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Title: Lower monumental spillway hydraulic model study
Authors: Wilhelms, Steven C.
Murphy, Thomas E.
Yates, Laurin I.
Keywords: Dissolved gas
Hydraulic model
Spillway and stilling basin
Spillway deflectors
Stilling basin scour
Publisher: Coastal and Hydraulics Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: ERDC/CHL TR ; 03-13.
Description: Technical report
A 1:40 Froudian Scale model was used to investigate the hydraulic performance of the Lower Monumental Dam spillway, stilling basin, and tailrace for dissolved gas reduction and stilling basin apron scour. The model reproduced a 2-1/2 bay section of the spillway and portion of the nonoverflow section between the spillway and navigation lock. Performance characteristics of two spillway deflectors were evaluated. The existing deflector (12.5 ft long horizontal with small fillet radius for transition from spillway to deflector) was recommended at el 434.0 because of its slightly wider tailwater range for operation in skimming flow. However, for fish passage over the deflector, the Type I deflector (12.5 ft horizontal with 15-ft radius transition) can likely be adopted with little degradation in dissolved gas uptake. Loadings on the deflector were estimated with pressure measurements on the horizontal and vertical faces. Instantaneous cavitation pressures were measured on the vertical face of the deflector due to flow separation. Only minor cavitation damage has been observed at other spillway deflectors, and thus, significant damage is not expected. Pressure measurements on the stilling basin flow show potential uplift pressure as high as 3,300 lb/ft2. If these pressures have a pathway beneath the stilling basin apron, significant uplift force could result, ultimately causing a catastrophic failure of the apron. Debris was transported from the tailrace into the stilling basin for discharges above about 6.7 kcfs per spill bay (4.0-ft gate opening), when skimming flow occurred in the stilling basin. A numerical model of flow in the stilling basin showed a significant circulation cell on the stilling basin floor near the site of apron erosion, when operating the outside bay without a deflector. With a deflector on the outside bay, the circulation cell was nonexistent, indicating potential for significant reduction in apron scour. Experiments in the physical model verified the numerical model indications. Even with the outside bay deflector, some movement of debris in the stilling basin occurred. Thus, debris should be excluded from the stilling basin to completely eliminate apron scour. Several alternatives were investigating including armoring or grouting the tailrace to stabilize the debris, stilling basin wide debris trap, elevated end sill, and stilling basin splitter walls. Any alternative should be investigated in a general model.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Technical Report

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