The forum article, published in the September 2000 issue of the Journal of Hydraulic Engineering ASCE, may interest you :
   CHANSON, H. (2000). "Forum article. Hydraulics of Stepped Spillways: Current Status", Jl of Hyd. Engrg., ASCE, Vol. 126, No. 9, pp. 636-637 (ISSN 0733-9429).

You may download the file from the JHE web site or from my own site :
    http://www.uq.edu.au/~e2hchans/reprints/jhe2000.pdf
    http://www.uq.edu.au/~e2hchans/reprints.html
The file is a .PDF file format to be read with Acrobat Reader.

A text (ASCII) version of the article (without figure) is at the end of this email.

Of interest this is the second Forum article written by a member of the new Hydraulic structures section of the IAHR in the last 12 months. Professor Alan ERVINE wrote a Forum article at the end of 1999.

Any comments will be most welcome.

Hubert Chanson
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____________________________________________________________
Hubert CHANSON
Dept. of Civil Eng., University of Queensland
Brisbane QLD 4072, Australia
Ph.: (61 7) 33 65 35 16 - Fax: (61 7) 33 65 45 99
Email: h.chanson@mailbox.uq.edu.au
Internet : http://www.uq.edu.au/~e2hchans
____________________________________________________________
FORUM ARTICLE - HYDRAULICS OF STEPPED SPILLWAYS: CURRENT STATUS

