INDIRECT ESTIMATION OF UNGAUGED PEAK DISCHARGES IN A BEDROCK CHANNEL WITH REFERENCE TO DESIGN DISCHARGE SELECTION

Geomorphological evidence and recent trash lines were used as stage indicators in a step-backwater computer model of high discharges through an unguaged bedrock channel. The simulation indicated that the peak discharge from the 26.7 km2 catchment was close to 150 m3 s-1 during the passage of Hurricane Charlie in August 1986. This estimate can be compared with an estimate of 130-160 m3 s-1 obtained using the Flood Studies Report (FSR) unit hydrograph methodology. Other palaeostage marks indicate that higher stages have occurred at an earlier time associated with a discharge of 200 m3 s-1. However, consideration of both the geometry of a plunge pool and transport criteria for bed-rock blocks in the channel indicates that floods since 1986 have not exceeded 150 m3 s-1. Given that the estimated probable maximum flood (PMF) calculated from revised FSR procedure is at least 240 m3 s-1, it is concluded that compelling evidence for floods equal to the PMF is lacking. Taking into consideration the uncertainty of the discharge estimation, the 1986 flood computed using field evidence has a minimum return period of 100 years using the FSR procedure. This may be compared with a return period for the same event in the neighbouring gauged River Greta of > 100 years and a rainfall return period of 190 years. In as much as discharges of similar order to FSR estimates are indicated, it is concluded (a) that regional geomorphological evidence and flood simulation within ungauged catchments may be useful as a verification for hydrological estimates of recent widespread flood magntiude and (b) that palaeohydraulic computation can be useful in determining the magnitude of the local maximum 'historic' flood when determining design discharges for hydraulic structures within specific catchments.