Consulting Engineers and Scientists

Monmouth Strategic Flood Risk Mapping

Keywords: Strategic Flood Risk Mapping FEH, ISIS & TUFLOW Modelling.

Client, Architect & Other Organisations

Environment Agency

Monmouthshire County Council

 

Description

The town of Monmouth has been subject to a long history of flooding with records dating back to the 11 Century. Monmouth sits in the fork and at the confluence between the rivers Wye and Monnow. The most significant events in the recent past are the 1947, 1960, 1974, 1980, 1998 and 2000 floods which affected large areas of the Wye Valley including Monmouth.

 

The overall aim of the project was to update the flood risk mapping for Monmouth to facilitate a better understanding of the flood risks and to Monmouth.

Scope of Work

In 2006 Edenvale Young (then JYA Ltd) undertook a flood consequrnce assessment for the Monmouth Visitor Centr in Monmouth. As part of this project a combined ISIS-TUFLOW model of the Wye and Monnow was developed from approximately 4km upstream the confluence between the Rivers Wye and Monnow in the River Monnow and 2km in the River Wye, down to almost 3km downstream the confluence, defining a 9km long 1D model linked to a 2D 4m grid domain of 5.1 km2.

 

The objective of the SFRM project was to re-evaluate the hydrology of the Wye and Monnow at Monmouth and update the Development Advice maps for Monmouth. The first phase of the project was to investigate the relability of the and undertake rating extensions at Redbrook, Grosmont, Cadora and Kentchurch gauging stations using ISIS-TUFLOW modelling in order to evaluate the reliability of the rating curves, QMED and AMAX series.

 

Using the revised AMAX series the hydrology for the Rivers Wye and Monnow was reassessed using FEH statistical pooling group, single site, rainfall run-off and revitalised rainfall run-off methods for a range of return periods. The respective influence of the two watercourses was evaluated by using the FEH joint probability approach. This comprises an assessment of the flood flows on both catchments by evaluating the historical hydrological record from gauging stations upstream on the Monnow and Wye for a particular event or events with the record of the Redbrook Gauging Station, downstream from Monmouth.

 

The calibration of the model was performed considering an extensive historic data for chosen significant events in 1947 and 1998 and proved to be within accuracy between 8 and 300mm. Flood risk mapping for the requested scenarios was then produced. This involved 16 scenarios that combine the inflows for 20, 75, 100, 200, 1000 yr and 100 plus 20% climate change allowance. The mapping was done for two joint probability scenarios and also mapped for the study area including the existing flood defences and without them.