Case study:River Darent at Hawley Manor

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Location: 51° 25' 30.45" N, 0° 13' 51.03" E

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Project overview

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Status	Complete
Project web site
Themes	Habitat and biodiversity, Hydromorphology
Country	England
Main contact forename	Tom
Main contact surname	Cook
Main contact user ID
Contact organisation	Environment Agency
Contact organisation web site	http://www.environment-agency.gov.uk/
Partner organisations
Parent multi-site project
This is a parent project encompassing the following projects	No

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Project summary

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The River Darent in the Dartford area has been heavily modified over many years,including changes to channel planform, the implementation of land drainage schemes and abstraction,leading to an over widened channel. Prior to restoration this section of river, two miles upstream of Dartford, was very uniform with a shallow gradient and uniform cross section. The natural substrate is dominated by pebbles and gravel but had become overlain by silt. Flow and habitat diversity was limited with negative impacts on fish and macroinvertebrate communities. The aim of the project was to demonstrate that the processes that sustain a healthy chalk stream could be restored and the habitat protected during drought periods. This was to be achieved in a cost-effective way by re-working the in-channel gravels to form a low flow channel. The restoration work contributed to Chalk Rivers Biodiversity Action Plan (BAP) targets and complemented the implementation of the Darent Action Plan (1992).

All of the bed modifications were carried out from within the channel, so the banks and trees were left untouched. The channel was re-profiled using a long reach excavator creating a sinuous channel, with pool and riffle sequences, still within the confines of the original channel. The excavator accessed the river where there were suitable gaps between trees and the creation of gravel berms at these points enabled the excavator to exit the river without damaging the banks.

A low flow channel was created by moving small amounts of gravel in a meandering path within the confines of the wider channel. Part of the bed was kept at a higher elevation to create a sequence of riffles. In other areas more significant quantities of gravel were redistributed enabling pools to be created. Pools, spaced at approximately 20m intervals, were designed to be self-cleaning. Gravels were placed upstream of each to narrow the flow and increase velocity to induce scour in the pools.

The whole of the construction phase was supervised by the designer who was on site throughout the work and provided instructions to the excavator driver. Existing bankside trees with large root systems acted as natural deflectors and provided a variety of marginal habitat. A small amount of planting was carried out, including water crowfoot which was sourced locally from the Darent.

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Catchment and subcatchment

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Site

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Name	River Darent at Hawley Manor
WFD water body codes	GB106040024222
WFD (national) typology
WFD water body name
Pre-project morphology	Over-widened, uniform cross section and shallow gradient
Reference morphology	Narrow, sinuous channel with flow variation
Desired post project morphology
Heavily modified water body	No
National/international site designation
Local/regional site designations
Protected species present	No
Invasive species present	No
Species of interest
Dominant hydrology	Groundwater
Dominant substrate	Chalk
River corridor land use	Semi urban
Average bankfull channel width category
Average bankfull channel width (m)
Average bankfull channel depth category
Average bankfull channel depth (m)
Mean discharge category
Mean annual discharge (m3/s)
Average channel gradient category
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Average unit stream power (W/m2)

Project background

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Reach length directly affected (m)	250250 m <br />0.25 km <br />25,000 cm <br />
Project started
Works started	2005/09/24
Works completed	2005/09/24
Project completed	2005/09/25
Total cost category	1 - 10 k€
Total cost (k€)	2,114.002,114 k€ <br />2,114,000 € <br />
Benefit to cost ratio
Funding sources	Environment Agency

Cost for project phases

Phase	cost category	cost exact (k€)	Lead organisation	Contact forename	Contact surname
Investigation and design
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Reasons for river restoration

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Mitigation of a pressure
Hydromorphology
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Measures

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Structural measures
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Floodplain / River corridor
Planform / Channel pattern
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Monitoring

Hydromorphological quality elements

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quality elements

Element	When monitored		Type of monitoring		Control site used	Result
	Before measures	After measures	Qualitative	Quantitative

Biological quality elements

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quality elements

Element	When monitored		Type of monitoring		Control site used	Result
	Before measures	After measures	Qualitative	Quantitative
Invertebrates: Taxonomic composition	Yes	Yes	No	Yes	No	Improvement
Macrophytes	Yes	Yes	Yes	Yes	No	Improvement

Physico-chemical quality elements

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quality elements

Element	When monitored		Type of monitoring		Control site used	Result
	Before measures	After measures	Qualitative	Quantitative

Any other monitoring, e.g. social, economic

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Monitoring documents

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Additional links and references

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Supplementary Information

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