Hydrologie and hydraulic research in mountain rivers

Transcription

1 Hydrology of Mountainous Areas(Proceedings of the Strbské Pleso Workshop, Czechoslovakia, June 1988). IAHS Publ. no. 190, Hydrologie and hydraulic research in mountain rivers R.D. JARRETT U. S. Geological Survey- Denver, Colorado, U.S.A ABSTRACT Although our current (1988) knowledge of hydrologie and hydraulic processes is based on many years of study, there are river environmets where these processes are complex and poorly understood. One of these environments is in mountainous areas, which cover about 25 percent of the United States. Use of conventional hydrologie and hydraulic techniques in mountain river environments produce erroneous results and interpretations in a wide spectrum of water-resources investigations. An ongoing U.S.Geological Survey research project is being conducted to improve the understanding of hydrologie and hydraulic processes of mountainous areas and to improve the results of subsequent hydrologie investigations. Future hydrologie and hydraulic research needs in mountainous areas are identified. INTRODUCTION Although our current knowledge of hydrologie and hydraulic processes is based on many years of study, there are river environments where these processes are complex and poorly understood. Most hydrologie and hydraulics techniques have been developed in lower gradient rivers (slopes less than meter per meter) and nonerodible channels and are unverified (Jarrett, 1984). Because of rapidly increasing suburban development in mountainous areas during the past 10 to 20 years, hydrologie, hydraulic and related studies in mountain areas have increased. These studies include determining the quality (low-,mean-and flood flows) and quality of streamflow, determining routing of flow, measuring sediment transport, determining the effects of acid precipitation, modeling precipitation-runoff, studying geochemical processes, and assessing the effects of climate change. Hydrologie and hydraulic techniques used in these studies generally are unverified and, in some instances, these techniques are inadequate for studying rivers in mountainous areas because they were developed for lower gradient rivers. This paper describes a U.S. Geological Survey research investigation that is underway to improve the understanding of hydrologie and hydraulic processes in mountainous areas. Mountainous areas cover about 25 percent of the United States. To improve the results of subsequent hydrologic investigations, basic research is needed to under- 107

2 R. D. Jarrett 108 stand the physical hydrologie and hydraulic processes. Much of the initial research has been completed in Colorado, which is one of the most mountainous areas of the United States. In Colorado, fifty-three peaks are greater than 4,260 meters above sea level and an additional 1,000 peaks are higher than 3,050 meters. Much of this research is applicable to other mountainous areas. Interrelated process research are discussed in this paper for hydrologie and hydraulic investigations, and sources of data errors are identified. HYDROLOGIC RESEARCH A multidisciplinary approach based on analyses of streamflow and precipitation data and the use of paleohydrologic techniques was used to improve the understanding of flood hydrology in Colorado (Jarrett and Costa, 1983; Jarrett, 1987a). The study area included the foothills and high mountains:.'- of Colorado. Flooding on rivers in these areas results from rapid snowmelt, excessive rainfall, or a combination of both; hence, flood peaks exhibit a mixed-population process. The need for this study was emphasized by the 1976 Big Thompson River flash flood in the foothills of eastern Colorado. This flood, the largest natural disaster in Colorado history, killed 144 people and resulted in over $35 million in property damages. The subsequent difficulties in interpretation of this and other catastrophic floods, using conventional techniques, indicate that new methods, or modifications to existing procedures, are needed. Separation and mixed-population analyses of annual peaks into rainfall-runoff and snowmelt-runoff flows were made for 6 9 streamflow-gaging stations in the foothills and mountains of Colorado. Four flood-frequency curves were developed and are shown in Fig. 1 for analyses of: annual peak flows; annual snowmelt peak flows; annual rainfall peak flows, and; a composite curve of the snowmelt and rainfall curves. When snowmelt- and raingenerated annual peak flows are examined separately, flood-frequency analyses (using guidelines of Interagency Advisory Committee on Water Data, 1981) indicate distinctive trends based on elevation. Above about 2,300 meters, low-magnitude snowmelt flows predominate; rain generally does not contribute to the flood potential as shown for a higher elevation site in Figure la. Maximum snowmelt flows at elevations above 2,300 meters have unit discharges of less than 2.2 cubic meters per second per square kilometer. Below about 2,300 meters, large magnitude rainfall-produced floods predominate, as shown for a lower elevation site in Figure lb. Maximum rainfall floods below 2,300 meters have unit discharges greater than 22 cubic meters per second per square kilometer.

