(A) General requirements.
(1) All storm sewer improvements/construction shall be constructed in conformance with Chapters 52 and 53 of the City Code.
(2) All development with the exception of the construction of a single family dwelling on an existing lot of record, shall incorporate adequate provisions for stormwater runoff consistent with the City of Kasson standard detail plates, engineering guidelines and this chapter.
(3) Drainage discharge shall be managed so post-development runoff is equal to or less than pre-development.
(4) No stormwater runoff or natural drainage water shall be so diverted or directed as to overload existing drainage systems or create flooding or the need for additional drainage structures on other private properties or public lands without proper and approved provisions being made for avoiding these conditions.
(B) Calculation methodology.
(1) A hydrograph method based on sound hydrologic theory shall be used to analyze runoff for the design or analysis of flows and water levels.
(2) The design of any permanent stormwater treatment system intended to meet the requirements of this chapter shall be verified by routing the design storm hydrograph through the proposed facility using the storage indication method or other methodology demonstrated to be more appropriate.
(3) A stormwater report which includes pre and post development plans, routings, hydrographs and any calculations required to demonstrate compliance with this this chapter shall be submitted to the city for approval.
(4) Runoff rates for proposed activities and development shall:
(a) Apply land cover conditions existing as of the effective date of this ordinance as the baseline for existing conditions in runoff calculations.
(b) Post-development runoff rates shall be less than or equal to pre-development rates for the two-year, ten-year and 100-year design storms, unless otherwise permitted by the City Engineer.
(c) Atlas 14 precipitation data shall be utilized for pre and post-development runoff computations or as approved by the City Engineer.
(C) Permanent stormwater treatment system.
(1) Current NPDES general construction permit requirements shall apply.
(2) Permanent stormwater treatment systems facilities shall be designed and constructed in accordance with the Minnesota Stormwater Manual current edition and current NPDES general permit requirements.
(a) All permanent stormwater treatment systems shall be designed with an emergency overflow to pass runoff in the event of a blocked outlet control structure.
(b) The emergency overflow shall be designed and constructed to prevent erosion to the spillway.
(3) Infiltration areas must be fenced or otherwise protected from disturbance before the land disturbing activity starts.
(4) Vegetation used in conjunction with infiltration systems must be tolerant of urban pollutants and the range of soil moisture conditions anticipated.
(D) Stormwater collection and conveyance system.
(1) Stormwater collection and conveyance systems shall be designed to convey the ten-year design storm.
(2) Permitted gutter flow width shall be determined based on the required roadway in conjunction with providing a 12 foot unobstructed travel lane for emergency vehicles. Swales shall be designed to convey the 100-year design storm.
(E) Stormwater collection and conveyance systems shall be designed to safely pass the 100-year design storm.
(F) Culverts, natural drainageways or other conveyance facilities shall be sized to accommodate the potential runoff from its entire developed upstream drainage area.
(Ord. 879, passed 10-28-2020)