Geographic information system-based assessment of mitigating flash-flood disaster from green roof systems

https://doi.org/10.1016/j.compenvurbsys.2017.04.008Get rights and content

Highlights

  • A framework of measuring flooding mitigation from green roofs was constructed.

  • The rainwater retention of eco-roof systems was analysed over regions.

  • The eco-roof mitigation on urban flash floods was evaluated and visualised via GIS technologies.

  • The results revealed performances of roof properties for urban flood-prone areas during storms.

Abstract

Urban flooding is a severe problem and a growing development challenge for many cities around the world. However, urban impervious areas have sharply increased owing to booming construction activities, and this land-use change leads to more frequent flood inundation in urban flood-prone areas. A green roof system is regarded as an effective mechanism to manage rainwater and reduce flooding disaster, as it is capable of retaining rainwater, thus reducing rainfall-runoff. There is still a lack of assessment of this stormwater management tool for flash floods. The issue of flood inundation associated with green roof systems needs to be explored and developed. To evaluate the effects of green roofs on urban flood inundation, this paper aims to construct a framework for modelling urban inundation integrating a hydrological model of green roofs. The approach addresses both urban rainfall-runoff and underground hydrological models for traditional impervious and green roofs. To accurately calculate the spatial variation, we have proposed a hydrological model to evaluate regional runoff on the basis of a catchment mesh. The Deakin University Waurn Ponds campus in Geelong was then chosen as a community-based study case. From geographic information system (GIS) simulation, the results reveal that the green roofs generated varying degrees of mitigation of urban flash floods with storms of different return periods.

Introduction

There has been consistent growth in urbanised areas because of the construction of buildings and infrastructure in urban environments. The ongoing expansion of impervious ground surfaces, which is replacing previous land covers such as grasslands, inevitably leads to serious problems such as urban heat islands (Rizwan, Dennis, & Liu, 2008) and air pollution (Moore, Gould, & Keary, 2003). Another consequence could be more frequent and longer duration of flooding in urban flood-prone areas.

Urban flooding is the inundation of land or property in a built environment and is a growing development challenge for many cities around the world (Ran & Nedovic-Budic, 2016). This global phenomenon can result in damage to buildings, personal injuries, business disruptions, environmental pollution and transport interruption. Flooding events in Australia over the past 40 years have resulted in personal injury and deaths, and have cost the economy hundreds of millions of dollars. In January 1974, a flooding event induced by 900 mm rainfall in Brisbane resulted in 16 deaths, rendered 9000 people homeless and cost approximately AU$700 million (SCARM, 2000). In February 2007, thunderstorms caused local flooding in western Sydney, washing cars off roads and flooding buildings. Approximately 25,000 homes lost power (Bureau of Meteorology (BOM), 2015a). In addition, a severe thunderstorm drenched Geelong, Victoria's largest provincial city, on 19 February 2014 such that transportation was interrupted by ponding water of about 1 m depth (Mills, 2014). An estimated annual cost of more than AU$377 million means that flood inundation is the most expensive natural hazard in Australia (Middelmann, 2009).

Therefore, there is an urgent need to introduce mitigation measures through managing rainwater to reduce storm runoff. To date, several feasible measures, such as retention ponds (Yang, Li, Sun, & Ni, 2015), rainwater harvesting (Wu, Yu, Chen, & Wilby, 2012) and green infrastructure (Zellner, Massey, Minor, & Gonzalez-Meler, 2016), have been used to address the problem. Among these, greening of roofs can be considered an effective solution in storm water management as green roofs are capable of retaining rainwater and reducing peak runoff discharge (Locatelli et al., 2014). It is thus worthwhile to explore the mitigation effects on urban flash floods of green roof systems. This paper aims to construct a framework for estimating mitigation effects from green roofs on urban floods and to provide an approach to visualising the mitigation effects using geographic information system (GIS) mapping technologies.

Section snippets

Research on urban flooding and green roof stormwater retention

An important step in evaluating mitigation measures is appropriately modelling urban flooding for the identification of vulnerable areas and prediction of flood extents through maps of potential floodwater distribution (Al-Sabhan, Mulligan, & Blackburn, 2003).

Flooding is roughly categorised into riverine floods and flash floods by the BOM, as most events are predominantly caused by heavy rains (BOM, 2013). Riverine floods occur following heavy rainfall when watercourses do not have the capacity

Modelling hydrological performances of green roofs on urban flood inundation

Rainfall is a key factor driving the process of floods and affecting the hydrological behaviours of green roofs. As a critical step towards designing a flood inundation program, green roofs need to be considered in the rainfall-runoff model and the 2D surface runoff coupled with a drainage system. Three models are interrelated, as shown in Fig. 2. The urban terrain indicates the urban environment, including buildings, roads and highways. It is here used to determine flow direction and

Data acquisition and pre-process

The main data sources used here include meteorological information, hydraulic structures and topography, as shown in Fig. 2. The study case site should be first determined. For the preliminary application of this method, a community, the Waurn Ponds (WP) campus of Deakin University, was chosen as the study area in order to examine and analyse the mitigation of green roofs on flash floods. As depicted in Fig. 3, the WP campus (38.1979°S, 144.2973°E) is situated in the southern part of the City

Rainwater reduction through green roofs

The study area has been discretised by catchments, as mentioned in the previous section. The 10- and 50-year rainfall events are seen as common and severe storms, respectively. Fig. 6 shows the rainwater retention capability of green roofs over each catchment. Clearly, greening all bare roofs can significantly decrease rainstorm runoff but causes varying degrees of reduction for different rainfall events. Heavier storms produce higher volumes of rainwater infiltration for a traditional roof

Flood mitigation under common storm conditions

To demonstrate the maximum mitigation of flash floods, this research has taken the thickest extensive green roof system into account to simulate greening of all bare roofs on the WP campus. The runoff delay has been neglected here as the rainfall duration was much longer than the delay time. Inundation maps in Fig. 7 exhibit flood extents induced by a 10-year storm event at specific times for the two types of roof systems.

Specifically, the cumulative rainfall depth reached 45.83 mm by 12:00.

Conclusions

Urban flooding is a severe problem and a growing development challenge for many cities around the world, and it is the most expensive natural hazard in Australia. Green roof technology is regarded as an effective mechanism for reducing flood disaster, as it is capable of retaining rainwater and reducing rainfall-runoff. Because of a lack of research on assessment of the hydrological performance of green roofs for flash floods, this paper has considered Deakin University's Geelong WP campus as a

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