Colloque SHF : "Valeurs rares et extrêmes de précipitations et de débits, pour une meilleure maîtrise des risques" - Lyon, 15-16 mars 2006

Abstract

Uncertainty on flood discharge estimates are linked to errors on flood stage, discharge measurement and rating curve extrapolation. According to a set of several hundred of French hydrometric stations, the standard uncertainties are of the order of 1% (i.e ± 5 cm) on flood water level, and between 3 to 5% on discharge measurement, with larger values when flow velocity or gradient of water raising are important. More than half of the rating curves have not been gauged more than a 2 year flood. Errors on non gauged discharges can be very large when the hydraulic properties of flow are not taken into account (non linear relationship between stage and discharge, discontinuity of flow between main channel and flood plain). The final uncertainty on discharge estimate based on hydraulic extrapolation of the rating curve is about ± 15% for the 10 year flood, except for complex hydraulic situations or without any gauging values.