A Water supply system (WSS) consists of supply catchment, storage reservoir, treatment plant and distribution system and each component has different characteristics because of various geographical region, climatic conditions and demand by consumers. The WSS complexity is also related to the dynamic nature of the various characteristics that influence the system. For example, climate change and increasing demand due to population can affect the reliability of supply and its resilience. Indeed, the risk posed by flooding on water supply system should be considered as much as the obvious and devastating impact that flood have on the people’s lives and the environment.
Nowadays, the water supply, along with the sanitation, is considered a main factor in environmental sustainability, human health, social services and resilience (Luh et al., 2017). Therefore, the interruption of the service due to disasters, such as floods, is a scenario that must be considered during the design phase and for the management of the WSS. The assessment of flood risk on a Water Supply Systems requires a comprehensive approach able to account for several processes potentially leading to interruption of water supply and/or water quality degradation, which in turns involve several scales of analysis, from the catchment area to the distribution network. Such an approach aims at capturing the dependencies between environmental forcing and WSS components and the inner dependencies of the WSS itself.
Fundamentally, a Water Supply System may be described through three basic components: the source of supply (e.g. surface water, groundwater), the processing or treatment of the water, and the distribution of water to the users.
Flood events can impact the quality of surface water and groundwater in multiple ways: contaminated water can enter groundwater through wells, overflow and contamination from sewerage systems can occur, high level of rainfall and runoff can increase loading of pathogens, chemicals and suspended sediments in surface waters.
The source of supply can be affected by reduction of surface water quality, due to erosion, high loads of turbidity, pollutants transport, overwhelm wastewater, organic matter load. Actually, during flood events the most frequently problems are scarcity of safe drinking water (McCluskey, 2001; Bariweni & Tawari, 2012), disruption of water treatment facilities and, as a consequence, disease outbreak (Shimi et al., 2010; Speranza, 2010). Moreover, the reduction of groundwater quality can be caused by pollutants transport and flood effect on groundwater recharge. In particular, the volume of recharge rate during flooding is highly variable depending on the catchment characteristics and it is difficult to be quantified from observations. Comte et al. (2018) monitored the impact on groundwater quantity and quality due to a flood event occurred in 2017 in Botswana. The results showed that the groundwater level rapidly rose following flood and remained high for months. Moreover, the groundwater mineralization temporarily increased in rural areas upstream dams, while mineralization increase continued in peri-urban areas downstream dams.
Nevertheless, sometimes the river overflows (caused by flood events) are used to increases the groundwater recharge, especially useful specially in arid and semi-arid areas. Mainly in the wet season, the additional storage capacity could be used (like the dams) to store the excess runoff and making it available during dry periods (Alam et al., 2020). Zhang et al. (2017) demonstrated through experimental results that when the flood water depth increases, the height water table rises. Similarly, Sorman and Abdulrazzak (1993) suggested that flood runoff volume and duration are the major factors influencing the infiltrated volume and recharge to groundwater tables.
As concerns the water sources, severe flood events can also cause interruption of abstraction from artificial reservoirs and deterioration in the quality of stored water due to increase of turbidity (Chou and Wu, 2010). Lastly, flooding could affect well fields and result in pump failure and/or ingress of chemically/microbiologically contaminated flood water into damaged wells (Joannou et al., 2019; Sweya and Wilkinson, 2020).
Flooding can cause damages also to the water treatment component producing interruption of the treatment/water quality control because of treatment facilities flooded and damage to water treatment plants often located close to the river (Hedera, 1987; McCluskey, 2001; Barnes at al., 2012; Koh et al., 2017).
Moreover, flooding can have effects on the water distribution system: damages to infrastructures (pipe bridges, exposed pipelines, etc.) can occur causing disruption of the supply service (pressure fluctuations, intermittent supply) and contamination of the water resources (Arrighi et al., 2017; Joannou et al., 2019).
The understanding of the impact of flooding on a WSS is a complex issue and, moreover, the WSS failure can be considered as direct impact of the flood as well as an indirect impact; the former is caused by physical contact with floodwater and the latter occurs far from the event in either space or time (Thieken et al.,2006). Specifically, direct impact leads to damage to equipment, while indirect impact affects the service distribution to population far from the event. The direct losses can be evaluated through damage curves, relating water depth and losses (Smith, 1994); the indirect impact requires the interconnection between the network infrastructures and the flooded areas outside the location of the event (Gil and Steinbach, 2008). In this context, Arrighi et al. (2017) studied the supply system of the city of Florence by identifying that inhabitants indirectly affected by the WSS failure are two or three times as much as those directly flooded.
The assessment of the interactions between flooding and WSS is also promoted by the World Health Organization (WHO, 2011) with the main purpose to find appropriated, suitable and effective method to connect hazard and risk assessment; namely the connection among floods and source water quality. The assessment of natural hazards impact on WSS has to be involved and managed in the framework of Water Safety Plans (WSP), whose comprehensive implementation is supported and advised by the WHO (2017) to consistently ensure drinking-water safety. Specifically, a WSP has to be implemented as a tool for addressing the risk assessment and management on WSSs, as recommended in the revised drinking water directive (EU Water Directive 2020/2184) giving specific attention to small-scale water supply and sanitation systems, and to multisector cooperation, including communication. Based on a review of experience and good practice in the European Region, the WHO also summarized proven adaptation measures for water utilities, drainage and sewerage, and wastewater treatment systems during extreme weather events (WHO, 2011; 2017).
In this context, the MUHA (Multihazard framework for Water Related risks management) project, funded by the European INTERREG V-B Adriatic-Ionian ADRION Programme 2014–2020 program (https://muha.adrioninterreg.eu/), developed a Toolkit for WAter Safety Planning Procedures Decision Support System (WASPP–DSS). MUHA aims to improve forecasting, prevention and mitigation capacities of natural and man-made risks in water supply systems, strengthening cooperation between civil protection systems and operators at national, European and international levels in the implementation of the Water Security Plans. The four hazards related to the supply systems considered in the MUHA project are drought, flooding, accidental pollution and damage to critical infrastructure due to earthquakes.
The MUHA toolbox, developed on the basis of some key hypothesis and fully considering the WHO guidelines (WHO, 2009), represents a valid support for the early-stage development of the WSP leading the description of the WSS components and links, the hazards identification and risk assessment.
MUHA toolbox is here described and used for flood hazard impact assessment through a specific analysis that incorporates the different components of the WSS.
In this work, section 2 describes the procedure to evaluate the flooding impact on WSS by using a comprehensive approach and the main characteristics of the MUHA toolbox; in section 3 the application of the WASPP-DSS to three case studies identified in the framework of the MUHA activities is presented and, lastly, sections 4 and 5 outline the discussion and the conclusions.