(A) Waters. All earth changes subject to review under this chapter shall be designed, constructed and maintained to provide for control of stormwater flow waters and to protect water quality.
(B) Factors. Measures required for stormwater flow control shall take into consideration adverse effects on natural features; proximity of the site to lakes, streams, and protected wetlands; adverse effects on endangered plant and animal species, including their habitat or other sensitive environmental features; extent of impervious surfaces; potential for soil erosion and flooding; and the size of the site.
(C) Facilities. Stormwater flow control facilities shall be designed to provide for non-erosive velocities of stormwater flow.
(D) Drainage. Alterations to natural drainage patterns shall not create downstream flooding.
(E) Watershed plan. When a proposed earth change is located in an area where a watershed plan has been approved by the City Council, the standards for stormwater detention and retention volumes, discharge rates, and stormwater flow control facility locations specified in the approved watershed plan shall be deemed to meet the requirements of this chapter.
(F) Guidelines. Stormwater flow control plans shall conform to the following guidelines:
(1) Stormwater flow control.
(a) Stormwater flow control facilities may include, but are not limited to: retention basins, retention ponds, wet basins, porous pavement with sediment diversion berms, grassed swales with sediment diversion berms, grassed swales with check dams, filter strips, direct connection to existing stormwater drainage systems, and other facilities.
(b) Stormwater control facilities shall be planned and designed to reproduce the predevelopment hydrology of the site to the maximum extent possible.
(c) Infiltration trenches, perforated pipe and infiltration basins may be utilized provided that: (i) sediment is removed from stormwater flow before flow reaches the infiltration facility, and (ii) adequate provisions for facility maintenance have been made.
(d) Infiltration basins and infiltration trenches shall be lined with a vegetative cover designed to slow the flow of the stormwater and to trap pollutants. Sediment traps or sediment basins shall be provided for the purpose of collecting sediment before stormwater reaches the infiltration basin or trench. Infiltration facilities shall be designed to distribute stormwater flow volume evenly over the floor of the basin or trench and to prevent ponding or standing water.
(e) Drainage wells, commonly known as dry wells, shall be discouraged as a stormwater control method. If the use of stormwater retention or detention basins, either on-site or off-site is not feasible, the installation of drainage wells may be allowed. All drainage wells must provide the following: (i) catch basins, sediment basins, silt traps, or vegetative filter strips to remove sediment from stormwater flowing to the drainage well; (ii) an approved overflow system which will adequately protect the environment and/or existing stormwater drainage system downstream from the point of overflow discharge; and (iii) adequate provisions for maintenance.
(f) Detention basins shall be designed as extended detention basins to detain stormwater on the site for an adequate period of time, dependent upon soil type, to allow for maximum settling and removal of suspended solids and other pollutants. Vegetation shall be installed and maintained in the basin to help absorb pollutants.
(g) At a minimum, detention, retention and infiltration basins shall have the storage capacity to hold the increase in stormwater flow volume generated by the earth change. The required volume shall be calculated by comparing the undeveloped conditions from a two-year, 24-hour frequency storm event at 2.2 inches per 24 hours to the developed condition for a 25-year, 24-hour frequency storm event at 3.3 inches per 24 hours. The rational method or the U.S.D.A. Natural Resources Conservation Service method shall be used to determine flow volume.
(h) For off-site discharge of stormwater flow, the rate of discharge shall be determined by the capacity of the receiving drainage system. Calculations, prepared by a licensed professional civil engineer, shall be provided to:
1. Demonstrate the flow capacity of the stormwater drainage system to its point of final discharge to a river or lake.
2. Demonstrate the volume of stormwater flow being carried by the stormwater drainage system at the existing state of development, using the 25-year, 24-hour frequency storm event, prior to adding the proposed development stormwater flow.
3. Demonstrate whether or not the stormwater drainage system has adequate capacity to receive the proposed development stormwater flow.
(i) For proposed developments where parking lots, drive lanes, loading/unloading facilities, and the like, exceed five acres in size, the stormwater flow system for the site shall incorporate soil and sediment collection and oil and gas separation facilities prior to discharge into the stormwater drainage system.
(j) Retention and detention basins shall have an emergency overflow system. The overflow system shall be designed to accommodate flow from the 100-year storm event at 4.4 inches per 24 hours.
(k) Side slopes of any stormwater retention or detention basin shall be no greater than 3:1 (horizontal to vertical) so as to prevent soil erosion and allow for basin maintenance.
(l) Stormwater basins with pools of water 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; or
3. Fencing to prevent unauthorized access to the basin.
(m) Stormwater control facilities that discharge to a stream, lake or wetland shall have an adequately designed discharge system to prevent scouring, flooding or pollution on-site or downstream.
(n) Stormwater retention/detention basins shall not be located in wetlands unless approved by the Department of Environmental Quality.
(o) A ten-foot undeveloped buffer area shall be provided around the perimeter of all detention, retention and infiltration basins, which are one-half acre or more in size.
(p) Stormwater 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, as administered by the Michigan Department of Environmental Quality.
(q) Stormwater retention, detention and infiltration basins shall be maintained by the property owner unless assurance of proper maintenance can be provided through a government agency program. A maintenance plan shall be submitted for approval that as a minimum include, but not be limited to: removal of accumulated sediment, periodic structural repairs, reseeding of replacement of vegetative cover and regular clipping.
(2) Stormwater conveyance facilities and receiving waters.
(a) Unless otherwise approved, stormwater flow shall be safely conveyed through swales, vegetated buffer strips, enclosed conduits, or other approved conveyances so as to control flow velocity and off-site damage, to remove pollutants, to allow suspended sediments to settle, and to encourage infiltration.
(b) If storm sewers are proposed, the applicant shall design the drainage system to mitigate any harmful impact on water quality by using structural devices or other methods to prevent accelerated soil erosion and by locating discharges to maximize stormwater flow control and minimize impact to the receiving drainage system.
(c) No direct or indirect discharge of stormwater to receiving bodies of water, including lakes, streams or wetlands shall be allowed unless sediment and/or greases and/or other physical objects are trapped prior to discharge and stormwater flows are limited to non-erosive velocities.
(d) Lakes and streams, together with their adjacent banks shall not be dredged, cleared of vegetation, deepened, widened, straightened, stabilized or otherwise altered without all applicable local and state permits. Approval from the Department of Environmental Quality is required for proposed alterations of lakes and streams below the ordinary high water mark. Approval from the County Soil Erosion Control Officer is required for proposed alterations of lakes and streams above the ordinary high water mark.
(e) Construction of floor drains, storm drains, drainage wells, septic systems or other conduits by which stormwater or washwater containing oil, grease, toxic chemicals or other hazardous substances may be discharged are prohibited. This does not include occasional and non-deliberate fluid release from vehicles in parking lots.
(3) Engineering design guidelines. Guidelines for soil erosion and sediment control and stormwater flow management facilities shall follow the Best Management Practices of the State of Michigan, Department of Management and Budget, Office of Design and Construction Soil Erosion and Sedimentation Control Plan under Part 91, PA 451, 1994, as amended.
(Ord. eff. 9-6-07; Am. Ord. eff. 12-15-08) Penalty, see § 156.99