Sustainable Drainage System

Stover Country Park is owned and managed by Devon County Council as a Site of Special Scientific Interest (SSSI) , Country Park, Local Nature Reserve and a listed historic park and garden covering 46.15 hectares, consisting of a mosaic of 6 habitats.

A 10 acre naturalised freshwater lake forms its centrepiece which is regarded as the best example of this type of open water community in South West England. In Devon, there is no other single inland water body equivalent in size, naturalness and diversity. It is designated an SSSI due to its rare dragonfly and invertebrate assemblage.

Stover Lake 2008

Stover’s lake is currently recognised by Natural England as unfavourable. Our vision is to restore the lake to a favourable condition and the Sustainable Drainage System (SuDs) is an essential step towards the lake’s recovery.

Over the last two decades the lake’s biodiversity has declined rapidly. This is evident by the disappearance of the lake’s iconic water lilies, floating macrophytes (water plants) and associated invertebrates.

Intensive studies of the lake’s ecosystem and the wider catchment have been commissioned by the County Council, Natural England and the Environment Agency over recent years.

A Natural England condition assessment of Stover lake (21 July 2009) reports:

“an unfavourable no change” condition. The reason given for this adverse condition is “water pollution – agriculture/runoff”. 1998 English Nature report for South Devon notes that “Siltation, natural succession and pollution by hydrocarbon runoff from the A38 pose ongoing management issues at Stover lake”.

Stover Lake 2017

A 2014 report by Royal Haskoning concluded that accumulated sediment is anoxic (lacking in oxygen) and is the significant reason for the biodiversity decline and the current state of the lake bed makes it unsuitable for supporting either an aquatic plant community or a diverse invertebrate fauna.

The report states that the removal of the sediment is a pre-requisite for the restoration of the lake’s biodiversity alongside action to improve water quality inputs by felling conifers in the vicinity and managing highways run-off.

The sediment layer’s toxic and relative oxygen-deficient conditions physiologically inhibit root development and survival. The very soft consistency of the sediment prevents firm rooting  and contamination of the sediments by metals and hydrocarbons add further to the unsuitability of the lake bed.

Submerged species and the white water-lily have relatively inefficient aeration systems and so, are less able to cope with the lack of oxygen at depth. In turn, these plants provide the structural habitat for dragonflies and other aquatic fauna.

The potential for sediment removal to address these problems has been evident at Stover where we have in the recent past removed sediment along the inlet channels with a rapid increase in macrophyte (water plant) species resulting along the dredged sections. Freshwater habitats are a priority for England and this project will help secure the single most important lake in Devon.

The problem

The topography in this area is predominantly flat, the catchment areas that feed into Stover Lake include 5.68km of A38 both east and westbound carriageway length, on/off slip’s and the roundabout, Heathfield industrial estate, great plantation and surrounding fields.

Currently, there is no treatment to highways drainage runoff with visible signs of possible oil contamination in the highways drainage ditch running along the edge of the A38 and Stover Country Park.

The network of drains are traditional gravity drainage systems with a combination of gullies and carrier pipes along with filter drains. These systems have limited pollution prevention and with multiple outfalls off the network leading into ditches with watercourses coalescing and discharging into Stover lake.

Highways England water risk assessment tool analysis calculates the risk of a pollution incident occurring due to routine road runoff.

Pollution concentrations by highways discharges are calculated based on a derived relationship linking pollutant concentrations to average traffic flows and climatic area.

Because Stover Lake is an SSSI, it is considered to be ranked ‘very high’ in terms of importance of the attribute. The magnitude of the impact identified is considered to be ‘moderate adverse’  due to failure on soluble and sediment grounds but passes for environmental quality standard values. This gives an overall significance of effect as a ‘large/very large’ adverse effect.

Due to the above, Stover Country Park, Highways England, Natural England and Keir have been collaboratively working over the past 5 years to resolve the issue.

Highways England have agreed to invest £1.6million in building two Sustainable Drainage Systems to catch the sediment and filter out any contaminants in the water before it enters Stover lake. The cross-section of the drainage system shows the flow of water through the system. This will remove contaminants before entering into Stover’s water network.