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Storm Water Management Facility Sites
STORM WATER MANAGEMENT FACILITY SITES are generally a best management practice integrated throughout land development projects, which provide for a volume of water storage, infiltration, and evaporation that mimics the natural rate of run-off or ground water recharge. The physical design; i.e., size, location, and depth, must be designed and constructed according to all applicable ordinances and under the direction of a professional engineer. Seeding of native plant species, in addition to planting trees and shrubs, is an economical way to vegetate storm water management sites. The use of erosion control fabric, mulch, or hydromulch is necessary to control erosion during and after construction.
Examples of STORM WATER MANAGEMENT FACILITY SITES:
- Detention basins - temporarily impounds run-off water, allowing for release at controlled rates.
- Retention basins - storm water management facilities with permanent impoundment or pool for improving water quality.
- Bioretention basins and rain gardens - dynamic living microbiological systems that enhance retention, infiltration, and evaporation of run-off water while remaining attractive to wildlife.
Plant Selection
The role of plants in all storm water management facilities is to prevent erosion and slow water movement, hold or convert pollutants, enhance infiltration and evapotranspiration, and encourage wildlife. The designer can select plant species or mixes that meet the critical objectives and extreme conditions under which plants must survive. Native grasses establish rapidly, with fibrous root systems that tolerate fast-moving water. Herbaceous species play the role of adding aesthetics, texture, and providing wildlife food and habitat. Woody species assist in evapotranspiration, while preventing erosion.
Site Preparation
Invasive species, particularly those that will adapt to wet conditions, should be removed or sprayed before they become incorporated into the site. Normal vegetation can be worked into the topsoil, which should be stockpiled until the final grade has been established.
With the engineer’s specifications and dimensions in hand, on-site construction of the berm and outlets must be executed carefully in order to maintain structural integrity. The infiltration and plant growth areas should be loose and friable, high in organic matter, and completed without compactions from heavy equipment. By using the “dig and drop” method, one can use an excavator to dig and drop each area of the bottom soil in a loose manner. At this point, one can incorporate lime, composted leaves, and/or grass clippings. The excavation machine does not move over the finished surface, thus avoiding unnecessary compaction. Native vegetation can be planted or seeded over this uneven, absorbent surface.
Seeding and Planting Methods
Seeding and planting should begin immediately upon completion of the structure when the soil is still friable and before invasive weeds emerge. Plan seeding and planting before the basin is flooded, or allow the basin to drain to a few inches before seeding. Broadcast seed evenly over each unit by hand seeding or hydroseeding. Seeding rates are generally low (1/2 lb per 1,000 sq ft). The use of a seed filler; i.e., PAM-12, can be used to create a mix of 10 lb per 1,000 sq ft (i.e., 9-1/2 lb of PAM-12 mixed with 1/2 lb of seed), which can be broadcast evenly over the area. Barley, oats, or rye can provide temporary vegetation to protect the soil in storm water management facilities until permanent vegetation can be established. The use of native species; i.e., Virginia Wild Rye, can create an intermediate vegetative cover that succeeds to native long-term vegetation. Straw mulch or straw coconut mats are frequently used to control erosion and protect emerging seedlings from extreme temperatures and drying out. Mulch should be sparse to allow sunlight to reach the ground.
Transplanted seedlings and shrubs need temporary water until they become well rooted. Irrigation of seeded areas is of value until seedlings become established.
General Maintenance
In addition to structural maintenance, siltation needs to be removed as needed. Plants need to be trimmed back to maintain aesthetic value and invasive species need to be controlled. Close mowing or extensive chemical use is not conducive to water quality improvement and wildlife habitat.