伊利诺斯州河流曼宁糙率系数的估算

230
tions. Part of the challenge also results because of the lack of understanding characteristic changes of n-values in natural environment settings. Filed-verified Manning’s roughness coefficients in Illinois streams from a current study by the U.S. Geological Survey are analyzed for the variation of n-values with discharge (and, therefore, stage). Study results indicate that n-values could be decreasing, invariant, or increasing as discharge increases. The pattern of decrease, invariance, or increase depends on the relative magnitudes of base n-values of the banks and bed. The pattern does not depend on whether the channel is natural or constructed. However, patterns of change for nondimensional n-value computed from constructed channels are appreciably different from those computed from natural stream in Illinois. Data analyses indicate that nondimensional n-values computed from constructed channels remain a functional relation with Reynolds number in the rough flow regime. INTRODUCTION Roughness coefficients, where magnitudes represent the degree of resistance to flow motion from fluid boundaries, are used to estimate flow velocity or discharge in conduits or open channels. The coefficients are used to estimate the frictional losses during hydraulic processes stipulated in the governing equations for erosion or sedimentation, or constituent-transport analyses. Although the magnitudes of the coefficients are numerically small, their range of reasonable values in application is important because results of these analyses directly affect the design, operation, and management of water-resources projects. Several roughness coefficients, including Weisbach’s f, Chezy’s C, and Manning’s n, have been used in various studies. However, Yen (1992) pointed out that our understanding of these coefficients is based mainly on steady uniform flow in simplistic channel geometry, axis-symmetric circular pipes, or widerectangular channels, with rigid boundaries and without lateral inflows. Among the roughness coefficients mentioned above, Manning’s n is used most by hydraulic engineers because of it is more reliable in most practical applications (Henderson, 1966). Many methods for estimating n-values have been proposed; but an appropriate method that can be applied to systematically estimate the n-values in natural channels still is lacking at present. The lack of better understanding of n-values results in part because of the dimension of Manning’s n, if the n-values were evaluated as point, cross-sectional, or reachwise values; or if the n-values were estimated by momentum or energy approaches (Yen, 19ogical Survey (USGS) Illinois District, in cooperation with the Illinois Department of Natural Resources – Office of Water Resources (IDNR-OWR), have collected field data for assessing n-values for Illinois streams. As of 2004, data have been collected from 29 study reaches including both natural and constructed streams, and methods for improving visual presentation for estimating n-values (Barnes, 1967) have been developed. Discharges ranging from base-flow to flood conditions at each study reach, bed and bank materials, cross-sectional geometries, and reach layouts are considered. The primary purpose of this paper is to describe the characteristic changes in Manning’s n with discharge (and, therefore, stage) based on data collected in Illinois streams. The n-values presented here are considered as one-dimensional and reachwise, and were computed using the energy approach. BACKGROUND In steady-uniform flow, Manning’s equation relates the average velocity (V) to crosssectional geometry parameter R (hydraulic radius) and slope (S) through Manning’s n as
Estimating Manning’s roughness coefficients for Illinois streams
D.T. Soong
U.S. Geological Survey, Water Resources Discipline, Illinois District, USA ABSTRACT Estimating the magnitudes of Manning’s roughness coefficient, n, in natural and constructed channels remains a challenge to the scientific community. Part of the challenge results because of the lack of clarification in distinguishing the derivation and subsequent use of nvalues in momentum or energy approaches, or for point, cross-sectional, or reachwise applica-
V =
k n 2 3 12 R S , n
(1)
where kn is a conversion factor equal to 1 m1/3/s in metric units (Yen, 2002). Both R and V are evaluated at cross sections, but S is estimated for the study reach and is the expression for flow resistance. The slope term can be determined using the momentum or energy definitions, which are different in concept and in numerical values (Yen, 1992). The numerical values of momentum and energy slopes are equal to the channel slope only for steady-uniform flows in prismatic and impervious channels without lateral inflows (Yen, 2002). The momentum and energy slopes also are different from the generally used channel bed slope, water surface slope, or total head slope. Using the energy approach, the total heads at cross-sections 1 (upstream) and 2 (downstream) of a study reach are balanced as