Date of Award:

1995

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Advisor/Chair:

David S. Bowles

Abstract

Every dam must be capable of safely passing a predetermined flood magnitude. For high-hazard dams, it is a common practice to require that this flood be the Probable Maximum Flood (PMF). The determination of the PMF starts with the determination of the Probable Maximum Precipitation (PMP). We used the generalized estimates of the PMP as outlined in Hydrometeorological Report (HMR) 49. In this study we used the storm event model approach to convert the PMP into PMF. Different synthetic unit hydrograph (SUH) techniques were then used in the conversion process. The Soil Conservation Service (SCS) curve number method was used to estimate the losses, and the U.S. Bureau of Reclamation (USBR) lag time relationship was used as the basis for estimating the time parameters for the different (SUH) methods.

The objectives of this study were 1) to evaluate the theoretical and empirical basis for the SUH methods that are commonly used for Utah PMF determinations; 2) to compare the PMF determinations for representative Utah watersheds based on alternative SUH methods using Geographic Information System (GIS) techniques, and to explore the effects of parameter uncertainty ; and 3) to make recommendations for the selection and use of SUH methods for Utah PMF determinations.

An interactive PMF modeling system was formulated. The modeling system processes the different databases and estimates the parameters required for HEC 1 model input to produce the PMF hydrograph. Five SUHs were used, tbe SCS, Clark, Snyder, USBR, and the Corps of Engineers (COE) LA valley S-graph. Seven representative Utah high-hazard dam sites were selected and used in the evaluation of the five SUH techniques, focusing on their procedure, practice and applicability, and analytical and empirical evaluations.

GIS procedures proved to be a very efficient and flexible means for obtaining rainfall-runoff model inputs. Deviation of the site-specific time-area curve from the default curve in HEC 1 leads to errors in the peak flow estimate. In the absence of suitable events for site-specific development of unit hydrographs, the USBR SUH technique is to be used, but careful consideration should be given to the appropriateness of the use of local storm K, values for Utah local storm PMFs.

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