Boggs Mountain Demonstration State Forest Draft ... - Cal Fire

Boggs Mountain Demonstration State Forest Draft Management Plan, June 2008
Culvert Sizing Procedure for the 100-Year Storm
The Rational Method of estimating flood discharge from small watersheds was used to determine
the proper size diameter culvert for the drainages on Boggs Mountain Demonstration State
Forest. To calculate the size of culvert which would handle the 100-year storm flood flow, the
volume of runoff which would occur at each stream crossing must first be estimated. The
Rational Method is based on the equation: Q=CIA
Where: Q= peak runoff at crossing (in cfs)
C= runoff coefficient (percent runoff)
I= uniform rate of rainfall intensity (inches/hour)
A= drainage area (in acres)
Information needed:
A= area of watershed (acres)
H= elevation difference between highest point in watershed and the crossing point (ft)
L= length of channel in miles from the head of the watershed to the crossing point
I= uniform rate of rainfall intensity. Obtained from precipitation frequency-duration data for
local rain gages .
C= runoff coefficient from table created by Rantz (1971)
When selecting an “I” value, one must consider the travel time for the runoff to reach the crossing
(Tc), and the precipitation conditions for the particular watershed. Time of concentration can be
calculated using the formula: Tc=[11.9L 3 /H] 0.385
Where: Tc= time of concentration (in hours)
Once the time of concentration has been determined, then that value is used to determine which
rainfall duration to use. For all of the calculations, a 10 minute duration was used. The runoff value
for a 10-minute duration of a 100-year flood flow was obtained from rainfall depth duration frequency
table for Clear Lake Highlands (.31), and then multiplied by six to get the hourly value (1.86). The
runoff coefficient (C) used for all of the drainages at Boggs was 0.4. This is representative of
woodland with a shallow impeding horizon.
Once the volume of runoff was estimated for each of the crossings, the Culvert Capacity
Nomograph was used to determine the needed culvert size that will accommodate the 100-year
flood flow. The culvert entrance type for all of the crossings is Type 1, with a headwall. The
headwater depth in diameters was calculated by dividing the distance from the road grade to the
bottom of the culvert by the culvert size in feet. By drawing a line from the headwater depth (in
diameters) through the discharge values (in cfs), one will be able to determine the needed culvert
size (in inches).
Lower Mill Creek:
The crossing at Lower Mill Creek has two 30-inch “shotgun” culverts mounted in concrete
measuring 4’ from the base of the culvert to the headwall, and 7’ to the road grade. The
headwater depth is 2.8 culvert-diameters.
A=152 acres L=3200 feet (.67 miles) H=680 feet
Tc= [11.9(.67) 3 /680] .385 = .133 hours (7.96 minutes)
82