FLOOD07755

• County: Statewide
• Basin: Statewide
• Title: Engineering and Design Runoff from Snowmelt
• Date: 1/5/1960
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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EM 1110-2-1406
5 Jon 60

CHAPTER 10
OPTDIUl\1 SNOWMELT RUNOFF FOR DESIGN FLOODS

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10-01. GENERAL. Design floods involving snowmelt fall within two general seasonal categories.
Winter rain-on-sllow floods involve the evaluation of snowmelt ns nn ndded component of runoff,
genernlly less than the rainfall amounts. Springtime snowmelt floods arc usually much longer
in duration than winter floods and involve the filcHing of the entire winter's accumulated snowpack.
Rainfall is usually of minor consequence in spring snowmelt floods. For some regions there may
be overlapping of flood characteristics within these broad groupings.
Optimum mcteorologie and hydrologic conditions (i.e., conditions most favorable to runoff)
for design flood synthesis arc estnblished in Corps of Engineer practice for two classes: (I) maximum
probnblc flood; and (2) standard project flood. A procedure used in dcveloping a design flood
should be rational and should represent all processes nffeeting runoff within the ranges thnt mny
be experienced for n given flood /llngnitude. For snowmelt design floods, the primnry problem
is evnluntion of snowmelt by n rntionnl procedure which essentinlly represents the physical processes
of heat transfer in terms of appropriate rnetcorological parameters, cOIlsidE'ring the characteristics
of the basin. In most caSl'S a simple temperature index cannot be relied upon for determining
snowmelt for- design floods, beeausc of: (1) the lack of uniformity aIhong basins of different
cnvironmcnt~; (2) the significant changes in snowmelt rates that may OCcur ,vithin a given basin
due to fnctors other than air temperature; and (3) the danger of extrapolating to conditions
beyond the limits to which the indcx applies. For snowmclt design flood", consideration must also
be given to the effect of the snowpack conditions on runoff.

l(}-02. FACTORS INVOLVED IN SNOWMELT DESIGN FLOODS. Evnluation of specific
conditions pE~rtincnt to pither winter rain-on-snow or springtime snowmelt design floods include
the following considerations:
A. Initi"l snowpaek characteristics.
1. Snow cover (area covered).
2. Snowpack watpr equivalent and distribution with respect to elevation.
3. SnolVpaek condition witb respect to temperature and free water, and their variation
with elevution.
4. Albedo of the snow surfuee (for basins with significant open areas).
B. Determination of critical sequenee of meteorologicnl factors affecting melt.
c. Determination of rainfall.
D. Determination of SnOWIlH'lt rates, utilizing appropriate general equations.
E. Determination of loss nnd runoff conditions.
F. Synthesis of all f~lctors affecting runoff into a design flood hydrograph, utilizing basin
storage or unit hydrograph routing methods.

10--03. OPTIMUM SNOWPACK CONDITIONS. For spring snowmelt design-flood hydrographs,
only the total water equivalent of tbe snowpack and its distribution with ~levation and area Jnust
be considered; the structural character of the snowpaek can be assumed constant. For winter
rain-on-snow floods, however, a reasonnbly likdy stage of metamorphism of the snowpaek must be
assumed, together with reasonably possible liquid water storage deficiencies, especially at the
high elevat.ion zones.
For spring snO\vmcIt design floods, the m,lximum possible snowpack water equivalent is
generally based upon detailed studies of the potential total winter-senson precipitation. The
studies may relate maximum \\.Tinter-season precipitation to sIze of drainage area and normal
anIluul prel'ipitation, as was done for the Columbia River basin by the lIydrometeorological
Section of the u.s. \\:'eather Bureau.51 Fronl such studies the rnaxirnurn winter snowfall for

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