1000 Hydraulics Chapter - Ministry of Transportation

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-ofway
drainage, which may include runoff 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 of 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 profile. 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 of 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 of 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 of highway ditches varies and is
dependent upon ditch shape, depth, slope, type of material
and maintenance requirements. The bottom width of 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 of 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