Stochastic Urban Pluvial Flood Hazard Maps Based upon a Spatial-Temporal Rainfall Generator

Nuno Simões,João Leitão,Christian Onof,Alfeu Marques,Rui Pina,Susana Ochoa-Rodríguez,Li-Pen Wang

doi:10.3390/w7073396

Abstract

It is a common practice to assign the return period of a given storm event to the urban pluvial flood event that such storm generates. However, this approach may be inappropriate as rainfall events with the same return period can produce different urban pluvial flooding events, i.e., with different associated flood extent, water levels and return periods. This depends on the characteristics of the rainfall events, such as spatial variability, and on other characteristics of the sewer system and the catchment. To address this, the paper presents an innovative contribution to produce stochastic urban pluvial flood hazard maps. A stochastic rainfall generator for urban-scale applications was employed to generate an ensemble of spatially—and temporally—variable design storms with similar return period. These were used as input to the urban drainage model of a pilot urban catchment (~9 km2) located in London, UK. Stochastic flood hazard maps were generated through a frequency analysis of the flooding generated by the various storm events. The stochastic flood hazard maps obtained show that rainfall spatial-temporal variability is an important factor in the estimation of flood likelihood in urban areas. Moreover, as compared to the flood hazard maps obtained by using a single spatially-uniform storm event, the stochastic maps generated in this study provide a more comprehensive assessment of flood hazard which enables better informed flood risk management decisions.

Highlights

Urban pluvial flooding is expected to become more frequent as a result of urbanisation and the envisaged effects of climate changes [1]
The use of such design storms is handy, the uniformly-distributed design storm concept neglects the impact of the spatial variability of rainfall fields, which has shown to have a significant impact on the hydrological response of urban catchments [3,4,5,6,7,8,9,10]
With the purpose of contributing towards a better estimation of flood hazard and risk in urban areas, this paper presents a methodology to stochastically assess urban pluvial flood hazard, based upon a stochastic spatial-temporal rainfall generator developed by McRobie et al [20]

Summary

Introduction

Urban pluvial flooding is expected to become more frequent as a result of urbanisation and the envisaged effects of climate changes [1] This type of flooding can happen virtually anywhere and has the potential to cause significant damage and disruption in highly urbanised areas, where the density of buildings, critical infrastructure and population is usually high. Spatially-uniform design storms, associated to a given return period usually estimated based upon point rainfall records, have been traditionally used in the design, planning and evaluation of drainage systems, including evaluation of urban pluvial flood hazard. Proposed a new approach to urban flood risk assessment, using fuzzy set theory to assess the spatial and temporal variability of urban flood damage, and Sun et al [18] proposed a general framework for a risk-based storm sewer network design that takes future risk into account. The main conclusions of the present study are outlined and future work is discussed

Pilot Location

Spatial-Temporal Stochastic Rainfall Generator

Hydraulic Model

Stochastic Flood Hazard Mapping

Conclusions and Outlook