Review of the Flood Risk Assessment (FRA) for the North Street Quarter, Lewes (February 2015)
By Paul Deane
(Chartered Civil Engineer having worked for the Environment Agency Flood Risk Management for 14 Years)
(Planning Document references: SDNP_15_01146_FUL-FLOOD_RISK_ASSESSMENT_AND_FLOOD_DEFENCE_PROPOSED_WORKS_PARTS 1 to 15)
The development proposed by Santon will increase flood risk in general, as a significant number of new dwellings are to be created behind the proposed flood defences. These flood defences will also marginally increasing the risk immediately upstream of the site as they will constrain the river within the river channel for higher river levels. This will affect the lowest properties in Landport (mitigated for by the flood defence scheme currently under construction). The new flood defences will however reduce risk to existing properties in Talbot and Pelham Terraces.
Whilst a major flood from the river is the most serious threat; relatively frequent surface water flooding from rainfall trapped within the site would decrease the quality of life for those future inhabitants of the development. The Santon FRA seeks to address this issue with a number of flood resilience measures which are welcomed, however the surface water will be trapped on the site when the river is high. This section of the River Ouse can remain high for several days following major rainfall events, and it is unlikely that the site will be completely free from surface water. (This issue is discussed in general in the All Party Group paper ‘Living with Water – March 2015).
If the South Downs National Park authority considers that development of this part of the River Ouse flood plan is permissible, then it is recommended that the probability of surface water flooding and consideration of remedial measures are addressed in greater detail than so far presented in the FRA.
Should the site be developed from a flood risk perspective?
Lewes District Council in association with South Downs National Park have produced a document “Lewes Joint Core Strategy – Sequential and Exception Tests (September 2014), which tests whether the benefits of the development outweigh the risks. It concludes that they do, whilst listing a number of requirements for management of the flood risk (see section 3.2 of the FRA).
Is there a better alternative to the proposed flood defences?
The Environment Agency produced the Sussex Ouse Flood Management Strategy in June 2004 following the Autumn 2000 flood. This considered a range of future flood management options for the river Ouse through Lewes including upstream storage to attenuate flows from extreme rainfall events. The strategy concluded that for the North Street and Talbot Terrace cell, the improvement to the existing flood defences to an indicative standard of 1:200years was the recommended option using the nationally adopted cost/benefit appraisal approach. This is economically driven, and does not necessarily produce the most desirable solution if money were no object.
In an idealistic world, it would be considered desirable by many to have the defences set back from their current location and ‘softened´ by constructing earth banks with biodiverse vegetation (including aquatic margins). This all comes at a price however as these measures all take up space which would otherwise be of commercial benefit. A flood alleviation scheme currently being developed for Derby is unique in England, and seeks to create a ‘blue corridor’ along the route of the river as it flows through the city centre which both reduces flood risk and greatly enhances the quality of environment. This is however a long term strategic approach for the regeneration of Derby, and will require many years to succeed.
The flood defences for the majority of Lewes flood cells are now complete and there appears little benefit in a localised ‘set back ‘solution to the North Street cell. The Environment Agency has expressed a wish to introduce bank vegetation margins which may gain the support of the SDNP, and would support biodiversity.
In conclusion therefore, the proposed flood defences are broadly considered to be the best viable solution for this location.
Are the proposed flood defences and dwellings at the right level to prevent future flooding?
The proposed flood defences are set at a level equivalent to the defences on the opposite river bank past the Tesco site (+6.1m OD). This is calculated to withstand a 1:200 year flood (reducing with climate change). Whilst this is of sufficient standard to currently receive ‘affordable’ insurance, it does not prevent the proposed dwellings from flooding, and it is essential that all future inhabitants are fully aware of this. Moreover the site flood mechanism is likely to be a combination of high/surge tide and extreme rainfall. In such an event, river levels will rise relatively quickly and water will cascade into the site to a depth of around 2 meters, where it is likely to remain trapped for several days. In order to protect human life, the flood warning measures will require detailed consideration, and all dwellings will require flood resilience measures. This is largely addressed in the FRA.
Are the proposed flood defences fit for purpose?
It should be recognised that the current flood defences are piecemeal, of various crest levels, and of dubious construction/foundation. The new defences are therefore a significant improvement. The solutions to the various sections of proposed defence appear well considered both from engineering and aesthetical perspectives. The Environment Agency promotes ‘passive’ solutions wherever possible, and opening barriers are discouraged. It would technically be possible to remove the proposed barriers from the return flood defences (Pells Pool to Talbot Terrace) and introduce passive solutions instead (rerouting of access needs/ramps/land raising). These should at least be considered during detailed design.
14 April 2015