Editing Flood (section)

===Coincidence===
Extreme flood events often result from coincidence such as unusually intense, warm rainfall melting heavy snow pack, producing channel obstructions from floating ice, and releasing small impoundments like [[beaver]] dams.<ref name="Abbett">Abbett, Robert W., ''American Civil Engineering Practice'', John Wiley & Sons, 1956</ref> Coincident events may cause extensive flooding to be more frequent than anticipated from [[100-year flood|simplistic statistical prediction models]] considering only precipitation runoff flowing within unobstructed drainage channels.<ref name="BR">[[United States Department of the Interior]], Bureau of Reclamation, ''Design of Small Dams'', United States Government Printing Office, 1973</ref> Debris modification of channel geometry is common when heavy flows move uprooted woody vegetation and flood-damaged structures and vehicles, including boats and [[railway]] equipment. Recent field measurements during the [[2010–11 Queensland floods]] showed that any criterion solely based upon the flow velocity, water depth or specific momentum cannot account for the hazards caused by velocity and water depth fluctuations.<ref name="Chanson_2011c">{{Cite book |last1=Brown |first1=Richard |url=http://espace.library.uq.edu.au/view/UQ:243550 |title=Turbulent Velocity and Suspended Sediment Concentration Measurements in an Urban Environment of the Brisbane River Flood Plain at Gardens Point on 12–13 January 2011 |last2=Chanson |first2=Hubert |author-link2=Hubert Chanson |last3=McIntosh |first3=Dave |last4=Madhani |first4=Jay |year=2011 |isbn=978-1-74272-027-2 |series=Hydraulic Model Report No. CH83/11 |page=120 |issue=CH83/11}}</ref> These considerations ignore further the risks associated with large debris entrained by the flow motion.<ref name="Chanson_2014">{{cite book |author=[[Hubert Chanson|Chanson, H.]] |url=https://eprints.qut.edu.au/88693/1/88693.pdf |title=Hydraulic structures and society - Engineering challenges and extremes |author2=Brown, R. |author3=McIntosh, D. |date=26 June 2014 |publisher=Proceedings of the 5th IAHR International Symposium on Hydraulic Structures (ISHS2014) |isbn=978-1-74272-115-6 |editor=L. Toombes |location=Brisbane, Australia |pages=1–9 |chapter=Human body stability in floodwaters: The 2011 flood in Brisbane CBD |doi=10.14264/uql.2014.48}}</ref>