Environmental Impact :: Flood Risk Assessments

Sustainable Drainage Systems (SUDS)

8.1 Siting & Present Condition

The site is currently occupied by a mixture of different uses including a holiday park, light industrial uses, and a tramway works.

8.2 Existing Drainage

The surface of the site is covered by mixed surfacing including patchy grass, gravel, concrete and tarmac.

The groundwater below the site in continuity with the estuary and is subject to saline intrusion.

8.3 Proposed Drainage System & SUDS

The objectives of sustainable drainage systems are to:

* recharge the groundwater resource as near to the source of surface water run-off as practicable, and to

* protect downstream watercourses and catchments from flooding.

In the present case there is a small downstream flood risk due to potentially increased rates of surface water discharge from the application site.

The ultimate aim will be to dispose of as much water as reasonably practicable by SUDS techniques. Appropriate methods which will be considered are:-

* increase in permeable areas through soft landscaping;

* roof water harvesting for residential units (e.g. water butts);

* permeable paving with water retention media below surfacing;

* soakaways; and

* swale / storm water attenuation pond (s).

Considerations which will influence the choice of SUDS are:-

* tide-lock flood defence requirements;

* potential ecological, environmental and amenity benefits or impacts;

* utility of harvested roof water to purpose onsite uses;

* permeability of the made ground;

* degree of hydraulic continuity with river water and reactivity to changes in river levels;

* degree and nature of any ground contamination found (the site once had gasworks on it);

* health and safety consequences.

The primary objective of SUDS techniques in this instance is to avoid increase flood risk downstream of the site. Another objective of SUDS techniques are to provide groundwater recharge. In this locality groundwater recharge is of low benefit due to groundwater’s hydraulic continuity with the local saline water tidal regime .

The degree of attenuation to be achieved would take into account the site’s location and discharge volumes during critical tide lock periods.

In terms of overall water management, storm water attenuation by means of a swale / berm adjacent to the existing main Rhyne, although possible ponds / basins in public open space areas are also under consideration.

8.4 Justification

The recommendation of PPG 25 (para. 56) for redevelopment of existing (brownfield") sites is to "to reduce run-off".

The aim of this recommendation is at promoting sustainable drainage systems to control the water as near its source as possible. This is to -

* conserve and reduce discharges to watercourses (and thus reduce flood risk downstream and erosion of channels and banks), and

* conserve and recharge groundwater resources.

As such, this second objective is inapplicable to the present proposal:

* there is no downstream watercourse at risk of capacity problems or flooding due to discharges from this site, and the estuarial reach of tidal water is certainly not at risk;

* a nominal increase in flood water depth in the tide-locked marshes is more than off-set by the additional storage volume to be provided adjacent to the Rhyne;

* discharges from this site will not increase channel or bank erosion risk ;

* the provision of typical SUDS (other than attenuation systems) confers little environmental benefit, but has potential to increase surface water flood risk (e.g. overflowing soakaway systems).

The recommendations of PPG 25 will be met by the provision of storm water storage and attenuation, and limited soakaways where these can be practicably employed.