3 109 Hydrologie and hydraulic research in mountain rivers Fig. la Flood-frequency curves for Clear Creek near Lawson, Colorado (modified from Jarrett, 1987a). Gage elevation is 2,463 meters and drainage area is 4.16 square kilometers Fig. lb EXCEEDANCE PROBABILITY Flood-frequency curves for Clear Creek near Golden, Colorado (modified from Jarrett, 1987a). Gage elevation is 1,748 meters and drainage area is 11.3 square kilometers

4 R. D. Jarrett 110 Separation of peak flows by météorologie cause provides an improved understanding of flood hydrology. The composite flood-frequency curves result in improved streamflow-gaging station flood-frequency estimates. This occurs primarily because the data are separated into homogenous populations. Large floods, previously identified as high outliers, generally are no longer classified as high outliers after are separated and analyzed as snowmelt or rainfall arrays. The upper elevation limit for significant rainfall flooding is latitude dependent (and probably worldwide); as the latitude increase, the elevation boundary between snowmelt- and rainfall-produced flood decreases. Because of the relatively limited number of streamflowgaging stations in the foothills and mountains of Colorado, paleohydrologic techniques were developed and incorporated into the hydrologie analyses. Paleoflood hydrology is the study of the movement of water and sediment in channels before the time of systematic streamflow-data collection (Costa, 1986). Historic and prehistoric floods in rivers are recorded in distinctive deposits and landforms in and along channels. Interpretation of these deposits and landforms and radiocarbon dating of paleoflood deposits provide supplemental information about the spatial occurrence, magnitude, and age of floods (Costa, 1978a, 1978b; Jarrett, 1987a). Extensive paleoflood investigations of channel features add supporting documentation to the theory that there is a total lack of large rainfall floods above about 2,300 meters. However, below 2,300 meters, evidence of multiple large rainfall floods is abundant in every channel. To understand runoff from mountainous areas, the causative hydrologie processes, particularly precipitation, need to be understood. Intense rainfall data for Colorado, for gaged and bucket-survey rainfall sites obtained since before 1900, were summarized. These rainfall data indicate that below 2,300 meters point rainfall commonly exceeds 25 centimeters in 6 hours, and for several storms has exceeded 50 centimeters in 6 hours. However, above 2,450 meters there has been no documented rainfall greater than about 5 centimeters in 6 hours. Flood-frequency relations at streamflow-gaging stations are well documented but also are needed at ungaged sites. Mixed-population analyses and paleoflood investigations helped define regionally homogenous flood regions of snowmelt and rainfall. Flood characteristics for ungaged sites were obtained by regionalizing streamflow data for sites in.each region on the basis of physical and climatic basin characteristics. The resulting regression equations dramatically improved the accuracy (standard error of estimate improved from 142 to 44 percent) when mixedpopulation processes are accounted for. By use of a multidisciplinary approach, this study led to improved

5 Ill Hydrologie and hydraulic research in mountain rivers flood estimates beyond 100-year recurrence intervals. Regional analyses, supported by radiocarbon dating of flood deposits (Costa, 1987b), indicate that the 1976 Big Thompson River flood had a recurrence interval of about 10,000 years (Jarrett, 1987a) The results of this mixed-population hydrologie and paleohydrologic research have improved the understanding and techniques for assessing precipitation and streamflow characteristics in the Rocky Mountains. The research has indicated that there are many different types of mixedpopulation processes in all parts of the United States. Additional research is needed to 1) determine the boundaries that separate between mixed-population regimes in other mountainous regions, 2) understand the different types of mixed-population hydrologie processes and to improve the methods of analyses, and 3) improve paleohydrologic techniques to extend hydrologie data spatially and temporally. HYDRAULIC RESEARCH Hydraulic investigations have been made for 32 higher gradient rivers (slopes greater than meter per meter) in Colorado to improve the understanding of hydraulic processes in mountain rives (Jarrett, 1984, 1985; Marchand and others, 1984). Mountian rivers are characterized by very turbulent flow and relatively coarse bed material. Investigations have been made to quantify flow resistance, evaluate velocity profiles for determining mean streamflow velocity and the shape of the velocity profile, and to evaluate the accuracy of current meters. This research has indicated that hydraulic processes in mountain rivers are poorly understood. Guidelines (Chow, 1959; Barnas, 1967) are available to aid in the selection of roughness coefficients of rivers. In the United States, Manning's coefficient n is most commonly used to quantify flow resistance. However, most flow resistance coefficients verified are for sites on relatively lower gradient streams. Data collected by Jarrett (1984) indicate that n values are much larger on higher-gradient, cobble- and boulder-bed rivers than on lower gradient streams that have similar relative roughness values. Flow regime, previously thought to be supercritical, has been determined to be subcritical (Jarrett, 1984). Because higher gradient stream data indicate that flow resistance varies considerably with depth of flow, equations were developed to assist in the assessment of flow resistance in mountain rivers. Onsite surveys and 75 current-meter measurements of discharge, were made on 21 mountain rivers in Colorado. Multiple regression analyses of Manning's n were related to measured hydraulic and sediment-size data. The analyses indicated that n varies directly with slope and inversely with depth of flow. The equation developed for

