1000 Hydraulics Chapter - Ministry of Transportation

SUPPLEMENT TO TAC GEOMETRIC DESIGN GUIDE
MoT Section 1020 TAC Section Not Applicable
BC MoT
Table 1020.A Maximum Runoff Coefficient Values For Coastal Type Basins
Surface Cover Impermeable Forested Agricultural Rural Urban
Physiography
mountain (>30%) 1.00 0.90 - - -
steep slope (20-30%) 0.95 0.80 - - -
moderate slope (10-20%) 0.90 0.65 0.50 0.75 0.85
rolling terrain (5-10%) 0.85 0.50 0.40 0.65 0.80
flat ( 25 years +0.10 +0.05 +0.15 +0.10 +0.10
snowmelt +0.10 +0.10 +0.10 +0.10 +0.10
For small interior drainage basins where the critical
runoff events are generally a result of summer rainstorms,
the runoff coefficients can be selected from the following:
• RTAC Drainage Manual Volume 1 (1982), Table
2.4.1-2.4.3, p. 2.22.
For information on runoff coefficients, refer to:
♦ RTAC Drainage Manual Volume 1 (1982), p. 2.22.
SCS Soil Groups and Curve Numbers
Hydrologic soil groups and soil/land use curve numbers
(CN) can be obtained from the following:
• Ministry of Environmnet (MoE) Soils Maps
• textural classifications provided by geotechnical
investigations
• CSPI Modern Sewer Design (1996), p. 67
• CSPI Handbook of Steel and Highway Drainage
Products (2002), p.109.
In areas where flooding is usually the result of winter
precipitation (e.g. coastal areas), curve numbers should
generally correspond to Antecedent Moisture Condition
III (AMC III) to reflect the highest runoff potential. In
areas where critical runoff values are the result of summer
storms (e.g. interior areas), Antecedent Moisture
Condition II should be assumed.
For information on SCS soil groups and curve numbers,
refer to:
♦
♦
CSPI Modern Sewer Design (1996), p. 68 &69.
CSPI Handbook of Steel Drainage and Highway
Construction Products (2002),p. 108.
1020.03 BASIN AND CHANNEL SLOPE
For small drainage areas, the slope of the drainage area
can be estimated using the following formula:
h1 − h2
s =
L
s is the average slope of drainage area, m/m
h 1 is the maximum elevation of drainage basin, m
h 2 is the minimum elevation of drainage basin, m
L is the maximum length of drainage path, m
Vertical drops such as falls and rapids, etc. should be
deducted from the calculations.
For large or complex drainage areas, the main channel
slope should be estimated using the Average Slope
Method or the Equivalent Slope Method.
Average Slope Method
The Average Slope Method is recommended for normal
use. It should give reasonable results for streams having
short rapids or falls. However, it is not recommended for
profiles which are strongly convex or concave for much
of their length.
For information on the Average Slope Method, refer to:
♦ RTAC Drainage Manual Volume 1 (1982), p. 2.11.
Page 1020-2 June, 2007