SUPPLEMENT TO TAC GEOMETRIC DESIGN GUIDE MoT Section 1020 TAC Section Not Applicable BC MoT This page is intentionally left blank Page 1020-8 June, 2007
BC MoT SUPPLEMENT TO TAC GEOMETRIC DESIGN GUIDE MoT Section 1030 TAC Section 2.1.2 1030 OPEN CHANNEL DESIGN 1030.01 DESIGN RETURN PERIODS For open channel design return periods, refer to Section 1010.02. 1030.02 OPEN CHANNEL CHARACTERISTICS Highway ditch designs typically accommodate right-<strong>of</strong>way drainage, which may include run<strong>of</strong>f from pavement areas, cut slopes and adjacent overland flow. Conversely, drainage channels are specifically designed for larger drainage basins and watercourses. Drainage channel design may incorporate the following considerations: hydraulic requirements, river engineering concepts, fisheries enhancement works, etc. For typical earth ditch sections, ditch sections in solid rock cuts and median sections, refer to: ♦ BC Supplement to TAC Geometric Design Guide, Fig. 440.A, B, C, D & G. For geometric properties <strong>of</strong> various open channels, refer to: ♦ RTAC Drainage Manual Volume 1 (1982), p. 3.4. Grades Roadside drainage ditch grades do not necessarily need to be the same as the road pr<strong>of</strong>ile. The desirable minimum sustained grade for channels is -0.5%, with -0.3% allowed as an absolute minimum to ensure drainage and prevent “standing water”. Steep channel grades should be checked for erodibility. For information on ditch grades, refer to: ♦ RTAC Drainage Manual Volume 1 (1982), p. 3.31. Channel Depth The roadside drainage ditch depth should be designed such that the ditch invert is a minimum 0.30 m below the bottom <strong>of</strong> the SGSB layer. The ditch should also be designed such that the flow does not frequently make contact with the SGSB layer. The maximum allowable depth <strong>of</strong> flow in minor ditches is 0.6 m. The recommended minimum freeboard is 0.3 m for small drainage channels, larger channels should have a greater freeboard allowance. For information on ditch depth, refer to: ♦ RTAC Drainage Manual Volume 1 (1982), p. 3.31. Channel Width The bottom width <strong>of</strong> highway ditches varies and is dependent upon ditch shape, depth, slope, type <strong>of</strong> material and maintenance requirements. The bottom width <strong>of</strong> a roadside ditch should not normally be less than 1 m. However, for major roadways, this may be increased for safety purposes to approximately 2 m. For information on channel width, refer to: ♦ RTAC Drainage Manual Volume 1(1982), p. 3.31. Sideslopes Typical channel sideslopes range between 1.5:1 (H:V) to 4:1. Ditch sideslopes steeper than 2:1 are generally difficult to maintain. For information on sideslopes, refer to: ♦ BC Supplement to TAC Geometric Design Guide, Fig. 440.A, B, C, D & G. Roughness Coefficients Manning’s roughness coefficients (n) are commonly used to describe channel and conduit characteristics. For Manning’s roughness coefficients, refer to: ♦ RTAC Drainage Manual Volume 1 (1982), Table 3.2.3, p. 3.12. For information on Manning’s roughness coefficients, refer to: ♦ RTAC Drainage Manual Volume 1 (1982), p. 3.10. Assessment <strong>of</strong> Existing Channel An existing channel should be analyzed to determine if there is sufficient capacity to accommodate the design flow. If channel capacity is insufficient, drainage problems may occur at unexpected locations during large flood events. In addition, channel stability and debris loads should also be assessed. June, 2007 Page 1030-1
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