Introduction

Rainwater harvesting (RWH):
Rainwater harvesting (RWH) describes the process of collecting and storing rainwater. The practice enables the storage of stormwater runoff from rooftops, courtyards, greenhouses, reservoirs, retention ponds or other built infrastructure. This is also the first element of most drainage systems. Water can be directly channeled to drains in the ground or harvested for further use. There are several possibilities to harvest rainwater such as by using water tanks, rain barrels, and cisterns. Cisterns can be as simple as large containers located on rooftops for rainwater storage. Commonly, the harvested water comes into use for irrigation, firefighting, toilets, sinks, showers, or laundry making. RWH is particularly useful in the context of humanitarian settlements, in rural areas, bothin (semi-) arid or tropical climatic conditions.

Retention basins:
Retention basins (also: wet ponds) are a special type of rainwater harvesting and/or infiltration basins (see Measure S.08 ). They show a permanent water level which, during heavy rain events, can store further amounts of stormwater runoff while improving the water quality based on natural processes. Mostly, the collected water in wet ponds is used for irrigation or watering livestock.

Benefits & Risks

Next to stormwater flood mitigation, RWH provides benefits such as food security and the conservation of local water resources. The idea of circular design stresses water reuse (and thus the use of greywater) for landscape irrigation, topsoil treatment, or toilet flushes. By reusing greywater there is less strain on freshwater supplies, septic tanks, and the overall use of chemicals. In contrast, the reused water may support land fertility by making use of nutrients that would have been most likely wasted.

Depending on the treatment of the harvested water, possible usages may be different (e.g., if chlorinated, it is useful for cleaning and sometimes can be even drinkable but is less adequate for irrigation). If greywater is reused for irrigation, they first need to be filtrated to remove soap and other pollutants. When installing such systems, it is important to properly inform the users about the possible usages of the water, and the uses to avoid.

Environmental Impact

RWH can have a positive impact on the natural environment since it helps with the conservation of local water resources, especially in times of water scarcity. In addition, RWH minimizes the need for complex water infrastructures such as piping systems. As a result, the overall environmental strains due to dams or treatment plants are reduced. The areas where greywater comes into use, should be carefully considered to avoid negative environmental impacts.

Good Practice

Filtered RWH in Mexico City, Mexico

In Mexico City, the non-profit organization Isla Urbana developed RWH systems for informal urban settlements. After examining the areas that are most prone to water scarcity and capable of rainwater catchment, the organization implemented around 20’000 systems for rainwater collection and treatment across the city. After harvesting the precipitation on the rooftops, the water is cleansed with chlorine for the use of bathing and cleaning. The solution not only reacts to the issue of water scarcity but reduces the waste of stormwater and its possible damages through flooding and sewage system overspill (Mseleku 2021).

Unfiltered/Greywater RWH in Guirhora Kello, Burkina Faso

The British NGO Water Aid works together with national and local governments to support marginalized communities with safe water and sanitation facilities. That is also the case in the village Guirhora Kello in Burkina Faso, where rainwater runoff from the roofs of public buildings has been collected in storage tanks based on the ground. In this case, the stored water is not filtered and, thus, used as greywater for services such as washing, toilets, or planting (Mseleku 2021).

References

GRAF Ireland Environmental Ltd    (2023):  How Do I Install A Rainwater Harvesting System?

Go Smart Bricks    (2019):  Top 7 Types Of Rainwater Harvesting Systems You Should Be Knowing

Mseleku, E.S.  (2021):  Guidelines for Integrated Flood Control Design in the Informal Settlements of Cape Town Municipality - A case study of Kosovo, Philippi District

Tasawwar, Sumbal; Kassaye, Rahel Birhanu; Schaldach, Ruth    (2018):  Traditional Ecological Knowledge (TEK) - Rainwater Harvesting Methods

Score Card

Criteria

Scale of Intervention

Shelter-Plot-Block Settlement Supra-settlement

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Type of Intervention

Engineered Nature-based Hybrid Non-structural

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Targeted Natural Hazard

Pluvial Flood Coastal/Riverine Flood

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Strategy Type

Relocate Reduce Hazard Magnitude Reduce Asset Vulnerability Reduce Casualties

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Implementation Time

Short (1 day ‐ 1 month) Medium (1 month ‐ 1 year) Long (> 1 year)

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Effect Duration

Short‐term ( <1 year ) Medium‐term (1 year to 10 years) Long‐term (>10 years)

  Water tanks usually last between 10 and 20 years.

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Targeted Vulnerable Assets

Buildings Transport Technical Infrastructure Land Cover

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Investment Costs

Low Medium High

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Maintenance Costs (yearly)

Low (<10% investment costs) Medium (10-50%) High (>50%)

Materials

Clay, Concrete, Filter Systems, Cistern container

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