Hydrological Theory
Canadian 1-Hour Storm Derivation
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From the MIDUSS Version 2
Reference Manual - Chapter 7
(c) Copyright Alan A. Smith Inc. |
Recent work by Watt et al (see
references) has suggested a simple two parameter design storm which
has a linear rising portion followed by an exponentially decreasing
recession curve. Watt et al also suggest the possibility of
reversing the linear and exponential segments but this option is not
supported by MIDUSS. Figure 7-5 shows a definition sketch of this
design hyetograph.
Figure 7-5 - The Canadian
AES 1-hour design storm.
The parameter values which you are prompted
to supply for this option are the total depth of rainfall (millimetres
or inches), the duration in minutes, the time to peak intensity
(minutes)
tp
and the decay coefficient
K.
The decay coefficient K is usually in the range 5 to 7. As with the
other options, the maximum duration and the time step used are as
defined in the Hydrology/Time parameters menu command.
It should be noted that the design storm as
suggested by Watt et al is intended to be used for storms of 1
hour duration only, since the data used for the work was limited to
this duration. However, MIDUSS allows you to define other values of
duration. Care should be taken if suggested values for the time to
peak are taken from Watt et al as these are intended
specifically for 60 minute storms.
The rising and falling limbs of the
hyetograph suggested by Watt et al are defined by
equations [7-11] and [7‑ 12] respectively.
for
t < tp
for
t > tp
For specified values of the parameters
td,
tp,
K
and the total depth of rainfall
Rtot
the peak intensity h
(see Fig. 7-5) can be obtained as follows.
The total depth for any time
t
can then be obtained by integration as shown in equations [7-14] and
[7-15].
for
t < tp
for t >
tp
By computing the volumes
Vk
and
Vk+1
at the beginning and end of a time step, the intensity during the
interval is then defined by equation [7-16].
Suggested values for
K
and
tp
are shown in two tables referenced below. These are based on data
published by Watt et al. Values in minutes of the time
to peak
tp
are for 60 minute storms only and should not be used for storms of
different duration. The time to peak as a proportion of duration is
provided for guidance. See the topics listed below.
Suggested K values for Canadian Provinces
Province |
K value |
B.C.(coastal
region) |
5 |
Yukon, New
Brunswick, Nova Scotia, Newfoundland |
6 |
B.C.(interior),
Alberta, Saskatchewan, Manitoba, Ontario, Quebec |
7 |
(from
Watt et al - see References)
Suggested tp values for locations in
Canada
Location |
tp
(minutes) |
tp
/Duration |
Yukon |
20 |
0.33 |
B.C.(coast) |
28 |
0.47 |
B.C.(interior), Prince George |
13 |
0.22 |
Alberta |
17-18 |
0.29 |
Saskatchewan |
23-24 |
0.39 |
Manitoba
(Brandon, Churchill) |
31 |
0.52 |
Manitoba
(Winnipeg) |
25 |
0.42 |
Ontario
(Timmins, Thunder Bay) |
24-25 |
0.41 |
Ontario
(Ottawa, Kingston, Windsor) |
26-27 |
0.44 |
Ontario
(Toronto, Sudbury) |
21 |
0.35 |
Quebec
(Montreal) |
27 |
0.45 |
Quebec
(Val D'Or, Quebec City) |
23 |
0.38 |
New
Brunswick (Fredericton) |
17 |
0.28 |
Nova Scotia,
Newfoundland |
26-28 |
0.45 |
(from Watt et
al - See References)
Note: These times to
peak were obtained from 60 minute duration storms.
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