Sustainable Drainage Systems

A Sustainable Drainage System (SuDS) is an approach to managing surface water runoff that aims to mimic natural processes and minimise the environmental impact of urban development. SuDS are designed to control and reduce the quantity and improve the quality of runoff, while also providing amenity and biodiversity benefits.

The Outline Planning Permission proposes use of a SuDS to manage the runoff water from the new development by diverting the water to the West of the site into a very large basin designed to hold water and allow a controlled drainage through a deep borehole – 17m deep

 

East Sussex County Council is the designated Flood Risk Authority and their deep borehole policy is:

In line with advice received from the Environment Agency, ESCC generally oppose the use of deep-bored soakaways as a means of managing surface water drainage on developments. The reasoning for this opposition is two-fold.

  1. The increased risk of groundwater contamination due to direct discharge of waters into groundwater.

  2. The potential impact on local groundwater levels and flows which can increase the likelihood of groundwater reaching the surface (and potentially increasing flood risk).

ESCC recommend that deep-bored soakaways are used as a last resort where the ground conditions are suitable, and the groundwater levels are known. The County Council requires that the following criteria are met when deep bored soakaways are proposed:

  • Investigations showing there are no other alternative surface water destinations.

  • The depth of the system should be no deeper than what is required for soakage.

  • A minimum of two water treatment stages using SuDS measures must be provided upstream of the soakaway.

  • An unsaturated zone must be provided below the base of the structure. There should be a minimum of 10m between the base of the structure and recorded maximum winter groundwater level.

 

The proposed SuDS does not address this policy at all, failing to address any of the points above.

The proposed SuDS is large although there is conflicting information in the application as to the depth of the SuDS basin.

The basin is stated as being an area of 0.159 hectares which equates to 1,590 square metres. To put this into perspective the swimming pool at Summerfields is 25m by 15m or 375 square metres. This basin will be over 4 times larger than the pool.

The depth of the pond is stated as being 0.6m deep but in the Flood Risk Assessment using height contours this is between 1.4m and 1.7m deep. This would mean that when full this SuDS basin would hold 2.7m litres of water (approximately 600,000 gallons).

There is no evidence that consideration has been given to the impact of building a new access road on the Ridge as even now water runs at speed down the road when there is heavy rainfall and this water will flow directly on to the green area and into this SuDS basin.

Any failure of this SuDS basin to contain this water could have drastic consequences for the surrounding areas.

Any failure of this SuDS basin to drain (become blocked) would result in overflow and again release significant volumes of water into the surrounding areas.

The ESCC policy talks about maximum winter groundwater level. As anyone who walks there in the winter will know this area of land already floods in winter to the maximum groundwater level is surface level so there is no way a SuDS can provide 10m of unsaturated zone.

With respect to the use of a deep borehole there is a claim that drilling has identified a fissure in the bedrock which they plan to push all the water into, however;

·         There is no information to suggest that this fissure is large enough to cope with the volume of water.

·         This water would enter this fissure under pressure due to the weight of water risking either

o   Washing away the layer of permeable bedrock and potentially causing landslips or subsidence in the local area

o   Reaching more solid rock and being forced upwards to the surface

·         A borehole this deep will be lower than the high point of the Ridge meaning this fissure could find it’s own route downwards and in any direction

o   Could flow towards the Sandhurst playing fields

o   Towards the garage potentially putting at risk the integrity of their fuel storage tanks

o   East or West under existing housing or under the new houses this is designed to protect

o   South in a fast flowing stream into St Helens Woods and onwards to the town centre.

This area of land is already seeing instability underground as a number of areas have seen subsidence and again these fill with water in the wetter  months.

The SuDS basin will, for much of the time, not be empty, how is safety maintained both for children and wildlife in an area that can be expected to have deep water.

Considerations of maintenance are dismissed with reference to appointing a maintenance company to oversee the SuDS with costs having to be borne by the owners of the new houses. The internet has many reports of this maintenance work not being carried out, done badly or residents being forced to pay ever increasing costs to protect their property from poorly designed and built flood protection defences.

Who will own this land if a development goes ahead and be responsible for the general upkeep and security of the SuDS?