6 R. D. Jarrett 112 predicting Manning's n in natural mountain channels with cobble or boulders bed material is: n = 0.32 S 0-30 R-0.16 (1) where S in meter per meter, is the slope of the energy gradient (or friction slope) R in meters, the hydraulic radius, is a measure of the boundary area that causes friction per unit of flow. If the channel is relatively uniform, water surface or bed slope can be used in Equation (1). Regime flow equations for predicting velocity and discharge in mountain rivers also were developed (Jarrett, 1984). The standard error of estimate of equation 1 was 28 percent for the Colorado data. The equation had the same accuracy when other data for higher gradient rivers of the world (slopes as high as 0,052 meter per meter) were used. Paul and Dhillon (1987) verified the accuracy of Equation 1 with other higher gradient, mountain river data of the world. Equation (1) is applicable for relatively clear water flow instable channels with minimal bank vegetation, regular banks, and few obstructions. Equation (1) is defined for slopes from to meter per meter and for hydraulic radii from 0.15 to 2.2 meters. Existing methods for determining the mean flow velocity in the vertical do not address the conditions present in high gradient, shallow-depth rivers that are common to mountainous areas. The Price AA current meter registers vertical-velocity components under turbulent conditions (Townsend and Blust, 1960); hence, recorded velocity is greater than the actual longitudinal velocity. However, the vertical-velocity component has been considered minimal for most measured rivers. Verticalvelocity profile, water-surface slope, and bed-material size were collected for 11 streamflow-gaging stations in Colorado using a standard Price AA current meter and a prototype Price PAA current meter (Marchand and others, 1984). The Price PAA current meter incorporates a Lexan polycarbonate polymer, solid-cup bucket wheel in place of the open-cup metal bucket wheel common to the Price type AA meter. The prototype current meter virtually eliminates the registration of vertical-velocity components in turbulent rivers. Vertical-velocity profiles (with 8 to 10 point velocities) were measured with the Price AA and Price PAA current meters at 3 to 4 locations at each river cross section for a total of 94 velocity profiles. The investigation included wading, cable, and bridge measurements. For vertical, velocity profiles were made using data from each type of meter. A typical profile, with a logarithmic velocity profile superimposed,- is shown in Fig. 2.

