Samson Brook Dam is an 8 GL, 32.7 m high earth fill embankment dam located on Samson Brook approximately 110 km south-east of Perth, initially constructed in 1938.
As reported in the March issue of ANCOLD News, the new spillway is the final stage of remedial work for the dam, which began with work on the outlet from 2010 to 2012 and on the embankment in 2014. The latter work included a 1.7 m raise of the embankment crest.
Previous geotechnical investigations indicated that the north western portion of the proposed stilling basin would be excavated in variably weathered dolerite while the remainder of the basin would be excavated within deeply weathered gneissic material. The configuration of the stilling basin was selected using a risk based approach where the hydraulic jump will be contained within the basin for intermediate flood events up to 145 m3/s, whilst maintaining overall flood security of the dam for events up to 1 in 500,000 AEP.
SRG has completed the upgrade of the Samson Brook Dam spillway for the Water Corporation. The Contractor had to withstand the usual rain related stand down days in addition to a bushfire that engulfed the site in January 2016 (as reported in the Mar 2016 News).
The new spillway is 15 m wide, with a straight approach ogee crest and has been designed to discharge the 1 in 500,000 AEP outflow of 280 m3/s. This replaced a 10 m diameter bellmouth ogee crest “Morning Glory” spillway with a capacity of 117 m3/s, which was decommissioned when the new spillway was completed.
Decommissioning the “Morning Glory” spillway included the following works:
- Construction of a concrete plug (approximately 10 m in length) in the unlined section of the spillway tunnel
- Grouting of the concrete plug to fill the soffit voids.
- Backfill of a section of the “morning glory” tunnel (upstream of the concrete plug) and the lower section of the bellmouth intake drop shaft, with cement stabilised fill
- Backfill of the “morning glory” bellmouth intake and drop shaft with select fill
The portion of the decommissioned “Morning Glory” spillway above ground has been retained for prosperity as it marks the one and only spillway of this type used by the Water Corporation in Western Australia.
Author: Jason Fowler