Introduction

Rainwater management is a topic of intense interest, with various aspects to be considered. On the one hand, rainwater if appropriately collected, processed and stored could be used t to enrich the available water reserves and contribute to the sustainability of the whole system. On the other hand, reduction of the inflow to the existing drainage systems allows them to service increased demands (e.g. due to further development of urban areas). Hence, an appropriate management of rainwater can have multiple benefits with regards to water management.

This resource will deal with (a) the reduction of inflow of rainwater to existing drainage systems and the design of Sustainable Drainage Systems, and (b) the reuse of rainwater for various activities.

Reduction of rainwater inflow to drainage systems

To study methods to reduce the inflow of rainwater to urban drainage systems, we first need to study factors that increase the runoff. Such factors are:

1. the regional topography / orography,
2. the geological composition of the subsoil,
3. the level of urbanisation,
4. the existence of potential vegetation in the area,
5. the alteration of the hydrographic network of a region,
6. deforestation, and
7. various problems and management errors related to human activities.

The topography of the area can significantly affect the runoff of rainwater. Water runoff happens naturally due to the effect of gravity. When rainwater falls on a slope, it starts moving downhill due to the horizontal component of its weight. Hence, the larger the slope, the bigger its acceleration (Figure 2.1). Because of the fast runoff, there is no adequate time for it to seep into the soil. So, when this takes place near or in urban areas, the rainwater flows into the existing drainage system, taking up part of its capacity. As rain typically falls on large areas, the total volume of the water that flows in the urban drainage systems can be excessive and often causes overflow, as their capacity is exceeded.

Fig. 2.1 Analysis of the weight of a water mass into perpendicular components (a) on a 45° angle, and (b) on a 60° angle

(image source: Papavasileiou, 2019)

The geological composition is another important factor. The property that mainly affects the overall runoff of rainwater is the permeability of the soil. The more permeable a soil is, the easier the water can seep into it, and so a smaller proportion of it will have a surface runoff. Another factor that can affect runoff is the level of the underground water table. The higher the water table is, the smaller the quantity of water that can seep into the soil. There have been multiple cases when very high water table (virtually at the surface level) combined with heavy rainfall resulted in floods which caused significant damage in urban areas, or even injuries and casualties.

The level of urbanisation of a region is often related to the reduced permeability of build surfaces. Typically, roads, parking lots, and buildings, cover large proportion of the land and, due to their very small permeability (or even zero in the case of buildings), the water falling over there areas is directed to the drainage system.

However, this is not the only way that urbanisation of a region can affect the rainwater runoff. Altering the hydrographic network of an area, can affect the environmental conditions and, so, the overall runoff. For example, blocking natural rivers and leading large bodies of water through different manmade paths can increase/decrease the level of saturation of the nearby soils and, consequently, affect the amount of surface water that seeps through them. When this happens without any planning, or based on incorrect designs, this can lead to catastrophic events, as the water will follow unpredicted routes.

Another way that human activities can affect rainwater runoff, is through the creation of artificial lakes. Due to the concentration and retention of large quantities of water in areas from which it would have naturally run off, the overall saturation of the soils in the areas can increase. This change can affect (a) the permeability of the soils, which is particularly reduced, (b) the bearing capacity of the soils, depending on the soil type, and (c) the micro-climate of the area.

Deforestation, whether it takes place intentionally, e.g. to collect timber, expansion of arable land, city development, or unintentionally, e.g. due to forest fires, or landslides, reduce the retention of water, which can run off more freely and, so, travels downhill.

Figures 2.2 & 2.3 show how increased urbanisation of an area can affect the overall runoff of rainwater, increasing the peak flow in the drainage system and, consequently, increase the flood risk of this area.

Fig. 2.2 Illustrative representation of the level of urbanization against the proportion of water that would evaporate, infiltrate the soil or run off.
(Image source: SCIRP)

Fig. 2.3. Illustrative representation of surface/interflow/base flow of rainwater if (a) a greenfield, and (b) a constructed area
(Image source: Researchgate)

Sustainable drainage systems

In the previous section, it became clear that various human activities, the overall increase in the population of cities, as well as climate change, could reach the existing drainage systems to their limits. When the total capacity of the urban drainage systems is exceeded, this leads to overflow and flooding of specific areas. So, how would it be possible to ensure the sustainability of our drainage systems?

Various methods have been proposed, which try to achieve:

• increased retention of the rainwater at the areas of rainfall,
• increased infiltration of rainwater in the subsoil (increased baseflow), and
• temporary storage of rainwater locally, to deter its flow downhill.

The design of sustainable drainage systems is an issue considered worldwide and local codes or guidelines have been developed for this purpose.

SUDS(UK)

Sustainable drainage is a concept that includes long-term environmental and social factors in decisions about drainage. It takes account of the quantity and quality of runoff, and the amenity value of surface water in the urban environment.

LID (US)

Low Impact Development is a comprehensive land planning and engineering design approach with a goal of maintaining and enhancing the pre-development hydrologic regime of urban and developing watersheds.

LID uses land planning and design practices and technologies to simultaneously conserve and protect natural resource systems and reduce infrastructure costs.

WSUD (AU)

Water Sensitive Urban Design (WSUD) is about integration of water cycle management into urban planning and design.

• The key principles of Water Sensitive Urban Design are:
• Protect natural systems,
• Integrate storm water treatment into the landscape,
• Protect water quality, Reduce runoff and peak flows,
• Add value while minimising development costs

A number of different methods have been proposed and realised in practice to achieve a more sustainable management of rainwater. One approach is to control the overall runoff at the source. Rainwater is collected with different methods (permeable pavement, green roofs, rainwater collection tanks, cisterns, dry wells), and so the overall runoff is reduced. Another approach aims at reducing the overall runoff rate and, so the amount of water that reaches the drainage system at its peak is smaller, reducing this way the potential for exceedance of the system’s capacity. Finally, another approach is to retain water on surfaces through which it will infiltrate the soil and, so, the runoff will take place subterraneously. Most of the applied methods, also achieve some filtration, enhancing the overall quality of the water that enriches natural aquifiers. Tables 2.1 to 2.3 present a number of the applicable methods used in practice.

Table 2.1. Rainwater collection methods

Table 2.2. Filtration and treatment methods

Table 2.3. Filtration methods

Application of the aforementioned methods/technologies, apart from the reduction of the runoff volume and/or the peak supply, can have additional benefits such as reuse of the water on-site (e.g. for irrigation purposes), improvement of the water quality, energy savings, or aesthetic enhancement. Depending on the intended outcomes, those properties need to be considered and assessed to select the most appropriate solution each time. Table 2.4 presents the available methods and their respective functions.

Table 2.4. Low-impact Development (LID) technologies and their functions.