North Fork Reservoir Sediment Release Plan REV 13OCT2023

7 Samuel Ashe Drive, Asheville, NC 28805 1 919-600-4790 1 [e-mail]
Technical Study in Support of a Sediment Release Program for the North Fork Reservoir
City of Hendersonville, NC
October 13, 2023
This technical study was conducted in support of implementing a long-term sediment release program
for the North Fork Reservoir located in the Pisgah National Forest near Mills River, North Carolina. The
City of Hendersonville withdraws water from the reservoir constructed in the 1920s under a Special Use
Permit authorized by the United States Forest Service as part of its potable water supply system serving
a population of approximately 80,000. The permitted water intake from this reservoir ranges from 0.5 to
2.0 million gallons per day which is mixed with water from Bradley Creek and Mills River at the Water
Treatment Plant in the Town of Mills River before being treated and delivered to customers.
Under the current reservoir operation program, excess water flows over the weir outlet on top of the
dam with no discharge from the bottom pipe outlet. The City must remove sediment accumulated from
the watershed every 15 to 20 years in order to maintain reservoir capacity. The City proposes to
implement a continuous bottom -discharge sediment release program to reduce the frequency of
reservoir sediment removal operations in addition to enhancing downstream ecological conditions with
more natural sediment transport and water temperatures.
The results of this technical study will be used in communications with the United States Army Corps of
Engineers in reference to Regulatory Guidance Letter No. 05-04 dated August 19, 2005, with the subject:
"Guidance on the Discharge of Sediments From or Through a Dam and the Breaching of Dams, for
Purposes of Section 404 of the Clean Water Act and Section 10 of the Rivers and Harbors Act of 1899."
This guidance applies to the releases of water and water -carried sediment that may result in the
transportation, reduction, or elimination of bottom sediment accumulations from or through dams.
RGL No. 05-04 states on page 4 that "Activities that are not usually considered regulated discharges of
dredged material and do not require DA permits include actions such as ... releases during times of high
water or flood stages for purposes of passing flood waters through the dam; and the lowering of lake or
pond levels that results in the release of only de minimis amounts of sediment." RGL No. 05-04 further
states that "When evaluating whether any discharge is de minimis, ... factors in this consideration should
include the time of year and normal seasonality of high volume flows, the size of incoming and outgoing
stream/river and the intended release volume, the natural hydrograph of the system, the speed of the
drawdown, the normal amount of sediment in the watershed, and the potential for environmental
Jennings Environmental PLLC is licensed with the North Carolina Board of Examiners for Engineers and Surveyors and is
authorized to practice engineering under the provisions of Chapter 89C and 55B of the General Statutes of North Carolina.
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harm." This report provides information on the factors listed above in two sections:
1. Hydrology and Sediment Modeling Analyses to estimate discharge and sediment release rates
that simulate natural rates expected for a range of watershed flow return periods.
2. Management Plan for Sediment Release that specifies outlet pipe operations that achieve de
minimis sediment impacts and provide cold -water bottom discharge through the dam. This
section also includes the monitoring plan to identity any harmful downstream impacts related to
excessive sediment accumulation.
1. Hydrology and Sediment Analyses
Hydrology and sediment analyses were performed for the North Fork Reservoir watershed to estimate
the flow and sediment release rates that simulate natural rates. These results were used to confirm the
appropriate hydraulic discharges from the pipe that would result in de minimis impacts downstream.
This information was used to estimate the appropriate sediment discharge rates from the lake to the
downstream river that would approximate natural sediment transport rates if the dam did not exist.
1.1. Hydrology Analysis
The North Fork Reservoir has a contributing watershed drainage area of 10.5 square miles, contained
entirely within the Pisgah National Forest. This includes 4.4 square miles from Fletcher Creek and 6.1
square miles from Big Creek, which meet to form the reservoir. The nearest stream gage (USGS
03446000) is located downstream on the Mills River, where the drainage area is 66.7 square miles of
mostly forest and agricultural land. Estimated stream flows presented in Table 1 for the outlet of the
lake were determined for multiple return periods using the on-line StreamStats application, from the
United States Geological Survey:
http://water.usgs.gov/osw/streamstats/north carolina.html
The 2-year return period estimated discharge of 685 cfs is similar to the estimated bankfull discharge of
601 cfs reported for the Blue Ridge Ecoregion on the NCSU Stream Restoration Program web site:
https://www.bae.ncsu.edu/extension/srp/technical-resources/
Table 1. Estimated Discharge by Return Period for North Fork Reservoir with a drainage area of 10.5
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square miles (USGS StreamStats).
