Introduction
Figure source: (uky.edu)
Stormwater is the general term for the rainfall-runoff collected from roofs, roads, and other surfaces before flowing towards low-lying land. This is the portion of rainfall that does not infiltrate into the soil.
Storm-water management means to manage surface runoff. It is essential in urban areas because run-off cannot infiltrate as the surfaces are impermeable. It can be applied in rural areas, too.
Traditional storm water management mainly focuses on draining high peak flows away. But this only dislocates high water loads. Modern approaches aim to store runoff water for a certain time, rebuild the natural water cycle, recharge groundwater, and use the collected water for irrigation or household supply. The cost depends on technology and the size of the systems. Planning, implementation, operation, and maintenance require expert knowledge.
Storm-water management is essential to prevent erosion of agricultural land and flooding of inhabited urban or rural areas. If sanitation facilities are flooded, it can cause severe damage and contamination of the environment. The result is a high cost and notably massive suffering for the local communities.
Storm-Water in rural areas
In rural areas, cumulative impact of countryside living subdivisions, roads and buildings causes an increase in peak flow rates, and the volume of water that is discharged after storm events. This leads to flooding and stream erosion. In rural areas and agricultural land it can lead to erosion.
Also, this water could be used for agriculture and drinking water. There are specific tools for rural areas and agricultural land. These include irrigation systems, called spate irrigation, or small-scale precipitation techniques such as micro basins, gully plugs, field trenches, bunds, dams, and rooftop harvesting. All of these techniques use important water sources while also reducing peak flows during rainy periods, storing water, and attenuating the power and velocity of floods.
Flooded agricultural land after a storm event. Source: ARC (2010)
Any urban development will make an impact on its environment. The hydraulic properties are changed by the construction of roads or buildings in an area. Previous layers are rendered less permeable or even impermeable. Depressions are raised to prevent ponding. Surfaces and conduits are constructed to drain runoff more efficiently. Natural vegetation is often removed, causing reduced interception and transpiration by plants. Limited vegetation cover exposes the soil to the impact of rain, which may lead to increased erosion. Natural meandering watercourses may be canalized to more effectively route flows through the development. Storm-water management is the science of limiting these negative impacts on the environment and enhancing the positive impacts, or catering for the hydraulic needs of a development while minimizing the associated negative environmental impacts.
In urban areas, flooding occurs very often because of impermeable surfaces as a human-made event. Runoff from such surfaces has a high velocity, which adds to stormwater drainage systems. This increases peak flow and overland flow volume and decreases natural groundwater flow (as no percolation is possible) and evapotranspiration. Furthermore, urban runoff has an increased pollution load, which leads to water pollution.
The traditional model of storm-water management is based on a misconception. It aims to drain of urban runoff as quick as possible with the help of channels and pipes, which increases peak flows and costs of storm-water management. This type of solution only transfers flood problems from one section of the basin to another section. Urban runoff contains a large amount of solids and a higher concentration of metals and other toxic components.
Integrated Urban Water Management (IUWM)
A more sustainable approach is Integrated Urban Water Management (IUWM), which refers to managing freshwater, wastewater, and storm water as links within the resource management structure, using an urban area as the unit of management (UNEP 2009).
IUWM includes the following activities:
💧 Improve water supply and consumption efficiency.
💧 Ensure adequate water quality for drinking water and wastewater treatment through using Environmentally Sound Technologies (ESTs) and preventive management practices.
💧 Utilize alternative water sources, including rainwater and reclaimed and treated water.
💧 Engage communities to reflect their needs and knowledge for water management.
💧 Establish and implement policies and strategies to facilitate the above activities.
💧 Support capacity development of personnel and institutions that are engaged in IUWM.
💧 Improve economic efficiency of services to sustain operations and investments for water, wastewater, and storm-water management.
New technologies for storm-water development in the sense of IUWM have been developed since the 1970s and include detention and retention ponds, permeable surfaces, infiltration trenches, surface and subsurface groundwater recharge, and other source control measures.
👉 Storage type devices
1. Detention Ponds
Detention ponds are excavated reservoirs or constructed in natural depressions, which are dry during low flow periods. These ponds provide temporary storage of storm-water runoff attenuation for both storm-water quality and quantity management. Ponds must be designed to allow storm water to sit long enough to settle out the solids to ensure the pollution removal role.
Figure: Extended Detention Basin Components. Source: NJDEP (2004)
2. Retention Ponds
Primarily, retention ponds are designed to improve water quality from storm-water flows. But they are often employed as flood control devices. They are designed not to dry out during dry periods, thus retaining water permanently as a part of their volume. The residence time is the basic design parameter, generally between 2 and 4 weeks. Water quality improves mainly through settling.
Figure : Retention pond (Source: winnipeg.ca)
3. Onsite Detention (OSD)
On-site Storm-water Detention (OSD) is a way of collecting the rain that falls on a site, storing it temporarily, and then releasing it slowly. So that it doesn’t worsen downstream flooding. It is mostly used in urban and residential areas, and it can be retrofitted in existing buildings.
4. Rainwater Harvesting
Interest in rainwater harvesting in urban areas is increasing as it provides the combined benefits of conserving potable water and reducing storm-water runoff. The rain that falls upon a catchment surface, such as a roof, is collected and conveyed into a storage tank. With minimal pre-treatment (e.g., gravity filtration or first-flush diversion), the captured rainwater can be used for outdoor non-potable water uses such as irrigation and pressure washing or in the building to flush toilets or urinals. It is also a widely used useful technique in rural areas.
