Admin Order 58-16-01 - Laserfiche WebLink

Home Browse Search

4. The report shall include related charts, graphs, calculations, summary tables and supporting narrative. B. Basin Boundaries Stormwater- systems shall be designed to convey the calculated runoff from the entire upstream drainage area during the applicable design storm event (See Appendix J). Basin boundaries shall be consistent with the City's Stormwater Basin Master Plan, unless otherwise approved by the City based on more current data. Runoff calculations shall be based on the. assumption of a full -build -out basin according to the land use designations of the most recent Metropolitan Plan. C. Hydraulic Grade Line (HGL) The hydraulic grade line at the outfall (downstream discharge point of the drainage area) shall be based on the most current water surface elevation information identified in the City's Stormwater Basin Master Plan or other data as approved. Hydraulic analysis shall account for energy losses (bend losses, manhole losses, other "minor" losses, friction losses, etc.) and differences in flow characteristics between surcharged systems and open channel systems. Minor losses shall be computed using formulas in the Oregon Department of Transportation hydraulics manual. Stormwater systems shall include a hydraulic analysis of all existing pipes and waterways downstream to the point of outfall. The water surface elevation from the City's Stormwater Basin Master Plan or a previously -approved storm study may be used as a starting pointforthe design. If the ultimate plan is for future construction of pipe between an existing pipe downstream and the current project, and if the current condition through this reach is an interim system, the design water surface elevation shall be based on the hydraulics of the future pipe rather than on the interim system, and shall be labeled "future HGL". The HGLfor the design storm event shall not be less than 6 inches below the gutter at any point where water could surcharge into the street (catch basins, manholes, curb inlets). If the hydraulic analysis of an existing downstream system is not available through the City's Stormwater Basin Master Plan, the engineer may choose to extend the XP-SWMM model using methods and assumptions consistent with the City's modeling work to determine the starting HGL for the system. Public Improvement Design Standards Manual, 2016 Page 2-11