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

The restored reach of the River Darent at Hawley Manor after two years

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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Image gallery

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River Darent at Hawley Manor before works

Immediately before works

During works

Excavator carrying out in-channel works

Immediately after works

The same reach after five years

In-channel vegetation after four years

Catchment and subcatchment Select a catchment/subcatchment Edit the catchment and subcatchment details (affects all case studies in this subcatchment) Catchment River basin district Thames River basin Darent Subcatchment River name Mid Darent Area category 100 - 1000 km² Area (km2) Maximum altitude category 200 - 500 m Maximum altitude (m) 244244 m <br />0.244 km <br />24,400 cm <br /> Dominant geology Calcareous Ecoregion Great Britain Dominant land cover Arable and Horticulture Waterbody ID GB106040024222 Other case studies in this subcatchment: Darent Valley Path, Lullingstone Castle, Lullingstone Castle phase 2 Site Edit site Name River Darent at Hawley Manor WFD water body codes GB106040024222 WFD (national) typology WFD water body name Mid Darent 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 Average channel gradient Average unit stream power (W/m2) Project background Edit project background 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 Stakeholder engagement and communication Works and works supervision Post-project management and maintenance Monitoring Reasons for river restoration Edit reasons for restoration Mitigation of a pressure Abstraction and land drainage schemes resulting in change to channel planform Hydromorphology Quantity & dynamics of flow, Substrate conditions Biology Invertebrates, Macrophytes Physico-chemical Other reasons for the project Measures Edit Measures Structural measures Bank/bed modifications Re-working of gravels to create riffles, Creation of pools, Creation of bars Floodplain / River corridor Planform / Channel pattern Channel narrowing, Adding sinuosity Other Non-structural measures Management interventions Social measures (incl. engagement) Other Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Substrate conditions Yes Yes Yes No No Improvement Biological quality elements Edit biological 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 Edit Physico-chemical 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 Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Monitoring documents Upload monitoring documents Additional documents and videos Upload additional documents Additional links and references Edit links and references Link Description Supplementary Information Edit Supplementary Information