(A) Storm sewers and inlets.
(1) Design methodology.
(a) All storm sewer piping systems, roadway ditches, and appurtenant pipe culverts shall be sized utilizing the “Rational Method” of design as described in Chapter 4, Hydrology, of the State Department of Transportation’s Drainage Manual for drainage areas less than 200 acres.
(b) Storm sewer pipes, pipe culverts and inlet devices shall be provided to satisfactorily accommodate the surface runoff incident to the 25-year design storm event.
(c) The hydraulic grade line of the system at any inlet or storm manhole shall not be higher than two feet below the inlet sill or the top of a manhole or junction box.
(d) An overland flow path shall be provided to safely pass flows which cannot be accommodated by the design storm event.
(2) Rainfall intensities. All design rainfall events for storm sewer design shall be based or the State Water Survey’s Bulletin 70 (refer to Table 13, Sectional Frequency Distributions for Storm Periods of 5 Minutes to 10 Days and Recurrence Intervals of 2 Months to 100 Years). The total rainfall value for the design storm shall be adjusted for the “St. Louis Urban Effect” as given in Table 4, State Water Survey’s Circular 172.
(3) Runoff coefficients. The proposed impervious surface (paved and building areas) runoff coefficient shall be 0.95 and the runoff coefficient for lawns shall be 0.20. Wooded and forested area runoff coefficients shall be 0.12. For a typical subdivision, the runoff coefficient shall be determined by calculating a composite street and lot coefficient for the area that flows toward the street. A separate composite lot coefficient may be computed for yard inlets. If the engineer wishes to use only one coefficient for the storm sewer design, the most conservative composite runoff coefficient shall be used.
(4) Storm sewer materials.
(a) Storm sewer pipes located under paved surfaces and within right-of-way shall be reinforced concrete pipe with rubber gasket joints and with sufficient wall thickness and reinforcement to carry the design loading, and with materials conforming to the requirements of ASTM C-76 reinforced concrete culvert, storm drain and sewer pipe with joints conforming to ASTM C-443 joints for circular concrete and culvert pipe utilizing rubber gaskets.
(b) The type of storm sewer pipe materials used when replacing or repairing existing storm sewer pipes shall be at the discretion of the Village Engineer.
(c) Storm sewer pipes and pipe culverts located outside of paved surfaces and right-of-way may be of any of the following materials:
1. Reinforced concrete pipe with sufficient wall thickness and reinforcement to carry the intended loading and meeting the requirements of ASTM C-76 reinforced concrete culvert, storm drain and sewer pipe;
2. Pre-coated, galvanized steel pipe meeting the requirements of AASHTO M245-78 and M246; provided, the plate thicknesses conform to IDQT standard specifications;
3. Aluminized steel Type 2 corrugated culvert pipe meeting the requirements of AASHTO M274 and M36 and conforming to IDQT standard specifications; and
4. Polyethylene pipe conforming to AASHTO M294-94. This pipe shall have an integrally formed smooth wail interior. Joints shall be elastomeric gasketed in conformance with ASTM F477-93. Pipe shall not be exposed to direct sunlight for more than six months. Pipe shall have metal end sections.
(5) Storm sewer construction.
(a) Properly sized storm sewer(s) shall be provided to carry surface runoff from streets and curb and/or gutters and shall be of sufficient length that they transport the runoff at least 60 feet from the street pavement back of curb.
(b) The minimum diameter of storm sewers shall be 12 inches and the minimum diameter of pipe culverts shall be 15 inches.
(c) The minimum and maximum gradients of pipes/storm sewers shall conform to the following criteria.
1. Minimum grade shall be 0.3%.
2. There is no maximum grade; provided that, erosion control devices shall be utilized for all outfalls of storm sewers or pipe culverts in accordance with the design requirements of HEC-11.
(d) All site drainage design, including outfalls of storm sewers and storm water detention facilities, shall take into account the effects of downstream tailwater conditions.
