The Hydraulics of Stepped Chutes and Spillways

The stepped channel design have been used for more than 3,500 years (chapter 2). A significant number of dams were built with overflow stepped spillways during the 19th century and early 20th century, before the design technique became outdated with the progresses in hydraulic jump stilling basin design. Recent advances in technology (e.g. RCC, polymer-coated gabion wire) have triggered a regain in interest for the stepped design, although much expertise had been lost in the past 80 years. The steps increase significantly the rate of energy dissipation taking place along the chute and reduce the size of the required downstream energy dissipation basin. Stepped cascades are used also in water treatment plants to enhance the air-water transfer of atmospheric gases (e.g. oxygen, nitrogen) and of volatile organic components (VOC). Research on stepped spillway hydraulics has been active between 1980 and 2000. For the period 1985-2000, the international database Science Citation Index® lists over sixteen journal papers and twenty-six discussions and closures on stepped chute hydraulics, all but two published between 1990 and 2000. A 1985 paper (SORENSEN, Jl Hyd Engrg) was cited seventeen times during the period; two papers published in 1994 (CHANSON, Jl of Hyd Res, No. 2 and 3) were cited fourteen and eleven times respectively (Ref.: Science Citation Index). The international database Global Books in Print® lists one book (CHANSON 1995, Pergamon). In 1998, Professor OHTSU and Dr YASUDA organised a workshop on the hydraulic characteristics of stepped channel flows in Tokyo, attended by over seventy participants. In 2000, Professors MINOR and HAGER organised an international workshop on hydraulics of stepped spillways in Zürich. The workshop attracted over forty participants from Europe, North America, Iran, and Australia; the sponsorship of the American Society of Civil Engineers (ASCE), International Association for Hydraulic Engineering and Research (IAHR) and Swiss national committee on large dams demonstrated the importance of the event. This book presents the state of the art in stepped chute hydraulics. It is based upon the research expertise of the writer, his professional experience as an expert-consultant, and his experience in teaching stepped spillway hydraulics to undergraduate students, postgraduate research students and professionals since 1982 (Fig. i). Results from more than forty five laboratory studies and four prototype investigations were re-analysed and compared. The book provides a new understanding of stepped channel hydraulics, and it is aimed at both the research and professional community. In the introduction (chapter 1), the basic concepts of stepped channels and stepped chute flows are described. A clear distinction is made between the main flow regimes (nappe flow, transition and skimming flow). A chapter presents the historical progress of stepped channels and spillways from the Antiquity up to today (chapter 2). Then the monograph reviews the hydraulic characteristics of stepped channel flows. Three different flow regimes may take place depending upon the flow conditions and chute geometry: nappe flow regime for small discharges, transition flow and skimming flow regime. The hydraulics of each flow regime is described in chapters 3, 4 and 5. The effects of flow aeration and air bubble entrainment are discussed. The gas transfer processes taking place above stepped chute are described : e.g., aeration, re-oxygenation, stripping, de-nitrification (chapter 6). Later practical examples of hydraulic design are presented : e.g. stepped fountains, stepped weirs, gabion stepped spillways, earth dam spillways with precast concrete blocks, roller compacted concrete (RCC) weirs, debris dams (chapter 7). The writer presents further a critical review of accidents and failures with stepped channels, highlighting that the hydrodynamic forces on the step faces are much larger than on smooth chute inverts (chapter 8). Wave phenomena and instabilities are reviewed and discussed in a separate section (chapter 9). In the last chapter (summary and conclusions, chapter 10), he summarises the key issues and he answers explicitly basic questions. At the beginning of the book, the reader will find the table of contents, a list of symbols and a glossary of technical terms and names. He will find also an index at the end of the book. After the conclusion (chapter 10), a detailed list of references is presented. It is followed by a list of physical and chemical properties of fluids (appendix I), The book presents results expressed in SI Units. A table of unit conversions is given in appendix II. Several appendices detail particular calculations : nappe trajectory at a drop structure (appendix III), bubble rise velocity calculations (appendix IV), modelling form drag and flow resistance in skimming flows (appendix V), void fraction distributions in chute flows (appendix VI), chute calculations in skimming flow (appendix VII), modelling air-water gas transfer in skimming flows (appendix VIII). Appendix IX presents a correction form. Readers who find an error or mistake are welcome to record the error on the page and to send a copy to the author. Corrections and updates will be posted on the Internet at : {http://www.uq.edu.au/~e2hchans/reprints/book4.htm} Lastly relevant Internet resources are listed below