1000 Hydraulics Chapter - Ministry of Transportation

BC MoT
SUPPLEMENT TO TAC GEOMETRIC DESIGN GUIDE
MoT Section 1050 TAC Section Not Applicable
Table 1050.B presents the spillway catchment width,
which is recommended for use with the
Spreadsheet/Calculator Method spillway inlet spacing
calculations:
Table 1050.B - Spillway Catchment Width
Inlet Type w (m)
Paved Spillway 0.600
Grates/Spillways Spacing on a Grade
Research conducted by the Washington State Department
of Transportation (WSDOT) has found that the capacity
of an inlet on a continuous grade can be estimated by
determining the portion of the gutter discharge directly
over the width of the inlet. The model assumes a
triangular flow cross section and is most accurate for
longitudinal slopes of -2% to -3%. The WSDOT model
has been modified to account for lateral inflow on mild
grades and high velocities on steep grades. Two methods
have been developed to provide approximate spacing
requirements and also suit calculator and spreadsheet
applications.
The inlet spacing calculations should be conducted
approximately where the inlet is to be located. At least
one iteration will be required to match the assumed inlet
location with the calculated inlet location.
For one or two lane roadways, a maximum
catchbasin/spillway spacing of 150 m is recommended.
The maximum median spacing of 250 m is recommended.
The maximum spacing criteria has been established to
facilitate maintenance operations and to prevent an
excessively long flow path in the event that one becomes
blocked. For one and two lane roadways, a minimum
catchbasin/spillway spacing of 20 m is recommended.
The minimum spacing criteria has been established to
prevent over-conservative designs.
Tabular Method
The Tabular Method provides a quick estimate of the inlet
spacings, but is limited in terms of crossfall and
longitudinal grade combinations. The tables were
developed using a runoff coefficient (C) equal to 0.95 and
a ponding width of 1.2 m. The tables are useful for
normal crossfall and longitudinal grades between 2% and
4%.
Inlet spacing coefficients have been provide in
Table 1050.C, Table 1050.D and Table 1050.E for
depressed Bicycle Safe grates, depressed Freeway grates
and undepressed Bicycle Safe grates. The notation for
the spacing tables are as follows:
s y is the longitudinal grade, m/m
s x is the crossfall, m/m
i is the rainfall intensity for t c equal to 5 minutes,
5 year return period, mm/hr
C1, C2 is the spacing coefficients for a single grate
C1, C2 is the spacing coefficients for twin grates
CB one is the initial inlet spacing, m
CB two is the consecutive inlet spacing, m
The following procedure shall be used to estimate
catchbasin spacings:
1. Select appropriate rainfall intensity.
2. Select the longitudinal grade (s y ) and crossfall (s x )
which closely matches the assumed inlet location. It
may be necessary to try more than one location.
3. From the appropriate table, select appropriate values
for C1, C2 (single) or C1, C2 (twin).
4. Determine effective/average pavement width (w)
from drainage patterns.
5. For single grates, determine CB one and CB two using
pavement width (w) in the following formulas:
CB = C1
one CB = C2
two
w
w
else, for twin grates, determine CB one and CB two using
pavement width in the following formulas:
CB = 1.2 C1
one
CB = 1.2 C2
two
w
w
Page 1050-2 June, 2007