20030147 Ver 2_DWR Comments_20070706

North Carolina
Michael F. Easley, Governor
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NCDENR
Department of Environment and
Division of Water Resources
July 6, 2007
MEMORANDUM
TO: John Dorney ~(
f~~
FROM: Jim Mead
SUBJECT: Determination of Minimum Release and Mitigation for
Tillery Dam FERC Relicensing and 401 Certification
William G. Ross Jr., Secretary
John Morris, Director
A settlement agreement has been reached between the North Carolina Department of
Environment and Natural Resources (NCDENR) and Progress Energy for the Federal Energy
Regulatory Commission (FERC) relicensing of Progress Energy's hydroelectric facilities on the
Pee Dee River. This agreement includes a release regime for Tillery dam and stream mitigation
through protection of riparian buffers. This memorandum describes the studies and analysis
involved in developing this part of the settlement agreement. A general outline of the study
process is also attached.
Instream flow studies were conducted at multiple locations by consultants for Progress Energy
(PE), who performed these studies in consultation with state and federal agencies using the
Instream Flow Incremental Methodology (IFIM). The reach of the Pee Dee River between the
Tillery dam and the headwaters of Blewett Falls reservoir was divided into three sub-reaches for
this study based on changes in hydrology and habitat type. Division of Water Resources (DWR)
staff were involved in study design, including habitat mapping and selection of study cross-
sections.
• Subreach 3 -Tillery dam to Rocky River; 5.35 miles; 8 transects representing
79% glide, 10% run and 11 % shoal habitat types.
• Subreach 2 -Rocky River to Browns Creek; 6.15 miles; 12 transects
representing 66% glide, 26% run, 4% pool, and 4% shoal.
• Subreach 1-Browns Creek to Blewett Falls reservoir; 9.0 miles; 3 transects
representing 92% glide and 8% pool habitat types.
One
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The consultants collected depth and velocity data at numerous points on each transect at
three distinct discharges, in addition to substrate and cover information at each point.
The three discharge levels for data collection varied by site and were approximately 500
to 900 cfs, 3000 to 3500 cfs, and 7000 cfs. Using this field data, a hydraulic simulation
model was calibrated for each transect that could simulate physical habitat conditions
over a wide range of discharges. DWR staff were heavily involved in this hydraulic
calibration.
Physical conditions simulated by the hydraulic model for each subreach were then
merged with Habitat Suitability Indices for the life stages, guilds and species of interest.
The result is a relationship of habitat (weighted usable area) to discharge (cubic feet per
second) for each organism being evaluated. For this IFIM study, 29 individual life
stages, guilds and species were evaluated.
The next step in the evaluation is referred to as "time series analysis." This entailed
converting a record of stream flows into a record of habitat values for each of the 291ife
stages, guilds and species at each of the three subreaches. Flow records for different
operational alternatives, as well as the unregulated or "natural" flow record, can be
converted to habitat values in this manner. The habitat records for different flow
scenarios can be compared using monthly duration curves and other statistical analyses.
One of the habitat metrics used for this instream flow study and all other major
hydroelectric relicensing studies in North Carolina is known as "Index C." The attached
IFIM study outline contains a -more detailed explanation of this metric.
To focus the analysis of voluminous output for multiple reaches and 29 species and life
stages, the technical work group for this Project undertook screening to determine the
"driver" or focus life stages, guilds and species. These are the 6 or 7 organisms that are
most responsive to changes in flow. A flow regime developed in consideration of these
species/life stages should also be suitable for other less flow-sensitive organisms. The
seven focus species for all three subreaches downstream of Tillery dam were: American
shad spawning life stage, shallow-fast habitat with higher velocity guild, golden redhorse
adult life stage, robust redhorse spawning life stage, deep-fast habitat with coarse mixed
substrate guild, deep-fast habitat with fine substrate guild, and deep-fast habitat with
gravel/cobble substrate guild.
DWR's preferred target level of habitat to be maintained is a flow regime that maintains
Index C values at 80% of the Index C value under unregulated flow conditions for the
focus species. However, this 80% of unregulated Index C is not a formal standard. The
final flow regime might consider several factors, including, but not limited to: the
amount and quality of habitat in the affected reach, fishery resource management
objectives, varying habitat results for different species and life stages, different levels of
habitat in the near-dam reach versus farther downstream after tributary inflow, extent of
improvement from existing flow regime, and other demands on water resources.
2

