DESIGN STANDARDS 

Minor Stormwater Systems and Major Stormwater Systems, as defined in the DuPage County Ordinance, shall be designed in accordance with the Illinois Department of Transportation’s (IDOT) Drainage Manual with the following Village-specific amendments. Whenever conflicts arise between the IDOT Drainage Manual and the DuPage County County-wide Stormwater and Floodplain Ordinance (DCSFPO), the latter shall take precedent.

Chapter 4 - HYDROLOGY

• TABLE 4-102a RUNOFF COEFFICIENTS
  • All LAWNS shall be 0.45.
  • All PAVEMENTS shall be 0.90 if asphalt, concrete or gravel (well-graded; many fines)
  • All PAVEMENTS shall be 0.75 if gravel (poorly-graded; few fines)
• 4-200 APPLICATION OF RAINFALL DATA TO HYDROLOGIC METHODS.
  • All references to ISWS Bulletin 70 shall be replaced with its 2019 update, ISWS Bulletin 75.

Chapter 8 - STREET GUTTERS AND STORM SEWERS

• 8-008.01 DESIGN STORM FREQUENCY. 
  • A 100 year design frequency shall be used whenever an overland flow path to the stormwater management facility cannot be provided.
• 8-008.02 TIME OF CONCENTRATION.
  •  If the time of concentration calculates to less than 5.0 minutes (All Roads) / 10 minutes (All Other Surfaces) for any inlet or storm sewer, a minimum time of concentration of 5.0 minutes / 10 minutes, (respectively) is used as the duration of rainfall.
• 8-008.03 MAXIMUM HIGHWATER.
  • The calculated water surface elevation must be kept below the top of the inlet grate and at least two feet below the manhole cover at least one foot below all rim elevations. The overland overflow calculations should be provided if the 100-year HGL surcharges (i.e. HGL is above the rim elevations).
• 8-008.06 TYPE OF MATERIALS.
  • See Construction Specifications.
• 8-009 GENERAL DESIGN PROCEDURE. 
  • An exhibit shall be provided showing the tributary areas to all the structures.
• 8-203 INLET LOCATIONS.
  • Roadways.
    • Maximum encroachment on local roads shall be 8 feet or a maximum spacing of 400 feet, whichever is more restrictive.
    • Locate to prevent surface water from crossing through or around an intersection. 
  • Parking Lots - Commercial/Office/Institutional
    • Maximum depth of ponding shall be 9 inches (100 year).
    • Minimum slope along drainage path shall be 1.0% (asphalt); 0.5% (concrete & gutters).
    • Maximum cross slope shall be 5%.
  • Parking Lots - Industrial
    • Maximum depth of ponding shall be 12 inches (100 year).
  • Residential (Single-units; attached & detached)
    • Side and rear lot inlets shall be placed such that no surface water shall be carried for a distance greater than 275 feet.
    • Channelized storm runoff in excess of 0.5 cfs (10 year) shall discharge into an inlet before crossing a sidewalk or curb unless slopes exceed five percent.  
• 8-204 CAPACITY OF INLETS.
  • When storm sewers are designed to convey the 100-year recurrence intervale, the inlet's grate capacity shall be shown to be sufficient for the tributary area when assuming 50% blockage.
• 8-305.02 TAILWATER.
  • When designing storm sewer that outfalls into a stormwater management facility, the tailwater in the stormwater management facility shall be assumed to be the following:
    • 10-year: Free outfall.
    • 100-year: Use the HWL elevation determined for the 10-year, 24-hour event.
  • When the 100-year discharge is into an existing storm sewer system that is not explicitly modeled, the tailwater in the outfall storm sewer shall be assumed to be the following:
    • <50 acres: Crown of outfall storm sewer.
    • 50-100 acres: Halfway between rim elevation and crown of outfall storm sewer.
    • >100 acres: Rim elevation of outfall storm sewer.

Chapter 12 - DETENTION STORAGE

• 12-003.03 GRADING AND DEPTH.
  • Structures adjoining stormwater management facilities shall be designed so that the finished floor or lowest opening to any habitable area, whichever is lower, is at least 12 inches above the calculated water surface during an emergency overflow situation.
  • Typical side slopes shall be no steeper than four (4) feet horizontal to one (1) foot vertical (4:1).
    • Slopes of three (3) feet horizontal to one (1) foot vertical (3:1) may be allowed, but shall be planted with native vegetation (i.e. no-mow grasses and forbs).
    • Slopes of two (2) feet horizontal to one (1) foot vertical (2:1) may only be used under special circumstances and its extent minimized. The slope's surface cover shall be designed to ensure minimal maintenance and long-term stability.
  • Stormwater management facilities intended to maintain a permanent pool of water (i.e. retention) shall be to a depth of at least 8 feet for a contiguous area no less than 25% of the normal water surface area.
    • A safety shelf shall be provided by maintaining slopes below the normal water line no steeper than four (4) feet horizontal to one (1) foot vertical (4:1) for a maximum depth of three (3) feet, then creating a minimum six (6) foot shelf with a maximum slope of 2% before sloping down again at the necessary steepness.
• 12-003.04 OUTLET WORKS.
  • The invert of the spillway at the outfall should be at an elevation 1 to 2 ft above the maximum design storage elevation. It is preferable to have a minimum freeboard of 2 ft. However, for very small impoundments (less than 2 acre surface area) either a minimum of 1 ft of freeboard may be acceptable or a minimum overflow elevation based on the 500 year storm water elevation shall be provided.
  • The emergency overflow shall be:
    • set 6-inches above the HWL when within an earthen berm to allow for future settlement.
    • constructed as a concrete weir when more than 20 acres of unrestricted, offsite area is tributary and conveyed through the facility.
    • no wider than 10 feet when the calculated depth of flow is less than 1 foot.

