20011729 Ver 1_Other Agency Comments_20021210

ACD-5; WA
E R
North Carolina Department of Environment and Natural Resources
Division of Water Resources
Michael F. Easley, Governor
December 10, 2002
MEMORANDUM
TO: James Caldwell
Division of Land Resources
FROM: John Sutherland
Division of Water Resources
SUBJECT: Dam Safety Permit Application
Proposed Rocky River Lower Reservoir Dam
Chatham County
William G. Ross Jr., Secretary
John N. Morris, Director
The Division of Water Resources (DWR) has reviewed the application for a dam safety permit for the
Town of Siler City's new dam to expand their lower reservoir on the Rocky River. We appreciate
this opportunity to comment and would be glad to discuss our recommendations with you or the
applicant. Our comments are as follows:
Minimum Flow Regime
During development of the environmental assessment for this project, the Town agreed to a minimum
release regime to maintain aquatic habitat downstream of the expanded reservoir. There are three
tiers of minimum flows - depending on the combined volume of usable storage available in both the
upper and lower reservoirs on the Rocky River. The triggers for each tier as follows:
• Tier 1 - releases to be made when the combined usable storage of both the upper and lower
reservoirs is greater than or equal to 70 percent of the combined total usable storage.
• Tier 2 - releases to be made when the combined usable storage is less than 70 percent, but greater
than or equal to 40 percent of the total usable volume.
• Tier 3 - releases to be made when the combined usable storage is less than 40 percent of the total
usable volume.
The quantity of the release varies by month and tier, and is shown in the table below. During initial
filling of the new reservoir the minimum release should be made at tier 1 levels, rather than following
the tiers in reverse order.
1611 Mail Service Center, Raleigh, North Carolina 27699-1611
Phone: [phone] \ FAX: [phone] \ Internet: www.ncwater.org
An Equal Opportunity\Affirmative Action Employer-50% Recycled\10% Post Consumer Paper

Memorandum from John Sutherland to James Caldwell, 12/10/02
Siler City Reservoir Expansion, Page 2 of 4
Minimum Release in
CFS
Month Tier 1 Tier 2 Tier 3
JAN 6.0 3.5 0.5
FEB 6.0 3.5 0.3
MAR 6.0 3.5 0.3
APR 9.0 4.5 0.3
MAY 8.0 4.0 0.3
JUN 5.0 2.0 0.3
J UL 2.5 1.0 0.3
AUG 2.0 1.0 0.3
SEP 2.0 1.0 0.3
OCT 3.5 2.0 0.3
NOV 5.0 2.5 0.5
DEC 7.0 3.5 0.5
Reductions in Withdrawals in Coniunction with Tier 2 or Tier 3 Minimum Releases
Water supply withdrawals from the Rocky River will be reduced by ten percent when the release is
reduced from tier 1 to tier 2, and by another ten percent (20 percent total) when minimum releases are
reduced from tier 2 to tier 3 levels. The amount of reduction in withdrawal will be based on the
average daily withdrawal for the 60-day period immediately prior to the first reduction in minimum
release. The water supply operator shall achieve each reduction in withdrawal within two weeks of
each reduction in minimum release.
The water system operator shall document reduction in water withdrawals by submitting reports of
daily water withdrawals to the Department. The reports should also include daily readings from the
minimum release metering system at the dam. These reports shall be submitted every two weeks for
as long as the minimum release is reduced below tier 1 levels.
Water Quality and Minimum Release Design
The plans for the minimum release structure submitted by the Town indicate that water would be
drawn from a depth of 5 feet off the bottom of the impoundment. We consulted with staff from the
Division of Water Quality (Diane Reed, DWQ) to see if there would be the potential for reservoir
stratification and release of water with low levels of dissolved oxygen using the proposed design.
After reviewing data for the upstream reservoir on Rocky River, DWQ concluded that there is a
definite potential for stratification and low dissolved oxygen levels from the proposed depth of the
release for minimum flows. The upstream reservoir has shown strong stratification and dissolved
oxygen levels as low as 1 milligram per liter (mg/1) at depths of about 6 feet from the surface (the
proposed design would be about 15 feet below the normal pool level). State water quality standards
set the minimum level for dissolved oxygen at 4 mg/1 instantaneous and 5 mg/1 daily average.
2

Memorandum from John Sutherland to James Caldwell, 12/10/02
Siler City Reservoir Expansion, Page 3 of 4
To avoid potential downstream water quality problems, it will be necessary to redesign the outlet
structure so that the minimum release can be drawn from at least three levels in the impoundment.
These levels should be: 2 feet below normal pool; 7 to 8 feet below normal pool; and from the
currently planned level of 5 feet off the bottom. It is anticipated that under most circumstances the
release would come from the uppermost level, with the others being used if impoundment levels are
temporarily reduced.
We discussed this briefly with Hubert Hawkins (DLR) and one possible modification of the current
design would be to create a separate chamber in the release tower containing the bottom drain and the
36 inch inlet pipe from the sedimentation box. The other part of the release tower would contain the
minimum release orifice and have gated inlets from the top and middle levels in the impoundment (2
and 7-8 feet below normal pool). A third gated inlet would connect to the chamber containing the
bottom level 36-inch pipe. Another option would be to create top and middle gated inlets in the
tower and install a valve in the 36-inch pipe leading from the sedimentation box. The consultant
should work with DLR on these or any other options to accomplish the goal of being able to
withdraw water from at least three levels (top, middle, and bottom) of the reservoir.
Downstream Galin
The Town submitted plans for metering the minimum release at the outlet from the dam. These plans
are acceptable as long as the metering system is calibrated with field measurements after the new
reservoir has filled, and at least every two years thereafter. Calibration measurements should be
provided to the Department within 30 days of completion.
During review of the environmental assessment for this project, concerns were raised regarding
downstream flows and habitat for aquatic species - including federally listed endangered species. It
will be important to monitor flows in the Rocky River to ascertain if the minimum release is
conveyed downstream without excessive channel loss, and to make sure that assimilative capacity
flows in the vicinity of the confluence with Loves Creek are maintained.
Because both the quantity and quality of water released downstream to the Rocky River are so
critical, the Town - in cooperation with the USGS - should install and maintain a continuous
recording gage in the vicinity of highway 64, upstream of Loves Creek. This gage should be
equipped to monitor both stream discharge and water quality. Water quality parameters to be
monitored should include dissolved oxygen, temperature, pH, and conductivity. After reviewing the
water quality data collected over a 5-year period following filling of the new reservoir, the agencies
will evaluate if it will be necessary to continue monitoring water quality parameters.
Plans for the gaging described above should be submitted to the Department for approval prior to
installation.
Memorandum from John Sutherland to James Caldwell, 12/10/02
Siler City Reservoir Expansion, Page 4 of 4
cc: Woody Yonts and Jim Mead, DWR
Danielle Pender, WRC
Steve Zoufaly, DWQ
John Ellis and Howard Hall, USFWS
Bill Lester and Angie Mettlen, Hobbs Upchurch
J.T. Grimes, Jewell Engineering
4