1. Introduction
Stormwater management in urban areas has been recognized as a serious challenge around the world [
1,
2,
3]. Increasing impervious areas as a result of urbanization is continually increasing the risk of flash floods [
4,
5,
6]. Due to occasional rainfall in semi-arid to arid environmental regions, proper consideration has not been given to stormwater management. In the recent past, climate change and environmental impacts, as a result of industrialization, have changed the predicted patterns and intensities of rainfall events in various regions (
Figure 1) [
7]. Therefore, this impact should be considered in the management of existing stormwater drainage infrastructure for safe collection and disposal of runoff. The problem becomes more significant in locations where natural wadies, which safely route the storm water, have been blocked by the expansion of cities and urbanization in arid regions [
8,
9].
In the Gulf region, the natural drainage patterns have been significantly altered due to urbanization. This problem is more significant in cities having rolling (presence of small hills) terrain where natural topographical constraints avert the runoff to be disposed-off to a nearby Wadi [
10]. As a result, surface ponds located within the city’s boundaries are being used as a final disposal point for stormwater. The problem has significantly been increased with expansion of cities, as a result of population growth, when the existing drainage systems overflow even during the smaller (than the design) return period floods. Presently, the municipality managers in arid environmental regions, are looking for answers of some critical questions for effective decision-making, such as (i) how much capacity of the existing storm-sewer infrastructure needs to be enhanced for a given return period flood, (ii) how to select the most feasible locations of additional ponds for safe disposal of the increased stormwater due to urbanization, and (iii) how to evaluate the technical, economic, and environmental sustainability of various stormwater drainage options. Therefore, there is a dire need to develop a framework that can answer these questions to facilitate the municipality mangers for effective decision-making.
In the past, various studies have been conducted on flood risk management for arid and humid regions. Chen et al. [
1] introduced simplified urban inundation simulation using storm water management model for a city in China. Thomas [
11] developed different coefficients to find storm water from urban areas in Sweeden. Ali et al. [
9] conducted a detailed review of literature on various aspects of flood risk. Al-Zahrani et al. [
12] developed a flood inundation and hazard map for flood management using Hydrological-Engineering-Centre, Hydrological-Modelling-System (HEC-HMS) in the Kingdom of Saudi Arabia (KSA). Almalki [
10] conducted interviews with people and municipality staff to highlight the technical and cost-related issues for flood management in KSA.
Some studies discussed general aspects of flood management in KSA [
13,
14,
15]. However, in-depth analysis addressing the impacts of urbanization and climate change on existing and improved (to handle longer return period floods) storm water drainage infrastructure in KSA have rarely been reported. In general, the storm sewer networks in arid regions were designed for shorter return period storms; consequently, storm water accumulates in streets during high return period floods and poses risk to life and property [
15,
16]. Floods of more than 20 years return period have been observed in Saudi Arabia in the recent past [
17]. This precious stormwater as a result of high return period floods also needs to be conserved for beneficial uses. Hence, the detailed design of the whole storm sewer network needs to be re-visited, using state-of-the-art methods, which has not been taken care of in the past studies. Finally, different management options may have varying technical, environmental, and financial considerations that should be evaluated in the decision-making process.
The framework developed in this research effectively answers all the three above posed questions. A methodology is established and implemented for optimal design of stormwater infrastructure by estimating the peak runoff through rainfall-runoff modeling in urban areas of high intensity/low frequency-flash floods. Present research also evaluates the impact of the geomorphological parameters (e.g., area of catchment, length of main drainage line, sub-areas under different land use etc.) obtained from two different data sets (30 m DEM and actual survey data) on the results of hydraulic modeling. HEC-HMS, geographical-information-system (GIS), and SewerGEMS software have been used for hydraulic modeling of different stormwater drainage options. Finally, Multi-criterion decision making (MCDM), consisting of Analytic Hierarchy Process (AHP) and Preference Ranking Organization Method for Enrichment Evaluation (PROMETHEE II), has been used to find the answer of the third research question.
4. Conclusions and Recommendations
Stormwater drainage infrastructure in urban areas of arid environmental regions, such as Saudi Arabia, cannot safely route the stormwater for more than five years of floods. However, truck sewers in some areas can accommodate up to 25 years of floods. This practice is not sustainable under the present situation of water scarcity in the region when this valuable water is primarily lost due to evaporation. A methodology is needed to evaluate different stormwater management options for urban areas of arid environments.
In this research, a methodology is developed for the design of the entire drainage networks in two urban catchments using state-of-the-art modeling techniques, such as HEC-HMS, GIS, and SewerGEMS. The proposed methodology revisits the design of drainage sub networks for higher return-period storms. The results of the hydraulic modeling of sewerage networks revealed that 30 m resolution DEM can be used with reasonable accuracy, particularly when expensive actual ground survey data is not available. Three stormwater drainage options, including the existing system, an additional ponds system, and micro-tunneling have been investigated for Buraydah, having wadies blocked due to city’s expansion and small hill barriers.
The stormwater drainage options have been evaluated using AHP and PROMETHEE II for multicriteria decision-making. Evaluation of all the options against the criteria of flood risk, environmental impacts, cost, and technical constraints revealed that the existing ponds system has high adverse environmental impacts regarding water quality, health, and being a source of mosquitoes. Based on the PROMETHEE II results, the micro-tunneling option outranked the other two due to minimal flood risk and environmental impacts (i.e., two criteria with higher importance weights established by AHP). Other sustainable options for flood risk management integrated with water conservation strategies can be investigated in future studies for arid environments. The study results should be validated before practical implementation of the micro-tunneling option by involving more stakeholders from water directorates, other relevant agencies, and public representatives.
Inclusion of a wider range of stakeholders (particularly the involvement of general public) can further improve the multicriteria decision-making process in the Gulf region. Such an initiative will enhance the sustainable development and implementation of water conservation strategies in KSA.