Constructed Wetland
Technical Standards for Rain Gardens
1. Consider permeability, soil types and ground water conditions in the sitting of the facility. Organic soils provide best nutrient retention and substrate for plant growth. Plant material used to colonize this wetland should be selected for hydro-period, resistance to contaminates, stem density, flowing and fruiting characteristics. Use native plants and harvest by trimming every few years to maintain plant vigor and access to sunlight. SA
2. The contributing drainage area must be a minimum of ten (1) acres. The BMP must remain hydrated. Minimum dry weather flow path of 1.5:1 (length:width) must be provided.
3. Depths in BMP should be 45% shallow (0 to 6 inches – high marsh), 35% medium (6 – 18 inches – low marsh), and 10% deep (greater than 18 inches – open water). Maximum depth of the deep water pool shall be ten (10) feet. • Safety ledges needed adjacent to permanent pool areas that are deeper than 3 feet. Maximum temporary ponding depth above permanent pool is 3 feet.
4. A sediment fore bay shall be used at points of concentrated inflows. Inflow energy dispersion is required to distribute flow and prevent short-circuiting. Rip rap may be used for this purpose using locally collect stone.
5. Wetland vegetation that covers 90 percent of the wetland area must be assured after one year through an approved vegetation and maintenance plan. Sediment must be cleaned out if the wetland vegetation coverage is less than 90 %.
6. Volume should accommodate a one (1) year, twenty-four (24) hour storm for the watershed that the wetland is constructed to serve. The wetland area should be comprise two (2) to four (4) percent of the watershed area being served. Length to width ratio shall be 3:1or greater with curvilinear edges to lengthen the distance from the entry point to the exit point. The fore bay is always on the upstream side of the wetland. Make this end accessible for maintenance and collected sediment disposal. Use long stem plants such as iris, cattail or black rush in the fore bay to prevent erosion and to allow sediment to fall out. If not possible use vegetative filters as baffles to lengthen, filter and slow the flow of water through the system. Also, by using islands and peninsulas in the design, the travel path of water can be increased. Vary water depth with at least fifty (50) percent being six (6) deep with some open water areas where the depth of water is at least thirty-six (36) inches to provide diverse habitat.
7. Limit destruction of existing wetlands in order to construct a facility of this kind.
8. Plant the constructed wetland with colonies of a combination of native wetland and facultative plants with such species as Iris, Buttonbush, Cypress, Red Maple, Pickerel Weed, Alligator Weed, Delta Duck Potato, Sedges and Rushes, No Cattails! Refer to Wetland Plants of the New Orleans District, U.S. A. C. E. 1977.
9. A grading plan, planting plan and drainage plan stamped by a landscape architect must be submitted for this comprehensively designed storm water facility that must be designed with naturalized landform shape.
