Case study:River Darent at Hawley Manor
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Project overview
| 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 |
Project summary
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.
Monitoring surveys and results
Lessons learnt
Catchment and subcatchment
Site
| 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 | |
| Average channel gradient | |
| Average unit stream power (W/m2) |
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
Measures
Structural measures
| |
|---|---|
| Bank/bed modifications | Re-working of gravels to create riffles, pools and bars |
| Floodplain / River corridor | |
| Planform / Channel pattern | Channel narrowing, Increase in sinuosity |
| Other | |
Non-structural measures
| |
| Management interventions | |
| Social measures (incl. engagement) | |
| Other | |
Monitoring
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
| 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
| Element | When monitored | Type of monitoring | Control site used | Result | ||
|---|---|---|---|---|---|---|
| Before measures | After measures | Qualitative | Quantitative | |||
Any other monitoring, e.g. social, economic
| Element | When monitored | Type of monitoring | Control site used | Result | ||
|---|---|---|---|---|---|---|
| Before measures | After measures | Qualitative | Quantitative | |||
Monitoring documents
Image gallery
Additional documents and videos
Additional links and references
| Link | Description |
|---|
Supplementary Information
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