STORMWATER SERVICES

Storm drain inlet protection devices remove sediment from storm water before it enters storm sewers and other downstream areas. Inlet protection devices are sediment barriers that can be constructed of washed gravel or crushed stone, geotextile fabrics and other materials that are supported around or across storm drains.

Due to their poor effectiveness, inlet protection on storm drains need continual maintenance during the site’s construction phases, and should always be used in conjunction with other sediment controls.

A construction entrance is a stabilized pad of stone underlain with geotextile, used to reduce the amount of mud tracked off-site with construction traffic. Construction entrances should be installed at any points of ingress/egress, and all construction traffic should adhere to using them. Since most of the mud is flung from tires as they reach higher speeds, construction entrances should not be the only practice relied upon to manage off-site tracking.

Common issues observed with installations of construction entrances are the lack of geotextile being applied, incorrect size of stones used, and thickness of stone layer not being sufficient. Construction entrances must be installed first, before any grading activities occur, and should be no less than 70 feet in length. An ODOT #2 stone layer at least 6 inches thick shall be applied over a geotextile layer. The use of geotextile under the stone helps to prevent potholes from developing and will save the amount of stone needed during the life of the practice.

Sediment basins are temporary settling ponds that release storm water runoff at a controlled rate. The basin is designed to slowly release runoff, detaining it long enough to allow most of the sediment to settle. The typical components of a sediment basin are the embankment, pool area for water and sediment storage, principal and emergency spillways, and a skimmer.

As with many other sediment controls, sediment basins should be constructed as a first step in any land disturbing activities, and shall remain in good working order until any area contributing runoff is stabilized with dense permanent vegetation. Often times, sediment basins are converted to detention basins for post-construction storm water management.

A sediment trap is a temporary settling pond formed by construction of an embankment and/or excavated basin with a simple outlet structure. The outlet structure is typically stabilized with geotextile and rip-rap. The purpose of a sediment trap is to detain sediment-laden storm water runoff, and allow the majority of the sediment to settle out. They are established early in the construction process using natural drainage patterns and favorable topography to minimize grading. Sediment traps are to be used for 5 acres or less of contributing drainage area (Larger areas require a properly constructed sediment basin with controlled outlet structure).

Temporary seeding or mulching must be applied to exposed soil where additional work (grading, etc.) is not to occur within 14 days to provide temporary soil stabilization. Temporary seeding and/or a protective layer of straw mulch protects bare soils from erosion by shielding it from raindrop impact. The most effective way to avoid using temporary soil stabilization is to phase construction activities. This is achieved by leaving at least half of the site undisturbed, or re-stabilizing areas of the site prior to additional grading operations.

Permanent vegetation stabilizes soil, reduces erosion, prevents sediment pollution, reduces storm water runoff by promoting infiltration, and provides storm water quality benefits offered by dense grass cover. Any disturbed areas that are at final grade or will remain idle/dormant for one year or more must have permanent seeding applied within 7 days of the most recent disturbance. If the area happens to be within 50 feet of a stream or other surface water of the state, seeding must be applied within 2 days of reaching final grade.

The most common issues we see are the timing of application of seed, and the lack of mulching or erosion control matting used. Permanent seeding includes site prep, seedbed prep, planting, AND mulching. On areas where soils may be easily erodible such as slopes greater than 3:1, it is necessary to use either an erosion control matting, or apply hydraulic seeding with a tackifier.

Erosion control matting is applied after seeding to stabilize the soil while vegetation becomes established. Erosion control matting is also useful in areas where straw mulch is difficult to hold in place due to wind or water, or in areas where the soil is prone to drying. Matting not only prevents erosion, but holds in moisture as well, accelerating seed germination.

Concrete trucks are not permitted to wash out directly into storm sewers, streams or drainage channels. Washing into a nearby storm drain is considered to be an illicit discharge. The purpose of installing a concrete washout pit is to collect and retain all the concrete washout water and solids in leak proof containers, so that this caustic material does not reach the soil surface and then migrate to surface waters or into the ground water. The washwater that is retained will evaporate, leaving only hardened cementitious solids which can then be recycled. As with all best management practices (BMPs), regular maintenance is required to ensure effectiveness.