Witham Third IDB Catchment Study

Challenge

The Witham Third District Catchment Study covers the following three catchments:

  • Dogdyke, Park Beck and Marsh Lane
  • Kirkstead
  • Bardney Manor Farm, Duckpool, Southrey, Stixwould and Woodhall.

Located to the south east of Lincoln the catchments drain into the left bank of the River Witham, are generally low lying and drain under gravity or with the aid of pumping stations.

We were tasked with modelling 70km of the IDB watercourse to analyse the interaction between the gravity and pumped systems that exist within the Witham Third District. The Board required an evidence base to aid in future maintenance, in the refurbishment of their pumping stations and in the analysis of their operating regimes.

Solution

The initial part of the study collated and reviewed all available data including a survey of the boards watercourse which comprised of 824 cross sections and 200 in channel structures covering culverts, bridges and sluices.

Design flood runoff hydrographs were then calculated for the catchment using the techniques described in the Flood Estimation Handbook (FEH). HEC-RAS (version 5.0.3) was used to create three standalone models of the catchments in the geo-referenced format.

The HEC-RAS models were set-up in two stages. First an in-bank model was developed making use of the topographic survey data. This included all the study watercourses and any significant structures including road crossings. Inflows were informed from the catchment hydrology.

Boundary conditions for the model were taken from the Environment Agency River Witham model. The models were calibrated for any observed flood events. Having setup the in-bank model and checked the schematisation the model was independently reviewed before being extended to include out of bank areas.

The models were constructed in 1D-2D to account for overland flow in the upper catchments. They utilised the sub-grid geometry function of HEC-RAS, as the 2D domain was constructed using a 20 grid, whilst still providing high resolution outputs.

Benefit

HEC-RAS now includes a 2D component for representing floodplain areas meaning the model can directly produce flood outlines without intermediate processing using Geographical Information Systems (GIS) software.

The models are now being using to adjust the operating rules of the pumping stations and operation of sluices within the catchment. This will improve the day to day efficiency of the pumps as well as reducing flood levels.

The project benefited from our experience and wide use of HEC-RAS on other IDB projects. The team has also been involved with beta testing HEC-RAS 2D for the US Army Corps of Engineers.

Want to know more?

Email Gavin Hodson for more information on IDB catchment modelling and HEC-RAS 2D. You can also visit our flood modelling web page. If you want to find out more about HEC-RAS sign up for the workshop.

HEC-RAS 2D Modelling Workshop

Following the success of the 2016 and 2017 workshops, we are excited to once again host the lead developers of the Hydrologic Engineering Center (HEC) software.

Gary Brunner, Senior Technical Hydraulic Engineer, and Cameron Ackerman, Senior Hydraulic Engineer, are coming over to the UK to deliver a workshop on using HEC-RAS v 5.0.4 for 2D Modelling.

The workshop is aimed at those with a good background in open channel hydraulics and you must be familiar with the HEC-RAS software. Basic HEC-RAS input and output data requirements will not be covered in this class. Some experience applying HEC-RAS using the Unsteady Flow modelling components is recommended.

Visit our website for more information and to book your tickets today.



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