Improving flash flood forecasting using a frequentist approach to identify rainfall thresholds for flash flood occurrence

Improving flash flood forecasting using a frequentist approach to identify rainfall thresholds for flash flood occurrence

Wu, Z. (TU Delft Hydraulic Structures and Flood Risk; Wuhan University; IHE Delft Institute for Water Education; Eco-Environmental Monitoring and Research Centre)
Bhattacharya, Biswa (IHE Delft Institute for Water Education)
Xie, Ping (Wuhan University)
Zevenbergen, C. (TU Delft Urban Design; IHE Delft Institute for Water Education)

2022

Flash Flood Guidance (FFG) is a rainfall threshold which initiates flooding in streams. It merely provides a binary output (yes or no) which has large uncertainties in forecasting. In this paper, we propose a new method by combining FFG with the Frequentist method to present the probability of flash flood occurrence based on historical rainfall events. We first calculated deviation from the log transform rainfall data leading to flash floods. Kernel Density Estimation (KDE) was used to describe the deviation. Normal Distribution Function (NDF) was chosen to fit the KDE output and to calculate probabilities of flooding as per the Frequentist FFG. In order to aid decision making, three probability thresholds (10, 20 and 60%) were used for defining four flood risk classes, namely very low, low, significant and high, and were colour coded respectively as green, yellow, orange and red. The proposed Frequentist FFG method was then applied to the Posina River basin in Italy. Comparison of forecasts from the conventional FFG (with probability 0 or 1) and Frequentist FFG for 94 6-hourly rainfall events, including 23 flood events, shows that the Frequentist FFG presented a probability of flooding varying from 0 to 100% and the corresponding risk class can be used to reduce false alarms while still reducing the disaster risk. The application of the developed approach to the Posina basin shows that decision making regarding flash forecasting is easier with the presented approach compared to the traditional FFG approach.

Flash flood
Flash flood guidance (FFG)
Frequentist
Posina
Probability

http://resolver.tudelft.nl/uuid:dd1eef66-2835-47c1-8cc6-eb041b1f9d45

https://doi.org/10.1007/s00477-022-02303-1

2023-07-01

1436-3240

Stochastic Environmental Research and Risk Assessment, 37 (1), 429-440

Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.

Institutional Repository

journal article

© 2022 Z. Wu, Biswa Bhattacharya, Ping Xie, C. Zevenbergen