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 WRD DISTRIBUTION: A, B, FO, PO