7 113 Hydrologie and hydraulic research in mountain rivers 0.0 \ Water surface Logarithmic curve 1 / o h > LU -. Q 3r Z ΠO UJ < > DC 0.4 Price A A Current Meter, (V = 0.82 meters per second) y/y=0.5, v Price PAA Current Meter (V = 0.74 meters per second) CO S o i o h- < Vv \/Y=0.6 Y = 0.67 meter _L _L j _ RATIO OF POINT VELOCITY TO MEAN VELOCITY IN THE VERTICAL (v/v) Fig.2 Velocity profiles, Lake Creek above Twin Lakes Reservoir, Colorado, at station 7 meters, August 16, 1983 (modified from Marchand and others, 1984) Intermediate bed-material diameter is 247 milimeters and the water-surface slope is meter per meter Mountain river velocity profiles are S-shaped and nonlogarithmic; velocities are lower near the streambed and greater near the water surface than for a logarithmically distributed profile. A logarithmic velocity profile does not develop because of the extreme drag from the cobble and boulder bed material and the high velocity flow near the water surface. The estimated mean velocity in the vertical, taken as the velocity measured at 0.6 depth (the conventional practice), consistently is smaller than the true mean velocity in the vertical. For Lake Creek, shown in Figure 2, the mean velocity needs to be obtained at about 0.5 depth of flow. Hence, the mean velocity determined by the 0.6 depth of flow will consistently underestimate the mean velocity in the vertical. The velocity profiles for all Colorado river data were averaged to provide a mean velocity profile for all sites for each current meter. The mean velocity profile for each meter, with a logarithmic velocity profile superimposed, is shown in Figure 3. The mean velocity profile also is S-shaped and nonlogarithmic. One method of measuring mean velocity in the vertical is to measure a near water surface velocity and divide by a coefficient

8 R. D. Jarrett 114 of about 1.18 from the logarithmic velocity profile (Fig.2). For mountain rivers, the near surface coefficient is about 1.5 (Fig.3): The commonly used coefficient of 1.18 will consistently result in overestimated mean velocity in the vertical in mountain rivers. 0.2 X -Price Meter << 5 Solid-cup price meter RATIO OF POINT VELOCITY TO MEAN VELOCITY IN THE VERTICAL Iv/V) Fig.3 Mean vertical-velocity profile for mountain rivers that have slopes ranging from to meter per meter The comparison of the mean velocity in the vertical data for each type of meter indicates the Price AA current meter consistently overregisters point velocity and mean velocity (Fig.2). For all Colorado data, the Price AA current meter overregistered mean vertical velocity by an average of about 5 percent. However, the data indicate that as stream slope increases, the percent overregistration increases to as much as 25 percent. Additional hydraulic research includes a) improving the understanding of hydraulic processes in other turbulent rivers (such as in alluvial rivers), and b) improving methods of measuring and estimating velocity and discharge in mountain rivers. Research of hydraulic processes of mountain rivers encompases an analysis of existing U.S.Geological Survey data collected as part

9 115 Hydrologie and hydraulic research in mountain rivers of the Colorado study, compilation of other available data, and use of data collected in the tilting flume at the U.S.Geological Survey Gulf Coast Hydroscience Center in Mississipi. Presently (1988) velocity profile measurements, comparative point-velocity data with different current meters, flow resistance measurements, and alternative methods for indirectly measuring peak discharge for ranges of slope, discharge, and bed-material configurations have been collected in the flume, but have not yet been analyzed. Additional configurations need to be run. Additional instream data for different conditions (for example, to identify where turbulence can affect the performance of the Price AA current meter) need to be collected and analyzed to assess the magnitude and range of stream conditions where velocity is not measured correctly. The data base for Colorado is primarily for shallow-depth, higher gradient rivers; hence, additional hydraulic data need to be collected (and analyzed) in mountain rivers having greater depths. HYDROLOGIC AND HYDRAULIC DATA ERRORS One of the basic premises of hydrologie and hydraulic investigations is that the data are accurate and representative. In studies of mountainous areas, precipitation and streamflow data may contain significant errors. Jarrett (in press) identified errors in precipitation and streamflow data in mountainous areas. Extremes of hydroclimatic data are of special concern because they commonly contain large and sometimes biased errors. Before a hydrologic/hydraulic investigation is started, it is essential to ascertain that data or the methods (and equations) to be used are valid and applicable to the study area. An understanding of the source, type, and magnitude of data errors is needed to mitigate their effect in hydrologic/hydraulic studies. It is important to identify data that have significant errors because use of these data may produce invalid conclusions in subsequent investigations. For example, an evaluation of 7 0 slope-area measurements (the most common indirect method of estimating flood discharge) was made for higher gradient rivers throughout the United States (Jarrett, 1987b). The analyses included an evaluation of flood measurements from other countries. Errors in flood measurements in higher gradient rivers are common. These errors were associated with underestimated in values, incorrect evaluation of scour, expansion and contraction losses, viscosity, unsteady flow, number of cross sections, state of flow, and large stream slope. Measurement errors typically were as large as 75 to 100 percent or more (Jarrett, 1987b). These common problems consistently resulted in overestimated (or biased) peak discharge in mountain rivers. Also,

