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<br />INTRODUCTION <br /> <br />An urban highway sideslope may be either bare, <br />paved, or grassed. A sideslope exposed with bare soil <br />is normally seen only during its construction or <br />shortly after its construction prior to grass growth. <br />Regardless of whether or not the soil surface of a <br />sideslope is covered with grass, infiltration plays a <br />significant role in the rainfall-runoff process which is <br />reflected in the total runoff from an urban highway <br />watershed. The successful modeling of surface runoff <br />porn such a small watershed thus hinges greatly on <br />how accurately one can evaluate the infiltration <br />amount or rate during and after a rainstorm. For <br />example, a poor estimate on the infiltration capacity <br />for a given soil-cover-moisture complex of a sideslope <br />may result in unrealistically low or high flow rates <br />computed for a drainage inlet by means of one of <br />existing surface runoff models. In view of the fact <br />that the sideslopes may have various soH strata <br />including topsoils and subsoils, several species of <br />grass, and different degrees of antecedent moisture <br />content, among many other factors which may <br />influence infiltration, development of a general in- <br />filtration model which accounts for all pertinent <br />variables is a formidable task, No attempt was made <br />to develop such a general model. Instead, existing <br />parametric (algebraic) infiltration equations were <br />used to formulate "standard" infiltration-capacity <br />curves for soil-caver-moisture complexes representing <br />highway sideslopes, It is noted that the standard <br />curves may also provide a basis for classifying or <br />grouping soil-cover-moisture complexes as related to <br />their fmal infiltration capacity (Musgrave, 1955; <br />Musgrave and Holtan, 1964). Therefore, time.varying <br />infiltration characteristics of a given soil-cover- <br />moisture complex on a highway sideslope can be <br />expressed in terms of a unique infiltration capacity <br />curve which, after being described mathematically, <br /> <br />can be integrated into a general surface runoff model <br />for inlet hydrograph computations. <br /> <br />Following a brief literature review on rain <br />infiltration, a mathematical model of the one~ <br />dimensional infiltration is formulated and solved <br />numerically. The primary objective of formulating <br />and solving such an idealized mathematical model is <br />to use its numerical solution as a basic testing tool in <br />the subsequent analyses of various parametric infiltra- <br />tion models. It was felt that this mathematical tool <br />was necessary in the validation of the parametric <br />infiltration models for lack of reliable experimental <br />data available in the present study, Laboratory <br />observations were made of the effects of various <br />properties of soil, rain, and grass and different bed <br />slopes on the infiltration capacity using a computer- <br />controlled rainstorm simulator. Conclusive results <br />were not obtained because of instrumentation failure <br />in some data acquisition systems. <br /> <br />A method was developed to relate the standard <br />infiltration-capacity curves for given soil-cover- <br />moisture complexes to the corresponding Soil Con~ <br />servation Service (SCS) runoff curve numbers (CN) <br />for hydrologic soil-cover-moisture complexes. The <br />parameters in the infiltration model were related to <br />the runoff CN so that given a CN value, the <br />corresponding infiltration model parameters could be <br />evaluated from such relationshIps and hence the <br />standard inf1J.tration-capacity curve constructed. The <br />practical use of this method is evident because some <br />easily applicable relationships between the infIltration <br />parameter values and runoff CN are readily obtained <br />or estimated from the SCS hydrologic groups of given <br />soil-cover-moisture complexes. <br /> <br />1 <br />