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<br />EM 1110-2-1406 <br />5 Jan 60 <br /> <br />specific basins may be derived. For standard project flood conditions, the snowpack wHt"r <br />equivalent dctermi::wtion is based on It'ss severe conditions than for the rnaximum possible and <br />conforms to the mccximum which is reasonably characteristic of the region involved. <br />For ,vinter rain.on-sllow floods the initial sIlowpack condition is important both from the <br />consideration of snowmelt and for E,torage and delay of liquid water in the slIowpaek. For max:- <br />mum probable rain-on-snow flood conditions! it may bp llssuffied in some cases that suffi(~i('nt <br />water equivalent exists to provide snowInrlt continuously through the storm period throughout <br />the entire range of e;(lvation. In other cases, a derived optimum snow wedge is required. Also <br />for the maximum prohable flood, it may be assumed in many cases that the preceding melt. or <br />rainfall has provided drainage channels through the snowpaek and has conditioned it to produce <br />runoff wit,hout significant delay, so that wairf excesses from rain and snowmelt during the stonn <br />period are immediatety available for runoff. In unusual cireumstnnces, howevpr, especially where <br />a significant portion of the basin is at high elevations, it. ma,v be necessary to ascribe snowpac:<: <br />storage and df'lay to a portion of the water excess. This sno\vpack condit.ion may be establishe,l <br />on the basis of mctporologieal events preceding the design storm. For standard project flooi <br />determinations, the storage and delay of liquid water in the snowp"ek should be evaluated for all <br />ranges in elevation, based on preceding met.eorological events. The maximum runoff condition <br />is one where there is (I) sufficient snow on the basin initially to provide melt contribution to rnnolI <br />over as large a portion of the area as possible for the storm pC'riod, and yet (2) a minimum depth <br />of snow, especjolly at high elevations, to provide the least possible storOll"r rrqnin'd in conditioning <br />the snowpack to produce runoff. <br /> <br />10-04. OPTIMlM METEOROLOGIC CONDITIONS. The critical iiood-producing mete- <br />orologic conditions for spring snowmelt floods ore those in \"hich the wintpr snowpu('k accum111a- <br />tion occurs with no significant melt, followed by a cold spring wit.h minimum snowmelt and <br />continued increase ir: the snowpa~k. After the maximum accumulation of the snowpack has <br />occurred, there is a conditioning period during which the melt is modt'rate; the snowpack and <br />underlying soil are conditioned t.o produce maximum runoff throughout) the basin, and thr snow <br />surface albedo may approach its minimum value. Finally, the mcteorologie factors atTpcting <br />snowmelt are a.llo\n'd to increase to their maximums, at a time when the heat input to the bnsin <br />can be near its seasonal maximum. The prolonged period of continuous high heat input witholl~ <br />cessation is importnlli~ in producing the maximum flood peak, because the runoff ra.tt's may tlwn <br />approach the snowmelt rates for the snO\v-covcred area contributing to runoff at the time of tho <br />flood peak, as an equilibrium inflow-<lutflow condition. Rainfall occurring near the snowmel-, <br />peak ma,' be superimposed upon the critical snowmelt, sequence to augment the maximum <br />probable flood peak discharge. For standard project flood conditions, a similar but less seve"" <br />sequence of snowmelt conditions may be assumed, which would be reasonably characteristic of tbe <br />maximum for the region involved. <br />For standard project rain-on-snow floods, the pre-flood meteorological sequence must be <br />carefully analyzed, to determine tlw initial sno\vpack condition. Air tempC'ratures may he such <br />that part of the precipitation falling during this period will be in the form of snow in the higher <br />elevations.. and part in the form of rain in tll(' lower fireas. Separatioll of these effects rnu<o;(. be <br />made in order to arrivf~ at a reasonable snowpack condition for the basin as a whole. During tlw <br />period of the design s;~orm, optimum meteorological conditions affecting snowmelt arc assumed a8 <br />those reasonably characteristic for t.hc n'gion. <br />:.\feteorological factors which are pertinent to the computation of snowmelt for design floods <br />are subdividC'd as follows: <br /> <br />34 <br /> <br />e <br /> <br />. <br /> <br />J <br /> <br />e <br /> <br />e <br />