Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
Powerline Plan and Environ. Assessment Jan. 2013 - Flood Control ...
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<strong>Powerline</strong> <strong>Flood</strong> Retarding Structure<br />
Pinal County, AZ<br />
Draft Supplemental Watershed <strong>Plan</strong><br />
<strong>and</strong> <strong>Environ</strong>mental <strong>Assessment</strong><br />
conveyed by the auxiliary spillway <strong>and</strong> would impact the CAP canal <strong>and</strong> downstream residential<br />
areas west of the dam. The dam would most likely remain designated as an “unsafe non-emergency<br />
elevated risk” structure. The Sponsors have stated that a constructed breach the dam is not an<br />
acceptable alternative.<br />
Alternative 2 would provide flood protection for greater than the 100-year event. Consistent with<br />
P&G, the alternative that has the highest positive net benefits would be identified as the NED <strong>Plan</strong>.<br />
However, per Section 502.2 of the NRCS National Watershed Program Manual, “In cases where<br />
human life is at risk in the event of catastrophic failure of an existing dam that does not meet<br />
current safety <strong>and</strong> performance st<strong>and</strong>ards, an overriding reason for an exception to the NED plan<br />
requirement exists. To avoid seeking individual exceptions in such cases, the NED plan is defined<br />
as the federally assisted alternative with the greatest net economic benefits.”<br />
The effects of the alternative plans in terms of the EQ <strong>and</strong> OSE accounts are summarized in Table<br />
6-2 <strong>and</strong> Table 6-3, respectively below.<br />
SOILS<br />
Geology<br />
Table 6-2 Summary <strong>and</strong> Comparison of C<strong>and</strong>idate <strong>Plan</strong>s – EQ Account<br />
Alternative 1 Alternative 2<br />
No Federal Action<br />
Decommissioning<br />
Sponsor's Preferred/NED <strong>Plan</strong><br />
IDSM design life<br />
15 years<br />
Hardened channel segments within<br />
earth fissure risk zones<br />
Prime <strong>and</strong> Unique<br />
Farml<strong>and</strong><br />
No effect<br />
No effect<br />
WATER<br />
Surface Water Quantity No effect Discharge to Vineyard Road FRS<br />
<strong>Flood</strong>plain Management<br />
Potential exposure the<br />
downstream floodplain to<br />
flooding risks from<br />
overtopping flood events<br />
or discharges through the<br />
auxiliary spillways above<br />
the 100-year event.<br />
<strong>Powerline</strong> flows directed to the<br />
Vineyard Road FRS <strong>and</strong> thus<br />
provide continued flood protection<br />
up to the 100-year event for<br />
downstream floodplain<br />
PLANTS<br />
Threatened <strong>and</strong><br />
Endangered Species<br />
No effect<br />
No effect<br />
USDA- NRCS Page 6-10 <strong>Jan</strong>uary <strong>2013</strong><br />
Kimley-Horn <strong>and</strong> Associates, Inc.