Stormguard floodplan considers:
“A pluvial flood is caused by the stores for rainwater underground becoming over capacitated and so the water begins to back up. The flood usually occurs in the weakest part of the pipe and spreads outwards. It sometimes backs up to street drains and causes a surface flood. This type of flooding is perhaps the least disastrous to urban life as usually they are only a few centimetres deep. However this is still enough to seep into your home and disturb a buildings foundation. If this happens frequently over a number of years it may begin to erode the mortar around your property and form cracks in the cement. This can be worsened if the water in the cracks freeze, as it will increase the size of the crack, and so in the event of a coastal or flash flood water will easily find a way in.”
Castle Point Borough Council’s Surface Water Management Plan (SWMP) document, dated April 2012 was produced by URS Scott Wilson in conjunction with Castle Point Council and the Environment Agency.
“As part of Phase 2 Risk Assessment, direct rainfall modelling has been undertaken for Castle Point…
The results of this modelling have been used to identify Critical Drainage Areas (CDAs) to denote an area or catchment where multiple or interlinked sources of flood risk cause flooding during a sever rainfall event, affecting houses, businesses and/or infrastructure and where mitigation measures may be implemented to reduce the impact of flooding.
Those areas identified to be at more significant risk have been delineated into Potential Surface Water Flooding Hotspots (PSFWHs) representing the key area at risk of surface water flooding, contributed to by the rainwater falling within the area of the wider CDA flooding which has the potential to have the deepest flooding and the most receptors affected.”
“Within the South Essex study area six CDAs within Castle Point Borough Council have been identified.
Due to the large number of potential CDAs (across South Essex) identified, in order to focus on the key flood risk areas and to develop and present options for Phase 3, the CDAs were shortlisted based on the following:
a) the frequency of historical flooding within the CDA and PSWFH;
b) the potential risk of groundwater flooding within the CDA;
c) the frequency of sewer flooding incidents within the CDA or PSFWH;
d) the presence of critical infrastructure at risk within the PSWFH;
e) whether significant future development is likely which could exacerbate surface water flooding;
f) the number of buildings and residential properties flooded at a depth greater than 0.3m within the CDA.”
The obvious weakness in this approach is the reliance on Castle Point Council officers and their failure to record and provide the historic surface water flooding evidence, the erosion of the drainage system by various means such as the infilling of dykes, and the planning of a densely urbanised townscape at Canvey Island.
In respect of Canvey Island the Castle Point Borough Council SWMP modelling found that:
“The pluvial modelling indicates that there are no extensive areas of surface water flooding, due to the flat topography, limited overland flow and the managed system across the CDA.”
“There is no PSWFH within the CDA.
The pluvial modelling does not highlight any areas of significant flooding and the only flood risk from surface water is associated with drainage system failure.”
The recent Essex County Council Report into the floods on Canvey Island during July 2014 is far more direct in identifying that:
“Canvey has been developed on an extremely flat and low lying estuarine island in the mouth of the River Thames with a land height of approximately 1m below the mean high water level, its current form being the result of nearly 500 years of land reclamation and drainage. The island has been historically susceptible to all sources of flooding, and is protected from tidal flooding by a sea wall on all sides.
As a consequence the water level on the other side of the sea wall is regularly higher than the land, meaning that the drainage of surface water is wholly reliant on pumps to discharge water into the estuary at these times.
These pumps are the final element of a long, incredibly complex and interlinked surface water drainage system comprising of drains, culverts, sewers, open watercourses, main rivers, pumps and storage areas all with varying capacity, which need to be operating efficiently in order to drain the island.
Rainfall on the island may flow a substantial distance before reaching the pumps, through infrastructure owned or managed by a large number of different organisations and individuals and in some cases without a clear understanding of ownership.
Any constriction on flow either due to blockage or insufficient capacity for the rainfall event can affect the effective operation of the entire drainage system.
The island has a history of surface water flooding, with several locations experiencing flooding issues on more than one occasion outside of the large magnitude events of 20th July 2014 and 24th August 2013. As a result of the relatively densely populated urban areas and large areas of impermeable surfaces the island is especially susceptible to intense rainfall events which result in flash flooding.
In combination with the flat topography of the island this means that Canvey is particularly dependent on the designed drainage infrastructure to mitigate flood risk.”
Quite clearly during the open debrief discussions between CPBC, Anglian Water, Environment Agency the “blue light” services and Essex County Council a clearer picture of the “complexities” of the drainage system on Canvey Island has been appreciated.
However the Surface Water Management Plan document, so readily accepted by the Castle Point cabinet as it appeared to support the further urbanisation of Canvey Island, encouraged a level of complacency and led to a degree of unprepardness ahead of the 2013 and 2014 flooding.
This SWMP document casts a shadow over the name of publisher URS Scott Wilson.
They deserve the opportunity to be engaged so that a decision can be made as to the value of the SWMP.
This document remains, unaltered, in support of the draft Local Plan and it’s proposal to develop 1600 dwellings at Canvey Island!
The Government Scientist is due to be engaged to consider the Essex County Council Flood Report.
Whilst Essex County Council are handed the responsibility of Lead Local Flood Authority, the weakness appears the number of agencies involved. Central funding has a way of being diluted as it makes its way down through the system.
A local, appointed single person responsible, is required to co-ordinate implimentation of flood recommendations and be able to source and direct future funding. A job for our local MP.