2017-02-21_REVISION - M1988112 ‎(3)‎

• Permit No: M1988112
• IBM Index Class Name: Revision
• Doc Date: 2/21/2017
• Doc Name: Request for Technical Revision
• From: Battle Mountain Resources, Inc.
• To: DRMS
• Type & Sequence: TR34
• Email Name: WHE
• Media Type: D
• Archive: No

decrease in tail water within the basin, the 15 percent margin of safety from using the Type III
basin with impact blocks should maintain basin functionality.
A basin length of 50.0 ft. was selected based on a calculated length of 49.6 ft. Baffle blocks and
chute blocks were calculated based on standard recommended ratios relative to the incoming
flow depth and the proportioned to fit the channel width.
Basin wall height was set at 20 ft. and freeboard was minimized during this extreme event based
on direction by BMRI. That decision was reached based on the cost of providing greater wall
height, the extremely rare event being evaluated and recognizing that some maintenance in such
a situation would be acceptable. It was also decided to address the potential overtopping by
providing armoring protection for the area adjacent to the stilling basin walls.
Freeboard is normally provided to prevent basin walls from being overtopped by surges, splash
and spray, and wave action due to turbulence of the jump. Based on an empirical procedure
established by USBR a reasonable approximation of appropriate stilling basin freeboard can be
estimated as Freeboard (ft.) = 0.1(vl+ d2). Using that formula the recommended freeboard and
wall height for the 500 -year event would be 6.6 feet resulting in 21.4 feet high walls. The 20.0
ft. wall height selected is sufficiently close to providing full freeboard at the 500 -year event.
To mitigate the risk of damage as a result of basin overtopping, grouted rip rap is being placed
alongside the basin walls. Up to 20 percent of the PMF flow was assumed to overtop the basin
side walls as a basis for determining associated rip rap. Overtopping flow on the north side of
the basin would flow west toward the access road and then north down the roadway.
Overtopping flow on the south side of the stilling basin would flow west and then around the
headwall and into the discharge channel.
Straight Drop Basin
To prevent scour at the basin outlet due to flow overtopping the 8.5 high wall at the downstream
end of the stilling basin, additional energy dissipation is required prior to release. Design of this
supplemental energy dissipation was generally based on concepts of design for a straight drop
basin with impact blocks. The structure was sized primarily for the 500 -year flow event with a
little oversizing of the impact blocks, apron length and end sill to partially address overloading in
greater events. Because of the tail water conditions previously discussed, the end sill wall height
at the secondary basin outlet was increased slightly beyond that otherwise recommended in order
to help maintain tail water within the basin. Although not all criteria are met, the basin should
provide sufficient energy dissipation to protect the structure even in the PMF event. Procedures
in Design of Small Dams for a Straight Drop Spillway with Impact Blocks were generally
followed. Calculated basin sizing is as follows:
Table 3.2. Straight Drop Basin Parameters
San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 21
500 -year
PMF
Designed
Basin Length (ft)
25.6
43.1
27.5
End Sill Height (ft)
1.62
2.55
3.0
Baffle Block Height (ft)
3.11
5.11
4.0
San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 21