Project Description

Newton Stewart Flood Study

Flood Risk Assessment

Client: Dumfries and Galloway Council

Date: 2015-2019

Following extensive flooding in November 2012 and 2015, Kaya Consulting Ltd. was commissioned by Dumfries and Galloway Council (DGC) to undertake a detailed flood study for the towns of Newton Stewart and Minnigaff, focusing on flooding risk to urban areas from the River Cree and the lower part of the Penkiln Burn.

Adjacent to the town, the River Cree has a combined catchment of approximately 367 km² and flows are monitored at a SEPA gauging station. To help calculate extreme flows required for the hydrology assessment, a detailed 1D-2D Flood Modeller Pro mathematical model was constructed. The hydraulic model underwent significant calibration using local photos (see below) and videos as well as data from the SEPA gauge. The model was used to predict extreme flows in the watercourse (i.e., out of bank flood flows which are difficult to accurately calculate based on simple extrapolation methods).

Newton Stewart calibration

Using the model results, Kaya Consulting undertook a detailed hydrological assessment to calculate the likely flow experienced during the 2012 event, and also derive a range of return period flows to be used in the assessment.

After agreeing the hydrology flows with SEPA and the Local Council, the hydraulic model was used to calculate flooding within the town for various return period flows. Model results indicated that during a 200-year flood event, over 120 properties could be at risk of flooding (split between residential and non-residential).

As part of the study, Kaya Consulting was also asked to undertake a Benefit-Cost Assessment. Damage cost estimates were calculated using the Multi-Coloured Manual and topographic survey of threshold levels.

An optioneering assessment was undertaken to evaluate potential flood mitigation measures to help reduce flood risk to the town. Various structural and non-structural measures were assessed, such as:

• Provision of upstream storage
• Reduce runoff through Natural Flood Management measures
• Direct defences within the urban reaches of the Cree
• Lowering of the weir at the Bridge of Cree
• Property Level Protection

Following the original study, DGC has progressed the scheme including design of the scheme and preparation of a flood order (a process required under the Flood Risk Management (Scotland) Act 2009, for local authorities to promote a flood protection scheme in their areas). DGC retained Kaya Consulting Ltd. to provide mathematical modelling support during the process.

The hydraulic model was used to consider a range of flood management options, proposed by DGC consultants. Over 300 model runs were carried out to assess the effectiveness of a range of mitigation options. All model results were provided to assist in the selection of the preferred option.

As part of the sensitivity testing of the proposed scheme, it was also required to assess the effect of future climate change uplifts taking into account the latest climate change predictions. This required design flows to be increased by 28% and 44% as well as increasing the water levels at the downstream boundary which is tidally affected. As a requirement for the final economic appraisal, a full suite of model runs was required, ranging from 2 year to 1000 year return periods.