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e TALK Inter 1 5 <br />Management of FI lairs <br />Continued from o t ~~• <br />The discharge capacity of the emergency <br />spigway of a dam may be ten to twenty tunes the <br />capadty of the ptirtclpd spigway of a daml Thus, if <br />conflgtxafloru <br />and flood <br />protectkon levels <br />9~dng <br />developmen4 in <br />the floodpbin <br />(mrTtedbtety <br />downstream <br />from a flood <br />contrd dam ore <br />based on <br />discharges from <br />the dam's <br />~~ <br />spillway but a <br />storm occurs <br />which causes <br />flow from the <br />emefQARCy <br />spigway ten to <br />tW@nty tl(r1eS <br />greater than <br />that occurring <br />from the <br />principal spigway, sigNflcant damage and even koss <br />of Nfe can occur in the developed downstream <br />fk>odplain (see Flgure 3). <br />How gkety ~ it that this situation could occur? <br />Design of principal spgiway outflow ~ usually based on <br />reducing inflow from a 100-year storm occurrng over <br />ttte upstream watershed to a little more than bank-full <br />flow downstream from the dam. Sbc inches of rainfaq <br />occun(ng over a 24-hour perked is representative of a <br />100-year storm in Minnesota. Design of emergency <br />Qmergency spillway ---_ ~ ~ ~ „~~ <br />spillway outflow from a flood conird reservdr ~ otter <br />based on a storm event representattve of mmdmum <br />e~erienced ralnfaq events which have occ~xred in <br />the region. In Mlmesota, l0 to 14 fiches of rain over a <br />10 to 12-hour period ~ representative of a ma~drrxxn <br />e~erienced <br />® Ft~~~e <br />~`oob // <br />a--Fioodway <br />open apaca usa only <br />X100 -YEAR FLOOD LEVEL <br />® Normal Water Level _ <br />event. During the <br />period of 1970 <br />ttxough 1985, the <br />State <br />Climatologist and <br />the University of <br />Minnesota Shc <br />Science <br />Department <br />reported that at <br />least 57 storm <br />events have <br />taken place <br />which could be <br />categorised as <br />°emergency <br />aplilway° design <br />events. So the <br />reCOrdS Show ttlat <br />these so-cooed <br />°emergency <br />spigway° events <br />do occur. <br />The premise of this essay is not that flood contrd <br />dams should not toe built. Although dams for food <br />contrd have been found to have negative <br />environmental impacts, such structures can be <br />effective in reducing damages, provtdod downstream <br />floodplain land tsse controls that guide new <br />development adequately reflect the floodwater <br />discharge potential of all discharge components of <br />#tte dam. <br />