An Assessment into the issues created by surface water flooding within the Rhondda Cynon Taf County Borough, Wales

Abstract

Recent flooding events including the February Storm Dennis and June 17thfloodshave shown that the situation faced by the RCT County Borough will only worsen with the advent of climate change, increasing the frequency and damage caused by the occurrence of repeated winter storms. One of the key factors contributing to the occurrence of surface water flooding is the mismanagement of culvert networks. RCT culvert policy indicates that culverting of watercourses should be carried out as a last resort, and constructed culverts should be able to handle a Q100+30 year event.

Analysis using digital Vector Maps and GIS software revealed numerous culvert networks that lie within vulnerable areas.Identified culvert inlets were recorded and compared to pre-existing data from the Environment Agency. EA culvert inlet numbers were found to be only a third of what was recorded in this study(1150 inlets compared to 3434 inlets), with the actual number expected to be much higher still due to a lack of sufficient documentation or archives. Observations of such culvert inlets fall to those who are responsible for their maintenance, with measures needing to be put in place to ensure that the culvert networks are not neglected and kept up to standard.Those culverts which lie under the responsibility of the RCTCBC should be prioritised based on their vulnerability and relative importance.Surface water risk within the RCT County Borough was found to be closely related to the steep topography combined with highly dense urban environments.

The environmental characteristics for the Rhondda and Cynon basins were analysed using a series of maps constructed from shapefile data. Percentages of slope angle and land use coverage was determined using spatial analysis within QGIS. Analysis showed that while manmade land use cover was similar for the two basins(11.48% for the Rhondda basin, 13.48% for the Cynon Basin), the slopes for the Rhondda basin were far steeper than those within the Cynon basin(coverage for angle of slope greater than 10 degrees being 56.44%and 35.71%respectively).This paired with the reports of surface water flooding during 2020 in areas like Pentre indicate a greater importance in general topography in the causation of surface water flooding in these areas.

The risk of surface water flooding within the Nantgarw and Pentre communities was observed using NRW flood models and simulation created using HEC-RAS software, then compared to existing images of the Storm Dennis floods.This indicated widespread flood risk within these key sites, and that existing flood defences were not sufficient in preventing the damages from a Storm Dennis scale flood. The vulnerability and Importance of 51 culvert inlets within the Pentre Community have been mapped using a custom-made index based on environmental observations, analysis within this study,and background research to compare their location patterns.The conditions of observed culverts were often inconsistent and prone to blockage, greatly increasing the risk of surface water flooding within the Pentre Community. Annual inspections are advised for key culvert inlets to prevent blockages occurring through the building up of debris or the presence of unauthorised structures.A complete comprehensive archive of culvert networks should be pursued by the relevant authorities;however, this is a project that could take many years to complete.

Date of Award	2022
Original language	English
Sponsors	Rhondda Cynon Taf County Borough Council
Supervisor	Tony Harris (Supervisor), Angela Morris (Supervisor) & Malcolm Thomas (Supervisor)