Figure- Rainwater harvesting (Source; sacleanwater.com)
5. Green Roofs
Green roofs, also known as “living roofs” or “rooftop gardens,” consist of a thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof. Green roofs are touted for their benefits to cities, as they improve energy efficiency, reduce urban heat island effects, and create green space for passive recreation or aesthetic enjoyment. To a water resources manager, they are attractive for their water quality, water balance, and peak flow control benefits. From a hydrologic perspective, the green roof acts like a lawn or meadow by storing rainwater in the growing medium and ponding areas.
Figure: Green roof (Source- ekagroup.com)
6. Constructed Wetlands
Beside constructed wetlands for wastewater treatment, there are also engineered stormwater wetlands to manage flood peaks. They improve water quality of surface run-off and restore some of the cities natural habitat and birdlife. They can be combined with surface and subsurface groundwater recharge systems or soil aquifer treatment.
Figure: Constructed wetland (Source: blogs.umass.edu)
👉 Infiltration Type Devices
1. Infiltration Trenches
Infiltration trenches are shallow excavations that are filled with uniformly crushed stone to create underground reservoirs for storm-water runoff. The runoff gradually filtrates through the bottom of the trench into the subsoil and eventually into the water table. To avoid sediment penetration the walls and the top are lined with geotextile. Trench designs may be modified to include vegetative cover and other features, establishing a bio-filtration area. They are often constructed beside outdoor parking lots or beside streets. Treatment occurs during infiltration into the soil.
Figure: Infiltration trench (Source: citywindsor.ca)
2. Grass Filter Stripes
Grass filter stripes (filter stripes or grassed filters) are densely vegetated, uniformly graded areas that treat surface flow from adjacent impervious areas. Grass filter stripes function by slowing runoff velocities, trapping sediment and other pollutants and providing a modest infiltration.
Figure: filter strips (Source: lakesuperiorstreams.org)
3. Grassed Swales
Grass swales (vegetated swales) are open grassed channels in which storm-water runoff is slowed down and partially infiltrated along their course. Check dams and vegetation in the swale slow the water to allow sedimentation, filtration through the root zone and soil matrix, evapotranspiration, and infiltration into the underlying native soil. Simple grass channels or ditches have long been used for storm-water conveyance, particularly for roadway drainage. Enhanced grass swales incorporate design features such as modified geometry and check dams that improve the contaminant removal and runoff reduction functions of simple grass channel and roadside ditch designs.
Figure- Grass swale with check dams (Source: Virginia Stormwater Management Program)
4. Pervious Pavements
Pervious pavement is a permeable pavement surface with a stone reservoir underneath. The reservoir temporarily stores surface runoff before infiltrating it into the subsoil or subsurface drainage and in the process improves the water quality. Porous materials such as ancient lime mortars and pervious pavements are made using relatively mono graded materials.
Figure: Pervious pavement (Source: www.thecooldown.com)
5. Infiltration Basin
An infiltration basin (infiltration pond) is a facility constructed within highly permeable soils. It provides temporary storage of storm-water runoff. An infiltration basin does not normally have a structural outlet (like detention basins) to discharge runoff from the storm-water quality design storm. Instead, outflow from an infiltration basin is through the surrounding soil. An infiltration basin may also be combined with an extended detention basin to provide additional runoff storage for both storm-water quality and quantity management.
Figure: Infiltration Basin (Source: megamanual.geosyntec.com)
Advantages of Storm-Water Management
📍 Proper drainage of surface run-off.
📍 Possibility to recharge groundwater and re-use precipitation water and surface runoff as irrigation or household water.
📍 Treatment of storm-water in a very early stage.
📍 Avoids damages on infrastructure (private properties, streets, etc.); flood prevention.
📍 Can be integrated into the urban landscape and provide green and recreational areas.
Limitations of Storm-Water management
📍 Expert planning, implementation, operation and maintenance required.
📍 Depending on the technique, a lot of operation and labor required.
📍 Risk of clogging infiltration system caused by high sedimentation rates.
Discussion
Modern storm-water management techniques mostly have some ecological treatment effects with the general aim to protect the health, welfare and safety of the public, and to protect property from flood hazards. It is achieved by safely routing and discharging storm-water from developments.
All storm-water management systems need a proper operation and management service. Regular maintenance extends the life of storm-water systems, improves site drainage, and reduces pollution entering surface waters and groundwater. Governmental facilities as well as private property owners are responsible for operation and management.
Theoretically, storm-water management is necessary in every settlement, rural or urban, to protect human health, prevent water pollution, re-use precipitation water (on agriculture or household level) and prevent damages to infrastructure. It is essential in urban areas where constructed surfaces change the hydraulic properties and prevent infiltration. The whole management has to be adapted to the local conditions (climate, topography, resources, etc.). Therefore, expert knowledge is required.
References
1. Pwdplanreview.org. (2019). 3.2 Stormwater Management Design Strategies | Philadelphia Water Stormwater Plan Review. [Online] Available at: https://www.pwdplanreview.org/manual/chapter-3/3.2-stormwater-managementdesign-strategies
(Accessed 18 Sep. 2019)
2. Sswm.info. (2019). Stormwater Management | SSWM - Find tools for sustainable sanitation and water management!. [online] Available at: https://sswm.info/waternutrient-cycle/wastewater-treatment/hardwares/semi-centralised-wastewatertreatments/stormwater-management (Accessed 18 Sep. 2019)
3.Stormwater management practices at EPA.
Available at : https://www.epa.gov/greenepa/stormwater-management-prctices-epa-facilities (Accessed on 18 sep.2019)
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