Return Period (years)
Estimated Discharge (cfs)
90% Prediction Interval (cfs)
2
685
393 — 1190
5
1220
701— 2110
10
1630
923 — 2870
25
2180
1190 — 4000
50
2590
1370 — 4890
100
3080
1570 — 6020
200
3500
1730 — 7100
500
4160
1950 - 8860
Exact values of low -flow, or baseflow, rates are not available, since a long-term flow record has not been
documented at the dam. However, these conditions can be estimated from the long-term record at the
downstream USGS gage. Daily mean discharge data were obtained from this gage on the Mills River
(USGS 03446000) for the twenty water years 2000 through 2019 (i.e., October 1, 2000 through
September 30, 2019). A running 7-day average of these flows was calculated throughout the 20-year
period. The lower values of these 7-day mean flow rates represent low -flow conditions at the Mills River
gage. With the assumption that the entire watershed at the gage (DA = 66.7 square miles) contributes
equally to baseflow, these low flows can proportionally be scaled to approximate low -flow conditions at
the North Fork Reservoir (DA = 10.5 square miles). Summaries of these data are presented in Table 2.
Over the 20-year study period, this gage analysis suggests that the lowest 7-day average flow at North
Fork Reservoir was 3.2 cfs, while 90% of the 7-day periods had mean flows over 9 cfs (101h percentile).
The lowest 7-day mean estimated discharge for each water year ranged between 3.2 and 16.9 cfs. For
13 out of the 20 years, that discharge was greater than 7 cfs.
This gage analysis is similar to that which would be used to calculate the low -flow 7Q10, which is the
annual minimum 7 day average flow that, over the long-term, would be exceeded 9 out of 10 years. The
USGS calculated 7Q10 (and the related 7Q2 and 30Q2) for 301 sites in the western Piedmont and
Mountains of North Carolina, and reported results in USGS Water Supply Paper 2403:
https://Pubs.er.usgs.gov/publication/wsp2403
Table 2. Summary of 7-day Mean Discharge (cfs) at Mills River USGS Gage and North Fork Reservoir
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(estimated), for twenty water years, 2000-2019.
Likelihood
of Occurrence
Mills River Gage (cfs)
[DA = 66.7 square miles]
North Fork Reservoir
(estimated) (cfs)
[DA = 10.5 square miles]
Minimum (August 2008)
20
3.2
10th Percentile
57
9.0
20t" Percentile
78
12
30t" Percentile
96
15
40t" Percentile
116
18
50t" Percentile
137
21
60t" Percentile
158
25
70t" Percentile
196
31
80t" Percentile
245
39
90t" Percentile
332
52
Maximum (September 2004)
1291
203
Based on the USGS analyses, the 7Q10 for a 10.5-square-mile watershed in the western Piedmont and
Mountains is expected to be between 0 and 11.2 cfs, with a median expected value of 3.3 cfs. Similarly,
7Q2 would be between 0.5 and 16.6 cfs, with a median of 5.8 cfs; while 30Q2 would be between 1.9 and
19.1 cfs, with a median of 7.0 cfs. Flows at the North Fork Reservoir could reasonably be expected to be
greater than the median values due to the absence of impervious surface in the watershed and annual
precipitation depths that exceed many areas within the USGS study area.
1.2. Hydraulic Analysis
The dam at the reservoir has the following outlets:
• Broad -crested weir along the top of the dam, approximately 100 feet in length
• Intake pipe to the water treatment plant: one round orifice with an assumed diameter of 16
inches, and invert located approximately 10.2 feet below the weir
• Bottom discharge pipe: one round orifice with a diameter of 16 inches, and invert located 16.7
feet below the weir
Relevant elevations for the reservoir are:
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2449.2 feet: full pond/crest of weir
2439.0 feet: invert of water intake pipe (approximate)
2432.5 feet: invert of bottom discharge pipe
The intake pipe to the water treatment plant is assumed to operate continually with a rate between 0.5
and 2.0 million gallons per day (MGD) to satisfy permit conditions, which equates to 0.8 to 3.1 cfs.
