Submitted to: American Society of Agricultural Engineers Meetings Papers
Publication Type: Popular Publication
Publication Acceptance Date: July 22, 1999
Publication Date: N/A
Interpretive Summary: In a principal spillway the intake structure collects and slowly releases excess water from a reservoir into a pipe that passes through the dam. This pipe normally exits into a stilling basin downstream of the dam where the force of the water can be safely dissipated without eroding the downstream channel. Standard designs for a principal spillway structure have been developed over time and are used reliably in most situations. For the new dam to be built in Franklin County, MS, however, the proposed structure was larger than normal and not of standard design. Thus, a model study of the intake tower, pipe, and stilling basin was performed to determine if the non-standard structure would perform efficiently, and to determine optimum configuration for the stilling basin. Design modifications resulting from this study allowed a construction cost savings of over $100,000. This model study identifes specific design improvements, and advances our knowledge about the hydraulic performance of principal spillways. These results should be of interest to dam owners, builders, and designers.
Technical Abstract: A 1:20.6 scale physical model study was conducted of the proposed Franklin County Lake principal spillway system for Franklin County, Mississippi. The objective of the study was to evaluate the hydraulic performance of the drop inlet, elbow transition, and stilling basin and plunge pool designs. The hydraulic performance of the drop inlet and elbow transition was very good. Two stilling basins, USBR Type III and SAF, were evaluated. At desig discharge, the plunge pool area with the USBR Type III stilling basin was stable with 165-mm (6.5-in) riprap. The plunge pool area with the SAF stilling basin was stable with 246-mm (9.7-in) riprap. The SAF stilling basin is less than one-half the length of the USBR Type III stilling basin and the better choice from an economic standpoint. Wingwalls are not required with either stilling basin for hydraulic performance.