Hydrological and Hydraulic Assessment in Lower Stondon,
May 23, 2019


RAB Consultants was commissioned to produce a hydrological and hydraulic assessment in support of a proposed residential development situated within Flood Zones 2 and 3.

The challengesite layout

The purpose of this work was to calculate design flows for the catchment relevant to the development site, which was then used to hydraulically model the watercourse and estimate design flood levels for the site.

The surveyed watercourse was 1,100 metres in length and ran the length of the southern boundary of the site. The catchment area was 6.05km2 and received mainly rural runoff from Upper Stondon, Windmill Farm and Shillington. Lower Stondon is located in Central Bedfordshire, 700 metres upstream of the River Hiz.

There were six structures along the modelled reach, proximal to the development site. These structures were all circular concrete culverts and varied both in length and size throughout. Variable data had to be inserted into Flood Modeller software, along with hydrometric data, in order to produce a schematisation that accurately represented the watercourse.

The solution

model schematicThe flooding mechanisms were examined in detail by obtaining a topographic survey of the channel bathymetry and key features, and developing a 1D hydraulic model of the watercourse using Flood Modeller.

Detailed modelling results showed the site did not flood during the 5% AEP and the 1% AEP flood events, with flow remaining below both banks within the vicinity of the site. However, the most eastern boundary of the site did flood during the 1% AEP (1 in 100 years) plus 35% climate change and the 0.1% AEP (1 in 1000 years) scenarios. With flood depths of up to 50 mm during the climate change scenario, and up to 150 mm during the 0.1% AEP scenario.

Sensitivity testing was then undertaken, which is the process of adjusting key parameters within the hydraulic model to assess the impact on the modelled outcomes and validate the data.

The developers were then able to use this report to inform their residential housing layout ensuring that more vulnerable uses have been steered away flood risk areas.