Hogsmill River – Fish Migration Success

Over the past 3 years, the South East Rivers Trust have been improving fish passage along the Hogsmill River, a tributary joining the River Thames at Kingston, as the waterbody is currently classed under the Water Framework Directive as having poor fish populations. At the start of the project there were 15 obstructions which prevented fish from passing along its six-and-a-half-mile length.

The situation

Earlier this year the Trust approached us and requested support to model the hydraulic impacts of placing low cost baffles (LCBs) at Kingston Gauging Station as a means of rendering the structure passable to fish migrating upstream. The key balance at this site was the position of the uppermost baffle, which if too high could compromise the rating curve at the site, and if too low could reduce the efficiency of the baffles as a fish pass.

Considerable research has been undertaken on the optimal baffle placement on crump weirs which have a slope of 1:5 on the face of the weir. However, the face of Hogsmill Weir has a slope of around 1:3.3 and so the baffle placement rules from the research undertaken on crump weirs would not be directly applicable.

How we can help?

JBA staff adapted an existing 1D HEC-RAS hydraulic model of the Hogsmill Weir to test impacts on the gauge’s rating curve of the placement of LCBs (in particular the uppermost baffle). Our work was able to clearly demonstrate that the LCBs could be placed closer to the weir crest than previously thought thereby making the weir more passable and proposing a solution that is acceptable to both the Fisheries and Hydrometry functions of the Environment Agency (EA).

This work follows on from LCB design work that we undertook for JBA Bentley and the EA under the EA’s Water Environment Management (WEM) Design and Build framework contract. We produced designs for a LCB installation at Lilford Weir on the River Nene where the LCBs were installed during planned weir repair works.

Want to know more?

For more information on how we can assist you with fish pass feasibility and design please contact David Mould, Senior Chartered Analyst. For further information on this specific project please contact Maxine Zaidman, Technical Director.

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