Mitigation measures

Practical mitigation measures have now been applied comprehensively across the two demonstration catchments to fully mitigate all impacts, as far as is practically possible. The results will provide a reality check determining the extent to which these measures, when fully applied, can be expected to deliver water quality improvements.

Combined effects of measures suggest that at the catchment scale we should be able to reduce phosphorus from diffuse sources by up to 50% at least in the short term (up to 5 years), reduce drain flow nitrogen by about 30%, and reduce sediments by as much as 75%. The project will aim to intercept about 65% of the demonstration area field drain and ditch drainage before it enters a Water Framework Directive classified watercourse.

80% of the streams already had buffering strips whether from existing woodland or agri-environment schemes, and non-inversion tillage continues on 25% of the arable land. The objective of flow mitigation measures is to reduce the peak hydrograph in a 1:100 year flood to a level equivalent to a 1:75 year flood. There is evidence that such a reduction brings about significant economic benefits in terms of cost-benefit ratios for flood defence work, as it is cheaper to construct hold back water in the catchment than defend downstream areas (Farming Floodplains for the future, 2012).

Precise locations for water quality mitigation measures were identified by a combination of the catchment modelling of risk areas and field-by-field walkovers. The location of flow interceptor ponds in this study was based on selecting the 25% of land that was most vulnerable to rapid water loss (i.e. the heaviest clays) with no more than one detention pond for every 8ha of land. Interception ponds have an average diameter of 22m and a depth of 2m, though area and depth varies according to the exact location. An average density of three ponds per km2 is required to achieve this. Clean water ponds have been created for biodiversity purposes which aim to maintain and enhance catchment biodiversity by creating supporting habitats for uncommon macrophytes and aquatic invertebrates.

The project team has considerable experience of all aspects of installation of practical water quality mitigation measures which include the implementation of conservation tillage, riparian corridor management (e.g. fencing), installation of buffer strips, creation of constructed wetlands and ponds, bunding of ditches, protection of river and stream crossings, and nutrient management practices in livestock enterprises.

 

Below is a map showing the different types of use the land in Eye Brook catchment was subject to in 2014.

eyeLU2014

eyemitigation2

The map above of the Eye Brook catchment shows the practical measures implemented and in operation by March 2014. New interception ponds are designed to capture sediment and are connected to a form of direct input from adjacent agricultural land.

 

Below is a map showing the different types of use the land in Stonton Brook catchment was subject to in 2014.

stontonLU2014

stontonmitigation2

The map above of the Stonton Brook catchment shows the practical measures implemented and in operation by March 2014. New wildlife ponds  are designed to increase biodiversity in this catchment and have no direct input from adjacent agricultural land.