FLOOD06021

• County: Larimer
• Stream Name: Big Thompson
• Basin: South Platte
• Title: Flood History of the Big Thompson River Basin: Flash Floods, Paleofloods, and Dam-Break Floods
• Date: 1/1/1979
• Prepared For: Larimer
• Prepared By: USGS
• Floodplain - Doc Type: Flood Documentation Report

Table 2. Comparison of peak discharge using different methods

Estimated peak discharge
(ft3/s)

Location

Drain~
age
area
(mi2)

Slope
(It/It)

Slope-
area
method
(McCain
and
others.
1979)

Rainfa.lI~
runoff
method
(Miller
and
others,
1978)

Paleo-
hydraulic
method
(Costa.
1963)

Crilical-
depth
method

Big Thompson River
tributary below
Loveland Heights.
Colo. .. .. .. . . . .. . . . . . .. . . . . . . .
Dark Gulch at Glen
Comfort. Colo. . . . ... . . . . .. . . ...

1.35

1.0

.125

7.200

3.390

4.630

subsequently eroded after the peak flow, and a cobble-
and boulder-covered streambed is left. According to
Henderson (1966), suspended sediment reduces flow re-
sistance, but that effect is small compared with the in-
creases in flow resistance due to bedforms induced by
sediment motion.
Costa and Jarrett (1981) stated that very high
sediment-laden streams or debris flows have been misin-
terpreted as waterfloods. Debris flows (one type of
mudflow) are a heterogeneous mixture of sediment sizes
and contain sufficient clay or silt to form a cohesive
non-Newtonian fluid mass. The main difference between
mudflows and debris flows is that debris flows have
larger size material than mudflows. The slope-area
method was developed for Newtonian fluids. Some in-
vestigators have misapplied the slope-area method and
have computed world-record-breaking waterfloods
where, in fact, the debris flows were initiated by only
moderate rainfall or, in Some places, snowmelt on steep
unstable slopes. Five of seven documented large water-
floods investigated in Colorado were, in fact, debris
flows (Costa and Jarrett, 1981).
The important point to remember is that no infor-
mation is available at present for adjusting the velocity
of flow or for estimating roughness coefficients of flows
having large concentrations of sediment. However, it is
doubtful that sediment-laden streams have no effect on
flow. Mudflows and debris flows take place in many
parts of the country, although they are most common in
the western third of the United' States. In most events,
slope-area measurements should not be used if the event
may have been a mudflow or debris flow because the
method may not be applicable, as there are no verified
n-values, and because of debris damming that affects !he
flow hydraulics (velocity, depth, and slope) in the chan-
nel. An indication of a mudflow is a thick (1{4-1 in.)
veneer of mud on the ground surface, obstructions, and

~<jJ'

0.077

8.700

4.700

5.400

3.740

3.~~0

vegetation. Evidence used to identify debris flows in-
cludes coarse lobate, poorly sorted, unstratified, pebbly
mudstonelike unconsolidated deposits having well-
defmed levees and terminal lobes, and the extent of
ground-litler disruption below high-water marks. Also,
debris flows generally end where slope decreases and,
from that point downstream, channel shows minimal or
no evidence of disruption. If the slope-area method is
used to compute the peak debris discharge, all assump-
tions and a qualification as to the unknown accuracy
needs to be incorporated in the report on the debris flow.
If no mudflow and debris-flow features are found,
then the slope-area method can be cautiously applied,
but the evaluation should note that sediment concentra-
tions are great. Because of the need for more informa-
tion on mudflows and debris flows, hydraulic and sedi-
ment data should still be collected for these events. This
documentation should include the source area and cause
of debris flow, length of reach affected, site characteris-
tics (width, depth, slope, and apparent roughness), dep-
ositional and sediment characteristics, and photo-
graphs.

Velocity Distribution

The vertical-velocity distribution in fully developed
turbulent flow has been shown to be approximately log-
arithmic (Chow, 1959). The logarithmic vertical-velocity
distribution for a stream is shown graphically in figure 3
and is considered applicable in computing the mean ve-
locity of a slope-area measurement. Measurements of
streamflow velocity are normally made by means of a
current meter, which measures the velocity of water
flowing through incremental subareas of a channel cross
section. The mean velocity of the flow through each sub-
area is obtained by measuring the velocity at 0.6 of the
depth of flow (V...> if the depth is less than 2.5 ft, or by

Evaluation of the Slope-Area Method for Computing Peak Discharge 19

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