PROGRAMS & PLANS--Estimating flow characteristics from channel size In Reply Refer To: March 26, 1975 EGS-Mail Stop 415 SURFACE WATER TECHNICAL MEMORANDUM NO. 75.16 Subject: PROGRAMS & PLANS--Estimating flow characteristics from channel size Channel size has been used by WRD in estimating mean flow of streams in arid and semiarid regions of western U.S. for 6 or 8 years. More recently the value of channel size for estimating flood-peak characteristics has been demonstrated. More than half a dozen Districts have used the method and several applications have been published. Most of this work was done by interested hydrologists working more or less independently of each other. Two years ago the Surface Water Branch began coordinating this effort. This coordination consisted of field studies, reanalysis of data, convening of a meeting on standardization of methods and terminology in Denver, April 22-24, 1974, and organization of a test in northern Wyoming to determine how consistently channel measurements could be made by different individuals. This memo (1) suggests appropriate uses for the method, (2) presents guidelines for data collection, analysis, and application, (3) emphasizes that field guidance must be obtained from a qualified consultant, (4) encourages improvements in these methods, and (5) reaffirms the coordinating function of the Surface Water Branch. The channel-width method is appropriate for use where flow characteristics are poorly related to drainage area and other basin characteristics. These include arid and semiarid areas and those areas where geology plays a dominant role in determining the flow characteristics. In areas having these characteristics, specific uses include: 1. Estimation of mean flows of perennial streams when time does not permit making monthly discharge measurements for a year. 2. Estimation of mean flows of ephemeral streams. 3. Estimation of flood-peak characteristics. 4. Presentation of a relation of a flood-peak characteristic to channel width in a regional flood-frequency report. The relation might be the only one presented for a particular part of the regional study, or it might be given as an alternate to regression on basin characteristics. Users of such a report should be advised to obtain field training before applying the relation, or to call on experienced hydrologists in WRD to estimate the characteristics at specific sites. Before making channel measurements the attached "Guidelines for Using Channel Size as an Indicator of Flow Characteristics, 1975" should be studied, and a few days of field instruction should be obtained from a man experienced in the technique. Districts should inform the Surface Water Branch when they begin a project of this type. And all reports using channel size for estimating flow characteristics are to be reviewed in the Surface Water Branch. The variability of channel measurements made by 7 experienced individuals was defined on 22 test reaches in Wyoming in Autumn 1974. A tentative analysis was based on the assumption that the average of the logarithms of the 7 widths in each reach is the best estimate of the width in that reach. Using this assumption the analysis indicates that variability in measuring channel width would result in a standard error in discharge of 30-35 percent if the model error were zero. Since model error is not zero, the standard error of a discharge estimate would be appreciably greater. Further research is needed; promising aspects include use of channel slope, an index of channel shape, and some index of bed and bank material in the relations. Districts are encouraged to review published work on channel morphology and to seek improved methods. Harry H. Barnes, Jr. Chief, Surface Water Branch Attachment WRD Distribution: A, B, FO Guidelines for Using Channel Size as an Indicator of Flow Characteristics, 1975 Selection of Reach The selection of a suitable reach in which to measure channel dimensions is important. The following guidelines represent the ideal reach and can seldom be satisfied; there can be no substitute for good judgment in selecting the best reach available: 1. A straight, narrow reach in which flows are approximately uniform is preferred. 2. Bed and banks in the reach should be stable but should be of material that has permitted the channel to develop into a normal size and shape for the flow regimen (but the flow regimen should not be known at time of reach selection and channel measurement!). Bedrock channels may not be representative; they should not be used. For large rivers, low-level aerial photography may be helpful in locating a suitable reach. Selection of Cross Section Channel dimensions at 3 levels in a channel may be measured. The section defined by the lowest channel bars is most commonly related to mean flows, and the sections defined by the active channel width and the main channel width are indices to flood characteristics. The terminology and descriptions of these 3 levels are still tentative. Section defined by lowest channel bars.--This section has been described by Moore (1968), Hedman (1970), and others. The reference level is defined by the tops of point or channel bars where these are (a) bars of which the particles are moved annually and (b) the lowest prominent channel bars. In some channels these bars do not exist. In some other channels these bars will be below the water surface for most of the year. The above description refers to perennial streams. In ephemeral streams, particles may not be moved annually but will be moved by significant flows. Care must be taken to avoid selection of erosional features in ephemeral streams. Consistency in identifying this section in ephemeral channels is difficult to attain. Active channel section.--This section has been described by Hedman and Kastner (1974) and by Hedman, Kastner, and Hejl (1974). The active channel is the lower portion of the channel entrenchment in the flood plain that is actively involved in the transportation of water and sediment during the usual regime of a stream. Depositional features within the active channel are altered and shifted regularly during the normal.fluctuation of streamflow. Beyond the boundaries of the active channel, the geomorphic features are relatively permanent and generally are vegetated. The sides of the active channel occur as steeply sloping banks in straight reaches and as stabilized point bars on the inside or convex side of a channel bend. The reference point used in measuring the geometry of the active channel is selected at the upper edge of the banks or point bars where they abruptly change to a flatter slope. This reference point is above and shoreward from the reference point defined by the lowest channel bars. Annual vegetation generally is present above the reference point and can often be used as a clue in identifying this point. However, judgment is required in using the vegetation line as a guide. In regions where high flows are infrequent, some grasses and sedges may grow part of the way down the banks or even into the water. On the other hand, in extremely arid regions the banks may be completely devoid of vegetation. If the selected reach meets the listed criteria, the active channel width will be the width of the low-water channel because there will be no. bars in a narrow channel. As the stream widens bars will form on one or both sides of the channel and the width between these bars or between one bar and the opposite bank (if only one bar is present) will be about the same as the width of the low-water channel in a restricted section. These bars will become vegetated in contrast with those describing the lower section. Main-channel section.