WA – Samson Brook dam – Spillway Upgrade Project

Authors: Bob Wark & Michael Somerford.

The upgrade of Samson Brook Dam spillway for the Water Corporation is well under way. The Water Corporation has engaged SRG for the construction of the spillway works, with concrete supplied by Hanson and batched on site.  As reported in the December 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.

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. The new spillway is a 15 m wide straight approach ogee crest, designed to discharge for a 1 in 500,000 AEP outflow of 280m3/s. This will replace a 10 m shaft diameter bellmouth ogee crest glory hole with a capacity of 117m3/s, which is to be decommissioned when the new spillway is complete.

The site was visited by the WA ANCOLD YP group on February 18 with representatives from a wide range of groups attending (see separate YP report).

Works on the spillway upgrade were progressing well, as reported in the last issue of ANCOLD news.  The dam is located in jarrah forest about 10 km from the towns of Yarloop and Waroona.  The fires in late January burnt out the town of Yarloop and almost 100000 hectares of forest and farmland.  The fires were started by lightning and burnt completely through the site which had been evacuated and the plant quarantined in a relatively secure clearing.  Work was delayed work for a week but there was no major damage to the site, plant or equipment.

The total volume of excavation for the works was around 50,000 m3 and surplus material from this excavation has been used to back fill the quarry used to supply rock for the earlier work on the embankment.

Placement of the chute floor and walls is well advanced.  The chute floor is designed as a continuously reinforced slab without joints, with underdrainage and foundation seepage cutoffs at 10 m intervals.  The overall length of the spillway is approximately 230 m and when finished it will contain about 3000 m3 of concrete.

Excavation for the stilling basin is well advanced; the main stilling basin floor slab is 2 m thick and will include almost 1000 m3 of concrete.

Work is expected to be completed by the start of winter.

Spillway excavation showing the forest burnt during the recent fires

Spillway excavation showing the forest burnt during the recent fires

Progress with construction of the chute floor and walls

Progress with construction of the chute floor and walls

 Forming the 15 m wide crest structure with reinforcement being placed prior to forming up.

Forming the 15 m wide crest structure with reinforcement being placed prior to forming up.

Excavation for the Stilling Basin – a weathered dolerite dyke can be seen in the background

Excavation for the Stilling Basin – a weathered dolerite dyke can be seen in the background