City of Charlevoix Storm Water Ordinance Design Standards
Temporary Storm Water Management Standards
   (A)   Temporary storm water management facilities shall be installed by the applicant and inspected by the agent before grading, filling or grubbing is initiated.
   (B)   Where permanent storm water management facilities, such as detention or retention basins are proposed for use during construction as a temporary storm water management measure, the construction sequence and grading plan shall be designed for the proper and effective implementation of these facilities.
   (C)   Temporary storm water management measures shall be maintained throughout the duration of the earth change, including the later stages of development. Maintenance activities may include, but are not limited to removal of accumulated sediment, structural repairs, and reseeding or replacement of temporary vegetative covers.
   (D)   Temporary storm water management facilities shall be designed in accordance with the Michigan Department of Environment, Great Lakes, and Energy (EGLE) Best Management Practices (BMP) Guide Book for Michigan Watersheds.
   (E)   At a minimum, during construction all regulated earth changes shall be required to provide temporary storm water management that either contains the volume of runoff generated from a 10-year, 24-hour design storm on-site for all disturbed area, or to provide silt fencing or other permeable barriers that will manage the flow of storm water discharging off-site, diffusing it and releasing it at reduced velocities, where such discharge will not adversely impact downstream properties.
Permanent Storm Water Management Standards - General
   (A)   Storm Water Management Plan Preparation.
      (1)   For regulated activities on sites that exceed one acre, the agent may require the Storm Water Management Plan is prepared by a registered professional as defined by this chapter.
      (2)   For regulated activities on sites that exceed one acre, the agent may request that the submitted site plan be reviewed by one or more registered professionals contracted by the agent. The costs incurred for such review(s) shall be the responsibility of the applicant. The applicant shall deposit with the agent such fees in an amount determined by the agent equal to the estimated costs. The fees shall be held in escrow in the applicant’s name and shall be used solely to pay these costs. If the amount held in escrow becomes less than 10% of the initial escrow deposit or less than 10% of the latest additional escrow deposit and review of the application is not completed, then the agent may require the applicant to deposit additional fees into escrow in an amount determined by the agent to be equal to the estimated costs to complete the review. Failure of the applicant to make any escrow deposit required under this chapter shall be deemed to make the application incomplete, thereby justifying denial of the application. Any unexpended funds held in escrow shall be returned to the applicant following final action on the application. Any actual costs incurred in excess of the amount held in escrow shall be billed to the applicant and shall be paid by the applicant prior to the release of a final decision on the application.
      (3)   If the applicant disputes the agent’s need for outside professional assistance in the review of the submitted plans, or the professional(s) selected, the applicant has the right to appeal the agent’s decision to the Appeals Board, who shall be responsible for making the final decision.
   (B)   On-site storm water management facilities which minimize adverse impact to downstream properties shall be required for all sites unless a proposal for off-site storm water management has been approved. Storm water management facilities may include, but are not limited to: retention basins/ponds, detention basins/ponds, wet basins, storm water treatment units, controlled outfall structures, and rain gardens or other bio-filtration systems.
   (C)   The Michigan Department of Environmental Quality “Urban Stormwater Best Management Practices Manual” will be used as a reference as well as the following manuals: “Controlling Urban Runoff” by the Metropolitan Washington Council of Governments; “Designing Stormwater Quality Management Practices” by the University of Wisconsin, Madison; and the “Design of Stormwater Filtering Systems” by the Center for Watershed Protection.
   (D)   Retention and detention basins shall have an emergency overflow system. The overflow system shall be designed to accommodate flow from a 100-year storm event, or as otherwise required by the appropriate State of Michigan agency.
   (E)   If the storm water facilities for a 50-year storm cannot discharge to a stream, lake or wetland without causing scouring, flooding or pollution on site or downstream, then the basin shall be designed to hold or infiltrate storm water from a 100-year, 24-hour frequency storm event.
