FLOOD04361

• County: Statewide
• Basin: Statewide
• Title: Techniques of Water-Resources Investigations of the US Geological Survey Measurement of Peak Discharge by the Slope-Area Method
• Date: 1/1/1984
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

2

TECHNIQUES OF WATER-RESOURCES INVE8TIGATIONS

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I"" Section 1 )
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PLAN VIEW

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Section 2.....:

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Energy gradient

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h.!

hI

Water su....
"Gee

h,+ k(l1 h.}

h'2

Datum

L

h2

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PROFilE VIEW

Figure 1.-Definition sketch of a slope-area reach.

The friction slope S to be used in the Man-
ning equation is thus defined as

S hI l1h+l1h,-k(l1h,)
L L '

(3)

where l1h is the difference in water-surface
elevation at the two 8ections, and L is the
length of the reach.
In using the Manning equation the q uan ti ty
(1.486fn)AR'I3, termed conveyance, K, is com-
puted for each cross section. The mean
conveyance in the reach is then computed as
the geometric mean of the conveyance at
the two sections. This procedure is based
on the assumption that the conveyance varies
uniformly between sections. The discharge
is computed by the equation

Q=.,JK,K,S,

(4)

where S is the friction slope as previously
defined .

(omputation of Friction Slope

The computation of the friction slope by
equation 3 involves the determination of the
water-surface elevation and velocity heads
at each section and an evaluation of the loss
due to contraction or expansion. Water-
surface elevations are taken from the pro-
me defined by high-water marks as described
in chapter Al by Benson and Dalrymple (1967).
The velocity head (h,) at each section is com-
puted a.s

aV'
h=-,
, 2g

(5)

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where V is the mean velocity III the section