(e) All trenches for storm sewer pipes and pipe culverts that are located under paved surfaces shall be properly backfilled and properly compacted to a density not less than 95% of maximum density, as defined in ASTM D2321. Manholes shall be constructed as depicted in Ch. 154, App. A, of this code of ordinances.
(6) Inlet/catch basins construction.
(a) Inlets/catch basins shall be constructed in accordance with the standards depicted in Ch. 154, App. A, of this code of ordinances.
(b) The spacing for inlets/catch basins shall be calculated so that the runoff from the design storm shall not encroach upon the pavement by more than five feet from the back of curb; provided, however, that, under no circumstances shall such inlet/catch basin spacing exceed 300 feet. Calculations should include curb and gutter and inlet capacity and consideration of bypass flows
(c) The intersection of inlet tops with sidewalks should be detailed for each location to ensure continuity in the sidewalk jointing, expansion joint placement and final surface elevation.
(B) Cross-road culverts, bridges and channels.
(1) Design methodology.
(a) In general, all cross-road culverts, bridges and channels shall be sized utilizing the USGS Regression Equation. The rural USGS Regression Equation shall be utilized for watersheds from 0.02 square miles to 10,000 square miles. In urbanized areas, the urban USGS Regression Equation methodology shall be utilized for watersheds from 0.7 to 630 square miles. Since the USGS Regression Equation may produce conservative results in watersheds less than 320 acres, it may be necessary to compare the results computed to other methodologies such as the Rational Method. SCS TR-55 or HEC-1 computer analysis. In complex watersheds, SCS TR-55 or HEC-1 Flood Hydrograph Computer modeling shall be utilized to compute peak storm water flows. A HEC-RAS analysis shall be developed to size the structures or channel and establish water surface profiles created by the proposed drainage system based on the aforementioned hydrologic models.
(b) Watersheds over one square mile are subject to review by the state’s Department of Natural Resources (IDNR), Office of Water Resources (OWR), and should be designed under the specific requirements of the INDR\OWR.
(c) All cross-road culverts, bridges and channels shall be provided to satisfactorily accommodate the runoff incident to the 100-year design storm event. A minimum of two feet of freeboard shall be provided below roadway shoulders to the 100-year flood elevation, taking into account the headwater and tailwater effects of the culvert.
(d) For a new culvert crossings, the water surface profile increase shall not exceed one-half foot at the structure, nor 0.1 foot at a point 1,000 feet upstream of the structure as determined by the horizontal projection of the maximum created head and the slope of the hydraulic grade line.
(2) Rainfall intensities.
(a) For HEC-1 and SCS TR-55 modeling of cross-road culverts, channels and bridges, the design rainfall intensities shall be based on State Water Survey Bulletin 70 (refer to Table 13, Sectional Frequency Distributions for Storm Periods of 5 Minutes to 10 Days and Recurrence Intervals of 2 Months to 100 Years). The total rainfall value for the design storm shall be adjusted for the “St. Louis Urban Effect” as given in Table 4, State Water Survey Circular 172.
(b) Rainfall shall be distributed utilizing the Huff distributions in State Water Survey Circular 173 (Table 3, Median Time Distributions of Heavy Storm Rainfall at a Point). The first quartile point rainfall distribution shall be used for durations less than or equal to six hours. The second quartile distribution shall be used for storms greater than six hours and less than or equal to 12 hours. The third quartile point rainfall distribution shall be used for durations greater than 12 and less than or equal to 24 hours. The fourth quartile distribution shall be used for durations greater than 24 hours. All quartiles should be run for each required frequency to determine the controlling peak storm duration for each frequency.
(3) Cross-road culvert materials.
(a) Cross-road culverts shall be reinforced concrete, pipe with rubber gasket joints and with sufficient wall thickness and reinforcement to carry the design loading, and with materials conforming to the requirements of ASTM C-76 reinforced concrete culvert, storm drain and sewer pipe with joints conforming to ASTM C-443 joints for circular concrete and culvert pipe utilizing rubber gaskets, or reinforced concrete box culverts conforming to ASTM C-789 pre-cast reinforced concrete box sections for culverts, storm drains and sewers, or ASTM C850 pre-cast reinforced concrete box culverts, storm drains and sewers with less than two feet of cover subjected to highway loadings.