   (F)   Sites that are located in areas serviced with a municipal storm system, and that have the prior approval of the municipal system owner, may discharge storm water to that system after it has been treated with an approved separator system that removes 60% of sediments. The applicant shall be responsible for all costs incurred to the municipal system to accommodate any storm water discharged to the system from the site.
   (G)   The rainfall amounts for Charlevoix County shall be the numbers given by the Natural Resources Conservation Service for a 24-hour duration and are as follows: one-year storm equals 1.8 inches; two-year storm equals 2.2 inches; five-year storm equals 2.7 inches; ten-year storm equals 3.0 inches; 25-year storm equals 3.5 inches; 50-year storm equals 3.9 inches; 100-year storm equals 4.2 inches.
   (H)   The maximum grade for the side slopes of any storm water retention or detention basin shall be no greater than 3:1 (horizontal to vertical) for vegetated basins. Where, due to site limitations, this maximum side slope grade cannot be met, the agent may grant an increase in the slope, provided additional stabilization (beyond seed and mulch) is proposed.
   (I)   Storm water basins with permanent pools of water of three foot depth or greater with side slopes steeper than one on six shall have one or more of the following safety features:
      (1)   Safety ledges at the basin perimeter which are at least ten feet wide.
      (2)   Aquatic vegetation surrounding the basin which discourages wading.
      (3)   Fencing to prevent unauthorized access to the basin.
   (J)   Storm water detention basins shall not be constructed in regulated wetlands unless approved by the appropriate State of Michigan agency and/or the Army Corps of Engineers.
   (K)   Storm water detention basins which impound five acres or more and have a head of six feet or more shall meet dam construction permit requirements in Part 315 of Act 451 of 1994, as amended, administered by the Michigan Department of Environmental Quality.
   (L)   Whenever possible, a created wetland or other bio-filtration area shall be incorporated into storm water management facilities to assist removal of soluble pollutants that cannot be removed by conventional settling. Sediment carried off by runoff shall be required to settle out prior to discharge into the created wetland or other bio-filtration area.
   (M)   Storm water management basins designed for retention, detention or infiltration shall be isolated from septic systems and water wells by 50 feet or more. Variations in the required setback may be granted by the Health Department of Northwest Michigan prior to the issuance of a Charlevoix County Storm Water Management Permit.
   (N)   New fueling stations will be required to install an approved separator system for sites that discharge storm water off-site. Existing fueling stations that are modifying more than 25% of their existing impervious surfaces will be required to install an approved separator system if they discharge storm water off-site.
Retention Basin Design
   (A)   Small projects in areas that have less than one-half (0.5) acre of impervious surface shall be allowed to have runoff retention stored at two inches of runoff from all impervious surface areas in lieu of detailed hydrologic calculations.
   (B)   At a minimum, retention basins created in soils with permeability greater than 1.3 inch per hour shall have the storage capacity to hold the increase in runoff volume generated by the earth change.
      (1)   The required volume shall be calculated by comparing the undeveloped conditions for a two-year, 24-hour frequency storm event to the developed condition for a 25-year, 24-hour frequency storm event. Soil permeability rates are listed in the following table:
Soil Texture & Structure
Permeability (Inches/Hour)
Coarse sand and medium sand
6 or more
Fine sand and loamy sand
3 - 6
Sandy loam
2 - 3
Loam, sandy clay loam
1.3 - 2
Clay loam, silt loam, clays, silts, muck, peat, marl
Less than 1.3
         The retention basin shall be designed to drain within 72 hours.
   (C)   At a minimum, retention basins, which are created in soils with permeability less than 1.3 inch per hour, shall be designed to store runoff from back-to-back 50-year, 24-hour rainfall events.
Detention Basin Design Standards
   (A)   When using the Natural Resource Conservation Service Method, the volume of a detention pond is to be calculated based upon a 50-year, 24-hour storm with the developed site conditions and with an allowable outflow of a ten-year, 24-hour storm based upon the pre-existing site conditions or 10% of the flow rate calculated by the 50-year developed site conditions analysis. [The TR-55 program does not accept lower values than 10% of the developed rate.]