EARTHWORK AND GRADING

• MASS GRADING
  • A mass grading plan shall be provided when the Development will exceed 20 acres of land disturbance. The plan shall ensure that overland flow routes to the stormwater management facilities are sufficient based on the elevations of the roadway's subgrade, building pads, etc. This plan shall be in addition to the Stormwater Pollution Prevention Plan (SWPPP).
• OVERLAND FLOW ROUTE.
  • All grading designs shall ensure that overland flow routes are provided to convey the 100-year design storm to the designated stormwater management facility. Cross sections and their supporting calculations shall be provided at all critical locations.
  • The grading plan shall ensure that the following maximum depths are not exceeded during the conveyance of the 100-year design storm:
    • Maximum depth of eighteen (18”) inches in rear yards.
    • Maximum depth of twelve (12") inches in a parking lot.
    • Maximum depth of nine (9") inches in a roadway.
• SLOPES.
  • Minimum swale grades shall be two percent (2%).
  • Maximum slopes within a lot shall not exceed 3:1 measured horizontal to vertical for a ten (10) foot horizontal distance. Special design measures shall be shown if greater changes in grade elevation are required.
• SOIL INVESTIGATION.
  • A soil boring report shall be submitted for all subdivisions that intend to construct public or private roadways. The reports shall include the following information:
    • Unconfined compression strength of the roadways' subgrade;
    • Indicating the expected water table elevation range;
  • Similar data relative to single lots may also be required during the review of grading plans for building construction.
• RESIDENTIAL LOTS.
  • Any openings below the top of foundation and less than three feet above the HWL must be designed to provide watertight integrity to the structure.
  • The finished grade elevation around the foundation shall generally be 0.5 feet below the top of the foundation and the lowest opening unless special conditions exist.
  • Final lot grading shall provide a relatively level area (1:20, 5% maximum slope) in the rear yard for residential use. A minimum distance before major grade changes in residential areas, is 25 feet behind a proposed structure.
  • Where specific house types are shown on the lot-grading plan, grading shall be done to fit a particular structure.  Where a specific house type is not shown, rough-grading elevations should be included and shall use a composite pad elevation.
  • Grading operations must anticipate all utility locations.
  • All unsuitable material must be removed from the proposed building area or as indicated on the soil’s report for the subject development.
  • The minimum slope between the edge of sidewalk and back of curb, shall be two (2) percent.
  • The maximum driveway apron slope or that area between the sidewalk and the public street, or private drive, shall not exceed four (4) percent.
  • The minimum slope between a building elevation and a side yard swale in residential areas is two (2) percent and the maximum slope is six (6) percent.
  • No building foundations will be permitted to extend below expected high water table elevations unless adequate provisions to drain the expected sump pump discharges are provided.
• RETAINING WALLS. 
  • Retaining walls that exceed 3 feet in height, or that are terraced more than 6 feet in height, or are in critical locations (i.e. stormwater management facilities, parking lots, etc.) as determined by the Village Engineer, shall be designed and sealed by a professional structural engineer licensed by the State of Illinois. 
  • Other design requirements may be applicable, such as retaining walls or terracing of the property, depending upon the land plan and topographic conditions of the site.

POST-CONSTRUCTION BEST MANAGEMENT PRACTICES

• NATIVE VEGETATED STORMWATER STORAGE FACILITY 
  • Depth-duration curves for the 2-year, 5-year and 10-year shall be provided and referenced when selecting the plant species.
• VOLUME CONTROL.
  • Void space in aggregate shall not be used for volume control storage unless the aggregate area is adequately protected from sedimentation.
• PERMEABLE PAVEMENT
  • Only interlocking concrete pavers specifically designed by the manufacturer as a permeable pavement is allowed.

CONSTRUCTION SPECIFICATIONS

The materials, installation, and testing shall be in accordance with the Standard Specifications for Water & Sewer Construction in Illinois, 8th Edition (SSWS). Local amendments to Division V are as follows: