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 in dicators in a step-backwater computer model of high discharges through an unguaged bedrock channel. The simulation indicated that the peak d ischarge from the 26.7 km2 catchment was close to 150 m3 s-1 during th e passage of Hurricane Charlie in August 1986. This estimate can be co mpared with an estimate of 130-160 m3 s-1 obtained using the Flood Stu dies Report (FSR) unit hydrograph methodology. Other palaeostage marks indicate that higher stages have occurred at an earlier time associat ed 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 i n the channel indicates that floods since 1986 have not exceeded 150 m 3 s-1. Given that the estimated probable maximum flood (PMF) calculate d from revised FSR procedure is at least 240 m3 s-1, it is concluded t hat compelling evidence for floods equal to the PMF is lacking. Taking into consideration the uncertainty of the discharge estimation, the 1 986 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 di scharges of similar order to FSR estimates are indicated, it is conclu ded (a) that regional geomorphological evidence and flood simulation w ithin ungauged catchments may be useful as a verification for hydrolog ical estimates of recent widespread flood magntiude and (b) that palae ohydraulic computation can be useful in determining the magnitude of t he local maximum 'historic' flood when determining design discharges f or hydraulic structures within specific catchments.