Timber Crib Weirs in Queensland, Australia
by Hubert CHANSON (h.chanson@uq.edu.au)
M.E., ENSHM Grenoble, INSTN, PhD (Cant.), DEng (Qld), Eur.Ing., MIEAust., MIAHR, 13th Arthur Ippen awardee
Dept. of Civil Engrg., Univ. of Queensland, Brisbane QLD 4072, Autralia


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

(1) In countries and regions where transportation is difficult and timber plentiful, timber dams are serious competitors of concrete dams. In Russia, a 12-m high rockfill dam with a timber crib overflow spillway was built around A.D. 1700 (CHANSON 2001). In America, the North-East benefited from the experience of Northern European settlers and timber dams were reported as early as AD 1600. Prior to AD 1800, timber dams were usually 3 to 4.5-m high. At a later date, some much bigger ones were built successfully to a height of 30 m but most timber dams were less than 6-m high. The Redridge timber crib weir, located upstream of the steel dam is a sturdy example. Completed in 1894, the dam was 16.1 m thick at base, 8.5 m thick at crest and 15.2 m high.

(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

CHANSON, H. (1995). "Hydraulic Design of Stepped Cascades, Channels, Weirs and Spillways." Pergamon, Oxford, UK, Jan., 292 pages (ISBN 0-08-041918-6).
CHANSON, H. (1997). "A Short History of Stepped Cascades in Australia." ANCOLD Bulletin, No. 106, Aug., pp. 101-111 (ISSN 0045-0731).
CHANSON, H. (1998). "Utilisation of Stepped Channels and Study of Stepped Channel Flows in Australia." Hydraulic Characteristics of Stepped Channel Flows, Workshop on Flow Characteristics around hydraulic Structures and River Environment, Nihon University, Tokyo, Japan, November, Invited keynote lecture, 21 pages. (PDF Version at EprintsUQ
CHANSON, H. (2001). "The Hydraulics of Stepped Chutes and Spillways." Balkema, Lisse, The Netherlands (ISBN 90 5809 352 2).
CHANSON, H. (2002). "Timber Crib Weirs in Queensland, Australia. Some Heritage Stuctures with a Solid Operational Record." Royal Historical Society of Queensland Journal, Vol. 18, No. 3, pp. 115-129 (ISSN 0085 5804). (PDF Version at EprintsUQ)
CHANSON, H. (2009). "Le Point du Vue de l'Ingénieur Hydraulicien. Préface." ('The Perspective of a Hydraulic Engineer. Foreword.') in "Ouvrages Bois dans les Cours d'Eau. Etat de l'Art, Applications et Dimensionnement. Guide Technique." ('Timber Structures in Rivers. State of the Art, Applications et Design. Technical Handbook.') by D. ROMAN, Office National des Forêts, Direction du Développement, Paris, France, pp. 7-8 (in French). (Record at UQeSpace) (PDF file)
CHANSON, H., YASUDA,Y., and OHTSU, I.(2000). "Flow Resistance in Skimming Flow : a Critical Review." Intl Workshop on Hydraulics of Stepped Spillways, Zürich, Switzerland, H.E. MINOR & W.H. HAGER Editors, Balkema Publ., pp. 95-102 (ISBN 90 5809 135X). (download PDF file)
CHANSON, H., and TOOMBES, L. (2000). "Stream Reaeration in Nonuniform Flow: Macroroughness Enhancement. Discussion." Jl of Hyd. Engrg., ASCE, Vol. 126, No. 3, pp. 222-224 (ISSN 0733-9429). (Download PDF file)
CHANSON, H., and TOOMBES, L. (2003). "Strong Interactions between Free-Surface Aeration and Turbulence in an Open Channel Flow." Experimental Thermal and Fluid Science, Vol. 27, No. 5, pp. 525-535 (ISSN 0894-1777). (Download PDF File)
CHANSON, H., and GONZALEZ, C.A. (2005). "Physical Modelling and Scale Effects of Air-Water Flows on Stepped Spillways." Journal of Zhejiang University SCIENCE, Vol. 6A, No. 3, March, pp. 243-250 (ISSN 1009-3095). (Download PDF file)
GONZALEZ, C.A., and CHANSON, H. (2004). "Interactions between Cavity Flow and Main Stream Skimming Flows: an Experimental Study." Canadian Journal of Civil Engineering, Vol. 31, No. 1, pp. 33-44 (ISSN 0315-1468). (Download PDF file)
TOOMBES, L., and CHANSON, H. (2005). "Air-Water Mass Transfer on a Stepped Waterway." Jl of Environ. Engrg., ASCE, Vol. 131, No. 10, pp. 1377-1386 (ISSN 0733-9372). (Download PDF file)

