Department of Civil
and Environmental Engineering
University College Cork
Rainfall Analysis and Flood Hydrograph
Determination in the Munster Blackwater
Catchment
By
Paul Guéro
A Thesis submitted for the Degree of Master of Engineering Science
October 2006
Abstract :
The Munster Blackwater catchment, in the South West of Ireland, is regularly subject to flooding, particularly in the towns of Mallow and Fermoy where it causes many disturbances for its inhabitants and sometimes severe economic losses. A good understanding of rainfall-runoff processes is therefore important in order to prevent such situations.
In the first part of this project, particular attention was given to rainfall data. The installation of a 32 tipping buckets network in the catchment, ranging in both longitude and elevation provides precise time-scaled information. Detailed analysis of spatial and temporal variation over the catchment was examined. The existence of an intensity gradient from West to East, and a neat correlation between elevation and rainfall depth were highlighted. It explains the higher runoff over catchment area observed in the West, which are responsible for rising of water level downstream in the East. Particular attention was also given to the 2006 spring and summer that appeared to be a significantly dry period. A drought assessment showed that 2006 was comparable to 1976, when the most important dry period was recorded in Ireland.
The Unit Hydrograph is the surface runoff hydrograph resulting from one unit of rainfall excess uniformly distributed spatially and temporally over a watershed for a specified duration. In the second part of the project, three different approaches of this concept (the synthetic Nash Instantaneous Unit Hydrograph, the analysis-based Ordinates Method and the “in between” Geomorphological Unit Hydrograph of Reservoirs) were studied. The Unit Hydrograph concept was incorporated in a rainfall-runoff model structure, which was applied at the outlet of the three nested sub-catchments along the river: Duarrigle (245 km2), Dromcummer (861 km2) and Killavulen (1265 km2). A comparison of the simulation results using several storms identified the Nash Instantaneous Unit Hydrograph as being the more efficient method, and the approach providing the best flood hydrograph determination. The model efficiency appears to be dependent on the catchment size and the model should not be applied to drainage areas greater than about 500 km2.
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