1000 Hydraulics Chapter - Ministry of Transportation

BC MoT
SUPPLEMENT TO TAC GEOMETRIC DESIGN GUIDE
MoT Section 1020 TAC Section 2.1.2
Step 5 - Determine Rainfall Intensity
The nearest rainfall gauging station is located at Comox
Airport (El. 24 m). Since the basin elevation varies from
El. 20 m to El. 760 m, a precipitation gradient is
expected. The 10-year rainfall intensity corresponding to
the time of concentration will be used due to the increased
reliability of rainfall data over more frequent return
periods (e.g. 2-year). A previous hydrological study
estimated the average intensity over the basin will
increase at a rate of 5% per 100 m rise in elevation.
i = (9mm / hr)(( 740m )( 05 . )( 005 . ) + 1) = 107 . mm / hr
100m
Step 6 - Determine Design Flow
There are numerous ways of estimating the design flow.
A few different methods will be used.
Method 1 - Rational Formula
Since the basin is small and there is limited data, the
Rational Formula will be used to determine the 10-year
flow. The 10-year flow will then be converted to a 200-
year flow. Studies have shown that the Q 200 /Q 10 ratio is
approximately 1.7 for this region.
Method 2 – SCS Peak Flow Method
For this creek, a 24-hour, Type 1A rainfall distribution
will be used in the analysis. The 10-year 24 hour total
rainfall volume will be obtained from the Comox Airport
IDF curve. The estimated 10-year flow will be converted
to a 200-year flow.
Total Rainfall = (3.2mm / hr)(24hr) = 76.8mm
Q 10 = 7.4m 3 / s
3 3
Q 200 = 1.7Q 10 = 1.7(7.4m / s) = 12.6m / s
Method 3 - Regional Frequency Analysis
Hydrological studies have resulted in regional frequency
curves for the area.
Q
200
= (A)(unit runoff)(peaking factor)
2 3 2 3
= (6.6km )( 18 . m / skm )( 15 . ) = 17.8m / s
Since the results do not vary significantly, an “average”
will be taken. The 200-year flow is estimated to be 15
m 3 /s.
Q = CiA ( 040)(107 . . mm / hr)(660ha)
10 =
360
360
3
= 7.9m
/ s
3 3
Q 200 = 1.7Q 10 = 1.7(7.9m / s) = 13.4m / s
June, 2007 Page 1020-7