Methods for Controlling Storm Water Flow
There are several methods of controlling storm water flow. All require site design that will slow and control the movement and disposal of storm water. Slowing run off of storm water diffuses the flow, infiltrates water into the ground or detains water allowing and increased amount to time for water to filter, percolate, evaporate or be used by plant materials. Designing storm water facilities is key to controlling storm water flow. The following design parameters have found as methods to better control storm water which must take place if run off is to be cleaned of its water borne pollutants prior to being disposed in fresh water bodies.
Diffuse flow means run off is conveyed in very shallow un-concentrated flow at velocities that do not cause erosion. Keeping the flow in small amounts through ground shaping allows the site to absorb more water. Flow diffusers are used in areas of topographic change to transform flowing water in grassed swales into shallow sheet flow into planted buffers, grass filter strips or riparian buffer. Flow diffusers prevent gully erosion. Diffused flow through a forested buffer or a grass filter strip achieves a significant removal of suspended solids.
Infiltration flow allows storm water to enter the top surface of the soil or underground BMP facilities allowing water from the surface to inter subsurface flow. This subsurface flow will further slow the disposal of water and allow for ground soils to clean the water by removing certain substances. Naturally occurring infiltration into sandy loamy top soils or directed infiltration into sand filters or French drains can remove a large percentage of surface runoff, sediments and certain chemicals that are picked up from parking lots, or site service areas that are then fixed to the site and removed from non-point flow.
Detention flow is created through site design that allows the temporary storage and slow release of storm water run off. Slow release occurs over a designed period of hours or days so that the facility returns to its dry condition before surfacing materials such as turf grass, meadow grass or other vegetative ground cover will perish. During detention, the storm water also has time to evaporate and infiltrate further reducing the amount of off site run off and increasing the percentage of the design storm captured on site.
The key design strategy for diffusing, infiltrating and detaining water is reducing slope, increasing surface roughness and increasing the time of concentration. These three methods of water control are used to manage run off volume, peak discharge and water quality performance into and through a series of BMPs on each development site. But there are two other methods that are used to reduce the amount of storm water being allowed to leave the development site. One method retains the water and the second reuses the water.
Water retention involves capturing a designed volume for both temporary storage and an amount that creates a permanent pool of water. This facility, often called a wet pond, has two volumes of water to manage through design. The temporary pool of water is released over a period of hours or days to a water volume that creates a permanent water line at a designed elevation. The second pool of water stays on the project site as a wet pond. This pond must have a depth sufficient enough to allow fish and permanent plantings of wetland plants. Wet ponds not only serve the function of storm water management but can be designed to be a significant aesthetic feature of any designed landscape through the introduction of wetland plants, wildlife, fishes and shape. Retention facilities must be designed with critical elevations to release the temporary pool while maintaining the permanent pool at a volume sufficient to allow living things to prosper.
Water reuse is not only a storm water management method but a water conservation tool as well. Re-use of water involves capturing storm water flows and using it for non-potable purposes such as turf grass irrigation, watering yards plants, growing food or cleaning. If captured from roof water this storm flow can even be used for toilet flushing. Since roofs generally occupy from thirty (30) to sixty (60) percent of any development site capturing water here can go along way toward meeting the capture rate percentage called for in the local on-site storm water management ordinance.
Rip rap erosion protection is an important part of sustainable drainage design that protects erodible soils following new construction.
Vegetation protection is the best tool used to protect the integrity of the edges of water bodies from the energy of moving water. Mature vegetation along the edges of streams, lakes, water bodies or sustainable drainage facilities such as bioswales, detentions, retentions and riparian buffers make the best erosion prevention tools available. However, following new construction the roots of plants have not yet formed the underground web that holds the soils in place. To allow vegetation to take root, it is important to stabilize high energy erodible places such as inflows, water ramps, outflows and bank edges with stone rip rap. This armoring will protect the integrity of new construction until vegetation takes over. Rip rap used on sustainable drainage projects such as these will be composed of various sized pieces of crushed limestone varying in size from 3” to 8” in diameter.
Filtration media also is used to control the flow of storm water. One of the methods used to control storm water flow is to allow it to percolate into the ground. Porous sandy soils do this easily. Silty, clay soil does not. In situations in which water will not percolate faster than ½ per hour it is helpful to improve soil infiltration rates by amending the soil with a blended mixture of imported soil that is more sandy and porous. Existing impermeable are extracted and replaced with a filtration media. Filtration media are columns of soil in twelve (12) inch lifts composed a surface growing medium composed of sand, sandy loam topsoil and organic matter over a porous base consisting of sand, fine aggregates, medium aggregates, and course aggregate. This improved soil mix is used to replace excavated slowly impermeable clays and silty clays for the purpose of increasing ground percolation and infiltration.
Evaporation is one of the best tools to use to control the disposal of storm water as part of the technology associated with storm water flow. In the hot South evaporation can be an important method of controlling storm water. Spreading water over hot concrete changes water from a liquid to a gas (a green house gas called water vapor) and is a flow control technology that can be used in the design of sustainable drainage systems. For instance, the irrigation system adapted to dispose of storm water can flood concrete parking lots on hot summer afternoons to evaporate water as a method of on site disposal.
Vegetation design is key to any on-site sustainable drainage program. Vegetation sucks up water through its root system and stores much of this water within cells of the plant and or converts the water to cellulose or releases it as a gas. This process is called evapotranspiration and a large tree will pass hundreds of gallons of water a day from the ground to the sky. All plants transpire water so an important factor in sustainable drainage design is to use vegetation as one of the disposal points in an on-site storm water system. Most common storm water BMP’s that can be used in site design such as detentions, retentions, rain gardens, rain groves, protected forest remnants and preserved wetland can be designed with well selected and planted landscape plants.
Storm Water Flow Characterization involves determining all factors associated with its flow from rain clouds to points of disposal. There are many factors involved for which some of the more common include rainfall source, intensity, frequency, amount, and flow rate and surface character. Any design work associated with storm water management requires calculations, estimates, and principles that are used to determine flow characteristics. The prime way to describe storm water flow is to be able to determine the amount of rain fall, the actually drainage area that is producing the water volume and how and at what speed (time of concentrations) that the water seeks the drainage basin outfall point. In addition, to design any facilities, one must have a design storm to work with and design a water conveyance system that will not flood during times of intense rainfall events that exceed the design storm. Finally, it is important to be able to document which part of the rain fall event gets managed on-site from that which leaves the site and enters a centralized city-wide drainage system.
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