APPLICATION: FLOOD FORECASTING MODEL FOR LOIRE RIVER (FRANCE) - Modelling Uncertainty in Flood Forecasting Systems - page 111

Chapter 6
APPLICATION: FLOOD FORECASTING
MODEL FOR LOIRE RIVER (FRANCE)
Summary of Chapter 6
This chapter presents the results of the uncertainty analyses carried out
on the flood forecasting model of Loire River in France. Three
methods are applied: FOSM, Improved FOSM and fuzzy set theory
and an expert judgment-based qualitative method. In the absence of
sufficient information to represent uncertainty in the model inputs and
parameters by probability distributions, the FOSM method is used as
an appropriate tool for uncertainty estimation in the forecasts.
Approximate estimates of the coefficient of variation of the discharges
produced by rating curves are also assessed comparing the confidence
intervals of the forecast water levels with the observed water levels.
Separate analyses are carried out for the rising and subsiding flows.
Although the FOSM method is found successful in most of the cases, a
problem was encountered at a station for a particular flow condition
near extrema due to the linearised assumption of the method. The
Improved FOSM method (detailed in Section 4.2) is used to solve this
problem. Results of the improved method are compared with the
results of FOSM method and MC method. The effect of the different
shapes of the PDFs and the size of perturbation ratios are also
analysed. The expert judgement-based qualitative method using fuzzy
set theory (detailed in Subsection 3.2.2) is also applied. The qualitative
approach can incorporate the sources of uncertainty, which cannot be
incorporated in the quantitative framework like FOSM method.
Independent assessments of four experts are used for the analysis. The
description of the Loire model and the sources of uncertainty are
presented in Section 6.1. The applications of the FOSM, Improved
FOSM and the qualitative methods are presented in Sections 6.2, 6.3
and 6.4, respectively. The conclusions and discussion are presented in
Section 6.5.
6.1 Loire River flood forecasting model
The Loire River is the longest river (about 1,080 km) in France, which drains an area of
about 117,000 square km. The Loire basin has a temperate maritime climate with heavy
precipitation, including winter snowfall in the highlands that occupy its upper basin. The
area of its headwaters is also subject to violent autumn storms from the Mediterranean.
The river water level is usually highest in late winter, but there is no reliable rule; floods
may occur in any month, though normally not in July and August (“Loire River.”