- 24th May 2018
- Posted by: Sophie Smith
- Category: Projects
Kings Mill reservoir is used for water sports and other leisure activities. Siltation over the years has been gradually reducing depths causing problems for these recreational users. Ashfield District Council commissioned us to investigate various options to encourage sedimentation at the upstream end of the reservoir where it has less impact on the water sports users and can more easily be managed, to support their application for Heritage Lottery Funding.
The preferred option proposed by the council was to undertake localised dredging and reform the sediment into submerged islands (mounds) as shown in the model results figure. Modelling is needed to demonstrate both the efficacy of this solution in terms of reducing velocities and encouraging sedimentation, and also to show that there are no adverse impacts on water levels upstream or flows downstream. One particular challenge is that there is very little knowledge of the actual sediment supply rates for different flow conditions.
A gauge further downstream in the catchment allowed low flows and flood flows to be calculated which were then scaled by catchment area for the various upstream inputs to the reach-scale model. Topographic survey of the upstream channel and reservoir outflows were used, along with LiDAR and approximate bathymetry of the reservoir to create a HEC-RAS model of the reservoir and upstream reach.
Different aspects of HEC-RAS were exploited:
- A 1D sediment transport model helped calibrate long-term (1-10year) sediment loadings and inform potential supply rates.
- In order to represent the proposed bathymetry changes, the reservoir was modelled in 2D and linked to the 1D reach to establish impacts on velocities.
- The floodplain upstream was also modelled in 2D for the added benefit of showing existing flood risk and that this is not altered by the proposals.
- The 1D-2D HEC-RAS models were run for various event hydrographs.
An openFOAM 3-dimensional computational fluid dynamics model of suspended sediment settling was then constructed for just the upstream part of the reservoir, using boundary conditions from the HEC-RAS model, in order to investigate detailed flow patterns and sedimentation locations.
- The HEC-RAS 2D results in the figure show that the dredging and mound formation create a zone of low velocities which will encourage sedimentation and reduce transport into the main lake body.
- The openFOAM 3D model confirmed preferential sedimentation in the dredged area, and in other zones of local recirculation.
- Velocities and shear stresses from model results can also be used to inform design of protection for the islands to prevent them being washed out in higher flows.
- The ability to utilise different types of simulation (1D long-term sediment transport and 2D hydrodynamic) within the HEC-RAS software allowed efficient modelling.
Want to know more?
Find out more about HEC-RAS v5.0.4 at the HEC-RAS 2D Modelling Workshop in London on 25-27 June. Following the success of the 2016 and 2017 workshops, we are once again hosting the lead developers of the Hydrologic Engineering Center (HEC) who will discuss the newly released HEC-RAS v5.0.4.