In Australia, although the average annual rainfall is about 420 mm, the spatial and temporal variability is high. The rainfall may vary from zero for several years to extreme hydrological events (e.g. 515 mm in 6 hours at Dapto NSW in 1984). High evaporation coupled with the variability of surface runoff make conservation and development of water resources more expensive and less effective in Australia than in many countries. The design of hydraulic structures (e.g. weirs) has necessitated the development of new design and construction techniques, particularly with the provision of adequate flood release facilities and safe energy dissipation. The latter may be achieved by the construction of steps on the chute (e.g. CHANSON 1995). The study reviews the design, construction and operation of Queensland timber stepped weirs. The purpose of the paper is to provide a critical review of the local expertise and experience, and to assist new designs.

Australian engineers has had a long experience of stepped spillway design. The earliest (large) stepped cascade, built in Australia, is the Eastern spillway of the Malmsbury dam completed in 1870 (CHANSON 1997). The Malmsbury cascade , built of crib and masonry is still in use. Timber crib weirs were also a more common design built in Australia and New Zealand since the early European settlements. The technique is well suited to countries and regions where transportation is difficult and timber plentiful (1).

A series of timber crib weirs was built in Queensland (Australia) throughout the 20th century up to the early 1960s (CHANSON 2002). The design of Queensland timber weirs was unusual because it was relatively standardised. Most weirs were timber crib structures held by timber piles with a downstream stepped face. A clayfill embankment was placed upstream of the weir for water tightness. Recent weir refurbishments included the replacement of the clayfill by a vertical upstream concrete wall. A scour outlet was usually installed next to a bank for easy access. A low-flow section was sometimes installed. The most common design was the simple staircase shape (Type I) (e.g. Greenup(2), Cunningham). Usually the weir was lean and h/l < 0.4 where h is the step height and l is the step length. The Type II was characterised by diagonal brass aligned in the flow direction and connecting the pileheads of different rows. The design was used mainly for steep weirs (e.g. Silverleaf, Whetstone(3)). The writer was told of overflow problems caused by the diagonal brassing, but the Silverleaf and Whetstone weirs have been used over 45 years without major problems. Altogether the timber stepped weirs have been a successful design. Several structures are still used after more than half-century (e.g. Fig. 4 to 7). Some sustained successfully very large overflows (e.g. Cunningham weir in 1956 (4)) and surge waves (e.g. Silverleaf weir in 1995 (5)). The presence of the stepped geometry was adopted for stability and simplicity of construction. Nevertheless the steps contributed to the dissipation of the flow energy and no stilling basin was required downstream of the weirs. At low flows, most of the energy was dissipated on the steps.

Hubert CHANSON inspected several Queensland timber crib weirs built between 1935 and 1958. They are still in use today, and they were in good condition at the time of inspection.
The writer approached a number of Queensland engineers involved in the design of timber weirs back in the 1950s and 1960s. Several people indicated that the timber weir design followed a gradual evolution with no structural nor hydraulic specific guidelines. In the early weirs, railway sleepers were used sometimes. The use of steel sheet-piles was introduced in the 1950s, sometimes in timber weir constrution. For example, Greenup weir toe was protected with steel sheet-piles.

The timber crib weirs in Queensland have had an excellent reliability record. Some have been in use for 80 years although 3 failure cases are known (e.g. Bonshaw). Overall the construction cost of a timber dam is lower than a concrete structure but the maintenance charges are substantial, particularly at sites where large floods, ice and debris runs are frequent. Timber crib weirs are porous structures and leakage is frequent. It might become significant but the dam porosity can be used as a mean to regulate the downstream flow. It can be reduced by use of geotextiles. A serious construction consideration is the tightening of the foundation.

Footnotes

(2) Greenup weir : timber crib stepped weir (H = 5 m) on the Macintyre Brook, completed in 1958, upstream of Whetstone weir.

(3) Whetstone weir : timber crib stepped weir (H = 5 m) on the Macintyre Brook, completed in 1951. A major flood occurred in 1956, the maximum recorded stream height being 11.8 m at Inglewood.

(4) Cunningham weir : timber-crib stepped weir (H = 4 m) on the Dumaresq river, completed in 1954. During a major flood in 1956, the maximum recorded head-above-crest reached 7.3 m. The weir was little damaged and it is still in use.

(5) The Silverleaf weir was completed in 1953. The 5.1 m high timber crib structures was refurbished in 1995. During refurbishment, an earthfill cofferdam was built upstream to de-water the weir. A major flood event took place on the 7 February 1995 when the cofferdam was overtopped. The resulting flood wave submerged the weir. Altogether the structure was overtopped for 24 hours and seepage through the timber cribs lasted further 12 hours before repairs could start. After the event, inspections showed that the weir sustained the surge wave without major damage (CHANSON 2001). The Silverleaf weir refurbishment was completed as planned and the weir is in use since.

References

Relevant links

Acknowledgments

License

This page was visited : 7,900 times between 28-01-2000 and June 2012.
Last updated on 24/07/2012