1.1 Introduction
Traditionally, stormwater systems, comprising stormwater drainpipes, curb inlets, manholes, minor channels, roadside ditches and culverts, are designed to remove stormwater from sites as quickly as possible to a main river channel or nearest large body of water to reduce on-site flooding.1 , 2
Many cities have implemented drainage systems as part of a larger sewer system that in addition to managing stormwater also regulates domestic and industrial wastewater. There are two types of sewer systems:
- Combined: Wastewater and stormwater are collected in one pipe network. Mixed water is then transported to a wastewater treatment plant for cleaning before being discharged into a river or large body of water.
- Separate: Wastewater and stormwater are collected in two separate networks. The wastewater is transported to a wastewater treatment plant, while the stormwater is conducted to the receiving waterway if it does not contain pollutants or needs to be treated separately before being discharged.3
While traditional grey infrastructure systems have, over many decades, proved to be effective in collecting stormwater runoff and draining it from the city, reliance on them has led to numerous unintended negative consequences relating to water quantity and water quality.
They include increased peak flows and total discharges from storm events; enhanced delivery of nutrients and toxins degrading aquatic habitats in urban waterways; and combined sewer overflows (CSOs) during wet conditions, exposing urban populations to health risks from waterborne pathogens and toxins.4 , 5 , 6
1.2 Impacts of Traditional Grey Infrastructure on Water Quantity
There are numerous impacts traditional grey infrastructure has on water quantity, including changes in hydrological cycles, increased peak flows and downstream flooding risks, changes in groundwater and surface water levels as well as inadequate dimensioning, resulting in increased climate change-related flood risks.
1.2.1 Changes in the Local Hydrological Cycle
In natural settings, only a limited amount of surface area is covered by impervious surfaces, resulting in most rainwater replenishing groundwater resources, filling rivers and lakes and being taken up by plants and trees. This process is assisted by infiltration, rainfall interception, evapotranspiration and soil retention. In cities, sealed surfaces including buildings, squares, streets and sidewalks act as a barrier to water, and instead of infiltrating through the soil, rainwater flows on the surface.7
1.2.2 Increased Peak Flows
Urban expansion, particularly in flood-prone areas, alters the natural path of waterways by increasing impermeable surfaces that reduce rainwater infiltration, thus increasing overland flows that typically exceed the capacity of drainage systems (Table 1.1).
Table 1.1
Change in watershed characteristics after urbanisation
Ground cover | Evapotranspiration (%) | Runoff (%) | Shallow infiltration (%) | Deep infiltration (%) |
|---|---|---|---|---|
Natural ground cover | 40 | 10 | 25 | 25 |
10–20% Impervious surface | 38 | 20 | 21 | 21 |
35–59% Impervious surface | 35 | 30 | 20 | 15 |
75–100% Impervious surface | 30 | 55 | 10 | 5 |
1.2.3 Downstream Flooding Risks
Traditionally, urban drainage systems are designed to prevent local flooding by conveying stormwater away from vulnerable sites, the aim being to drain stormwater as fast as possible out of the city. However, if urban districts upstream drain stormwater too quickly it may cause urban flooding downstream.8 In addition, downstream flood risks may be amplified due to ageing systems that cause sewers to overflow, block natural flow paths and increase runoff.9 This issue is exacerbated with many cities facing financial challenges of developing new infrastructure while also operating, maintaining, rehabilitating and ensuring environmental compliance of the current ageing infrastructure.10
1.2.4 Changes in Groundwater and Surface Water Levels
Stormwater systems can impact negatively on the local climate as infiltration and evaporation are reduced, resulting in cities’ climates becoming warmer and drier compared to the surrounding areas. The result of warmer, drier climates is lower groundwater recharge rates, which can reduce the availability of drinking water in cities. In addition, lower groundwater levels can potentially lead to lower stream base flows, decreasing habitats and cover available for instream inhabitants, therefore increasing competition and vulnerability to predators. With reduced flow, there is also the likelihood of increased water temperatures and lower dissolved oxygen levels, both of which will cause additional stress to instream inhabitants.11 , 12
1.2.5 Increased Climate Change-Related Flooding Events
In many urban settings stormwater drains are typically designed for a one in 30-year flood occurrence. However, this dimensioning is likely to be inadequate when confronted with extreme weather events caused by climate change.13 Heavy downpours have increased in frequency and magnitude in the past 50 years and are expected to become more frequent and intense as global temperatures continue to rise, leading to unmanageable stormwater runoff. In the United States the average 100-year floodplain is projected to increase by 45 percent by the year 2100.14 Adapting to these changes will lead to higher running costs and investments, which will place capital ...