(b) All cross-road culverts shall be constructed with appropriate end sections conforming to the roadway embankment slope or be constructed with headwalls.
(c) The type of culvert pipe material used when replacing or repairing existing cross-road culverts shall be at the discretion of the Village Engineer.
(4) Drainage swales.
(a) Natural drainage swales may be utilized to accommodate surface runoff providing they are located near lot lines and the flows induced therein do not pose health or safety hazards for residents.
(b) In general, all drainage swales having less than 1% slope shall be paved with portland cement concrete or shall utilize storm sewers to transport storm water flows.
(c) The minimum and maximum gradients of earth drainage ways and/or swales shall conform to the following criteria.
1. Minimum grade shall be 1%, which may be reduced to 0.35% if paved with concrete, or as approved by the Village Engineer.
2. Maximum grade shall be 3% with no protection other than sod, or 12% with paving, rip rap and/or energy dissipaters.
(d) All new or improved storm water drainage swales created in new developments shall not allow overland drainage to exceed 250 feet without being captured by a storm sewer system. The velocity of flow in these drainage swales shall not exceed five fps unless measures are taken to avoid erosion.
(e) Area inlets shall be required to intercept flows greater than four cfs in paved or un-paved swale areas.
(C) Storm water detention facilities. In order to protect downstream property from potential damages by increased flow rates or greater velocities, the village may require the installation of drainage detention facilities, where such facilities are deemed necessary in the interest of public safety and welfare. All detention facilities shall be located in outlots.
(1) Design methodology.
(a) Storm water detention systems for new developments or re-developments as a minimum shall be designed to control the peak rate of discharge from the property for the two-year, 24-hour and 100-year, 24-hour storm events to discharge rates at or below those which existed prior to development utilizing approved runoff hydrograph methods as outlined herein. In addition, the storm water detention system shall be designed to control the peak rate of discharge from the property for the one-hour, two-hour, three-hour, six-hour, 12-hour and 24-hour storm event, whichever is determined to be the most critical. Additionally, the discharge from a storm water detention facility shall not cause an increase in flooding or channel instability downstream, when considered in aggregate with other developed properties and downstream drainage capacities.
(b) All hydrologic/hydraulic drainage calculations shall be accompanied by the “Engineer’s Hydraulic/Hydrologic Drainage Summary and Certification”, as listed in Ch. 154, App. A, of this code of ordinances.
(c) For detention basins with drainage areas less than 200 acres the modified rational method as described in the IDOT Drainage Manual in Chapter 4, Hydrology, and Chapter 12, Detention. In addition, the existing and proposed impervious surface (paved and building areas) runoff coefficient shall be 0.95, and existing and proposed agricultural, pastures, wooded areas and lawns shall be 0.20. Wooded and forested area runoff coefficients shall be 0.12.
(d) Detention basins with more complex watersheds or watersheds exceeding 200 acres shall be analyzed utilizing a hydrograph routing method that is generally acceptable to the State Department of Natural Resources, Office of Water Resources, such as the Soil Conservation Service TR-20 and TR-55 Methodology, or the Corps of Engineers HEC-1 computer model to develop inflow and outflow hydrographs for the existing and proposed condition runoff and for routing through the detention basin.
(2) Rainfall intensities.
(a) Design rainfall events for storm water detention design utilizing the “Rational Method” of analysis shall be based on the State Water Survey’s Bulletin 70 (refer to Table 13, Sectional Frequency Distributions for Storm Periods of 5 Minutes to 10 Days and Recurrence Intervals of 2 Months to 100 Years). The total rainfall value for the design storm shall be adjusted for the “St. Louis Urban Effect” as given in Table 4, State Water Survey Circular 172.
