Flow Process Recognition for Floods in Mountain Streams
In Reply Refer To: July 21, 1992
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 92.11
Subject: Flow Process Recognition for Floods in Mountain Streams
Whenever floods are investigated, especially in small, mountainous
basins, one of the most important tasks is to properly identify
the flow process that occurred in the basin. This responsibility
may be overlooked despite the fact that debris flows can occur in
steep terrain throughout the country. Incorrect identification
and documentation of flow events can result in questionable
discharge estimates. Historically, some debris flows in mountain
drainage basins have been analyzed as water floods (Costa and
Jarrett, 1981). As noted in Office of Surface Water Technical
Memorandum 92.10, recent review of the peak-flow data base in
California has emphasized concerns that some peaks computed as
water floods were actually debris flows. Misrepresentation of
such peak flows could have profound impacts on flood-frequency
estimates. The purpose of this memorandum is to bring the
importance of flow-process identification to the attention of
Division personnel and to provide guidance on how to properly
identify flow processes.
When debris flows occur, water is mixed with sufficient volumes of
sediment to cause water and sediment to no longer behave as two
separate phases; they move as a single, almost plastic-like body.
Peak discharge determinations based on traditional indirect
measurement methods will be flawed when debris flows occur. Not
only will a significant amount of the cross-sectional area below
high-water marks represent sediment rather than water, but the
hydraulic properties of a plastic-like water-sediment mixture (a
non-Newtonian fluid) differ significantly from those of water (a
Newtonian fluid), on which traditional hydraulic theory is based.
The shear strength of debris-flow material results in diagnostic
sedimentology and landforms. Two papers are attached that
summarize sedimentology and landforms associated with water floods
and debris flows. Attachment 1 is entitled, "Rheologic,
Geomorphic, and Sedimentologic Differentiation of Water Floods,
Hyperconcentrated Flows, and Debris Flows," by Costa (1988).
Attachment 2 is an informal summary on "Describing and Naming
Detrital Sediments," which was prepared by Tom Pierson in
Vancouver, Washington, to guide field work in basins of known
debris-flow activity. The debris-flow process can be viewed on a
videotape (Open-file Report 84-606, "Debris-flow Dynamics" by
J. E. Costa and G. P. Williams (1984), 22.5 minutes) available for
loan from any Regional Surface-Water Specialist, the National
Training Center videotape library, or the Office of Surface Water.
In reading the attached papers, you will notice that the term
hyperconcentrated flow is assigned to flows that are intermediate
between clear-water floods and debris flows. It is generally
thought that traditional hydraulic computational methods can be
applied to hyperconcentrated flows.
Following is a synopsis of the two attachments. A glossary of
terms is printed at the end of this memorandum to assist readers
who are unfamiliar with terminology used in these descriptions.
Deposits usually associated with water flooding:
o Deposits typically are stratified. Beds are likely to show
horizontal stratification, cross-bedding, and/or imbrication (see
fig. 5 in Attachment 1 and fig. 1 in Attachment 2 ).
o Individual beds within deposits usually are thin: a few mm (a
mm equals about .003 ft) to several tens of cm thick (1 cm equals
about .03 ft).
o Deposits are very loose and friable. There are often voids
between clasts in gravel deposits (an open-work structure).
o Sizes of individual sediment grains are generally characterized
by a log-normal frequency distribution. Particles can be of a wide
range of sizes. Clasts are usually rounded (see fig. 2 in
Attachment 2). Sediment can be moderately to poorly sorted,
particularly for flash floods (see fig. 4 in Attachment 2).
o Sediment usually is deposited as bars, fans, or sheets.
o Sediment tends to be deposited on the inside of meander bends.
o Channels have large width-to-depth ratios.
Deposits usually associated with debris flows:
o Deposits usually are massive and unstratified. Weak
imbrication is sometimes present. Inverse grading often is found
near the base of deposit (see fig. 5 in Attachment 1 and figures 1
and 3 in Attachment 2).
o Beds are typically thick; 0.3 to 1 or 2 meters (1 m equals 3.28
ft).
o Deposits often consist of muddy, sandy gravel or muddy,
gravelly sand. Some mud--silt or clay--is nearly always present.
