Urban Drainage and Intersection Design

1. Urban Drainage and Intersection Design Image: http://www.dhi.dk/Consulting/WaterInTheCity

2. Urban Street Design - Drainage • Drainage is a secondary function of an urban street system • Runoff in areas is actually brought to the street and the gutter (versus water either being intercepted or brought to the ditch in rural areas) • Goals with Drainage on Street • Preserve safety and convenience of traveling public • Provide economical means of stormwater transport • Prevent water from leaving street system except where appropriate

3. The Stormwater System • Minor system: • Role – collect stormwater and discharge to major system • Design Storm – 2 to 10 years typically (some use 25 year) • Use inlets and conduits (small stuff - most of the system) • Major system: • Role – carry stormwater from minor system to natural watercourses • Design storm – typically 100 year • Use culverts, bridges, retention basins (detention basins?) – big stuff • If fails, property damage or loss of life

4. Intersection Drainage Design - Driveways • Avoid changes in longitudinal drainage due to driveways (ponding, etc. result) • Prevent water entering driveway by: • High point of drive should exceed maximum inside curb flow (top of curb for arterials/collectors and 0.1 m over curb for locals) • Contour land to flow into roadway

5. Intersection Drainage Design - Intersections • Avoid ponding on approaches – use minimum grades, and avoid sag immediately before intersections (Hazel and 4th Street) • Locate inlets upstream of intersections (limits flow in intersection)

6. Intersection Drainage Design - Intersections • Consider impact on drainage due to intersection cross slope adjustments (for smooth travel) – alternatives • Adjust minor street only (for local/collector and collector/arterial) • Adjust gutter line of both streets (only appropriate if intersection area is small – local/collector) • Adjust both centerlines but maintain cross section of both – generally avoid

7. Calculations Given Q, find d (depth) and T (width) of flow Use Q=0.557(Z/n)S1/2 d8/3 where: S = longitudinal grade in ft/ft Z = reciprocal of the cross slope d = depth at the curb in ft. d = [n Q/.56Z S1/2]3/8 See Iowa DOT Design Manual (excellent)

8. Sources: Green Book and SUDAS

9. Typical plan and profile for storm sewer

18. nQ

19. Source: SUDAS http://www.iowasudas.org/documents/Ch2Sect5-05.pdf