Project Details

Client: Cardiff County Council, ARUP

Categories: Hydraulic Modelling, Hydrology, Outline Business Case, SuDs

When: 2020

Location: Glarmorgan, UK

Whitchurch Brook is a heavily modified urban watercourse which runs from the M4 through the Cardiff suburbs of Rhiwbina and Whitchurch before joining the River Taff via a culvert at Lydstep Park, north west of the city centre. In 2008 Edenvale Young developed an FMP-TUFLOW model of the watercourse Cardiff City Council and in 2015 worked with Mott MacDonald to deliver the Rhiwbina Flood Alleviation scheme. This scheme won the ICE (Wales) Roy Edwards Award for smaller projects in 2016.  In 2019 Edenvale Young, in conjunction with Arup, were appointed to prepare an outline business case to alleviate flooding from the Whitchurch Brook to the south of Rhiwbina and Cadelyn Parc.

Edenvale Young converted the existing FMP-TUFLOW model to a Distributed Hydrological Model (DHM) to ensure that fluvial and surface water flooding were both represented in the modelling (see below). Modelling was undertaken for a range of return periods for the “walk away”, “business as usual” and three doing something options.

The Challenge

Run-off from the urban environment is a significant factor in the response of rivers within all towns and cities in the UK. Traditionally, lateral inflows (calculated using the Flood Estimation Handbook (FEH)) are used to represent the movement of surface water into an urban river. This approach is valid in many circumstances but has limitations particularly for smaller rivers where:

  • Surface water run-off provides a significant proportion of the total flow in the watercourse.
  • There are discrete storm sewer catchments which discharge to specific locations to the watercourse
  • Source control measures such as SuDs are proposed to alleviate flooding

To overcome these limitations for the Whitchurk Brook study, the existing FMP-TUFLOW model was converted to an ESTRY-TUFLOW DHM model using a rainfall boundary. This has the advantage that the urban environment including roads, building, the storm sewer network and infiltration through permeable surfaces are explicitly represented. DHM models reflect the complexity of the urban environment and gives a combined representation of surface water and fluvial flooding which is important in Whitchurch. It should be noted that DHM models represent a significant advance on direct rainfall modelling which have been used for surface water flood mapping.

Outcomes

The hydraulic model was sucessfully calibrated to three events and gave very good agreement with historical flooding within Whitchurch.  The options modelling will be incorporated within the Outline Business Case and presented to the Welsh Government in 2020.

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