Admin Order 58-16-01 - Laserfiche WebLink

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Table 3-1 Storm Recurrence Intervals for Planning and Design of Drainage Improvements (a) Includes roadside ditches and drainage swales (b) Storm sewer systems or a closed conduit whose length exceeds that of a normal culverted crossing of a single roadway (c) Includes local or residential streets, local collectors, and any other roadways up to a major arterial (d) Major arterial or better within the City's right-of-way maintenance (e) Assuming ultimately planned development conditions (Le., impervious cover) within the City's Urban Grovnh Boundary (UGB) and existing development conditions outside of the City's UGB (t) The 5 -year recurrence interval can be used in unusual situations involving sufficient topographical conditions that result in an exceptionally high cost differential between the 10 -year and 5 -year improvement design (e.g., 40%) (g) Closed pipe systems should not be used on waterways draining more than 640 acres (i.e., I square mile) (h) The 5 -year storm maybe used when the Rational Method is applied to calculate the design flow rate. The 10 -year storm should be used for closed pipes with X40 acre drainage areas when using the City's SWMM modeling results or when extending the City's SWMM model using consistent methods and assumptions as used for the City's SWMM modeling work. Type of Drainage Improvement Culverts and Bridges - Design Type of Roadway Storm Drainage Area (acres) Open Closed Major Major Recurrence interval in Channel Pipe Collectors and Arterials Years Neighborhood and Collectors Minor Arterials <40 40 TO 640 >640 (a) (b) (c) (d) (e) X X 5110 (h X X X X 10 (f) X X 10 X X X 1.0 (f) X X 25 X X X(g) X 25 X X 50 All improvements on waterways with FEMA 100 -year flood lain 100 (a) Includes roadside ditches and drainage swales (b) Storm sewer systems or a closed conduit whose length exceeds that of a normal culverted crossing of a single roadway (c) Includes local or residential streets, local collectors, and any other roadways up to a major arterial (d) Major arterial or better within the City's right-of-way maintenance (e) Assuming ultimately planned development conditions (Le., impervious cover) within the City's Urban Grovnh Boundary (UGB) and existing development conditions outside of the City's UGB (t) The 5 -year recurrence interval can be used in unusual situations involving sufficient topographical conditions that result in an exceptionally high cost differential between the 10 -year and 5 -year improvement design (e.g., 40%) (g) Closed pipe systems should not be used on waterways draining more than 640 acres (i.e., I square mile) (h) The 5 -year storm maybe used when the Rational Method is applied to calculate the design flow rate. The 10 -year storm should be used for closed pipes with X40 acre drainage areas when using the City's SWMM modeling results or when extending the City's SWMM model using consistent methods and assumptions as used for the City's SWMM modeling work.