--The reference level for this section is variously defined by breaks in bank slope, by the streamward edges of the flood plain, or by the lower limits of permanent vegetation. On perennial streams it is virtually the same as the flood-plain level described by Emmett (1972, 1973) and by Leopold, Wolman, and Miller (1964). In selecting the main channel width, one should avoid a high reference level (such as a terrace) that does not reflect the present flow regimen. This is most often a possibility on ephemeral streams, especially if the channel has been downcutting. Measurements 1. Measurements should be made by someone with field experience in selecting the features; and measurements should be made without knowledge of flow characteristics at the sites. 2. At selected reference level for each type of section measure width to two significant digits. 3. Measure depth at sufficient verticals to define shape of section. 4. Measure at 2 or 3 cross sections separated by at least one channel width if suitable ones are found. 5. If practical, measure local channel bed-slope or water-surface slope and record how determined (hand level, transit-stadia, etc.). 6. Photograph the reach and cross section, including the feature defining the edges of the section. 7. Record type of bed and bank material; sand, gravel, broken rock, clay, etc. For gravel or rock, determine approximate size range. 8. Rate the suitability of the section. 9. Record location of the section. Analysis of Data The purpose is to define an estimating relation. 1. For preliminary work, graphically relate the flow characteristic to width. If the flow characteristic is not well defined, plot the probable range rather than the point value. Identify on the graph any width measurements that are of questionable reliability. Draw the relation line so that most weight is given to the reliable data; a least squares fit to the data points is not necessarily the best interpretation. Channel depth is ordinarily not a significant variable but the ratio of depth to width may be if channels of unusual shapes are included in the data. Flood characteristics should be related to active-channel width or to whole-channel width, not to the width of the lowest channel feature. 2. Compare relation obtained in (1) with relations applicable to nearby regions. If the relations and the streams are similar, consider using one relation applicable to a large region. A single relation for flood characteristics has wide areal applicability whereas a relation to mean flow is much more limited. 3. Define separate relations of flood characteristics to widths of active-channel and whole-channel if data are available. 4. Do not combine basin and channel parameters in one relation. 5. Include basic data in the report. 6. Identify channels that don't fit the relations and look for reasons. Keep a record of these. 7. Original ideas and analyses are encouraged, but a user report should only document defined relations; it should avoid describing how to measure a channel. 8. Reports for field use will be reviewed in the Surface Water Branch to maintain some uniformity. Application of Relations 1. This should be done by someone with field experience in selecting the features; if this experience is not available consulting services should be sought. 2. Look for suitable reach. If there is not a suitable one near the point of interest, it may be possible to find ones some distance upstream and downstream; then the discharge at the point of interest can be interpolated. 3. Select one to three cross sections in each reach and measure as previously described at one or more reference levels in each cross section if these levels can be identified. Commonly it will not be possible to identify all three levels at one cross section; in some sections only one reference level can be identified with confidence. 4. Determine discharge from the appropriate relation. SELECTED REFERENCES (Some describe recent work but these should not be used as models because they do not conform strictly to the guidelines.) Emmett, W. W., 1972, The hydraulic geometry of some Alaskan streams south of the Yukon River: USGS open-file report, 102 p. Emmett, W. W., 1973, The channels and waters of the upper Salmon River area, Idaho: USGS open-file report, 467 p. Hedman, E. R., 1970, Mean annual runoff as related to channel geometry of selected streams in California: USGS Water-Supply Paper 1999 E. Hedman, E. R. and Kastner, W. M., 1974, Progress report on streamflow characteristics as related to channel geometry of streams in the Missouri River basin: USGS open-file rept., 24 p. Hedman, E. R., Kastner, W. M., Hejl, H. R., 1974, Selected streamflow characteristics as related to active-channel geometry of streams in Kansas: Kansas Water Resources Board Tech. Rept. no. 10, 21 p. Hedman, E. R., Moore, D. 0., Livingston, R. K., 1972, Selected streamflow characteristics as related to channel geometry of perennial streams in Colorado: USGS open-file rept. Langbein, W. B., 1971, Estimation of annual runoff from channel surveys: USGS Water Resources Bull., Apr. - Sept. 1971, p. 3-5. Leopold, L. B. and Maddock, Thomas, Jr., 1953, The hydraulic geometry of stream channels and some physiographic implications: USGS Prof. Paper 252. Leopold, L. B., Wolman, M. G., Miller, J. P., 1964, Fluvial processes in geomorphology: San Francisco, W. H. Freeman, 522 p. Moore, D. 0., 1968, Estimating mean runoff in ungaged semiarid areas: IAHS Bull., V. XIII, no. 1, p. 28-39. Moore, D. 0., 1974, Estimating flood discharges in Nevada using channel-geometry measurements: Nevada Highway Dept., Hydrologic Rept. no. 1, 43 p. Riggs, H. C., 1974, Flash flood potential from channel measurements: IAHS Publ., no. 112, Proc. of Paris Symposium, p. 52-56. Wahl, K. L., 1974, Relating streamflow characteristics to channel cross-section properties - a literature review: Unpublished but available on request from Surface Water Branch, 36 p.