Water in Celtic Countries: Quantity, Quality and Climate Variability (Proceedings of the Fourth InterCeltic Colloquium on Hydrology and Management of Water Resources, Guimarães, Portugal, July 2005). IAHS Publ. 310, 2007, 201-210.
Mechanisms of flooding in
the Mawddach catchment
GRAHAM HALL & ROGER CRATCHLEY
Centre for Arid Zone Studies and
School of Agricultural and Forest Sciences, University of Wales, Bangor, UK
Abstract
Rainfall distribution across the Mawddach catchment recorded at 22 raingauges
does not correlate well with ground elevation, instead suggesting that maximum
precipitation is generated at valley heads where maximum uplift occurs. Streams
within the Mawddach catchment have their sources in upland blanket bogs. Water
table monitoring shows the water storage capacity of older humified peat to be
very limited, with saturation possible during a single storm event—throughflow
is concentrated in zones of younger Sphagnum peat. Large thicknesses of
glacial and periglacial deposits infill river gorges in the catchment.
Hillslope throughflow monitoring indicates that saturation of these deposits
can create conditions for rapid runoff and flooding downstream. Temperature
monitoring of river bed sediments has identified resurgence of groundwater
after prolonged rainfall, further increasing river discharge. Computer
modelling predicts that flood plain forestry within the catchment can provide
significant temporary storage for peak discharge, reducing flood levels
downstream.
Key words blanket bog; flooding; flood plain forestry;
groundwater; North Wales; rainfall distribution