BOARD01013

• Board Meeting Date: 7/26/2006
• Description: ISF Section - Injury with Mitigation Proposal - Basalt Water Conservancy District - Case Nos. 5-01CW305 and 02CW077
• Board Meetings - Doc Type: Memo

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The Slate records for these structures offer IDtaI monthly diversions (in AF) through each structure.
Utilizing these records, an average dally tranlHlasin diversion value was compu1e<l, and added to the
flow at (3).

In order to facilitate this initial exploratory analysis, several simplifying assumptions were applied:

. The daily average differential between (1) and (2), the gages bracketing Ruedi Reservoir, was
assumed to accurately refteGt daily average Ruedi operations. This does not account for the
buffering effects of ReseIVOir storage and transit time issues.
. The same daUy average differential between (1) and (2) was applied instantaneously to the
gage (3), again not accounting for transit time and transit loss eflecls between (2) and (3).
. Daily average tranlHlasin diversion effects were compu1e<l from monthly records, and also
applied without consideration for transit time or loss eflecls.

Over the 34 year period of record, the average adjustments are ouUined following:

Aug Adj. . Sep Adj. Oct Adj.
structure (CFS) (CFS) (CFS)
Ruedi -53.0 -65.8 -75.7
Boustead 34.7 6.7 4.9
Twin lakes 33.3 9.7 6.5
Busk-lvanho 4.4 1.9 1.4
Total 15.1 ....eA -64A

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SUbsequenUy, the inferred Ruedi-adjusllld flow values for the Roaring Fork River at Glenwood Springs,
reftecling a 97 -year period of record, were subjected to a Log-Pearson III statistical analysis, evaluating
the retum Intervals for the 3O-day average, late season low flows. The results are ouUlnec:l as follows:

2002
30 Day Avg
Flow (CFS)
161
162
366

August
September
October

Recurrence
IntervBI(Yeare)
316
478
12

Notably, flows in October ioo Based substanlially due to a rainy period. This is refteded in the
comparatively low recurrence interval in October. Nonetheless, the findings of the frequency analysis
for August and September, the time period bracketing RESOURCE's 2002 flow measurements within
the crilIcal reach of ooncem, support the conclusions ouUinec:l In our Seplember 10", Ieller to DavId
Hallford, Esq., BWCO Counsel, whlch evaluated flow ,,~ for USGS 09073400, Roaring Fork
River near Aspen. The staUslical evaluation of this gage, which provides only 39 years of record,
supported the conclusion that 2002 was approximately a 1-in-100 dry year on the upper Roaring Fork.
An evaluation of the return period of Inferred flows at USGS 09080500, reftecling trends for the entire
Roaring Fork watershed, and offering a 97 -year period of record, supports the conclusion that August
2002 was a 1-in-316 dry-year event, and September was a 1-in478 dry year event

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