Duluth UDC § 1016.02

1. The preliminary design (initial pipe sizing and profile design) of piped collection systems required under Section 1016.01(1) herein shall be based upon conveyance of the peak flows associated with a fully developed 25-year storm with the hydraulic grade line (HGL) being one foot or more below the top of each structure, gutter line or proposed final ground surface elevation, whichever is lowest. 2. Once the preliminary design of a piped collection system has been prepared, it shall be analyzed for its behavior during conditions of 100-year flow, with the objective of this analysis being to ascertain the quantities of flow and flow paths followed by flows exceeding the capacity of the system, whether these pond at inlets or flow along the ground's surface. 3. Based on the analysis of 100-year conditions, the preliminary design shall be revised where necessary to produce a final design for which the likelihood of dwelling flooding, major property damage, or substantial public access and/or utility interruption shall be less than one chance in 100 years. 4. The minimum allowable pipe diameter shall be 15 inches. 279 | P a g e Article Ten. Erosion Control and Stormwater Management 5. Catch basins shall be spaced so that the spread in the street for a 10-year design flow shall not exceed half the outside travel lane, as measured from the face of the curb. Gutter spread calculations shall be submitted to the City for review and approval prior to issuance of a Development Permit. 6. Complete flow, velocity, and hydraulic grade line computations shall be provided for all portions of a piped collection system. Hydraulic grade lines shall be shown on the storm drainage profiles and associated calculations shall be submitted for the 25-year storm with the Development Plans.