FiveRivers were contracted by Herts and Middlesex Wildlife Trust to undertake an options and detailed designs appraisal and plan for river restoration improvements to a 7km reach of the River Ash in Hertfordshire.
The River Ash, one of the 240 chalk rivers in the UK, suffers from the pressures of over-abstraction, agriculture, impacts from sewerage, historic modifications/alterations and degraded habitat.
Project requirements
The problem
The 7km reach identified for options and detailed design works was owned and managed by the Easneye Estate, who with HMWT, wanted to see the river improved and detailed designs undertaken to provide an ‘off the shelf solution’.
The river, as mentioned, suffers from the pressures of over-abstraction, agriculture, impacts from sewerage, historic modifications/alterations and degraded habitat.
Using our extensive knowledge of designing and delivering improvements on degraded chalk stream habitats, FiveRivers were tasked with designing challenging solutions and alterations that would improve the habitat of the Ash.
Both specialist and smaller more volunteer friendly installation activities were requested to allow for a range of mechanisms for delivery of the scheme when designed.
Our approach
The solution
Dividing the 7km reach into easily identifiable sub reaches, that could be designed and submitted for consent independently, FiveRivers started by undertaking catchment walkovers with all stakeholders to determine key drivers and pressures along the 7km reach as well as understanding any constraints and previous works that may have already be undertaken. Following this initial phase technical details for improvements were drawn up and an options appraisal report produced with suggestions for enhancements on each sub reach as well as detailing findings on constraints from a desktop analysis (including hyrdology, flood risk, designated sites, heritage and archaeology, geomorphic character, ecology) and budgetary construction costings for each technique. Multiple options were suggested for each sub reach with both challenging and conventional ideas put forward for discussion. These included:
- Bank regrading
- Creation of inset floodplains
- Creation of wetland scrapes and wetland pond features
- Ditch blocking
- Backwater creation,
- In channel berm creation
- Large Woody Material installation
- Gravel riffle installation
- Undertaking tree works to reduce shading
- Undertaking tree planting to increase shading
- Removal of concrete in channel structures
- Reinstating old paleo channel
- Hibernacula creation
Project outcomes
The results
Following agreement with stakeholders of the proposed alterations and improvements in each sub reach, the outline designs were formalised, drawn up and presented for approval. Qualitative risk assessments and, where required, quantitative flood risk modelling was also undertaken to ensure the designs had no impact on localised or downstream water levels. Construction costs were also provided to ensure any future funding bids undertaken to allow for installation were accurate. All designs, once finalised, were drawn to CDM2015 regulations to include information important to pass on in the design phases to the contractor.
All designs were submitted for consent with the Environment Agency and approved.
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Luke Roberts
Design & Construction General Manager
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Telephone: 01722 783 041
sales@five-rivers.com
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