Hydrology & Hydraulic Modelling
Kerry Gilfach
Keywords: Civil Engineering Design & Costing, Flooding Project Appraisal Study, River Modelling (FEH, ISIS & -TUFLOW).
Kerry 2008
Client, Architect & Other Organisations
Powys County Council (PCC) The Welsh Assembly Government
Description
The town of Kerry to the east of Newtown is flanked by a series of steep hills which are known to be subject rapid run-off from thin soils in the form of sheet flow which can result in flooding to land and property particularly following long duration storms. This is a characteristic flooding mechanism for many sites in the Brecon Beacons and Black Mountains. The River Mule flows to the west and east of Kerry and is an ordinary watercourse which feeds into the Severn at Abermule.
In 2002 Edenvale Young Associates (then JYA Ltd) completed a pre-feasibility study for the site which recommended that a full project appraisal study (PAS) should be undertaken. The requirements for the project appraisal were required the development of a careful programme of work which included: procurement of LiDAR & topographic Information; hydrological modelling; interviews have with flood affected residents; development of a rainfall model in TUFLOW; preparation of engineering options and cost benefit analysis.
Scope of Work
The project initially focused on the conversion of a HEC RAS model to ISIS in order to model the River Mule. Unfortunately, the flooding experienced to houses within the estate in 1998 could not be recreated even by using very high inflows to the model. The modelling and hydrology were audited and the site re-visited. As a result of this work it was confirmed that the ISIS model and hydrology were correct. Accordingly, a TUFLOW model of the region was constructed using LiDAR information supplied by the Environment Agency. The TUFLOW model was then subject to a number of direct rainfall events which demonstrated that flowing to the urban areas of Kerry was the direct result of blockage to a bridge on the Mule, pluvial flooding and run-off from the surrounding fields. The direct rainfall model also confirmed eye witness accounts of the 1998 event and flood depths on the estate.