Stepped-channel spillways, stepped spillways, staircase
waste waterways, or stepped chutes have been used for more
than 3,000 years (Fig. 1). In recent years they are experiencing
renewed attention. The present forum article discusses briefly
the current status of stepped spillways, and it draws attention
to a recent international workshop on the subject.
A significant number of dams were built with overflow
stepped spillways during the 19th century and early 20th cen-tury,
before such spillways became outdated by progresses in
hydraulic jump stilling basins (Fig. 2). Recent advances in
technology (e.g., roller-compacted concrete, polymer-coated
gabion wire), however, have triggered a renewal of interest in
stepped spillways. Unfortunately, though, much expertise has
been lost in the past 60 years. Research on stepped-spillway
hydraulics has been active for the past 10 years (Fig. 3). During
the period 1985–2000, the international database Science
Citation Index (The Web of Science) lists 14 papers and 21
discussions and closures on stepped-spillway, or stepped-chute,
hydraulics, all but two having been published between
1990 and 2000. A 1985 paper (Journal of Hydraulic Engineering)
was cited 17 times during the period, and two papers
published in 1994 (Journal of Hydraulic Research) were cited
22 times altogether. The database Global Books in Print lists
one book (Chanson 1995).
An international workshop on hydraulics of stepped spillways
was held recently at the Eidgeno¨ssische Technischen
Hochschule (ETH) in Zurich, Switzerland, in March 2000. The
workshop was organized by Drs. H. Minor and W. Hager, and
it attracted over 40 participants from Europe, North America,
Iran, and Australia. The participants were professionals, academics,
and researchers involved in stepped-spillway design.
Sponsorship by ASCE, IAHR, and the Swiss national committee
on large dams demonstrated the workshop’s profes-sional
importance. The workshop was organized into five sections:
(1) case studies; (2) aeration characteristics and
cavitation risk; (3) energy dissipation; (4) internal flow features;
and (5) design. Altogether 22 papers were presented,
plus an introductory and an invited lecture. There were nine
papers on skimming flows, and two on nappe flows. Two pa-pers
discussed the gas-transfer process (or mass transfer) at
stepped spillways and chutes, and seven articles dealt with
design experience.
Overall, the workshop presentations and the associated dis-cussions
were an useful exercise. They showed a general
agreement on a number of issues. It is acknowledged that the
waters flow as a succession of free-falling nappes at low flow
rates and as a skimming flow at larger discharges for a given
stepped-chute geometry. Yet there are some arguments about
a transition flow region between nappe and skimming flow
regimes, a theory supported by some researchers. All the work-shop
participants agree that air entrainment is significant on
stepped chutes. In nappe flow, one paper highlighted the com-plexity
of the air-water flow while several papers demonstrated
that air entrainment in skimming flow is similar to the self-aeration
process observed on smooth-invert chutes (Fig. 3). In
skimming flow, it is generally agreed that the cavity recirculation
contributes to significant form drag and that the dimensionless
friction coefficient f (or Darcy friction factor) is about
0.1 to 0.3. One analytical development implied f = 0.2. Yet
different research facilities yield different results, and people
agreed to disagree on the reasons for these differences!
Experimental studies suggest that cavitation is not an issue
on stepped spillways, because the flow velocities remain low.
Step damage caused by pressure fluctuations in the step cav-ities,
however, may be a problem. It is understood that scale
effects may be significant when the geometric scaling ratio
relating prototype and model dimensions is greater than 10 to
20.
Discussions during the workshop highlighted some impor-tant
questions. Is an Ogee crest the most suitable crest design?
What is the order of magnitude of the pressure fluctuations on
the step faces? Is there a drag reduction process induced by
air entrainment as observed on smooth-invert chutes? How
much mass transfer occurs along a stepped cascade? Is there
an optimum stepped design for reoxygenation purposes?
The writer attended the workshop and was surprised by the
absence of two topics: failures and education. No paper dis-cussed
accidents and failures with stepped spillways, although
over 21 major accidents have been documented (e.g., Chanson
1995). The writer is concerned that too few engineers are will-ing
to share their engineering-failure experiences with peers.
Yet doing so would be a great service to the profession. The
teaching of stepped-channel hydraulics is another issue. Of the
over 200 RCC dams built to date in North-America, only 60
were equipped with a stepped spillway. Why? It is believed
that most engineers, young and senior, have never been ex-
posed to the complexity of the stepped-spillway design. The
writer has lectured stepped-spillway hydraulics at postgraduate
and undergraduate levels since 1992 in Australia (e.g., Chanson
1999) and overseas (e.g., Ohtsu and Yasuda 1998). Could
it be that some researchers and engineers do not fully appre-
ciate the need for continuing education and undergraduate
teaching of quality in hydraulic structures?
In summary, research on stepped-spillway hydraulics is very
active. Although progress has been achieved in the past decade,
more research is needed to gain a sound understanding
of the complex flow patterns. The organizers of the recent
workshop did a service to the profession by attracting a diverse
group of professionals and researchers actively involved in
stepped-spillway design. A review of the proceedings book
(Minor and Hager 2000) resulting from the workshop will be
forthcoming in an later issue of the Journal of Hydraulic En-gineering.
The writer, as a participant in the workshop, has
two minor criticisms of the proceedings. Readers of the pro-ceedings
may not entirely grasp the number of issues for
which there is incomplete consensus. Second, as frequently
happens with good meetings, the informal discussions held
during the workshop added substantially to the understanding
of the subject of stepped spillways but the discussions are not
included in the book. The hydraulic design of stepped spill-ways
and chutes is an illustration of loss of hydraulic expertise
by professional engineers during the 20th century. It is hoped
that a lesson will be learned and that the profession will not
‘‘rediscover the wheel’’ every 60 years.

REFERENCES
Chanson, H. (1995). Hydraulic design of stepped cascades, channels,
weirs and spillways, Pergamon, Oxford, U.K.
Chanson, H. (1999). The hydraulics of open channel flows: An introduc-tion,
Edward Arnold, London.
Ohtsu, I., and Yasuda, Y., eds. (1998). ‘‘Hydraulic characteristics of
stepped channel flows.’’ Workshop on Flow Characteristics around Hy-draulic
Structures and River Environment, University Research Center,
Nihon University, Tokyo.
Minor, H. E., and Hager, W. H., eds. (2000). Hydraulics of stepped spill-ways,
Balkema, Rotterdam, The Netherlands.