   (B)   The allowable peak discharge rate from a permanent storm water management measure may be a staged rate. The maximum allowable peak discharge rate shall not exceed the peak discharge rate from the project site prior to the proposed development for all of the following 24-hour storm events: two-year, five-year, ten-year, 25-year, and 50-year. In no event shall the discharge exceed the ability of the downstream condition to convey the flow without damage to abutting properties.
   (C)   All sites with greater than one acre of impervious surface will require the detention outflow to be directed to approved storm systems or have the approval of adjacent property owners, with documented easements, or one can release at a two-year before construction rate if it can be determined that there is not a flooding hazard on the adjacent property. Low porosity in the soils in the area of discharge and depressions in the land would be examples of reasons to deny detention out-flowing at a two-year rate. Sites that have three acres or more of parking area must in addition have an approved separator system to remove impurities before discharging to the detention/retention pond or install an approved treatment forebay.
Storm Water Separator Design Standards
   (A)   Approved separators are to remove a minimum of 60% of sediments.
   (B)   Treatment forebay criteria - the treatment forebay is designed to store the “first flush” of pollutants typically found in urban storm water runoff, and to capture initial flush pollutant loads.
      (1)   The treatment forebay shall be a wet basin or approved structure with an impermeable bottom and sides to the design high water level.
      (2)   Sizing - the treatment forebay shall be sized to store the water quality volume (Vwq) defined as one-half (0.5) inch of runoff from the directly connected impervious area. This volume can be included in the overall flood control volume.
      (3)   The minimum required water quality volume is given by the equation:
         Vwq = 1815 A 1
         Where:   Vwq   =   Water quality volume (cft)
            1815   =    0.5 inch of runoff x 3,630 to convert ac-in to cft
            A   =   Contributing drainage area (ac)
            1   =   Percent impervious expressed as a ration
      (4)   Capacity for the water quality volume shall be provided above the normal water level.
      (5)   The overflow structure from the treatment forebay shall be sized for the peak inflow from the design rainfall event.
      (6)   The top-of-berm elevation between the treatment forebay and the infiltration basin shall be a minimum of one foot below the outer berm elevation.
      (7)   The treatment forebay shall have a minimum one-foot-deep sump below the inlet pipe for sediment accumulation.
      (8)   The outlet structure from the treatment forebay shall be designed to draw water from the central portion of the water column with the forebay to trap floatables and contain sediments. The top of the inlet structure shall be located a minimum of one foot below the normal water level, and the invert shall be a minimum of one and one-half (1.5) feet above the bottom of the treatment forebay.
      (9)   Material - treatment forebays shall be lined with impermeable materials extending up to the design high water elevation. A minimum 18-inch-thick clay layer, or an impermeable liner protected with a minimum of 12 inches of soil cover are acceptable alternatives. Maximum allowable permeability shall be 0.0001417 inch/hour as determined by a geotechnical engineer for clay placement, or manufacturer’s certificate for line products.
Underground Storm Water Management Facilities
   (A)   If the use of storm water retention or detention basins, either on-site or off-site is not feasible and the permeability of the soils is greater than 1.3 inch per hour, the installation of underground drainage systems (catch basins / manholes with open bottoms with stone and/or run(s) of perforated piping) may be allowed if they provide for detention or retention volumes as stated in these Charlevoix County Storm Water Ordinance guidelines. The perforated piping and dry basin structure(s) cannot be considered to provide for any outflow when calculating volumes for the detention system design. All underground drainage systems must provide the following:
      (1)   Catch basins or separator systems, sediment basins, silt traps for storm water flowing to the underground drainage system.
      (2)   An approved overflow system.
      (3)   Adequate provisions for maintenance.
   (B)   The required detention volume may be reduced by the agent by an amount not to exceed 50% if rain gardens are implemented and demonstrate the ability to accommodate an equivalent amount of storm water.
(Ord. 810, passed 11-18-2019)