PERMFILE70400

• Permit No: M2006046
• IBM Index Class Name: Permit File
• Doc Date: 1/8/2007
• Doc Name: Adequacy Response Clarification Letter
• From: Banks and Gesso
• To: DRMS
• Media Type: D
• Archive: No

01!0812007 18:19 3032748329
BANKS AND GESSO LLC
PAGE 03!06
Adequacy Clarfication
Grdrtd River Park Project, M•2006-046
January 8, 2007
Page 2
2. Clanfication of Comment 24.
As noted above, certain aspecks of Comment 24 are revised and
superceded by the most recently developed water management strategy.
Basin Confrgurafion. The dimensions of the basin at the north end of
Phase 1 B are mod~ed as noted in Comment 10, as explained above.
The applicant examined various options for locating the storage capacity
in the clarification/regulating basin, placing the volume both above
groundwater and within the alluvial flow (using special impervious
construction). Given the revised estimate of volumetric requirements, the
basin will be broad and shallow, with all storage capacity located above
the existing groundwater table.
Outlef to Colorado River. Previous plans assumed a gravity outlet from
the basin, such as a lined trapezoidal channel, but no rafional scenario
results in a grading plan conducive to a gravity outlet to the Colorado
River. In addition, scenarios for a gravity drain to the Colorado River
involve introducing a break in the floodway levee; as a flood risk
avoidance strategy, the applicant will avoid any activity that would threaten
the integrity of the floodway levee, including avoiding the construction of a
spillway. Therefore, discharge to the Colorado River is by pumps and
piping, which will draw water from a sump within the regulating pond and
return water to the Colorado River at an outlet safely located to prevent
erosion of the levee.
Spillways. Flood waters in the main channel of the Colorado River have
no route to surcharge into the Subject property by virtue of the floodway
levee constructed along the northeastern property boundary. The limits of
the flood fringe do include the proposed mining cells, and flood waters
would migrate into the site by a circuitous route well removed from the
main channel. The effectiveness of spillways is highly situational, and, as
a result of recent analysis, only one spillway is called for (between Phase
16 and Phase 3A) as shown on the current Revised Exhibits C-2 and F.
To avoid the unnecessary acceleration of flood waters, a spillway is not
desirable at the fringe location where the 100-year flood would first
encroach upon the site. Furthermore, based on additional data regarding
the groundwater table across the site, showing groundwater in mining cell
areas barely below the land surtace, the strategy of turning off pumps at
early flood stage will result in almost no void on site that would be subject
to large differential head pressures from an adjacent flooded area. Only
the interface between Phase 1B and Phase 3A, because of the presence
of underground gas infrastructure, is deemed appropriate far a spillway.