Relevant links

Air entrainment on chute and stepped spillways ... {http://www.uq.edu.au/~e2hchans/self_aer.html}
Current expertise and experience on stepped channel flows {http://www.uq.edu.au/~e2hchans/dpri/topic_2.html}
Minimum Energy Loss (MEL) weir design: an overflow earthfill embankment dam {http://www.uq.edu.au/~e2hchans/mel_weir.html}
Photographs of stepped spillways {http://www.uq.edu.au/~e2hchans/photo.html#Step_spillways}
Sabo check dams {http://www.uq.edu.au/~e2hchans/sabo.html}
Overflow embankment stepped spillways ... {http://www.uq.edu.au/~e2hchans/over_st.html}

 CIRIA {http://www.ciria.org.uk/about.htm} CIRIA Report : http://www.ciria.org.uk/publications/pubscat/sp142.htm
USBR Concrete Step Overtopping  Protection{http://www.usbr.gov/wrrl/steps/}


Acknowledgments

Hubert CHANSONr thanks the following people for their help : Professor C.J. APELT, Brisbane, Australia; Ms CHOU Y.H., Brisbane; Mr J. MITCHELL, Brisbane, Australia; Mr R. NILSSON, Brisbane; Mr Damien ROMAN, CEMAGREF, France; Mr A. SERRE, Inglewood, Australia; The Texas Historical Society, Texas QLD, Australia (Mrs E. CRAIG and Mrs C. GLASSER); Mr R. WHEELER, Brisbane; Mr A. WICKHAM, Brisbane; Dr Y. YASUDA, Nihon University, Japan

License

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.Hubert CHANSON is a Professor in Civil Engineering, Hydraulic Engineering and Environmental Fluid Mechanics, at the University of Queeensland, Australia. His research interests include design of hydraulic structures, experimental investigations of two-phase flows, applied hydrodynamics, hydraulic engineering, water quality modelling, environmental fluid mechanics, estuarine processes and natural resources. He has been an active consultant for both governmental agencies and private organisations. His publication record includes over 600 international refereed papers and his work was cited over 3,500 times since 1990. Hubert Chanson is the author of several books : "Hydraulic Design of Stepped Cascades, Channels, Weirs and Spillways" (Pergamon, 1995), "Air Bubble Entrainment in Free-Surface Turbulent Shear Flows" (Academic Press, 1997), "The Hydraulics of Open Channel Flow : An Introduction" (Butterworth-Heinemann, 1st edition 1999, 2nd editon 2004), "The Hydraulics of Stepped Chutes and Spillways" (Balkema, 2001), "Environmental Hydraulics of Open Channel Flows" (Butterworth-Heinemann, 2004), "Applied Hydrodynamics: an Introduction of Ideal and Real Fluid Flows" (CRC Press, 2009), and "Tidal Bores, Aegir, Eagre, Mascaret, Pororoca: Theory And Observations" (World Scientific, 2011). He co-authored two further books "Fluid Mechanics for Ecologists" (IPC Press, 2002) and "Fluid Mechanics for Ecologists. Student Edition" (IPC, 2006). His textbook "The Hydraulics of Open Channel Flows : An Introduction" has already been translated into Spanish (McGraw-Hill Interamericana) and Chinese (Hydrology Bureau of Yellow River Conservancy Committee), and the second edition was published in 2004. In 2003, the IAHR presented him with the 13th Arthur Ippen Award for outstanding achievements in hydraulic engineering. The American Society of Civil Engineers, Environmental and Water Resources Institute (ASCE-EWRI) presented him with the 2004 award for the Best Practice paper in the Journal of Irrigation and Drainage Engineering ("Energy Dissipation and Air Entrainment in Stepped Storm Waterway" by Chanson and Toombes 2002). Hubert Chanson edited further several books : "Fluvial, Environmental and Coastal Developments in Hydraulic Engineering" (Mossa, Yasuda & Chanson 2004, Balkema), "Hydraulics. The Next Wave" (Chanson & Macintosh 2004, Engineers Australia), "Hydraulic Structures: a Challenge to Engineers and Researchers" (Matos & Chanson 2006, The University of Queensland), "Experiences and Challenges in Sewers: Measurements and Hydrodynamics" (Larrate & Chanson 2008, The University of Queensland), "Hydraulic Structures: Useful Water Harvesting Systems or Relics?" (Janssen & Chanson 2010, The University of Queensland), "Balance and Uncertainty: Water in a Changing World" (Valentine et al. 2011, Engineers Australia). He chaired the Organisation of the 34th IAHR World Congress held in Brisbane, Australia between 26 June and 1 July 2011.
 His Internet home page is http://www.uq.edu.au/~e2hchans. He also developed a gallery of photographs website {http://www.uq.edu.au/~e2hchans/photo.html} that received more than 2,000 hits per month since inception.

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Applied HydrodynamicsEnvironmental hydraulics of open channel flowHydraulics of open channel flow (2nd edition)The Hydraulics of Stepped Chutes and SpillwaysThe Hydraulics of Open Channel Flow: an IntroductionAir bubble entrainment in turbulent shear flowsHydraulic design of stepped cascades, channels, weirs and spillwaysFluid Mechanics for Ecologists  McGraw-Hill Interamericana 13th Ippen award (IAHR)