In March, 2005, the technical work group began reviewing results of the instream flow
studies. In June, 2005 the group began discussing and evaluating various alternative flow
regimes downstream of Tillery dam. The work group included representatives from PE,
DWR, the NC Wildlife Resources Commission (WRC), the South Carolina Department
of Natural Resources, the US Fish and Wildlife Service, American Rivers, The Nature
Conservancy, and Alcoa Power Generation Inc. The group made extensive use of an
interactive spreadsheet that could calculate the Index C values for a particular flow option
for each of the 291ife stages, guilds and species. The Index C values for each flow
option could then be contrasted to the unregulated or natural Index C values, the Index C
values for the existing hydroelectric operations, and the maximum achievable Index C
values. The interactive spreadsheet also calculates these habitat metrics for each of the
three subreaches separately and in combination.
In addition to habitat evaluations, consultants for PE were also using the CHEOPS model
to determine the effect of different flow options on hydroelectric generation during on-
peak and off-peak periods. This modeling took into account the effect of a continuous
release on the availability of water being stored for use during periods of peak demand.
Another factor included was the hydraulic capacity of the turbines in the Tillery
powerhouse. The lowest operational flow for any single turbine is approximately 2000
cfs. Continuous releases much below this level must be spilled or released through a gate
to avoid damage to the turbines -and thus continuous releases less than 2000 cfs cannot
be used to generate electricity. Powerhouse and penstock configuration does not allow
addition of a smaller turbine for minimum releases. Replacing one of the 2000 cfs units
with a smaller unit was also evaluated, but the reduction in capacity to meet higher peak
demands for electricity more than offset the gains from being able to generate power at
minimum releases less than 2000 cfs. PE's consultants also evaluated the possibility of
using the small "house unit" in the powerhouse to make the minimum release and
generate power. However, this unit is intended only for occasional operation to re-
energize the powerhouse in the event of a "black start" after a total power outage. It
would not stand up to continuous operation. In addition, the house unit and its piping
could only pass approximately 200 cfs maximum.
In May, 2006 Progress Energy proposed a minimum release of 300 cfs and stated that the
CHEOPS model indicated continuous minimum flows higher than that resulted in
unacceptable losses in peak power generation. PE proposed stream buffer protection to
offset the difference between the proposed release of 300 cfs and the preferred flows that
would produce habitat at 80% of the unregulated Index C values. The PE proposal also
included a 6-week release of 745 cfs starting in mid- to late February to coincide with the
American shad spawning run.
3

