An Integrated Meteorological /Hydrological Model for the Mawddach Catchment, North Wales Volume 1
MetadataShow full item record
This project makes a study of meteorological and hydrological processes operating in the Mawddach river catchment ofNorth Wales, with the objectives of recommending catchment management options to reduce the severity of flooding and to produce a design for a high resolution flood forecasting model for the catchment. An array of rain gauges installed across the catchment has allowed the detailed mapping of rainfall distributions. Two patterns are identified, with axes of high rainfall running NW-SE and N-S respectively. Within these zones, the locations of rainfall maxima do not necessarily correspond with the highest altitude. The approach direction of weather systems and the funnelling ofair flows along deep valleys appear to control rainfall distribution. A series of flood events are examined, particularly the convective squall line storm of3 July 2001 and the period ofintense frontal rainfall of3-4 February 2004. Modelling of rainfall is carried out using the MM5 meteorological model. It is found that frontal rainfall can be forecast to a high degree of accuracy. Convective thunderstorm events are less predictable, and different convective physics schemes within the MM5 package had differing degrees of success in forecasting the July 2001 Mawddach storm. The catchment is an area of hard, low permeability Palaeozoic rocks. Thick deposits of glacial and periglacial materials are locally present, particularly in valleys. Experiments to monitor hillslope throughflow and runoff show that these superficial deposits playa crucial role in controlling the antecedent conditions necessary for saturation-excess flood events. Deep blanket peat is found at a number of upland sites in the catchment. Watertable monitoring indicates that older peat has a low water storage capacity, with saturation possible within a few hours of heavy rainfall. Areas ofyoung Sphagnum peat can act as regulating reservoirs for flood water, and should be conserved. Field monitoring ofriver bed temperatures shows that resurgence of groundwater can occur in the deep river valleys of Coed y Brenin during storm events. However, resurgence occurs after the flood peak has passed and is not thought to influence the severity of flooding. Flood scenarios for the town ofDolgellau are investigated, including the effects of continued gravel deposition in the River Wnion. Gravel supply should be controlled through planting of native broadleaf woodland on riverbanks of(peri)glacial materials. A flood reduction scheme is proposed, with establishment of wet woodland and creation ofa flood interception basin in the lower Wnion valley. Field monitoring ofriver and tidal flows at the head of the Mawddach estuary indicates no additive effect ofriver flood and tidal peaks. Flooding at the head of the estuary is generally caused by river flows. Further reclamation of salt marsh could worsen upstream flooding. A new hillslope model has been written which allows for changing antecedent soil moisture conditions. The model generates a soil distribution based on the HOST (hydrology of soil types) scheme. A flood forecasting system is developed by combining the hillslope model with MM5 and existing river routing and floodplain modelling components. The system operates by a combination of parallel and distributed processing, to produce forecasts within an operationally useful timescale.