Understanding sediment transport in a modified chalk stream environment Location plan including Located east of the Yorkshire community, rich invertebrate Wolds, the River Hull is the and diverse breeding bird key structures most northerly chalk stream community as well as in the UK. Its headwaters are grassland, fen and alder and As shown in the above plan, designated as a SSSI, for its willow carr. there are six major weir characteristic aquatic plant and sluice structures on The SSSI is in unfavourable the headwaters, used for recovering condition, with current or former fish farm restoration and diffuse water abstractions, water flow pollution plans in place gauging and mills. There is a under the Water Framework need to determine whether Directive. These are aiming to these structures could tackle in-channel structures, be removed, modified or channel modifications, water their operating procedures abstraction and siltation in changed for the restoration partnership with others. plan and Restoring Sustainable Abstractions programme.
BRIDLINGTON
DRIFFIELD The Environment Agency commissioned Royal Haskoning to develop a model to assess how the six YORK structures impacted on sediment transport and deposition, and how modification options could change this. An existing 1D ISIS flood risk model was adapted to cover normal and low flows, and additional topographic data was collected to improve the model’s representation of key structures.
A 2D model was then created and used to map bed shear stresses for a range of flows and weir removal scenarios. Areas of high shear stress (yellow / red) indicate potential significant erosion; HULL areas of low shear stress (blue / purple) indicate low erosion or even deposition. Existing Situation
Driffield • Q50 (left) • Qmed (below) The East Yorkshire Chalk Rivers Trust and Yorkshire Wildlife Trust are already carrying out work, in partnership with the Environment Agency and Natural England, to narrow the river channel in places and remove minor weirs.
Before After
Partial Sluice Removal
For each of the structures, the study gives us a better • Absolute (left) understanding and greater confidence in what could be achieved • Difference (below) for the benefit of the SSSI and what the constraints are. It is being used to determine the locations for Restoring Sustainable Abstractions, and will be valuable for forthcoming discussions with river owners about what it is feasible to do to each of the structures in order to improve the SSSI towards favourable condition, connecting with other restoration plan actions to provide a holistic strategy for the restoration of this river.
Granville Davies Liz Chalk Royal Haskoning Environment Agency Westone Coverdale House 114 Wellington Street Amy Johnson Way Leeds York LS1 1BA YO30 4GZ Tel: 0113 3884849 Tel: 01904 822674 Email: [email protected] Email: [email protected]