Evaluating natural infrastructure for flood management within the watersheds of selected global cities

Kelly Gunnell,Mark Mulligan,Robert A Francis,David G Hole

doi:10.1016/j.scitotenv.2019.03.212

Kelly Gunnell, Mark Mulligan + Show 2 more

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https://doi.org/10.1016/j.scitotenv.2019.03.212

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Abstract

Cities are dependent on their upstream watersheds for storage and gradual release of water into river systems. These watersheds act as important flood mitigation infrastructure, providing an essential ecosystem service. In this paper we use metrics from the WaterWorld model to examine the flood management-relevant natural infrastructure of the upstream watersheds of selected global cities. These metrics enable the characterisation of different types, magnitudes and geographical distributions of potential natural flood storage. The storages are categorised as either green (forest canopy, wetland and soil) or blue (water body and floodplain) storages and the proportion of green to blue indicates how different city upstream basin contexts provide different types and levels of storage which may buffer flood risk. We apply the WaterWorld method for examining flood risk as the ratio of accumulated modelled annual runoff volume to accumulated available green and blue water storage capacity. The aim of these metrics is to highlight areas where there is more runoff than storage capacity and thus where the maintenance or restoration of further natural infrastructure (such as canopy cover, wetlands and soil) could aid in storing more water and thus better alleviate flood risks. Such information is needed by urban planners, city authorities and governments to help prepare cities for climate change impacts.

Highlights

We demonstrate the use of natural infrastructure flood metrics that are globally applicable and require low-data inputs for the upstream basins of five global cities: Chennai, Jakarta, Bogotá, London, and Guayaquil, by 1) mapping the magnitude and types of ‘natural’ storages in these basins, 2) determining how much of this storage is found in protected areas and upstream of/near to urban areas, and 3) using the WaterWorld realised flood mitigation metric to determine areas of potential annual flood risk
We used WaterWorld's (Mulligan, 2013) calculation of green storage, the MODIS Urban Land cover 500 m dataset (Schneider et al, 2009) and a protected areas dataset (IUCN and UNEP-WCMC, 2015) to show and calculate where and how much blue storage lies in urban areas and how much green storage lies within protected areas
Changes to land use associated with increasing urbanisation and agricultural intensification, are likely to significantly negatively impact water storage capacity, flood regulation and other ecosystem services, and the results presented here indicate that key areas of concern around major cities can be highlighted for further consideration and potential conservation and restoration actions

Summary

Introduction

Flood risk results from heavy or prolonged precipitation events leading to reduced infiltration into the soil and increased surface runoff across the land surface into surface waters (streams, rivers, lakes and reservoirs). Ecosystems can regulate floods by preventing their occurrence, such as by redirecting or absorbing precipitation, reducing surface runoff and river discharge; or mitigating their impact by providing retention space for surplus water and thereby lowering flood volumes and destructive power (Nedkov and Burkhard, 2012). The ecosystem service of flood regulation (prevention and mitigation) is of growing importance due to the projected increases in frequency, intensity and duration of extreme precipitation events under current and future climate change (Frei et al, 2006; IPCC, 2014; Rao et al, 2014; Revi et al, 2014) and due to increased exposure of increasingly valuable assets in urban areas near to rivers

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