Chemical Tracing Techniques for Evaluating Rill Hydraulics

Development of water erοsiοn and surface water quality control practices requires information concerning the hydraulic characteristics of upland areas. The relatively small flow rates normally found within rills make measurement of hydraulic parameters difficult. Chemical tracing procedures, originally developed for stream and river systems, have been successfully used to measure rill flow properties. A chemical tracer of known concentration is added to the rill and by knowing the degree of dilutiοn at a downstream sampling point, flow rate can be calculated. Rill flow velocity can be measured by determining the time required for a slug of tracer material to travel a designated distance. Measurements of flow rate and velοcity can be used tο calculate οther hydraulic variables. The ability to understand and properly mοdel rill flοw will improve as additional information [is gathered]

Beginning of Motion for Selected Unanchored Residue Materials

Conservation tillage systems help to maintain residue materials from the previous crop on the soil surface. The potential for serious erosion may exist if crop residues are removed by overland flow. This study is conducted to identify the hydraulic conditions required to initiate residue movement by overland flow. Corn, cotton, peanut, pine needles, sorghum, sunflower, and wheat residue are placed in a flume on smooth and sand surfaces, and flow is then introduced in progressive increments. The discharge rate and flow velocity required to initiate residue movement are identified. Hydraulic measurements are used to calculate the ratio of critical flow depth to residue diameter, critical Reynolds number, critical shear stress, dimensionless shear stress, and boundary Reynolds number. Regression equations are developed to relate dimensionless shear stress to boundary Reynolds number. Close agreement is found between predicted and actual dimensionless shear stress. If residue diameter is known, the regression equations can be used to estimate the beginning of motion for other residue materials. Information obtained in this study can be used to help identify proper residue management practices for conservation tillage systems

Maximum Surface Storage Provided by Crop Residue

Small ponds created by crop residue serve to store water on upland areas. The present study is conducted to identify the maximum surface storage provided by crop residue. Equations for estimating surface storage are derived assuming that residue elements were oriented end to end, at uniform intervals, in a direction perpendicular to flow. Separate equations are developed for conditions where average slope was greater than or equal to residue cover, or less than residue cover. Both equations relate average surface-storage depth to residue cover, residue diameter, and average slope. Data to test the reliability of the equations are obtained in a laboratory investigation. Artificial residue elements are glued onto a 1- m 2 test section positioned at slopes of 1%, 10%, or 20%. Four sizes of residue elements and three surface-cover conditions are examined. Surface-storage depth for each experimental condition is measured. Close agreement is found between predicted and measured surface-storage values. Surface storage occurring under field conditions may be substantially less than the estimates obtained using the predictive equations

Darcy-Weisbach Roughness Coefficients for Surfaces with Residue and Gravel Cover

Several types of hydraulic resistance factors may be present on upland agricultural areas. It is not known whether roughness contributions from individual elements are additive or if interactions between resistance factors may occur. In this study, Darcy-Weisbach roughness coefficients were measured on surfaces containing corn-soybeans, sorghum-cotton, and sunflower-wheat residue in addition to gravel cover. Varying rates of flow were introduced into a flume in which residue and gravel materials were securely attached. Roughness coefficients were calculated from measurements of discharge rate and flow velocity for Reynolds number values varying from approximately 1,200 to 13,000. The laboratory data were then used to identify the contribution to total hydraulic resistance provided by the different types of resistance elements. For most of the experimental treatments, the addition of smaller diameter residue materials (soybeans, cotton, or wheat) to surfaces containing larger resistance elements (corn, sorghum, or sunflower) did not significantly affect hydraulic resistance. However, smaller diameter residue materials did influence hydraulic resistance when they substantially increased the total volume of resistance elements. Existing roughness coefficient values were not significantly affected by the presence of gravel materials with diameters similar to the larger residue materials. The experimental results suggest that total hydraulic resistance cannot be predicted by simply adding the contributions provided by individual resistance elements. When estimating total hydraulic resistance on upland agricultural areas, the relative size, number, and volume of resistance elements must be considered

Darcy-Weisbach Roughness Coefficients for Gravel and Cobble Surfaces

A laboratory study is conducted to measure Darcy-Weisbach roughness coefficients for selected gravel and cobble materials. Varying rates of flow are introduced into a flume in which a given size class of gravel or cobble material is securely attached. Roughness coefficients are calculated from measurements of discharge rate and flow velocity. The laboratory data are used to develop regression equations for relating roughness coefficients to surface cover and Reynolds number. Accurate prediction of roughness coefficients for gravel and cobble surfaces will improve our ability to understand and properly model upland flow hydraulics

Roughness Coefficients for Selected Residue Materials

Analysis of surface runoff on upland areas requires identification of roughness coefficients. A laboratory study is conducted to measure Darcy-Weisbach and Manning roughness coefficients for corn, cotton, peanut, pine needles, sorghum, soybeans, sunflower, and wheat residue. Varying rates of flow are introduced into a flume in which selected amounts of residue are securely attached. Roughness coefficients are calculated from measurements of discharge rate and flow velocity. The laboratory data are used to derive regression equations for relating roughness coefficients to Reynolds number and either percent residue cover or residue rate. Separate equations are developed for Reynolds number values from 500 to 20,000, and from 20,000 to 110,000. Generalized equations are presented for estimating roughness coefficients for other residue materials not used in this investigation. Accurate prediction of roughness coefficients for residue materials will improve our ability to understand and properly model upland flow hydraulics

Closure to Darcy-Weisbach Roughness Coefficients for Gravel and Cobble Surface

The writers appreciate the interest expressed by the discussers in this manuscript, and are pleased to have the opportunity to further discuss this material. The discussion states that the writers have generally examined a condition already investigated in other previous studies. Reynolds number values and roughness element size for the articles referenced by the discussers are shown in Table 5. Since flow rate and Reynolds number values were not given by Ferro and Giordano (1991), data from this study are not included in Table 5. It can be seen from Table 5 that the roughness element sizes examined by Bathurst (1978) were much larger than those used by the writers. Each of the other studies was conducted to obtain information for use on river systems. The focus of this paper, in contrast, was upland areas. Thus, Reynolds number values employed by the other authors were substantially larger than those used in this investigation. The discussers reference material presented by Colosimo et al. (1988) to characterize flow conditions occurring in this study. It can be seen from Table 5 that the smallest Reynolds number value used by Colosimo et al. (1988) was 25 times greater than the largest Reynolds number value employed in this investigation. Certainly for river systems, with flow depths much greater than roughness element heights, Reynolds number may have a minimal effect on friction factor. However, for upland areas with much smaller water depths, friction factors may be substantially affected by Reynolds numbers. This influence is clearly shown in Figs. 2, 3, and 4