(A) General.
(1) All subdivisions shall be provided with drainage facilities having a design capacity adequate to carry off surface water falling or coming on the streets and developed areas in a ten year return period storm event under the projected final state of development of the entire upstream watershed.
(2) The minor drainage system components shall consist of storm drains, street gutters, small open channels, and swales designed to store and convey the peak rate of runoff from the ten year return period precipitation event of critical duration.
(3) The major drainage components such as open ditches and streams draining 40 acres or more in area shall be designed to store and convey stormwater flows beyond the capacity of the minor drainage components. They shall be designed to convey the peak rate of runoff from the 25 year return period precipitation event.
(4) (a) An excess stormwater passageway shall be provided for the floodplain of all natural water courses and such manmade water courses and storm drainage systems as the Village Engineer may direct, which shall have adequate capacity to convey the excess stormwater runoff from the tributary watershed. The capacity of this excess stormwater passageway shall be adequate to transport the peak rate of runoff from the 100 year return period storm, assuming all upstream areas are fully developed for uses specifically permitted by existing zoning, and antecedent rainfall in the tributary watershed is average (e.g., Antecedent Moisture Condition II).
(b) An excess stormwater passageway shall be provided within the design of all parts of a subdivision with a watershed area of ten acres or more. The excess stormwater passageway through the subdivision shall be adequate to transport the peak rate of runoff from the 25 year return period storm.
(c) No buildings or structures shall be constructed within an excess stormwater passageway;
however, streets, parking lots, playgrounds, park areas, pedestrian walkways, open green space, and utility and sewer easements may be considered compatible uses.
(5) All storm drainage facility construction activity shall conform to the requirements of the Village of Mahomet Infrastructure Design Manual.
(6) All stormwater drainage facilities shall comply with the requirements of the village stormwater management ordinance. When required by the stormwater management ordinance, stormwater detention facilities shall be provided for new subdivisions.
(7) Standard drainage systems shall consist of storm drains, inlets, manholes, and other related underground drainage facilities.
(8) Reduced impact development drainage systems may utilize surface drainage components such as roadside swales, culverts, subsurface drain tiles, and other related surface drainage facilities.
(9) Alternate drainage systems may be approved by the Village Engineer if a detailed engineering study performed by the subdivider’s engineer demonstrates that the alternate drainage system provides a level of service at least equal to that provided by the above requirements without increasing maintenance and operating costs to the village.
(10) All single family and two-family residential lots shall be provided with the ability to connect a private sump pump discharge line to a storm drain, tile drain, or sump pump discharge line.
(11) All multifamily residential, commercial, and industrial lots larger than 1 acre in size shall be provided with a subsurface service connection to the storm drainage system.
(12) The storm drain system in each new subdivision shall continue and extend existing, connecting storm drains in adjoining subdivisions, where appropriate.
(13) The storm drain system in each new subdivision shall be extended to any property adjacent to the subdivision and shall accommodate the tributary watershed from adjacent property.
(Ord. 15-12-01, passed 12-15-2015)