1000 Hydraulics Chapter - Ministry of Transportation

SUPPLEMENT TO TAC GEOMETRIC DESIGN GUIDE
MoT Section 1020 TAC Section Not Applicable
BC MoT
Kirpich Formula
This method can be used to estimate the time of
concentration for natural basins with well defined
channels, for overland flow on bare earth, and mowed
grassed roadside channels. For overland flow, grassed
surfaces, multiply t c by 2. For overland flow, concrete or
asphalt surfaces, multiply t c by 0.4.
0.77
0.00032 L
t
c
=
0.385
S
t c is the time of concentration, hr
L is the total stream length from the most remote part
of the basin as extended from the stream source to
the divide, m
S is the average slope of the total stream length, m/m
For information on the Kirpich Formula, refer to:
♦ CSPI Handbook of Steel Drainage and Highway
Construction Products (2002), p. 116.
Hathaway Formula
This method can be applied to small urban or agricultural
catchments and to small interior basins with light forest.
0.467
(rL)
t
c
=
0.234
1.65 S
t c is the time of concentration, hr
L is the total stream length from the most remote part
of the basin as extended from the stream source to
the divide, km
S is the average slope of the total stream length, m/m
r is the roughness coefficient
The table below presents roughness coefficients which
are recommended for use with the Hathaway formula.
Surface Cover r
smooth, impervious 0.02
smooth, bare packed soil 0.10
poor grass, row crops 0.20
rough, bare soil 0.30
pasture, range land 0.40
deciduous timber land 0.60
coniferous timber land 0.70
timber land with deep litter 0.80
Other Methods
Other methods of estimating the time of concentration for
small and large watersheds are:
• Uplands Method
• SCS Curve Number Method
• Bransby Williams Formula
Time of concentration in channels and conduits can be
estimated using Manning’s Equation, the Continuity
Equation and first principles.
For further information on time of concentration and
estimating the time of concentration, refer to:
♦ RTAC Drainage Manual Volume 1 (1982), p. 2.23.
♦
CSPI Handbook of Steel Drainage and Highway
Construction Products (2002), p. 114.
1020.05 PRECIPITATION
Intensity Duration Frequency Curves
Rainfall intensities can be obtained from Intensity-
Duration-Frequency (IDF) curves which are published by
the Atmosphere Environment Service (AES) for urban
centers in Canada. AES offers software and data for
printing and plotting IDF data.
IDF curve data is not recommended in high elevation,
mountainous areas or areas where snowmelt is a
significant contributing factor to flood events.
For general information on IDF curves, refer to:
♦ RTAC Drainage Manual Volume 1 (1982), p. 2.15.
Remote Locations
For remote locations where IDF curves are not available,
the Rainfall Frequency Atlas of Canada may provide the
best interpolations of extreme rainfall statistics in BC.
Design Storm
A design storm or rainfall pattern, rather than a single
point from IDF values, is required for many unit
hydrograph methods and simulation models. The design
storm pattern may be either historical (e.g. as actually
recorded) or synthetic (e.g. as recreated from statistical
summaries).
Page 1020-4 June, 2007