2014-01-22_PERMIT FILE - M2013064 ‎(10)‎

• Permit No: M2013064
• IBM Index Class Name: Permit File
• Doc Date: 1/22/2014
• Doc Name: Adequacy Review Response
• From: Varra Companies, Inc.
• To: DRMS
• Email Name: PSH
• Media Type: D
• Archive: No

Hydraulics
Hydrology
Water Resources
P.O. Box 6100
Breckenridge, CO 80424
970 - 547 -3823
www.flowtechnologies.biz
be heavily compacted due to months of heavy equipment moving on it. Thus, a critical shear
stress of slightly greater than zero (0.1 lb /ft2) was assumed and applied.
Provide Some Discussion As To How These Parameters Are Related To The Erodibility Factor,
Kd. Hanson et al. (I) provides guidance for approximate values of Kd relative to compaction and
percent clay. Note that representative particle size, plasticity index, total unit weight, undrained
shear strength, and critical shear stress are not factors in estimating Kd. A table is presented that
lists approximate values of Kd against percent clay, modified compaction, standard compaction,
low compaction, and greater or lesser than optimal water content.
Because the WinDAM modeling procedure is not an exact science — as with any other model of
natural processes — engineering judgment and sensitivity analyses were also performed to
estimate Kd. It was assumed that water content during a berm overtopping event would be
greater than optimal due to river overbank flooding and heavy rains. Also, compaction of the
berm down to several feet would be considerable due to movement of heavy equipment over
months on a daily basis. As stated in sub - section 3.2.2 of the subject document, "because of the
many assumptions and uncertainties, values of Kd were applied having two orders of magnitude
from 10 for "Weakest" scenario, to 0.1 for "Strongest." Such a ran eg of magnitude was chosen to
cover a very large range of head cutting and erosion possibilities due to the many uncertainties of
such modeling with the intent that the actual values would fall somewhere within the range of
model results."
Comment 4
WinDAM B Model Input /Output Data, p. 44: Please explain why the Maximum Pool Elevation
for the Worse (Kd =10) case is 0.96 feet lower than it is for the Best (Kd =1) and Strongest
(Kd ptl) cases.
During the Worse case failure scenario, headcutting and erosion of the berm occurs much sooner,
and at a significantly faster rate than for the other scenarios resulting in the berm breach to widen
and deepen at an earlier and faster rate. With a wider and deeper breach, discharge will be higher
consequently resulting in a lower water surface (pool) over the breach because a higher
percentage of the flood inflow is in the breach rather than overtopping the berm.
Comment 5
WinDAM B Model Input /Output Data, p. 55: Similar to Comment 4, please explain why the
Maximum Pool Elevation varies between the four cases analyzed.
Similar to Comment 4, the four cases model different berm breach headcutting and erosion rates
due to variations in the erosion parameter Kd. With different breach headcutting and erosion
rates — and consequent breach initiation time, and varying widening and deepening — the breach
discharges will also vary considerably. The maximum pool elevations then become a function of
the percentage of discharge in the breach relative to that overtopping the berm.
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