Grains are typically angular but can include some rounded clasts
picked up from the channel bottom.
o Deposits typically are compact and difficult to dig out. There
are seldom voids between gravel clasts in the deposits.
o Sediment usually is deposited as marginal levees or fronts
(snouts) of very coarse material. The snout is usually steep and
lobate in form and contains a large concentration of boulders (see
figure 5-E in Attachment 2).
o Channels tend to be semicircular or even "U"-shaped.
o Vegetation in the channel can sustain great damage. Damage to
vegetation may be minimal at the edges of the flow (see figure 5
in Attachment 2).
o Because the water-sediment mixture has some strength, sediment
tends to be "pushed" to the outside of meander bends where it may
be deposited. This is the opposite of water floods where sediment
tends to be deposited on the inside of meander bends.
All personnel responsible for investigating floods in mountainous
basins need to be aware of the importance of correctly identifying
flow processes and should have or be given adequate training and
experience in flow-process identification using geomorphic and
sedimentological evidence. Training classes that discuss
indirect-discharge methods sponsored by the National Training
Center and Regional Offices now include instruction on ways to
correctly identify flow processes. Personnel are encouraged to
take advantage of this training. The Office of Surface Water and
the Regional Surface Water Specialists are available to answer
questions about processes associated with any flood event.
Glossary of terms used in this memo:
(Except where noted, definitions are from "Glossary of Geology" by
Bates and Jackson (1987).
Alluvial fans--low, outspread, relatively flat to gently sloping
mass of loose rock material, shaped like an open fan or a segment
of a cone, deposited by a stream at the place where it issues from
a narrow mountain valley.
Bar (streams)--ridgelike accumulation of sand, gravel, or other
alluvial material formed in the channel, along the banks, or at
the mouth, of a stream where a decrease in velocity induces
deposition.
Bed--informal term for strata that are incompletly known (after
Bates and Jackson, 1980)
Clast--individual constituent, grain, or fragment of a sediment or
rock, produced by the mechanical weathering (disintegration) of a
larger rock mass.
Debris flow--flows in which solid particles and water move
together as a single viscoplastic body (after Johnson, 1970).
Detrital sediment--sediment formed by the accumulation of
detritus, especially that derived from pre-existing rocks and
transported to the place of deposition.
Friable--said of a rock or mineral that crumbles naturally or is
easily broken, pulverized, or reduced to powder, such as a soft or
poorly cemented sandstone.
Hyperconcentrated flows--flows with concentrations greater than 40
percent by weight (20 percent by volume)(after Beverage and
Culbertson, 1964).
Imbrication--sedimentary fabric characterized by disk-shaped or
elongated clasts dipping in a preferred direction at an angle to
the bedding.
Inverse grading (reverse grading)--type of bedding that displays
an increase in grain size with distance up from the base.
Lobate--having or consisting of a long tongue-like projection.
Newtonian fluid--substance in which the rate of shear strain is
proportional to the shear stress.
Rheology--the study of the deformation and flow of matter
Stratified--formed, arranged, or laid down in layers or strata.
Sheet (deposits)--deposit that is generally stratiform, more or
less horizontal, and areally extensive relative to its thickness
References:
Bates R.L. and Jackson, J.A., 1980, Glossary of Geology, Second
Edition: American Geological Institute, Alexandria, Virginia,
749 p.
Bates R.L. and Jackson, J.A., 1987, Glossary of Geology, Third
Edition: American Geological Institute, Alexandria, Virginia,
788 p.
Beverage, J.P. and Culbertson, J.K., 1964, Hyperconcentrations of
suspended sediment: Journal of Hydraulics Division, American
Society of Civil Engineers, v. 90(HY6), p. 117-128.
Costa, J.E., 1988, Rheologic, Geomorphic, and Sedimentologic
Differentiation of Water Flood, Hyperconcentrated Flows, and
Debris Flows, in Flood Geomorphology edited by V.R. Baker, R.C.
Kochel and P.C. Patton: John Wiley and Sons, Inc., New York,
p. 113-122.
Costa, J.E. and Jarrett, R.D., 1981, Debris flows in small
mountain stream channels of Colorado and their hydrologic
implications: Bulletin of the Association of Engineering Geology,
v. 18, p. 309-322.
Costa, J.E. and Williams, G.P., 1984, Open-File Report 84-606,
Debris-flow Dynamics, 22.5-minute videotape.
Johnson, A.M., 1970, Physical Processes in Geology: Freeman and
Cooper, San Francisco.
Charles W. Boning
Chief, Office of Surface Water
Attachments
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