(A) The minor drainage system shall provide for capacity to pass the two-year peak flow rate without flooding/ damaging property during the critical 100-year storm event.
(B) The major drainage system shall provide for capacity to pass the ten-year peak flow rate with safe passage of the critical 100-year storm event. Storm sewer design shall be sized on the assumption that they will flow full under the design discharge but will not be placed under the pressure head. Hydraulic grade line calculations shall be performed that demonstrate that sewer rims are not inundated at the design storm.
(C) Culvert and storm sewer design and construction shall conform to the IDOT Drainage Design Manual, unless defined otherwise in this section.
(D) All drainage systems shall provide an overland flow path that will pass the 100-year event flow at a stage at least one foot below the lowest grade, adjacent to a structure, in the vicinity of the flow path. If the drainage area is less than 20 acres, the major drainage system pipes and inlets may be sized for the 100-year flow instead of providing an overland flow path.
(1) The overland flow path shall be protected from any development, such as fencing, landscaping, storage sheds, pools, or other obstructions which could impair its function by impeding flow. This protection shall be established through a properly recorded covenant running with the land, restricting the use of the overland flow path area.
(2) Structures adjacent to an overland flow path shall have a lowest opening elevation six inches above the base flood elevation (BFE) for tributary areas 20 acres or less or one foot above the BFE for tributary areas greater than 20 acres.
(E) Street ponding and flow depths shall conform to IDOT Drainage Manual allowable encroachment requirements but in no case shall exceed curb heights.
(F) The minimum diameter for storm sewers shall be 12 inches.
(G) The minimum diameter for road culverts shall be 15 inches. A minimum of two feet of freeboard shall be provided below the roadway shoulder to the 25-year flood elevation.
(H) All storm sewer pipes under roads or sidewalks shall be reinforced concrete pipe, Class II minimum, conforming to applicable sections of the IDOT standard specifications for road and bridge construction, latest edition. Under grass areas the use of corrugated polyethylene (PE) pipe with a smooth interior is allowed conforming to the applicable sections of the IDOT standard specifications for road and bridge construction, latest edition. The use of any other storm sewer pipe or materials must be approved by the City Engineer.
(I) Culverts and storm sewers constructed in residential subdivisions shall be extended to the rear of the lots. Storm sewers will not be allowed to terminate in the front or middle of a lot.
(J) Permanent erosion control protection must be provided at the ends of discharging storm sewer pipes in which the storm event velocity exceeds two feet per second.
(K) Storm sewers designed with greater than 10% slope or with discharge velocities greater than five feet per second (fps), shall have approximately 20 feet of the pipe constructed at 0.5% slope at the outlet and shall have an erosion control system provided.
(L) Curb inlet and catch basin spacing shall not exceed a maximum gutter drainage flow distance of 250 feet. Inlet and basin spacing shall also be provided as needed to limit stormwater encroachment into the driving lane to six feet during the design storm for a minor drainage system.
(M) All new or improved stormwater 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.
(N) No paved surface overland flow grades and/or slopes shall be greater than 1.0%. Paved surface overland flow grades and/or slopes for swales and ditches shall not have a slope less than 0.3%.
(O) Reinforced concrete paved swales and ditches a minimum of four feet wide are required to be designed and constructed for any drainage swale with a slope less than 1.0%.
(Ord. 2017-03, passed 3-6-2017)