(A) The developer shall provide the development with a storm drainage system that is acceptable to the Village Engineer, which shall include storm water detention. The subdivider, through the subdivider’s engineer, shall submit drainage calculations supporting the storm drainage and detention system signed and sealed by a licensed professional engineer in the state. Included in the drainage calculations shall be a signed statement by the licensed professional engineer that the construction plans account for changes in the drainage of surface waters in the subdivision, as required in the Illinois Plat Act, 765 ILCS 205/2. Language for this statement shall be as follows:
“To the best of my knowledge and belief the drainage of surface waters will not be changed by the construction of such subdivision or any part thereof, or, that if such surface water drainage will be changed, reasonable provision has been made for collection and diversion of such surface waters into pubic areas, or drams which the subdivider has a right to use, and that such surface waters will be planned for in accordance with generally accepted engineering practices so as to reduce the likelihood of damage to the adjoining property because of the construction of the subdivision.”
(B) The storm drainage system shall be adequate to properly drain the development and all other upstream areas that are tributary to the development. Runoff from such upstream areas shall be calculated as if they were fully developed according to their existing zoning classifications.
(C) The storm drainage system shall be designed using a minimum five-year storm at the upstream end of the system and a minimum 25-year storm for the remainder of the system. Drainage calculations to support the storm sewer and storm water detention facility design shall be submitted along with the construction plans. Drainage calculations shall include a narrative and summary of the existing and proposed drainage patterns, surface cover types, times of concentration, flow rates, rainfall intensities and pipe and storage capacities. Said calculations shall be signed and sealed by a state licensed professional engineer. The subdivider is strongly encouraged to utilize recognized best management practices (BMPs), such as conserving open space, reducing imperviousness, utilizing bio-swales, creating depression storage, installing permeable pavement and the like, for reducing the quantity of storm water runoff and improving the quality of storm water runoff to mimic the natural watershed hydrology.
(D) The storm drainage system discharge onto adjacent downstream properties shall be designed so as not to increase the rate of runoff in conformance with the drainage laws of the state.
(E) The storm drainage system discharge points shall be provided with erosion control and velocity dissipation structures adequate to prevent damage to downstream properties, with structures to protect unauthorized entry into piping.
(F) Drainage structures shall be as specified by the village or as approved by the Village Engineer. Storm drain structures shall utilize resilient connectors between storm sewer pipes and laterals as specified in ASTM C 1478.
(G) All storm sewer construction shall be performed in accordance with the State Department of Transportation Standard Specifications for Road and Bridge Construction (latest edition) and supplemental and recurring special provisions. All storm sewers shall be backfilled with approved trench backfill material and compacted by approved methods. All storm sewer pipe shall be gasketed pipe.
(H) All field tiles encountered during construction shall be repaired as directed by the Village Engineer, with the size location, pipe material and depth indicated on the record drawings.
(I) Where field tiles are damaged and it is determined by the Village Engineer that it cannot be repaired, provisions shall be made to re-route field tile to the nearest available storm sewer.
(J) All storm sewer grates, catch basin grates, curb and gutter grates shall be stenciled with “DUMP NO WASTE! DRAINS TO WATERWAYS” message. Any variations in the wording of the message shall be approved by the Village Engineer.
(K) Sump pump discharge lines shall connect to a storm sewer, where available. When a storm sewer is not available, a sump pump collector line shall be provided by the subdivider. All sump pump discharge lines are to discharge groundwater only. The desired location of the sump pump collector line is the rear yard drainage easement. The subdivider’s engineer shall be responsible for sizing the sump pump collector line so that it maintains a minimum velocity of two feet/second. Collector pipe material shall be a minimum four-inch diameter PVC Schedule 40. Sump pump collector lines shall be a minimum depth of 30 inches. Clean outs shall be provided at dead ends, bends greater than eleven and one-fourth degrees and lengths longer than 300 feet. Sump pump discharge line taps that disturb the street pavement are prohibited. Sump pump discharge line details, dimensions and locations shall be provided on the record drawings.
(L) Rear yard drainage swales shall be provided, as needed and as determined by the Village Engineer, to convey storm water runoff from individual lots to the storm drainage structures provided by the subdivider. The rear yard drainage swales shall have a minimum two feet bottom width, four to one (4:1) side slopes and laid at a minimum slope of 0.5%. Permanent elevation monuments shall be established by the subdivider at the summit of each swale for the purposes of recording the swale starting point and elevation, prior to certification of completed construction of the subdivision by the village. Any modification of rear yard drainage swales and resulting conflicts, after conditional acceptance by the village, shall be addressed by the entity that caused the modification.
(M) For tributary areas of five acres or more, storm water detention calculations shall utilize a hydrograph method acceptable to the Village Engineer that generates runoff volumes through inflow hydrographs that account for the watershed features being drained. Inflow hydrographs shall be generated by the design formula using a design storm with a range of rainfall durations (one to 24 hours) associated with a recurrence interval of 100 years (1% chance of occurrence in any given year) and the post development watershed runoff conditions. The critical rainfall duration shall be used to size the detention basin. Rainfall depths and durations shall be as provided by the State Water Survey Bulletin 70 - Frequency Distributions and Hydroclimatic Characteristics of Heavy Rainstorms in Illinois, latest edition. Rainfall distribution shall be as provided for in the Illinois State Water Survey Circular 173 - Time Distributions of Heavy Rainstorms in Illinois, latest edition. The allowable release rate from the detention facility shall be computed using a recurrence interval of 100 years (1% chance of occurrence in any given year) for the storm period calculated by the time of concentration and the pre-developed watershed runoff conditions. The Village Engineer shall have the discretion to require the use of a recurrence interval less than 100 years (1% chance) to protect inadequate downstream infrastructure or sensitive downstream areas.
(N) For tributary areas of less than five acres, the rational method may be used to calculate runoff volumes and allowable release rates as outlined in the latest version of the Illinois Department of Transportation Drainage Manual. The allowable release rate from the detention facility shall be calculated using a recurrence interval of 100 years (1% chance of occurrence in any given year) for the storm period calculated by the time of concentration and the pre-developed watershed runoff conditions. The Village Engineer shall have the discretion to require the use of a recurrence interval less than 100 years to protect inadequate downstream infrastructure or sensitive downstream areas.
(O) Detention ponds serving tributary areas greater than five acres shall include a forebay (sediment trap) to capture sediment and pollutants contained in the storm water runoff. The forebay shall be sized for a range of 15% to 25% of the design capture runoff volume and designed to reduce the incoming channel velocity to less than four feet/second. Where a sediment trap is not practical or feasible as determined in concert with the Village Engineer, other means shall be utilized to control sediment.
(P) Detention ponds shall include an emergency spillway that is designed to safely pass the 100-year critical duration storm event to protect the integrity of the pond embankment.
(Q) Development covenants shall provide village officials or their representative’s access to the detention pond for inspection immediately after completion of construction and annual inspections as required by their national pollutant discharge elimination system (NPDES) permit issued by the Illinois Environmental Protection Agency (IEPA).
(2009 Code, § 41-5-5) (Ord. 10-09-01, passed
10-09-2001
; Ord. 07-10, passed 8-13-2007; Ord. 14-02, passed 4-14-2014)