appendix D below, page (5).
The Posford Haskoning Ltd report indicates a once-in-200 year extreme tide level of 3.51m AOD, (inclusive of surge effects and wind set-up, but not wave setup) for the year AD 2002. Using a derived mean annual rate of sea level rise of 6mm/year, 75 years from AD 2007 (the presumed year of initial beneficial occupation), the extreme once-in-200 years tide level will be 3.96m AOD. This then is the figure used as the basis for assessing risk to future residential development. (A lower level of 3.81m AOD, based on a 50 year design life from AD 2007 could be adopted for commercial / retail development).
From BS 6349:1:2000, and using wind speed data from BS 6399:2:1997, the significant wave height is calculated to be 5.68m high out in Seaton Bay for the surge tide return period of 200 years and a corresponding wind event return period of 1.37 years (with +10% increase in wind speed and +10% increase in wave heights due to climatic change, and +10% further increase in wind speed when converting the BS6399:2:1997 level based wind speed data to off-shore wind speeds). The wave period is 9.9 seconds. Inshore, taking into account sea bottom drag, wave run-up and “average” wave breaking depth limitations, the significant wave height impacting sea defences is reduced to 1.125m, but the wave period is (conservatively) taken to remain at 9.9 seconds, to calculate maximum overtopping rates.
Existing sea wall defence and beach levels are taken from The Admiralty Chart for Seaton Bay and additional survey work carried out for Jubb Consulting Engineers and reproduced in Appendix E1 below.
Employing a standard hydrological calculation technique (the Wallingford Procedure Modified Rational Method, adapted by reference to other data sources such as Chow’s Applied Hydrology) surface water run-off due to rainfall is calculated for the combination of all catchments draining to the sector of the Marshes into which the subject site also discharges. (These are represented on the drawings reproduced in Appendix E2 below). The total 1-in-100 year rainfall run-off surface water discharge is then added to the sea overtopping
volume to obtain a total volume discharging to the marshes during the tide-locked period.
The tide-locked period is determined for this purpose for two conditions:
i) high surge tide lasting over a single tide’s duration (i.e. the effect is to lift the highest astronomical tide profile by the surge height), and
ii) high surge tide lasting over two tides’ duration (i.e. the surge level does not drop sufficiently to allow for the Marshes to
discharge to the River Axe between tides – an event reported to currently occur at least once a year).
The method of calculation of the tide-locked periods is quite involved and iterative but is essentially based on determining the period for which the tide level is lower than the water level in the Marshes, using tidal profiles for Exmouth approaches and Lyme Regis to derive a profile for Seaton. Using that data, an assessment is made of the capacity of the existing tidal outfalls to discharge water impounded in the marshes during drowned outlet and free discharge conditions.
Existing Drainage Systems
Surface water drains from the main population areas of Seaton eastwards towards the River Axe and the marshes via a system of combined foul / surface water sewers (augmented by some single duty surface water sewers) and an open ditch / Rhyne system (as indicated on the SWWIM plan reproduced in Appendix E2).
Whilst some single duty surface water sewers discharge directly into the ditch / Rhyne system, the bulk of the surface water drainage is routed into the combined sewer in Harbour Road. This sewer also receives discharges from the properties between Harbour Road and the Esplanade.
The sewer in Harbour Road is an egg-shaped culvert which is routed to a pumping station in Riverside Way. Discharges from the pumping station are pumped north via a rising foul main to the sewage treatment works located in the marshes on the north side of Staffords Brook.
In the event of the surface water or combined sewers’ being overwhelmed by surface water inflows, they surcharge and overflow overland to the marshes north of the Regeneration Area: sewers west of The Underfleet Brook overflow mainly into The Underfleet and thence into the Rhyne system; some areas of Seaton nearer the southern end of The Underfleet will, however, flow down The Underfleet carriageway, overspilling both west into The Underfleet Brook and east through The Underfleet car park into the east-and-north flowing Rhyne system adjacent to the Tramway.
Surcharge flows to the Harbour Road combined sewers and to the streets south of Harbour Road will flood Harbour Road to a depth of up to 150mm in places, rising to the threshold levels of the accesses to the holiday village and former caravan park on the south side of Harbour Road, then overspilling and draining through these properties into the lower lying Rhyne and marshes north of the Regeneration Area.
The Rhyne system is routed to and through the marshes to discharge into the River Axe through flap-valved outfalls. In the event of their overtopping (due, for example, to tide-locked conditions or extreme peak discharge incapacity scenarios), they overflow into the southern marshes to the north of the Application Site and Regeneration Area, and drain back to the Rhyne system as the Rhyne incapacity abates.
The Axe Riverside land within the Regeneration Area east of the Application Site is currently occupied by a mixture of light industrial and boat building uses. It is slightly higher in elevation than Harbour Road so that shallow flooding in Harbour Road does not impact it significantly.
The combined surface water / foul water sewer in Harbour Road is routed through this land via Riverside Way (as noted above), to the South West Water pumping station at the edge of the Regeneration Area.
Surface water drainage in the Axe Riverside land to the east of Riverside Way generally drains directly to the River Axe, either overland (car park areas etc) or via two piped outlets. All other drainage is routed either into the combined sewer noted above, or into the drainage ditch behind (north of) the Riverside Workshops. In the event of combined or surface water incapacity, or in the event of pumping station failure, surcharge flows discharge into this drainage ditch behind the Workshops.
This ditch drains west to east and is then routed north to the main Rhyne system via a culvert running along the “Red Line” boundary of the Application Site, entering the Rhyne system via a boundary ditch immediately west of the Tramway depot.
Discharge from the Marshes to the River Axe is through 3 Rhyne outfalls fitted with tidal flap valves: two of 575mm diameter and one older one of approx 18 inches diameter. The northern-most outfall of these three is a 575mm diameter pipe located on the north side of Staffords Brook bank and which also accepts a discharge from the Northern Marshes catchments. (These northern marshes are not shown in Appendix E2.) A flap valve prevents the northern marshes draining through the Staffords Brook bank into the southern marshes (into which the subject site drains), but it also ensures that when the floodwater level in the northern marshes is higher than that in the southern marshes, the northern outfall is closed to discharges from the south. Accordingly, the worst case is when only one 575mm diameter outfall is operating in conjunction with the 18 inch diameter outfall.
Calculations reproduced in Appendix D determine that in the worst case up to 322mm of surface water (average depth) can accumulate in the southern marshes above existing mean ground level (1.25m AOD), and that it can be held for two successive tides before being released through the two outfalls over the following three inter-tidal periods.
Should the combined sewerage Pumping Station in Riverside Way become inoperable or overwhelmed by surface water inflows, it will overflow and the discharge will drain to the Marshes via the Rhyne system behind Riverside Workshops. The rise in water level in the Marshes is estimated to be approximately 40mm above the 322mm depth given above and tabulated below (i.e. a total depth of water about 360-370mm above average marsh surface level).
The net result of overflows to either the existing sewer in Harbour Road, or to the Pumping Station in Riverside Way would be a small increased depth of flooding in the Marshes. The knock-on effect would be a slightly prolonged drainage period as the Marshes discharge at low tide into the River Axe. In neither case is the situation any different from what it would be if the subject site remained undeveloped.
The existing principal flood escape routes are shown in Appendix E3, drawings P8877-G202C and G205B.
