- Floods replenish water reservoirs and create an impression of abundance, yet are often accompanied by water shortages.
- Such flood events disrupt municipal water supply in three ways: Damaging water supply infrastructure, elevating turbidity and transmitting waterborne diseases and contaminants.
- Failure to properly anticipate and mitigate the highlighted flood risks will lead to continued municipal water supply disruptions.
When my friend asked me why there seemed to be plenty of water in the reservoirs serving Nairobi city in Kenya following heavy flooding, yet not a single drop of water flowing from her tap, I decided to establish what was going on. I gathered that while floods and torrential rains give the perception that there is plenty of water, in many instances taps run dry during heavy flood seasons. This piece delves into the three major ways through which floods curtail continuous supply of piped water. The first section addresses how flooding can lead to the destruction of vital water supply infrastructure, the second explores situations where torrential downpours exacerbate turbidity levels while the third section looks at supply disruptions resulting from the transmission of waterborne diseases.
One of the most common effects of flooding is the damage of critical infrastructure such as energy grids, communication networks, transportation infrastructure, and water supply structures. Flash floods have the capacity to uncover buried pipe systems and accelerate their destruction by other elements. Major flood incidences can also dislodge and wash away installed water transmission networks and destroy components such as meters, valves, and pumps. Inundation of water treatment facilities can lead to the damage of important electrical and mechanical components.
During the ongoing floods in East Africa, some residents of Nairobi lost their piped water supply for almost a month due to the shut-down of an integral water treatment plant serving the city. According to the local utility, a major water transmission pipeline was damaged by a landslide. Reports indicate that it was challenging to make timely repairs to the system owing to the ongoing torrential rains and additional landslides in the area. Beyond the immediate damage to water distribution infrastructure and resulting supply disruptions discussed above, is a more threatening long-term impact. Floods accelerate the degradation of water supply networks and increase leakages across water distribution systems. This drives up non-revenue water losses which in turn perpetuate numerous supply disruptions.
The second way floods cause water shortages, is through the increased turbidity of water sources. Flash floods in combination with riverine flooding tend to wash away the soft mud found on riverbanks, increasing the turbidity levels of water sources feeding into water treatment plants. Turbidity refers to the cloudiness of water and is caused by suspended particles, particulate organic matter, and chemical precipitates. It is reported in nephelometric turbidity units (NTU). A World Health Organisation (WHO) report recommends that turbidity levels for drinking water should not exceed 5 NTU and should ideally be less than 1 NTU. During normal conditions, rivers and other waterbodies should ideally have an NTU of less than 10 according to the United States Geological Survey (USGS). However, during flood conditions, waterbodies exhibit highly elevated levels of turbidity which may complicate the water purification process by necessitating the use of more coagulants and water disinfection chemicals.
Water utilities that lack the means to treat water with high levels of turbidity often opt to shut down their treatment plants for a short period to allow the water turbidity to reduce. This in turn disrupts water supply services. Others may choose to continue supplying water whilst issuing their consumers with the requisite warnings. In Austin, Texas, following the 2018 floods, the water utility issued a boil water notice – a warning to boil or disinfect tap water prior to consumption – due to elevated levels of siltation. In Nairobi, Kenya, the water utility serving the city shut down its main water treatment facility for two days in March 2020 following heavy downpours in the catchment area serving the plant. The downpours increased the turbidity of the water entering the treatment plant to the point where the utility could not effectively reduce the turbidity through regular treatment.
The most adverse effect of flooding on water supply is the transmission of waterborne diseases or contaminants. This happens either through direct exposure to contaminated water in municipal water systems or the consumption of water from unsafe sources when normal supply is disrupted. A 2001 research study indicates that over 50% of waterborne disease outbreaks in the United States between 1948-1994 occurred after flood events. Rapidly flowing floodwaters collect various harmful pollutants including microbial contaminants, chemical compounds, carcinogenic compounds, faecal waste from humans and livestock, among others. These contaminants may find their way into the distribution system through leaking, dilapidated pipes which allow ingress of untreated floodwaters. In such cases where the distribution network is contaminated, utilities often issue boil water notices as they clean out the network through flushing and disinfection. They might also be forced to halt piped supply for hours or days to eliminate the pollutants. During the torrential rains experienced in New South Wales province, Australia in February 2020, the water reticulation system in Narrabi town was contaminated. While supply was not disrupted, the city water utility released a boil water notice warning their customers to treat their tap water before use.
These examples from various regions across the world illustrate the disruptive effects of flooding on continuous municipal water supply. As highlighted at the beginning of this piece, floods can present a false picture of the availability of water, masking the nuanced complexities involved in the abstraction, treatment, and distribution of drinking water. Floods damage vital water structures including treatment plants and conveyance infrastructure which in turn curtails supply as repairs are conducted. They also elevate turbidity levels of water sources that feed into water treatment plants which makes water purification more difficult and might damage water treatment equipment. Flood waters can also contaminate water distribution networks, forcing utilities to issue boil water advisories or halt supply as they flush and disinfect their networks. Failure to properly anticipate and mitigate the three highlighted flood risks will always lead to municipal water supply disruptions.