West Cumbria Rivers Trust - Introduction · Web viewThere are no statutory or non-statutory...

Dovenby beck Walkover Survey

Draft Report

Caitlin PearsonCatchment Officer

September 2017

Supported by:

Draft Report- Dovenby beck Walkover Surveys

Contents1 Introduction..............................................................................................................4

1.1 Project aim and Development..........................................................................41.2 Project Development........................................................................................4

2 Dovenby beck............................................................................................................42.1 Catchment Overview........................................................................................42.2 Geology and soils..............................................................................................52.3 Designations and status....................................................................................62.4 Waterbody condition........................................................................................6

3 Walkover survey summaries......................................................................................74 Recommendations....................................................................................................85 Conclusion and next steps.........................................................................................9


1 Introduction

1.1 Project aim and Development

1.1.1 The village of Dovenby has repeatedly suffered from flooding. The aim of this project is to:

- Increase understanding of issues and pressures affecting the watercourse and catchment;

- Identify measures that could reducing flood risk, improve water quality and river habitat and increase sustainability of land management practices;

- Promote best practice catchment management;

- Obtain funding to implement identified measures.

1.2 Project Development

1.2.1 The Woodland Trust funded West Cumbria Rivers Trust to work with Dovenby Flood Action group to undertake walkover surveys of the whole catchment and identify potential measures to reduce flood risk to Dovenby village and improve the water quality and biodiversity of the beck and surrounding area.

1.2.2 A full report of the walkover surveys was produced and has been used to support funding applications and inform the project.

1.2.3 This document summarises the identified potential measures.

2 Dovenby beck

2.1 Catchment Overview

2.1.1 Dovenby beck is a tributary of Broughton beck, which itself is a tributary of the River Derwent. Situated in West Cumbria between Cockermouth and Maryport, the catchment is approximately 2.7 km2 and the river is approximately 3.5 km in length with three main branches (Figure 1).

2.1.2 Land is split between around ten landowners in which livestock farming predominates, including beef and sheep enterprises. There are several small patches of woodland.


Figure 1: Map of Dovenby beck.

2.2 Geology and soils

2.2.1 The whole catchment, apart from the most downstream stretch before the confluence with Broughton beck, is underlain by limestone with subordinate sandstone (Figure 2). As limestone is relatively impermeable, flow occurs primarily through fractures and discontinuities and the area is a large karstic aquifer. This means response to rainfall is rapid.

2.2.2 Soils in the catchment are acidic loamy and clayey soils which are slowly permeable and have poor drainage. These soils result in a high risk of overland runoff and pastures and woodlands that stand wet for long periods of the year.

2.2.3 In combination, the geology and soils in the catchment result in rapid generation of overland runoff in response to rainfall and a ‘flashy’ flood peak.


Figure 2: Geology of Dovenby beck catchment.

2.3 Designations and status

2.3.1 The whole of the Broughton beck catchment is designated as a surface water Nitrate Vulnerable Zone. There are no statutory or non-statutory conservation designations within the area.

2.4 Waterbody condition

2.4.1 For the purposes of the Environment Agency monitoring and Water Framework Directive reporting, Dovenby beck, Broughton beck and Brides beck are considered as a single waterbody. This waterbody is classified as ‘Good’ compared to reference conditions that would be expected if there was no or very low human pressure being placed upon the river. There are, however, known issues with sedimentation and nutrient inputs.

2.4.2 Fish and habitat surveys conducted by West Cumbria Rivers Trust in both 2016 and 2017 identified the habitat within the Broughton beck catchment as being poor and the number of salmonids was significantly lower than expected.


3 Walkover survey summaries

3.1.1 A large area of land drains into Dovenby beck. The stream is on average between 1 and 2 m wide with earth banks. The bed material is predominantly silt with some small patches of gravel and lacks diversity.

3.1.2 The course of the beck has historically been altered and straightened to run along the field margins.

3.1.3 The land use is predominantly pasture used for sheep grazing and tree cover along the stream is mostly low.

3.1.4 Some watercourses are fenced to prevent livestock access but in other areas livestock have access to the watercourse and mild poaching (trampling of the river banks) is evident. Livestock access to watercourses will increase the amount of sediment and nutrients in the water, reducing water quality and habitat quality for a range of species.

