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 />
5.4.2 ALTERNATIVE 2 –DECOMMISSIONING: CONVERT THE POWERLINE FRS INTO A CHANNEL TO<br />
DISCHARGE INTO THE VINEYARD ROAD FRS<br />
Alternative 2 would decommission the <strong>Powerline</strong> FRS <strong>and</strong> <strong>Powerline</strong> IDSM <strong>and</strong> replace the dam<br />
with a channel (for the purposes of nomenclature this channel is termed the <strong>Powerline</strong> Channel)<br />
designed for the 100-year flood event. The <strong>Powerline</strong> Channel would collect runoff from the<br />
upstream watershed including the <strong>Powerline</strong> North Diversion Channel, Weekes Wash, <strong>and</strong> Siphon<br />
Draw <strong>and</strong> would discharge to the Vineyard Road FRS. The <strong>Powerline</strong> Channel would discharge<br />
into Vineyard Road FRS east of the north Vineyard Road auxiliary spillway. There would be no<br />
impoundment of floodwaters with the <strong>Powerline</strong> Channel from the <strong>Powerline</strong> watershed. The<br />
existing dam embankment would be removed or breached <strong>and</strong> reduced in height to be a nonjurisdictional<br />
dam. The FRS would no longer meet the jurisdictional definition of a dam <strong>and</strong> then<br />
be removed from ADWR dam safety jurisdiction. The <strong>Powerline</strong> Channel would convey the 100-<br />
year storm <strong>and</strong> would provide a 100-year level of flood protection. A conceptual plan layout of<br />
Alternative 2 is provided in Appendix C as Figure C-5.<br />
The FRS is currently under the jurisdictional purview of ADWR. To remove the FRS from<br />
jurisdictional status would require decommissioning per Arizona Revised Statutes Section 45-1209.<br />
The Arizona Administrative Code Section R12-15-1209 Application to Breach or Remove a High<br />
or Significant Hazard Potential Dam outlines the requirements to reduce a high or significant<br />
downstream hazard potential dam to non-jurisdictional size. These regulations will need to be<br />
considered during final design of this alternative.<br />
The proposed <strong>Powerline</strong> Channel was sized using existing conditions 100-year, 24-hour peak<br />
discharge, as the 24-hour peak discharge was greater than the 6-hour peak discharge. The<br />
<strong>Powerline</strong> watershed features two proposed channels. The first is a collector channel running along<br />
the existing FRS alignment to capture inflow that would otherwise reach the structure. The second<br />
channel is specifically for Siphon Draw, the largest wash within the watershed.<br />
The <strong>Powerline</strong> Channel was initially sized using normal depth routing within HEC-1. The<br />
preliminary channel sizes were then input into a FLO-2D model for the alternative. The <strong>Powerline</strong><br />
Channel was sized to convey a peak discharge of 12,500 cfs into Vineyard Road FRS. Where<br />
breakouts occurred from channel overtopping, the channel size was increased in width or depth in<br />
FLO-2D to provide additional capacity. The <strong>Powerline</strong> Channel bottom widths <strong>and</strong> geometries are<br />
summarized in Section 8.2. A typical section of the <strong>Powerline</strong> Channel is provided in Appendix C<br />
as Figure C-6.<br />
A construction cost for Alternative 2 is presented in Section 8. The cost estimate assumed that the<br />
entire embankment for the existing <strong>Powerline</strong> FRS <strong>and</strong> <strong>Powerline</strong> IDSM would be removed for a<br />
total of 704,000 cubic yards of material. The material is in the ownership of the Arizona State L<strong>and</strong><br />
Department. For the purposes of the construction cost the material would be stockpiled on site.<br />
Locations of stockpiles would be determined in final design.<br />
The initial hydraulic analyses of the <strong>Powerline</strong> Channel indicate several locations may require<br />
small earthen dikes to provide the 1-foot minimum freeboard required for a 100-year channel.<br />
These dikes may be required at the Channel confluence with Siphon Draw <strong>and</strong>/or at the discharge<br />
location into the Vineyard Road FRS. This will be determined in final design. These embankments<br />
may be considered levees. If considered as levees, the embankments would be required meet<br />
USDA- NRCS Page 5-11 <strong>Jan</strong>uary <strong>2013</strong><br />
Kimley-Horn <strong>and</strong> Associates, Inc.