(A) Capacity.
(1) Minor stormwater conveyance systems shall be sized to convey the 10-year runoff.
(2) Major conveyance systems shall be sized to carry the 100-year runoff.
(B) Storm sewers and swales.
(1) The 10-year design storm shall be used as a minimum for storm sewers and minor swales.
(2) Storm sewers shall contain the 10-year storm within the pipes and manholes.
(3) Roadside ditches and swales shall contain the 10-year storm without water encroaching on the pavement.
(4) Storm sewer design shall be based on full flow conditions; otherwise, hydraulic grade calculations shall be performed that show that the rims are not inundated at the design storm.
(5) Storm sewers and swales shall not connect to sanitary sewers.
(C) Existing systems.
(1) Field tile systems disturbed during construction must be reconnected by those responsible for their disturbance unless an approved drainage plan includes provisions for disturbing system. All abandoned field tiles shall be removed in their entirety.
(2) Storm sewers and swales may connect to existing drain tiles or storm sewers only if the applicant submits a maintenance agreement, recorded easements and a report that indicates the existing system has adequate hydraulic capacity and structural integrity. The recorded easement and maintenance agreement must extend from the connection to the discharge point in an open channel.
(D) Easements.
(1) All storm sewers and swales shall be located in a public road right-of-way or maintenance easement.
(2) The following easement widths shall be used to proved sufficient maintenance access:
Pipe Diameter (inches) | Easement Width (feet)
|
Less than 18 | 10 |
18 to 36 | 23 |
36 to 60 | 15 |
Greater than 60 | 10 feet plus the diameter of the pipe |
(3) The city engineer shall require additional easement widths if the storm sewer depth is greater than average.
(4) All drainage easements shall be accessible to vehicular equipment; however, vehicles do not have to be able to drive down the entire length of easement.
(E) Erosion protection.
(1) All stormwater conveyance systems shall be designed and constructed to withstand the expected velocity of flow from all events up to the base flood without erosion.
(2) Stabilization adequate to prevent erosion shall be provided at the inlets and outlets for all pipe and channel transitions.
(F) Sewer design.
(1) The minimum storm sewer shall be 12 inches for the first pipe and 12 inches for subsequent reaches unless approved by the city engineer.
(2) The minimum design velocity for a storm sewer shall be 2.5 fps. The maximum design velocity for a storm sewer shall be 8.0 fps.
(3) All flared end sections for 12-inch or larger storm sewers shall have grates to stop people, animals or large debris from entering. The grates shall have openings no larger than 3:8 inches (horizontal to vertical).
(G) Streams and channels. Where an existing stream or channel is proposed as part of the conveyance system of the development's increased runoff, the following requirements shall apply:
(2) Clearing of channel vegetation shall be limited to that which is necessary. A re-vegetation plan is required using the Native Plant Guide for Streams and Stormwater Facilities in Northeastern Illinois (NRCS, et al.) as a minimum standard.
(3) A minimum maintenance easement of 12 feet from the top of bank is required along on side of all channels draining 20 or more acres.
(H) Overland flow paths.
(1) The development must have an overland flow path that will pass the 100-year flood flow without damage to buildings or property.
(2) If the upstream drainage area is less than 20 acres, the storm sewer pipe and inlet systems sized for the base flood can be constructed in lieu of providing an overland flow path.
(3) The critical duration 100-year flood flow shall be used to design the overland flow path limits and it shall include all on-site and off site tributary areas.
(4) Overland flow paths shall be protected from fencing, landscaping, or storage shed placement, which could impair its function. This protection shall be established through an easement, deed or plat restriction as required in § 151.05.
(5) All principal buildings on parcels containing or adjoining an overland flow path or other high water level designation shall have a lowest adjacent grade elevation:
(a) One half of one foot above the overflow elevation for tributary areas of 20 acres or less.
(b) One foot above the overflow elevation for tributary areas greater than 20 acres.
(Ord. O-14-03, passed 2-18-03; amend. Ord. O-16-19, passed 10-15-19)