Example of livestock poaching

3.1.5 Through Dovenby village, the channel is confined by stone walls and has a bedrock substrate, underneath a thin layer of gravel and silt.

3.1.6 The beck flows underneath two road bridges, the second of which is significantly under capacity and causes pooling of the water and sediment deposition. A large bank of sediment builds up upstream of this bridge against the front wall of Limetree house and there is abundant vegetation within the channel. The deposited sediment reduces the capacity of the channel and causes flood water to overtop the walls sooner than would occur if the channel had greater width/depth and was less blocked by vegetation.

3.1.7 Downstream of the village the stream passes through a short section of broadleaved woodland. At the upstream and downstream extent of the woodland there are water gates which retard flows during high flows.

3.1.8 In addition to the risk of flooding from the beck, there is significant risk of surface water flooding from the track behind Crofthouse Farm. The track runs the length of the village and joins the main A594 road. A significant amount of runoff comes from the fields to the north of the village and down this track an into a small culvert which flows


under three houses to join the main beck. The culvert has insufficient capacity during heavy rainfall events and upwells into properties.

4 Recommendations

4.1.1 Ideally all watercourses would be fenced off from livestock and a riparian strip of long grass and trees established to reduce overland runoff and sediment entering the stream. This would require alternative drinking sources to be provided for livestock.

4.1.2 Areas of overland flow suggest that in some fields the soil may be compacted. Subsoiling fields would reduce compaction, improving soil quality and increase the ability of the soil to hold water, thus reducing runoff.

4.1.3 Most field boundaries have hedgerows but these have frequent gaps. Establishing dense hedgerows and filling these gaps could reduce the volume and rate of runoff.

4.1.4 In several areas upstream of the village, including through areas of woodland, the channel is deep. There is potential to use these areas to store and slow flood waters by installing a series of leaky dams to hold water back within the deep channel.

4.1.5 The beck runs through a lake within a marshy woodland dominated by goat willow (Salix caprea) and sedge reeds. The possibility of increasing the storage capacity of the lake and marsh requires further investigation.

4.1.6 Remeandering the stream would increase the length of the channel, allowing it to hold a greater volume of flood water and increasing the time it takes for a flood peak to reach the village. These works would be prohibitively expensive for the scale of the current project and may require compensation to landowners if they resulted in a loss of productive land. Instead, the capacity of the channel could be increased on watercourse corners by creating a pond area, or a two-stage channel, which would provide additional storage capacity for flood flows without sacrificing productive farmland.

4.1.7 Dovenby Flood Action Group periodically remove the build-up of silt and vegetation from the stream at the road bridge to increase capacity of the channel. These sediment deposits are a result of the water pooling behind the under capacity bridge and removing the deposits provides a slight reduction in flood risk to properties and prevents mobilisation of this silt in very high flows which would reduce habitat quality in downstream areas. Hand removal of this sediment is labour intensive and provides strong justification for reducing sediment inputs higher in the catchment.

4.1.8 This is the only location through the village where silt removal would significantly increase capacity as the rest of the channel has a very thin layer of deposited silt over bedrock substrate.

4.1.9 Dovenby Flood Action Group have discussed increasing the capacity of the bridge with Cumbria County Council but there are not enough properties at risk of flooding to justify changes to the road infrastructure.

4.1.10 Placing a series of small wooden dams or an earth bund across the track behind Crofthouse farm could reduce and slow runoff, whilst trapping sediment. This would


reduce flood risk to the properties around the small culvert and improve downstream habitat quality.

4.1.11 Increasing the height of the flood gates downstream of the village would increase conveyance and may reduce flood water backing up into the village.

5 Conclusion and next steps

5.1.1 A large area of land drains into Dovenby beck and ultimately through the village. The walkover survey provided a great deal of information to help identify possible interventions to reduce flood risk to the village whilst improving habitat and water quality.

5.1.2 Discussions with landowners will help to prioritise actions and create an action plan. Funding will be sought to deliver some of the identified measures, subject to agreement from landowners and the Flood Action Group.

5.1.3 Given the extent of the catchment and large number of potential interventions identified, this project can only seek to begin what will be a very long process of restoring the catchment to improve habitat and reduce flood risk.

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