Runoff control structures to temporarily store rainfall

Objective
Adaptation
Sectors
Water
Organisation
UNEP-DHI Partnership – Centre on Water and Environment
Asia Pacific Adaptation Network
Technology group
Urban storm water management

Description

Runoff control structures temporarily store rainfall. When a heavy rainfall event occurs, the volume of runoff into rivers increases dramatically, increasing the danger of flood damage. A water storage design can release the rainwater to the river over a longer period of time.

Runoff control structures are designed to capture runoff during peak flows, and can function as temporary storage sites. These structures are typically built with a discharge component to slowly release water into the nearby waterway to avoid it overflowing from the storage basin. It can be artificially built, for example by excavating a large area so that it is lower than the surrounding land.

Also, it may only require minor moderations to the existing landscape, such as constructing a raised barrier or mound around a flat, low-lying area (e.g. sports field, park, parking lot), or designing buildings on pillars to create a basin for storage underneath. Green spaces such as wetlands may also be utilized as temporary runoff storage structures. The structure can be utilized for other purposes when not needed, for example as a playground, football field, recreational area, etc.

Major sites for consideration:

  • Public facilities such as parks, schools, city hall, municipal offices, etc.
  • Single-family homes, housing developments, high-rise residential buildings
  • Large commercial facilities (e.g., supermarket parking lots in suburban areas)

Types:

  • Excavation type: The water storage area is excavated to create a water storage area. Example: Excavate the ground surface of a park deeper than the surrounding area to create water storage capacity. During regular times it can be used as a playing field or playground. When holding water it functions as an excavated pond.
  • Berm type (see Image 1): Create water storage capacity by building a low berm around the water storage area. Example: A low berm was constructed around the perimeter of a school ground, creating a basin.
  • Pilotis type: Build buildings on pillars or pilotis, creating a water storage space below.

    Example: A gymnasium is built on pillars, creating space on the ground level for vehicle parking, and for water storage. 

runoff_structure_1.png

Image 1. Left photo shows normal conditions, right photo shows water storage.

Source: http://www.arsit.or.jp/

Implementation 

Elevation and land use maps, along with hydrological modelling inputs, can be used to identify potential sites for the structure. Typically these will be flat, low-lying areas that are not always in active use. In very flat areas, excavation may be required to create a sufficient ’basin’ formation for storage. As an alternative to excavation, a berm can be built around the desired storage area.

Additional infrastructure components (e.g. pumps and drains) may be needed for excess water flow diversion, as well as training to manage the runoff structures. Maintenance includes removing sediments and debris to avoid system blockages, in addition to cleanup after rain events to remove excess debris and silt brought in by runoff. Further assessments may be needed to evaluate potential health risks associated with use of the structures, particularly those that are for public purposes, such as parks and playgrounds.

Costs

Initial costs: Initial costs vary greatly depending on the size of the storage area and materials used.

Subsidy programs: In Japan, legislation provides tax incentives, and low-interest financing is available from the Development Bank of Japan.

Energy source

None required

Ease of maintenance

Cleaning is required after release of stored water, as the storage area, drainage channels and orifices can become blocked by a buildup of soil and debris. One solution is to build a pit to remove debris upstream of the storage area, and to install a screen at the outflow point. 

Considerations (technology transfer criteria, challenges, etc.)

Technicians from consulting and construction firms are required at the design and construction phases.

Co-benefits, Suitability for Developing Countries

  • Accessible to developing countries, as runoff control structures can be built to store rainwater using the local terrain in its natural form, without the need for materials such as concrete. 
  • Traps sediments and pollutants, reducing contamination of natural waterways.
  • Provides ecological benefits, such as biodiversity, water quality, groundwater recharge, etc.
  • Minimizes flood damage risk, including risk of infrastructure damage and sewage overflows (in turn providing health benefits).
  • Creates multifunctional use of land during normal conditions, for example as a park, parking lot, sports field, etc.
  • Recycles water for other uses

Opportunities and Barriers

Opportunities:

- The terrain in its natural form can often be used to create runoff structures, requiring few materials for construction

 Serves as typically low-cost investment, not requiring major expansion of existing infrastructure, or new infrastructure construction.

Barriers:

- For optimal functioning, the structures require frequent maintenance to remove sediment and debris

- Some structures may not be immediately available for other uses after flood events as they require cleanup and maintenance.

Implementation considerations*

Technological maturity: 5

Initial investment: 2-4 (depending on the size and materials required)

Operational costs: 1

Implementation timeframe: 2-3

* This adaptation technology brief includes a general assessment of four dimensions relating to implementation of the technology. It represents an indicative assessment scale of 1-5 as follows:

Technological maturity: 1 - in early stages of research and development, to 5 – fully mature and widely used

Initial investment: 1 – very low cost, to 5 – very high cost investment needed to implement technology

Operational costs: 1 – very low/no cost, to 5 – very high costs of operation and maintenance

Implementation timeframe: 1 – very quick to implement and reach desired capacity, to 5 – significant time investments needed to establish and/or reach full capacity

This assessment is to be used as an indication only and is to be seen as relative to the other technologies included in this guide. More specific costs and timelines are to be identified as relevant for the specific technology and geography.

Case studies

Example 1: Excavated type (Japan)

  • A park in Hirosaki City (Image 2.), Aomori Prefecture, Japan, became a runoff control structure by excavating the park ground deeper than the surrounding area. Rainwater stored on the ground can be released from the discharge area into sewers and the river.

runoff-structure-2.jpg

Image 2. Example of excavated type of water storage structure

（http://www.arsit.or.jp/(link is external)

References

Asia PFAN 

Association for Rainwater Storage and Infiltration Technology 

Introduces a variety of rainwater storage and infiltration designs (in Japanese only).

http://www.arsit.or.jp/

UNEP-DHI Partnership- Runoff control structures to temporarily store rainwater