Hydrology/hydraulic calculations must be submitted showing how the runoff coefficient was derived, the total tributary area(s), volume derivation, flow rate computations, etc. Any coefficients used for calculations must be justified. At no time will parking lots be used in capacity calculations of basins.
Storage basins that have no outlet except infiltration and evaporation shall be designed to store a twenty-four (24) hour duration one hundred (100) year storm event. In Calhoun County this is a five point five (5.5) inch storm event. Calculations proving the runoff coefficients used that include the entire tributary area(s) (including off-site) and verify the volume of the proposed basin are required. The maximum allowable tributary area to a single retention basin shall be fifty (50) acres. The "developed" tributary area should match the "pre-developed" tributary area(s) as close as possible. No volume shall be credited until the basin bottom is one (1) foot above the seasonal high groundwater level.
Soils are very critical to the success of a retention basin. Soil borings logs for borings taken to at least two (2) feet below the proposed bottom of the retention basin must be supplied with calculations. A minimum of one (1) boring per surface acre of the top of the basin is required. These borings shall be done by a professional regularly engaged in soils investigations and must classify each soil type according to the United States Department of Agriculture classification system. Ground water level(s) must be provided. Soils with a saturated hydraulic conductivity rate < 0.52 inches per hour will disqualify the area for a retention basin. To determine the saturated hydraulic conductivity, tests can be run by a geotechnical company or literature can be provided justifying this number if the literature is acceptable to the Water Resources Commissioner.
Credit for soil infiltration may be granted for retention basin volume calculations only if they are based on one-half (1/2) of the most limiting saturated hydraulic conductivity of the soils in the retention area.
Evaporation is dependent upon many uncontrollable variables such as wind speed, vapor pressure, humidity, etc. Since it is so difficult to practically predict evaporation it will be ignored for retention and detention basin volume calculations.
Detention basins shall be sized by assuring that the post-development runoff flow rate at the point of outlet does not exceed the pre-development runoff flow rate for a 10 year storm. Hydrology calculations showing the pre-development runoff flow rates area required. Hydraulic calculations for the outlet structure are also required. A two (2) foot freeboard is required for detention basins and no volume shall be credited until the basin bottom is one (1) foot above the seasonal high groundwater level.
Drainage facilities within the road right-of-way will be the responsibility of whom ever the roads are dedicated to maintain.
If the development is under a 433 Agreement with the Water Resources Commissioner, the Water Resources Commissioner will have a right-of-way of the drainage facilities within the road right-of-way. The drainage facilities will meet the minimum requirements that the Water Resources Commissioner has established. If another municipality has more stringent rules set then those rules will take precedence.
The design engineer shall supply hydraulic calculations showing how runoff coefficients were derived. Calculations must be easy to follow, legible and accurate. Each individual tributary area to the inlet structures must be designated.
All storm sewer shall be designed to pass a ten (10) year design storm, gravity flow. The roughness coefficient used for the Hazen-Williams formula calculation shall be n=0.013 unless the engineer can prove differently.
2. Open Ditches (Swales)
All open ditches shall be designed to pass a ten (10) year design storm. No overflow is allowed. Erosion concerns must be addressed with erosion control blanket, or rip-rap with a geotextile underlay unless calculations can be provided to prove the full flow velocity will not carry sediments away.
Tributary areas to individual intake structures located in the road pavement should not generate more than one (1) cubic foot per second peak flow for a ten (10) year storm. Multiple inlets may be necessary if the flow exceeds one (1) cubic foot per second.
4. Erosion Protection
All end sections shall have erosion protection. The typical erosion protection in Calhoun County is rock rip-rap with a geotextile underlay. Calculations justifying the minimum size rock will be required.