DWR then calculated the mitigation need in bank miles for the proposed Tillery release
of 300 cfs, including 6 weeks in the spring at 745 cfs. Bank miles represent the length of
protected buffer along one side of the channel. For example, 4 miles of river channel
protected on both sides is equivalent to 8 bank miles. The first step in determining the
bank miles needed for mitigation is to calculate how much "credit" is given for the
release of 300 cfs. The process used to evaluate the proposed minimum release and
mitigation need was as follows:
1. The steps below are performed separately for subreach 3 and subreach 2. (For
subreach 1, the proposed release of 300 cfs resulted in Index C values that were
more than 80% of the unregulated Index C values.)
2. Determine the flow regime that would produce 80% unregulated index C values
for all life stages, guilds, and species.
3. Compare the Index C habitat values from this preferred flow regime to those
produced by a release of 300 cfs (with 6 weeks at 745 cfs). Do this for each
month for each of six focus species. The seventh focus species was not included
in the analysis because it had significant outlier results from the three other deep-
fast habitat guilds and because there are unresolved questions regarding the
habitat suitability indices for this particular organism.
4. Average the monthly percentage shortfall in Index C to generate an overall yearly
value for each of the six organisms.
5. Average the yearly percentage shortfall for the six organisms to generate one
overall average value for the percentage shortfall produced by the proposed
release of 300 cfs.
6. Multiply this percentage shortfall times the length of the subreach.
7. The Division of Water Quality (DWQ) uses mitigation guidelines that stipulate a
mitigation ratio of 4:1 if the selected approach is stream preservation. Multiply
the result from step 6 above by four.
8. To convert to bank miles, multiply the result from step 7 above by two.
9. Add the resulting bank miles for subreaches 3 and 2 together to yield the total
bank miles of mitigation needed for a release of 300 cfs.
A spreadsheet is attached showing the various buffer protection lands proposed during
settlement discussions and how much they contribute towards the overall mitigation need.
This calculation indicated that the total mitigation package proposed by Progress Energy
was not sufficient to offset the habitat shortfall for a minimum release of 300 cfs. The
spreadsheet also calculated the mitigation results for a minimum release of 350 cfs.
Comparing this to 300 cfs and interpolating resulted in an increase in minimum release to
330 cfs to go along with the total package of 28.7 bank miles of protected buffer.
Subsequent to this evaluation of mitigation needs, the work group had further discussions
regarding the release for American shad spawning in the spring. It was decided to reduce
the release to 725 cfs, but extend it's duration by 2 weeks. This modification in flow was
not reflected in the calculation of mitigation needs. However, the habitat shortfall is
much more influenced by the release of 330 cfs during the rest of the year than by the
higher release for two months in the spring.
4

It is important to recognize that the amount of mitigation needed was only determined
after the agencies and Applicant spent a year reviewing various flow scenarios for their
effects on habitat and hydroelectric generation. We did not start out with a package of
protected lands and "back fit" a release offset by the proposed mitigation. In fact once
we arrived at a tentative release of 300 cfs and actually calculated the mitigation need, it
was necessary to increase the release to 330 cfs.

GENERAL OUTLINE OF IFIM STUDY PROCESS
Field Data Collection and Physical Habitat Simulation
1. The affected stream reach downstream of a water control structure is
examined to determine habitat types present and presence of any major
tributary inflow.
2. The affected reach may be subdivided according to changes in aquatic
habitat or hydrology.
3. Transects (stream cross-sections) are selected within each subreach to
represent the range of available aquatic habitat.
4. The bottom profile of each transect is surveyed and individual points
(cells) on every transect are coded for substrate and habitat cover type.
5. Hydraulic data (depth and velocity) is collected at every submerged cell of
every transect at three discrete discharges.
6. The hydraulic data provides input to models that are used simulate depths
and velocities across each transect over the range of discharges being
simulated.
7. The end result is a set of physical conditions (substrate/cover, depth and
velocity) at every cell of every transect at every discharge being modeled.
• DWR staff participated in habitat mapping, transect selection, field data
collection, and calibration of hydraulic models for this relicensing.
Aquatic Habitat Modeling
1. Species and life stages are selected for evaluation based on field sampling
and fishery management interests.
2. Habitat suitability indices are developed for each species and life stage
selected. These are a preference scale of how a given species/life stage
responds to different substrates, cover types, depths and velocities.
3. The aquatic habitat modeling component of IFIM merges the output set of
physical conditions with the habitat suitability indices. For each flow of
interest, the combination of substrate, cover, depth and velocity is
evaluated at every cell and transect, and totaled for the whole study reach.
4. The end result is a table and graph of weighted usable area versus stream
discharge for every species and life stage being evaluated.
• DWR staff were involved in selecting species, reviewing habitat
suitability indices, and reviewing the habitat versus flow relationships for
this relicensing.
6