10 R. D. Jarrett 116 debris flows commonly have been misinterpreted as water floods, resulting in excessively overestimated discharges in mountain streams (Costa and Jarrett, 1981; Jarrett, 1987a). Knowledge of data errors, problems with application of measurement techniques, and overestimated peak discharge is essential because the values of the largest floods (in terms of unit discharge) commonly are the most erroneous. The most accurate flood measurements (based on hydraulic and hydrologie criteria) on higher gradient rivers probably will result from critical-depth measurements (Jarrett, 1984, 1987b; Jarrett and i^alde, 1987; Trieste and Jarrett, 1987). CONCLUSIONS The goal of this U.S.Geological Survey research is to improve the understanding of hydrologie and hydraulic processes in mountain rivers. Continued hydrologie, hydraulic, and paleohydrologic research on mountainous areas is needed and will benefit a broad range of hydrologic research projects and investigations. An improved understanding of basic hydrologie and hydraulic processes will improve methods of assessing the quantity and quality of surface water. These related studies rely on accurate data and hydrologie methods. Improved hydraulic methods can be incorporated into numerical simulation models of surface-water systems to be used to improve the analyses of hydrologie processes. The results of this research are applicable to other mountain rivers. REFERENCES Barnes, H.H.,Jr. (1967) Roughness characteristics of natural channels: U.S.Geological Survey Water-supply Paper 1849, 213 p. Chow, V.T. (1959) Open-channel hydraulics: New York, McGraw Hill, 68 0 p Costa, J.E. (1978a) Holocene stratigraphy in flood-frequency analyses: Water Resources ^search v.14, no.4, p Jarrett, R.D. (1978b) Colorado Big Thompson Flood, Geologic evidence of a rare hydrologie event: Geology v.6, p Jarrett, R.D. (1986) A history of paleoflood hydrology in the United States, : America! Geophysical Union, v.67, no.17, p Costa, J.E. and Jarrett, R.D. (1981) Debris flows in small mountain stream channels of Colorado and their hydrologie implications: Bulletin of the Association of Engineering Geologists v.18, no.3, p Interagency Advisory Committee on Water Data (1981)

11 117 Hydrologie and hydraulic research in mountain rivers Guidelines for determining flood-flow frequency (2d éd., revised): Reston, Va., U.S. Geological Survey Office of Water Data Coordination, Hydrology Subcommittee Bulletin 17B Jarrett, R.D. (1984) Hydraulics of high gradient streams: Journal of Hydraulic Engineering, American Society of Civil Engineers, v.110, no.11, p Jarrett, R.D. (1985) Analyses of vertical-velocity profiles in higher-gradient streams in Colorado: EOS, American Geophysical union, v.66, no.46, p.912 Jarrett, R.D. (1987a) Flood hydrology of foothill and mountain streams in Colorado: Fort Collins, Colorado State University, Department of Civil Engineering, Ph.D. dissertation, 239 p. Jarrett, R.D. (1987b) Errors in the slope-area method of computing peak discharge in mountain rivers: Journal of Hydrology, v.96, p Jarrett, R.D. (in press) Hydroclimatic data errors and their effects on the perception of climate change, In: Workshop on Monitoring Climate for the Effects of Increasing Greenhouse Gas Concentrations, Pingree Park, Colorado, 1987, Proceedings: Fort Collins, Colorado State Univesity, Cooperative Institute for Research in the Atmosphere Jarrett, R.D. and Costa, J.E. (1983) Multidisciplinary approach to the flood hydrology of foothill streams in Colorado, In Johnson, A.I., and Clark, R.A. (eds.) International Syposium on Hydrometeorology, Denver, Colo., 1982, Proceedings: Bethesda, Maryland, American Water Resources Association, p Jarrett, R.D. and Malde, H.E. (1987) Paleodischarge of the late Pleistocene Bonneville Flood, Snake River, Idaho, computed from new evidence: Geological Society of America Bulletin, v.99, no.l, p Marchand, J.P., Jarrett, R.D. and Jones, L.L. (1984) Velocity profile, water-surface slope, and bed-material size for selected streams in Colorado: U.S. Geological Survey Open-File Report , 82 p.

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