FLOOD06195

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Measurment of Peak Discharge at Culverts by Indirect Methods
• Date: 6/1/1968
• Prepared By: G.L. Bodhaine
• Floodplain - Doc Type: Educational/Technical/Reference Information

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MEASUREMENT OF P~;AK DISCHARGE AT CULVERTS BY INDIRECT :l-IETHODS 45

heel:lllse it may mean the difference between
" culvert. f1..wiu!!; full or JII1rt.ly fllll.
Remember thu.t the effect of side contraction
hecomes negligible for flow types 4, 5, and 6
and that vertical contraction is very important.

General Remarks
Storage

The storage effect of pondage upstream from
a culvert reduces the peak discharge that
normally would result if there were no embank-
ment actin!!; as a dam. If small-area sites are
bein!!; selected for regional hydrolo!!;ic stlldies,
it is advisable to select sites where t.he ponded
area is a ne!!;ligible Imrt of the tol,al draina!!;e
area.
The two main factors t,o be considered
in storage are the ra.te of rise in t.he pond und
the relationship of size of clllvert to size of
pond. For any given rate of rise, the size
(surface area) of the pond required 1,0 reduce
the outflow a selected percent can be comput,ed,
If the surface area is computed for several
sta!!;es, hoJdin!!; the rate of rise and percent
difference between inflow and outflow constant,

10
Curves are for corrugated-metal pipe having
_~rojectjng entrance and inlet control.
Values tor concrete pipe are about
10 percent larger

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B. CUf\"e can be dcyeloped. The cunoe.s of fig-ure
26 were de\'eloped for corrugated-metal pipes
with projectin!!; entrances and inlet control.
For concrete pipes the surface areas of figure
26 should be increased about 10 percent,.
The reduction in flow is directly proportional
to the surface area of the pond and to the rate
of rise. The curves of figure 26, therefore, can
be used for ul,her rates of rise or percentage
reductions in flow if treated correctly mathe-
matically. For example, if the surface area is
doubled at a given stage and rate of rise held
constant, the reduction in flow will be doubled,
or increased to 2 percent. If the rate of rise is
doubled, the reduction in flow at a given sur-
face area will be doubled.
These curves may be used in two ways: (1)
u) determine how much the dischar!!;e would be
llffected by a pond of known surface area and
(2) to determine the allowable surface area for
a ~hren reduct,ion in discharge. In considerin~
item 2, if II reduction in flow of less than ]
percen t is desired, a site should be seJected
where the surface area of the pond is less than
the allowable value shown on figure 26.
All the above factors except rate of rise can
be determined easily at most sites. Rat.. of rise

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20 50 100 200 500 1000 2000 5000 10.000
SURFACE AREA. IN SQUARE FEET. WHICH WILL CAUSE A I-PERCENT REDUCTION
IN DISCHARGE FOR RATE OF RISE OF I FOOT PER HOUR

Fi.gure 26.-Variation of headwater with surfoce area of pond for determining reduction in discha'ge for various
. sizes of culverts.