Storm Water Performance Standards
For any storm water design methodology to be enacted into local storm water design codes there must be methods to measure performance. These standards are generally enacted to meet local storm water conditions as well as the economic and political climate of the community. Common metrics to measure the performance of an on-site storm water management procedure can include a. site analysis, b. storm water, flow characterization, c. TSS control, d. Volume control, e. Capture Rate, f. Run off rate, g. Non-point pollutant abatement and h. Land disturbance limitation.
Site Analysis (SA) for storm water management requires the use of a topographic map. This drawing must be prepared by a registered land surveyor in which all critical site information must be accurately located. All high points, low points, site in-falls and out-falls must be identified. Critical topographic spot elevations must be found for existing features and along each property line. Slopes must be plotted with topographic lines at a one foot contour interval and slopes to steep to developed must be noted. Important site features such as drainage ways, wetlands, and unique or regulated habitats must be defined. Trees, tree groves and other important site feature mentioned in the landscape code must be identified by species, health and dbh for all trees over six (6) inch caliper at four and one half feet above grade. Finally surface conditions according to standard run off coefficient amounts must be noted.
A pre-application discussion with the city storm water Engineer is encouraged to determine the limits of storm water site analysis and to define all storm water design criteria. Several elements of the site analysis include but are not limited to the following are helpful.
- Land use, density, impervious surface area, and total area of the development site;
- Location, topography, on-site and off-site drainage conditions;
- Upstream and/or downstream volumes, discharges and velocities;
- Backwater impacts, effects on existing upstream and/or downstream drainage conveyance facilities;
- Ability of natural drainage channel to convey additional volume, discharges and velocities;
- Adjacent public storm water conveyance devises, sizes, volumes and storm water disposal sites;
- Potential mitigation measures; and
- Delineation of perennial and intermittent streams, jurisdictional wetlands, water table elevation, soil types, and regulatory FEMA Special Flood Hazard Areas.
This site analysis and the conversation becomes the basis of the design of the site plan, grading plan and drainage plan. Elevations and slopes can be used to predict how the existing storm water train operates and the amount of storm water that flows onto and off from the site. The drawing and its elevations, slopes and surface conditions can be used to calculate run off both during predevelopment, and after development. This volume in gallons or cubic feet per second (cfs) becomes the mathematical base for all storm water design calculations.
TSS Control.(TS) Storm water treatment methods and or facilities must be designed to achieve a standard for the capture and elimination of total suspended solids, TSS. TSS is listed as a conventional pollutant in the U.S. Clean Water Act and includes such elements as solids in water that can be trapped by a filter of a designated size. TSS can include a wide variety of material, such as silt, decaying plant and animal matter, industrial wastes, and sewage all of which impair water quality. A removal rate of 85% is recommended to apply to the volume of post development run off resulting from the design storm’s one inch of precipitation. Storm water BMPs can capture various amounts of TSS including:
- Dry Ponds 47%
- Wet Ponds 80%
- Sand Filters, French Drains 95%
- Vegetative Filter Strips 86%
- Planted Storm Water Buffers 86%
- Bioswales 81%
- Wetlands 76%
- Preserved Forest Floor 95%
Volume Control. (V) Most community drainage ordinances require that storm water run off from developed sites not exceed the amount from the pre-developed site for a design storm of a specified frequency or duration. This may be a storm of a two (2) year frequency for a twenty-four (24) hour storm event. But a community landscape code adapted to on-site storm water management can set a higher standard by requiring a specific capture rate (CR) that will hold a specified percentage of all rainwater falling on the development site for a specified rainstorm. This water can be captured in various storm water BM Ps including:
- Parking Lot detentions 20%
- Rooftop Recycling 40%
- Rain Gardens or Rain Groves 20%
- French Drains and Sand Filters 15%
- Wet Ponds 45%
- Preserved Wetland 75%
Capture Rate.(CR) The capture rate is the amount of rain fall that shall be retained and infiltrated, evaporated or transpired through plant material from each storm. This water quality criterion can be achieved with the use of storm water BMPs. If rain fall from a roof and parking lot from a one (1) inch design storm is captured and retained on site easily sixty (60) percent of a one (1) or two (2) year design storm can be captured. At the very minimum, each development site must retain a minimum of thirty (30) percent of the design storm and remove it from the storm water train to prevent the movement of non-point pollutants to down stream locations.
Run- off Rate.(RO) Obviously, if the minimum capture rate is thirty (30) percent the run off rate is seventy percent. This amount of run off should be filtered in storm water BMPs such as grass channels, bioswales, storm water diffusers, grassed filter strips and riparian buffers prior to its release at the site out-fall to the storm water train. The post-development storm water runoff rate leaving the site shall not exceed the pre-development (existing conditions) storm water runoff rate leaving the site for the local 2-year, 24-hour storm event.
Non-point Pollutant Abatement. (NPA) Other chemicals, bacteria, compounds and even viruses are contained within storm water run off. The EPA has defined over one (1) hundred components of urban waste that can have detrimental affects on man kind if collected in large concentrations or are allowed to enter potable water of the food chain. Each development site in urban areas shall be required to interrupt the flow of these pollutants. Each community should set standards for the capture and retention of the following;
- TSS
- Phosphorus
- Nitrogen
- Nitrate and Nitrate Nitrogen
- Copper
- Zinc
These elements can be removed in significant amounts with the use of the Dry Ponds, Wet Ponds, French Drains, Dry Wells, Sand Filters, Riparian Buffers, Micro-detentions, Bioswales and Constructed Wetlands
Land Disturbance Minimization. (LD) Limiting the disturbance of land is one of the best methods of managing storm water on development site. If a site can be developed using low impact development strategies that preserve trees, wetlands, grassy meadows and native habitat then nature’s system of storm water management can provide benefits to the community. A community landscape code should require as a minimum that the following critical landscape features not be destroyed in the development of property;
- Wetlands
- Unique habitats of mixed and layered forest
- Steam side edges
In addition, a community landscape code based upon storm water management should require that a minimum of twenty(20) percent of the site not be disturbed through clearing, grubbing, regarding, cut or fill and that this land be set aside as property line buffers, street side buffers and connected site open spaces that can be used for the introduction of vegetation based storm water BMPs such as landscape storm water buffers, riparian buffers, bioswales, rain gardens, and rain groves or the preservation of existing tree canopy coverage.
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