20011729 Ver 1_Other Agency Comments_20040810

A74LA.
NCDENR
North Carolina Department of Environment and Natural Resources
Division of Water Resources
Michael F. Easley, Governor
William G. Ross Jr., Secretary
John Morris, Director
August 10, 2004
MEMORANDUM
TO: John Dorney, Wetland/401 Unit - Division of Water Quality
FROM: John Sutherland, Water Projects Section Chief - Division of Water Resources
SUBJECT: US Army Corps of Engineers Public Notice No. 200220234
Town of Siler City, Rocky River Lower Reservoir Expansion
The Division of Water Resources (DWR) has reviewed the public notice for the section 404
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.
DWR's previous involvement with this project includes: overseeing the development of an
environmental assessment to meet requirements of the Division of Environmental Health (DEH),
Public Water Supply Section; and recommending downstream release requirements for the
Division of Land Resources' dam safety permit. For the past several months we have worked
with DWQ staff to help address concerns raised in the public comments on the water
reclassification and 401 permit for the proposed project.
DWR's comments on the section 404 permit are included below and are consistent with
conditions recommended for the dam safety permit. DWR and DWQ staff are finalizing
additional requirements regarding high flow pulse releases, and we expect them to be included in
the section 401 permit conditions that are also attached to the section 404 permit.
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Minimum Flow Regime
During development of the environmental assessment for this project for DEH, 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.
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
JUL 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
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Reductions in Withdrawals in Conjunction with Tier 2 or Tier 3 Minimum Releases
Water supply withdrawals from the Rocky River shall 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 initially submitted by the Town during review of the
dam safety permit indicated 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/l daily average.
To avoid potential downstream water quality problems, it will be necessary to install the outlet
structure so that the minimum release can be drawn from at least three levels in the
impoundment. These levels shall 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.
Downstream Gaging
During review of the dam safety permit, 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 shall be provided to the Department within 30 days of
completion.
During review of the DEH 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 - shall install and maintain a continuous
recording gage in the vicinity of highway 64, upstream of Loves Creek. This gage shall be
equipped to monitor both stream discharge and water quality. Water quality parameters to be
monitored shall 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.
cc: Jim Mead, DWR
Danielle Pender, WRC
Bob Zarzecki and Boyd Devane, DWQ
John Ellis and Howard Hall, USFWS
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