Urban hydrology in mountainous middle eastern cities

T Grodek,J Lange,S Husary,J Lekach

doi:10.5194/hess-15-953-2011

Abstract

Abstract. The Mediterranean climate together with the type of urban setting found in mountainous Middle Eastern cities generate much lower runoff yields than previously reported and than usually estimated for urban design. In fact, a close analysis shows that most of the rainwater remains within the cities as a possible source for urban groundwater recharge. The present study examined two locales – Ramallah, an old traditional Palestinian Arab town, and Modiin, a new township in Israel – both situated on the karstic Yarkon Taninim aquifer. This aquifer supplies the only high-quality drinking water in the region (one quarter of the Israeli-Palestinian water demand), which is characterized by dense populations and limited water resources. This paper provides the first measured information on the hydrological effects of urbanization in the area. It was found that the shift of the mountainous natural steep slopes into a series of closed-terraces with homes and gardens create areas that are disconnected from the urban runoff response. Roofs drained into the attached gardens create favorable recharge units. Mainly low-gradient roads became the principal source for urban runoff already following 1–4 mm of rainfall. Parallel roads converted single peak hydrographs towards multi-peak runoff responses, increasing flow duration and reducing peak discharges. The remaining urban area (public parks, natural areas, etc.) generated runoff only as a result of high-magnitude rainstorms. All of the above conditions limited urban runoff coefficients to an upper boundary of only 35% and 30% (Ramallah and Modiin, respectively). During extreme rainstorms (above 100 mm) similar runoff coefficients were measured in urban and natural catchments as a result of the limited areas contributing to runoff in the urban areas, while natural terrain does not have these artificial limits. Hence, the effects of urbanization decrease with event magnitude and there is significant potential for urban groundwater recharge. However, frequent low-magnitude rainstorms often generate highly polluted stormwater in urban sewer systems and this water should only be used with great caution.

Highlights

Urban hydrology is ambiguous and depends on the combined effect of local climate and urban characteristics (Trowsdale and Lerner, 2007)
Impervious surfaces control urban stormwater and cause: (i) an increase in peak discharges and volumes (e.g., Dunne and Leopold, 1978; Cheng and Wang, 2002) that in turn increases flooding and erosion risks compared to pre-urbanization hydrology (e.g., Wolman and Schick, 1967; Trimble, 1997); and (ii) deterioration of the water quality by pollution from various sources (Smullen et al, 1999; Lee and Bang, 2000; Gnecco et al, 2005; US-NRC, 2008; Piguet et al, 2008)
In the hyperarid town of Elat, Israel, roads and parking lots were found to retain most of the rainwater (Grodek et al, 2000)

Summary

Introduction

Urban hydrology is ambiguous and depends on the combined effect of local climate and urban characteristics (Trowsdale and Lerner, 2007). Impervious surfaces (roads, parking lots and roofs) control urban stormwater and cause: (i) an increase in peak discharges and volumes (e.g., Dunne and Leopold, 1978; Cheng and Wang, 2002) that in turn increases flooding and erosion risks compared to pre-urbanization hydrology (e.g., Wolman and Schick, 1967; Trimble, 1997); and (ii) deterioration of the water quality by pollution from various sources (Smullen et al, 1999; Lee and Bang, 2000; Gnecco et al, 2005; US-NRC, 2008; Piguet et al, 2008). Roads are primary runoff generators (Ramier et al, 2006) They replace natural drainage systems with relatively high runoff coefficients, limited flow capacity and higher drainage densities, contrary to natural catchments.

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Discussion

Conclusion