(b) For SCS-TR-55 and TR-20 methodology and HEC-1 methodology, rainfall shall be distributed utilizing the Huff distributions in State Water Survey Circular 173 (Table 3, Median Time Distributions of Heavy Storm Rainfall at a Point). The first quartile point rainfall distribution shall be used for durations less than or equal to six hours. The second quartile distribution shall be used for storms greater than six hours and less than or equal to 12 hours. The third quartile point rainfall distribution shall be used for durations greater than 12 and less than or equal to 24 hours. The fourth quartile distribution shall be used for durations greater than 24 hours. All quartiles should be run for each required frequency to determine the controlling peak storm duration for each frequency.
(3) Storm water detention basin construction.
(a) Where detention or retention basins are to be used as part of the drainage system for a property, they shall be constructed as the first element of the initial earthwork program. Any eroded sediment captured in these facilities shall be removed by the applicant on a regular basis and before project completion in order to maintain the design volume of the facilities.
(b) When practical, flows from off-site upstream areas should be bypassed around detention basins, whenever possible. The piping should be designed to pass the 100-year storm event based on maximum land use that is consistent with the current zoning or adjacent land use trends. If off- site flows are directed into the detention basin, allowable release rates for the pertinent watersheds and sub-watersheds shall not be modified above existing condition runoff rates.
(c) The placement of a detention basin in the flood fringe area shall require compensatory storage for one and one-half times the volume below the base flood elevation occupied by the detention basin including any berms. The release from the detention storage provided shall still be controlled consistent with the requirements of this section. The applicant shall demonstrate its operation for all stream-flow and floodplain backwater conditions. Excavations for compensatory storage along watercourses shall be opposite or adjacent to the area occupied by detention. All floodplain storage lost below the existing ten-year elevation shall be replaced below the existing ten-year elevation. All floodplain storage lost above the existing ten-year elevation shall be replaced above the existing ten-year flood elevation. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse.
(d) Detention basins should have an emergency spillway for conditions that exceed the storage volume. The maximum ponding elevation shall be calculated based on a routing of the 100-year design storm assuming the low-flow outlet is blocked with water ponded to the overflow structure’s sill. A minimum of one foot of freeboard shall be provided from the maximum ponding elevation to the top of the basin.
(e) Where a single pipe outlet or orifice plate is to be used to control discharge, it shall have a minimum diameter of four inches. Smaller basins may install a smaller rectangular or v-notch weir to control discharge. If this minimum orifice size permits release rates greater than those specified in this section, and regional detention is not a practical alternative, outlets, structures such as perforated risers, or flow control orifices shall be used.
(f) All ends of pipes discharging into a dry basin shall be connected with the low-flow pipe (outflow structure) by means of a paved swale. The paved swale shall be non-reinforced concrete, four inches thick, with a minimum 2% slope to the center and a minimum 0.35% longitudinal slope. Paved swales shall be a minimum of four inches deep and three feet wide or 1.3 times the diameter of the pipe entering the basin, whichever is greater, and be keyed to the structure. The bottom of the basin shall be sloped a minimum of 2% towards the concrete swale.
(g) The maximum planned depth of storm water stored shall not normally exceed four feet.
(h) The maximum side slopes for grassed dry basins shall not exceed one foot vertical for three feet horizontal (3H:1V slope). The maximum side slopes for grassed wet basins shall not exceed one foot vertical for five horizontal (5H:1V slope). All wet basins shall have a level safety ledge at least four feet in width two and one-half to three feet below the normal water depth.
(i) In no case shall the limits of maximum ponding be closer than 30 feet horizontally from any building, and less than two feet vertically below the lowest sill elevation.
(j) In no case shall the limits of maximum ponding be closer than ten feet, plus one and one-half times the depth of the basin from the right-of-way. If there is an easement behind the right-of- way, then this distance shall be measured from the edge of the easement.
(k) The maximum storm water ponding depth in any parking area shall not exceed six inches for more than one hour.
(Prior Code, § 19-327) (Ord. 1448, passed 4-3-2006; Ord. 1700, passed 3-17-2014)