- If other agencies having authority over the proposed development have more stringent specifications than these rules and regulations then the more stringent specifications will supersede.
- Submit three (3) sets of plans covering the entire area requesting approval.
- Plan size shall be twenty-four (24) inches x thirty-six (36) inches.
- The minimum scale to be used for all but the overall sheet shall be 1" = 50’ (1" = 60’ or more not acceptable). The overall grading and drainage plan shall have a minimum scale of 1" = 100’.
- Plans shall be sealed by a licensed Professional Engineer registered in the State of Michigan.
- An overall plan shall be provided showing all phases of the development, topography, total tributary area(s) and a general indication of proposed improvements for the phase requesting approval.
- The plans must include the proprietor’s name, telephone number and address.
- Each sheet in the plans shall have the following:
- Designer’s name, telephone number and address
- A proper engineer’s seal
- At least one bench mark with a reference to the datum used
- Sheet number, date and revision date
- North arrow and bar or graphic scale
- Existing structures, utilities and easements
- Any inflow into a storage basin shall have a minimum fall of eighteen (18) inches to the bottom of the basin (invert-bottom'18").
Grading And Drainage Plan
- Show all existing and proposed contours at a minimum of two (2) foot intervals.
- Show flow arrows to designate runoff direction.
- Show inlet and manhole locations.
- Show erosion control measures.
- Designate all 100 year floodplain levels with a heavy dashed line. DEQ regulated floodplains usually are predetermined. All other 100 year floodplains will need be calculated. This includes retention/detention basins and low areas that have no outlet.
- Show specifications for establishing vegetation in all areas disturbed by construction. Seasonal limitations may require seeding during the following spring. If this happens a silt fence will be required in area prone to erosion.
- Show the entire tributary area boundary on this plan for each stormwater storage area. Retention basins, detention basins and self contained low areas all have their own tributary areas.
- Unless a special variance is granted as a result of extensive engineering proof, no single retention basin tributary area shall be larger than fifty (50) acres.
- Show flood routing information; designate what would happen if the storm runoff conveyance systems were overloaded and backed up.
- Show locations of soil borings and provide soil boring logs. A minimum of one boring per acre of storage basin is required. Borings in the stormwater storage basin(s) must extend to at least two (2) feet below the proposed bottom.
- Soil classifications using USDA terminology is required.
- Provide at least one bench mark with a datum definition.
- For all stormwater storage areas show the volume, top elevation, bottom elevation inlet structure details, outlet structure details and freeboard.
- Indicate on the plans where energy dissipation (rip-rap, gabions, etc.) will be constructed.
- Inlets/manholes or intake structures shall be placed so that surface road runoff does not travel more than 300 feet. Exceptions to this rule will only be allowed if the project engineer provides proof that the 1 year storm peak flow to a particular structure is < 1 cubic foot per second (cfs).
- HDPE (High Density Poly Ethylene) smooth interior corrugated plastic pipe may be used as long as it is installed according to the pipe trench detail (Exhibit H) on Page 40 and is inspected during installation by the project engineer. HDPE pipe may not be acceptable under pavements if there is less than three (3) feet or more than eight (8) feet of cover. Class III RCP (reinforced concrete pipe) and CSP (corrugated steel pipe) gauge minimum are acceptable pipes for storm sewer.
- Inlets, manholes, piping and end section shall be specified according to Appendices C, D, E, F, G & H.
- Minimum pipe cover shall be eighteen (18) inches for twelve (12) inch pipe and more as pipe size increases. The project engineer will need to provide proof that the cover proposed is acceptable under Calhoun County Road Commission specifications.
- If side slopes of a storage basin are steeper than 4 to 1 (4 horizontal to 1 vertical) or if the stormwater is one (1) foot or deeper for more than twenty-four (24) hours a fence around the entire basin is required. Any proposed fence must be shown on the grading plan.
Construction Details And Specifications
If additional information describing the proposed construction is necessary a detail sheet is recommended.
1. On-site retention will be required of all subdivisions unless a positive outlet is acquired to a natural stream approved by the DEQ or to a county drain approved by the Water Resources Commissioner. Retention basins can be man-made or natural. If the area is a wetland, a DEQ permit will be required. All basins must utilize the following specifications.
2. Retention basins will be designed to hold a 100 year storm event in a twenty-four (24) hour period. In Calhoun County this is five point five (5.5) inches of rainfall within 24 hours.
3. To property size the basin the following items will be taken into consideration:
a. Soil borings "required to groundwater".
b. Soil type.
c. Total tributary area.
d. Coefficiency runoff calculations of the tributary area.
e. Infiltration rate calculations.
f. Floodplain elevation for area.
g. Design of basin natural or constructed.
h. Basin must have eighteen (18) inches of fall from the invert of the inlet pipe to bottom of basin. Bottom of basin has to be at least one (1) foot above the seasonal high groundwater level set by project engineer.
i. Evaporation is not considered.
j. Use of parking lots for retaining water is prohibited.
A detention basin can be used when a suitable outlet can be obtained that does not run water off from the development more than the pre-development runoff.
All drainage basins that are subject to water standing over 1 foot deep for twenty-four (24) hours and/or less than 4 to 1 slopes shall be fenced with approved fence. A sixteen (16) foot gate will be provide at an approved location.
Water Quality Design
All stormwater management systems proposed for developments or redevelopments of one (1) acre or more in size shall be designed so that stormwater is managed based on the following criteria:
- The minimum treatment volume shall be the first half (½) inch of rainfall on impervious surfaces. Water quality treatment methods shall be designed to achieve a minimum of 80% removal of Total suspended Solids (TSS) as compared with uncontrolled runoff, and so that discharge concentrations of TSS do not exceed 80 milligrams per liter (mg/l).
- Detention basins and any other system proposed to discharge stormwater off-site must restrict the peak discharge rate of the post-development 25 year storm whose duration equals the time of concentration, to the peak discharge rate of the pre-development 25 year, 24 hour storm event.