Time Series Analysis
1. IFIM is a suite of analytical approaches. A complete IFIM study always
includes time series analysis.
2. Time series analysis relates the habitat versus flow relationships to the
availability of water in the stream. The output from the aquatic habitat
model is used to convert a record of stream flows into a record of habitat
events.
3. Statistical analysis of the habitat record can be conducted to develop
various habitat metrics and other analytical products. These analyses are
usually done on a monthly basis to reflect seasonal differences -spawning
behavior, for example.
4. One output product is a habitat duration curve. Similar to a flow duration
curve, it represents the percentage of time a given habitat level is equaled
or exceeded.
5. There is one key difference between flow and habitat duration curves.
The habitat versus flow relationship is not linear, and in fact is often bell-
shaped, with lowest habitat levels occurring at low AND high flows.
Therefore, the habitat duration curve is not directly comparable to a flow
duration curve -since habitat levels are based on the shape of the habitat
versus flow relationship particular to each species and life stage.
6. Time series analysis is used to compare habitat availability for different
flow scenarios. For example, one could overlay the habitat duration
curves for a given species/life stage for "natural", existing with-project
flows, and proposed flow regime alternatives.
7. Habitat metrics are often used to allow a more quantified comparison of
different flow scenarios -percentage differences, for example.
8. "Index C" is one of the habitat metrics used in analyzing and interpreting
results from instream flow studies of aquatic habitat conducted using the
Instream Flow Incremental Methodology (IFIM). DWR staff were first
introduced to the use of this metric at an IFIM training workshop
conducted in 1992 by the developers of IFIM who were then part of the
National Biological Service (now part of the US Geological Survey).
Index C has been used to evaluate instream flows and aquatic habitat for
every major hydropower relicensing in North Carolina since the early
1990's.
9. Index C is determined on a monthly basis for each species and life stage.
It is calculated as the average of all habitat events in that month that are
less than the median (50% exceedance) level of habitat for that month.
For example, if you had only one year of daily stream flow data converted
to daily habitat events, there would be 31 values for January. The Index C
value for January would then be the average of the 15 lowest habitat
values. Note that these 15 lowest habitat events would not necessarily
occur on the 15 days of lowest flows. Some of them would be attributable
to high flow events if the species/life stage has a preference for lower
velocities.
7

10. "Index C assumes that low habitat events in a time series are the most
important biologically. By using averaging interval from median to 100%
exceedance values, all low habitat events are assumed to be important.
Values above median are considered excess habitat that cannot be used
effectively due to previous limitations created by low habitat values.
Index[CJ is responsive to any change, whether magnitude or duration of
low habitat events or change in absolute minimum. " (from: Problem
Analysis and Ne otg iating Solutions Usin IFIM, training course reference
material, December, 1992.)
11. A value of Index C is calculated for every flow scenario being considered.
DWR always includes the without-project scenario for evaluation, also
referred to as natural or unregulated flows.
11. To focus the analysis of voluminous output for multiple reaches and
numerous species and life stages, the technical work group for this Project
undertook screening to determine the "driver" or focus species. These are
the 6 or 7 species/life stages that are most responsive to changes in flow.
A flow regime developed in consideration of these species/life stages
should also be suitable for other less flow-sensitive organisms.
12. DWR's preferred target level of habitat to be maintained is a flow regime
that maintains Index C values at 80% of the Index C value under
unregulated flow conditions for the focus species. However, this 80% of
unregulated Index C is not a formal standard. The final flow regime might
consider several factors, including, but not limited to: the amount and
quality of habitat in the affected reach, fishery resource management
objectives, varying habitat results for different species and life stages,
different levels of habitat in the near-dam reach versus farther downstream
after tributary inflow, extent of improvement from existing flow regime,
and other demands on water resources.
13. IFIM is not a "standard setting" approach to determining instream flows.
There is no single flow output as a result. Unlike chemical water quality
standards, there is no one standard for the level of habitat to be
maintained. Rather, IFIM allows various alternatives to be compared in
hopefully reaching an acceptable solution.
DWR staff were very actively involved in determining the focus species
that are most responsive to changes in flow, selecting output products to
be provided, suggesting alternative flow scenarios for evaluation, and
reviewing the results of time series analyses for this relicensing.

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