A. Required Stormwater Management Facilities: The controlled release and storage of excess stormwater runoff shall be required in the DuPage County stormwater ordinance and technical manual. All stormwater submittals shall be accompanied by the DuPage County stormwater permit application and applicable sections as required by the permit.
B. Timing Of As Builts And Restoration: All stormwater management areas shall be as constructed field verified immediately upon fine grading, with signed and sealed survey submitted. Upon written approval by the village engineer, all sod, seed and soil erosion blanket to be placed.
C. Unhindered Conveyance To Excess Stormwater Facility: A natural or manmade channel system shall be designed with adequate capacity to convey the stormwater runoff from all tributary upstream areas through or around the development. This system shall be designed to safely carry the 100-year runoff peak generated from a 100-year, 24-hour type II storm. Design of this system shall also take into consideration control of stormwater velocity to prevent erosion or other damage to the facility which will restrict its primary use. In the event that the area within this bypass channel is reshaped or restricted for use as a floodway, the appropriate governmental agency will cause removal or require the removal of such restrictions at the expense of the party or parties causing said restriction.
Should the development contain an existing natural waterway, this land configuration shall be preserved as part of the bypass channel system. All channels or ditches used in the bypass channel system shall have a maximum side slope ratio of four horizontal to one vertical (4:1), and a bottom width adequate to facilitate maintenance and carry the flood or storm runoff without eroding velocities and shall be included in the plans for land development.
D. Method Of Calculating Stormwater Storage Volume: The required volume for stormwater detention shall be calculated by means of an event hydrograph and flood routing method per the DuPage County stormwater ordinance. This volume shall be provided for the excess stormwater runoff that is tributary to the area designated for detention purposes. The stormwater release rate and tailwater condition shall be considered when calculating the stormwater storage actual volume requirement. The control structure shall be designed to not exceed the stormwater release rate. Storage capacity obtained by excavating the natural ground within the 100-year floodplain may not be considered effective for stormwater management. Incremental pond volumes shall be calculated using the conic method (frustum of general cone volume) as follows:
Incremental Volume - 1/3 (Elevation 1-Elevation 2) * [Area 1 + Area 2 + sq rt (Area 1 + Area 2)]
Calculations shall be submitted for approval with the preliminary plat showing the method and parameters used in the determination of the basin size and calculations of the volume and area intended to be utilized on the site. The preliminary plat shall delineate the area to be used for the detention basin as set forth in the submitted calculations.
Public access by easement to and over all detention areas, and a perimeter easement shall be granted to the village and recorded in the subdivision plat, as the village may deem necessary for access. In lieu of a public entity accepting ownership of the detention facility, such detention areas shall be maintained by a special service taxing district or homeowners' association comprising the entire subdivision.
E. Retention: Retention shall be provided below the outlet elevation and shall be planted with native wetland plants per the DuPage County stormwater ordinance or other BMPs provided.
F. Safety Ledges: Safety ledges shall be provided for all wet detention ponds deeper than three feet (3') and shall be a minimum of ten feet (10') in width.
G. Secondary Use Of Detention Basin: Normally, dry detention basins may be designed to serve a secondary purpose such as recreation, open space, or other types of uses that will not be adversely affected by occasional flooding. Dry portions of detention ponds shall have a minimum ground slope of two percent (2%).
H. Maximum Depth Of Water Storage And Minimum Freeboard: Maximum depths of planned stormwater storage within detention ponds shall not exceed the allowable fluctuation in water depth due to site topography and the outlet elevation. The minimum freeboard (elevation difference between 100-year design water level and top of berm) shall be one foot (1'). It shall be the responsibility of the design professional engineer of record to provide slope stability analysis. The elevation at the storage facility high water line shall not create a tailwater which inundates the adjoining upstream property owner greater than predevelopment conditions and shall be a minimum of two feet (2') below any adjacent structures' lowest opening. The storage facility normal water line elevation shall be a minimum of two feet (2') below the basement floor elevation of any nearby structure.
I. Side Slope And Emergency Overflow: The maximum interior and exterior side slopes of the pond berm shall be four (4) units horizontally to one unit vertically. If slopes greater than four to one (4:1) are necessary to meet storage requirements or area restrictions, approval shall be obtained from the engineer for the village, and erosion control shall be provided in addition to the protection required to ensure public health, safety, and welfare. Design of shoreline shall be per the village standard detail.
J. Protection Of Detention Basins: Detention basins next to roads that will exceed four feet (4') in depth shall be protected from vehicular traffic entering the basin by wooden guardrails or nine inch (9") barrier type curb.
K. Outlet Control Structures: Outlet control structures installed shall require little or no maintenance or attention for proper operation. Each stormwater storage area shall be provided with a method of emergency overflow in the event that a storm in excess of the 100-year return frequency storm occurs. This emergency overflow facility shall be designed to function without maintenance or attention for proper operation and shall become part of the natural or surface channel system described in the preceding paragraphs. Hydraulic calculations shall be submitted to substantiate all design features. Both outlet control structures and emergency overflow facilities shall be designed and constructed to fully protect the public health, safety, and welfare. Stormwater runoff velocities shall be kept to a minimum and turbulent conditions at an outfall control structure will not be permitted without complete protection for the public safety.
L. Storage On Paved Surfaces: Paved surfaces such as parking lots or streets that are to serve as stormwater storage areas shall have minimum grades of one percent (1%) and shall be restricted to storage depths of six inches (6") maximum. Rooftop or street storage shall not be permitted.
M. Submission Of Approval Documents: Plans, specifications, and all calculations for stormwater runoff control as required hereunder, shall be submitted to the village as part of the review and approval of the improvement plans.
N. Required Stormwater Management Submittals: All submittals as required by the DuPage County stormwater ordinance and technical manual shall be provided along with the following:
1. Storm sewer sizing tabulation;
2. Storm sewer subbasin drainage exhibit with labeled areas, structures, storm sewer sizes and slopes;
3. A disk or CD containing all computer generated input and output files.
O. Sequence Of Construction: The facilities for stormwater runoff control shall be constructed prior to any site grading, street, or drainage construction and shall include provisions for siltation control.
P. Fee In Lieu: A fee in lieu of detention per article 13 of the DuPage County stormwater ordinance may be considered. (Ord. 2015-02-05, 2-5-2015)