(A) General. All developments, whether public or private, where curb and gutter is required and at other locations determined by the Director of Public Works and Engineering or authorized designee, shall include provisions for the construction of storm sewers and appurtenances designed in accordance with this chapter. Storm sewer systems and pipe culverts shall be designed accordance with the IDOT Bureau of Design and Environment Manual and installed in accordance with the IDOT Standard Specifications for Road and Bridge Construction, latest edition.
(B) Service areas. All storm sewers, culverts and open channels shall be designed to accommodate all areas which naturally flow to the area of the development and also any additional areas which are planned to contribute in the future to the drainage area, as identified by the Director of Public Works and Engineering or authorized designee. If extending the ultimate service area beyond the natural drainage area limits served by the proposed subdivision results in additional construction costs within the subdivision, a written agreement may be made with the corporate authorities for the recapture by the owner or developer of the additional cost when future system extensions are made. Recapture payments will be made to the owner or developer only after the person benefitting from the drainage area adjustment has made payment to the city.
(C) System extensions. The location of proposed extensions to the existing storm sewer system shall be approved by the Director of Public Works and Engineering or authorized designee.
(D) Basic design standards. Unless otherwise approved by the Director of Public Works and Engineering or authorized designee, the following basic design calculations shall utilize the following methods and criteria:
(1) Hydrology (design flows). Design runoff rates for conveyance shall be calculated using the rational method for watersheds of ten acres or less. Design runoff rates for conveyance shall be calculated using an event hydrograph method for watersheds greater than ten acres. Design runoff rates shall be calculated using the requirements of the Will County Stormwater Ordinance, latest edition, or Illinois State Water Survey Bulletin 75 Northeast Sectional rainfall statistics, whichever is most recent. The elapsed duration time used in determining rainfall intensity shall be equal to the time of concentration (inlet time) plus the time of flow between the most distant inlet and the point in the system under consideration (travel time). Inlet time shall be calculated using TR-55 or similar methodology, but shall not be less than ten minutes. For the rational method, the composite runoff coefficient shall be calculated using 0.95 for all impervious areas and 0.35 for all pervious areas.
(2) Hydraulics (conveyance capacity). Storm sewers and open channels shall be designed to provide adequate capacity using Manning's formula. Culverts shall be designed using inlet control and outlet control culvert design hydrographs, HY-8 or a similar program. Design mean velocity for pipes and open channels shall be at least 2.5 feet per second but shall not exceed ten feet per second, while flowing full.
(3) Storm sewer criteria. Storm sewers serving inlets shall not be less than 12 inches in diameter. Storm sewers serving sump pumps, roof drains and service lines shall not be less than six inches in diameter. Storm sewers shall be designed to convey the runoff from a 10% annual probability (ten-year) storm while flowing full (not surcharged); and shall be designed to convey the runoff from a 1% annual probability (100-year) storm in a surcharged condition, provided that the hydraulic gradeline is not more than six inches above the rim grade at each structure. Storm sewers shall be laid straight in both horizontal and vertical planes between manholes. Storm sewers of different diameters shall join only at structures, and to maintain a uniform energy gradient, invert elevations shall be adjusted to match the pipes at their 80% depth.
(4) Culvert criteria. Culverts shall not be less than 15 inches in diameter. Culverts shall be designed to convey the runoff from a 2% annual probability (50-year) storm with a HW/D value of 1.5 or less.
(5) Inlet criteria. Surface drainage inlets shall be provided so that surface water is not conveyed across any street intersections or parking lot driveways. Surface runoff shall not extend a distance of more than 400 feet along the surface of the ground and shall not create a design flow rate of more than two cubic feet per second for a 10% annual probability (ten-year) storm before being intercepted by drainage inlets. Inlets shall discharge into storm sewers, which shall not discharge into side lot or rear lot drainage ditches or swales. Inlets shall be provided at all low points.
(6) Lot drainage. Tops of foundation shall be six inches above the adjacent grade. The lot grade shall fall six inches within the first ten feet of the building line or lot line, whichever distance is less. Sump pumps shall be connected to the storm sewer system and the point of discharge for sump pumps shall be shown on the subdivision plan for each building served having a basement or crawl space. The overland flow route of stormwater runoff away from each building into swales and storm sewers, and to the location where it leaves the site, shall be shown on the subdivision plan. Positive drainage shall be established for each lot whether or not it is the intention to construct a building on that lot.
(7) Manhole placement. Manholes shall be located at the following locations:
(a) At the termination of all sewers which do not terminate at a catch basin or inlet.
(b) Changes in direction, horizontal or vertical.
(c) Changes in pipe shape or pipe size.
(d) Junctions with other storm sewers.
(e) A storm structure (manhole, inlet or catch basin) shall be located contiguous, abutting or within five feet of the rear lot line of each residential lot unless that lot is contiguous to a detention facility or swale that is at least three feet lower than lowest point on the lot.
(f) Maximum manhole spacing:
1. For six inch to 24 inch pipes, maximum structure spacing equals 350 feet;
2. For 27 inch to 36 inch pipes, maximum structure spacing equals 400 feet; and
3. For 42 inch or larger pipes, maximum structure spacing equals 500 feet.
(8) Estate residential zoning districts. In E-R zoning districts, storm sewers are not required. Culverts shall be reinforced concrete pipe (RCP). The length of culverts shall be at least the width of the pavement crossed plus eight feet plus eight times the depth of the ditch.
(9) Swale criteria. All swales shall be sodded and limited to a maximum water depth of 12 inches. Side slopes of swales shall not be steeper than four horizontal to one vertical.
(10) Open channel criteria. Open channels may be provided on an optional basis in lieu of enclosed storm sewer pipe when the channel will drain an area 80 acres or larger, or in the E-R Estate Residential District when located in the public right-of-way, and as hereinafter specified:
(a) All open channels located within a subdivision or located on public property or easements to the subdivision and other open channels within 150 feet of the subdivision, in the E-R Estate Residential District, shall be improved as follows:
1. Maximum slopes shall be six horizontal to one vertical.
2. A six inch diameter underdrain shall be constructed with a minimum cover of six inches along the flowline of all open channels.
3. An easement for drainage, access and municipal utilities, shall be provided along the open channel with a width adequate to include the area covered by the runoff from a 1% annual probability (100-year) storm.
(b) All open channels located within a subdivision classified in the R-1 Single Family Dwelling District shall be improved as follows:
1. Maximum slopes shall be four horizontal to one vertical.
2. Minimum ditch gradient shall be 0.50%.
3. Minimum ditch depth shall be 24 inches below the adjacent street pavement centerline.
(E) City of Lockport standard drawings. Storm sewer system structures shall meet the requirements shown on the City of Lockport standard drawings.
(Ord. 18-008, passed 2-7-18; Am. Ord. 21-021, passed 8-18-21)