Hydraulic analyses were conducted to estimate flow rates through the bottom discharge pipe that
would allow for natural rates of sediment transport and a cold -water discharge to the receiving stream,
while maintaining the reservoir at full pond and allowing for continued operation of the water intake.
Flow through the bottom discharge pipe was approximated by applying the orifice equation:
where:
Q = CdA(2gh)o.s
Q = discharge from the orifice (cubic feet per second, cfs)
Cd = orifice coefficient of discharge (dimensionless)
A = cross -sectional area of orifice (square feet)
g = acceleration due to gravity (32.2 feet/second2)
h = driving head acting on horizontal centerline of orifice (feet)
The following assumptions were made for this analysis:
• The water level in the reservoir can be approximated by the top elevation of the dam (i.e., the
reservoir remains at full pond)
• No tailwater control conditions exist downstream of the dam
• Orifice discharge coefficient (Cd) equals 0.62 based on standard empirical conditions
• The outlet pipes are not obstructed
Applying the orifice equation results in the following discharge rates from the bottom discharge pipe:
• For a vertical opening of 16 inches (i.e., the pipe is fully open): 28 cfs
• For a vertical opening of 8 inches (i.e., the pipe is half open): 14 cfs
• For a vertical opening of 6 inches: 9.7 cfs
• For a vertical opening of 4 inches: 5.5 cfs
• For a vertical opening of 2 inches: 2.0 cfs
The required opening height can also be calculated for any flow of interest. For example, an outflow of 8
cfs could be achieved by opening the pipe approximately 5.2 vertical inches.
This analysis produces a range of discharge scenarios that can be adjusted based on the rate of inflow to
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the reservoir. Examples of these discharge scenarios include, but are not limited to, the following:
• During times of relatively high inflow (i.e., greater than 31 cfs):
o Water intake: 3.1 cfs (maximum allowable)
o Bottom discharge pipe: 28 cfs (fully open)
o Dam weir: any additional inflow above 31 cfs
• During times of moderate inflow (i.e., between 17 and 31 cfs):
o Water intake: 3.1 cfs (maximum allowable)
o Bottom discharge pipe: 14 cfs (half open)
o Dam weir: any additional inflow above 17 cfs
• During times of low inflow (i.e., below 17 cfs):
o Water intake: 0.8 to 3.1 cfs, provided that 8 cfs remains for baseflow
o Bottom discharge pipe: 8 cfs (open 5.2 inches; satisfies minimum permit condition)
o Dam weir: any additional inflow
1.3. Sediment Transport Analysis
Since the most recent dredging operation in 2007, the accumulation of sediment in North Fork Reservoir
is approximately 5,700 cubic yards based on bathymetric survey. This volume represents an
accumulation rate of 438 cubic yards per year (570 tons per year, or 54 tons per square mile of drainage
area per year). This accumulation rate is in general agreement with measured long-term suspended
sediment yields between 31 and 58 tons per square mile per year reported for North Carolina mountain
forested basins in USGS Water Supply Paper 2364:
http://Pubs.usgs.gov/wsp/2364/report.pdf
The release of stream sediment through the bottom discharge pipe can be considered to have a de
minimis impact on the downstream ecosystem if the concentration of suspended sediment generally
mimics the concentration found in nearby natural streams without impoundments. A recent study in the
Chatooga River Watershed was published by the Coweeta Hydrologic Laboratory of the USDA Forest
Service:
http://www.srs.fs.usda.gov/pubs/la/ia riedel005.pdf
This study reported highly variable total suspended solids (TSS) measurements related to discharge rates
during storms for four non -impounded and generally undisturbed tributaries. Due to similarities in land
uses, the most relevant relationship for North Fork Reservoir is the result for the recession limb of the
hydrograph for the Addie Branch tributary to Chattooga River. Of the four study streams, Addie Branch
had the most undisturbed watershed and lowest TSS concentrations. For Addie Branch, the following
regression equation was reported:
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where:
TSS = 0.87Qo.67
TSS = total suspended solids (mg/L)
Q = stream discharge (L/s)
The maximum sediment concentration measured in Addie Branch during the 15-month study was
approximately 200 mg/L. This suggests an upper limit for the acceptable sediment concentration
downstream of the North Fork Reservoir. It is assumed that TSS concentrations in flow going over the
dam are negligible, due to settling occurring within the reservoir. During the highest flow scenario for
the bottom discharge pipe at the North Fork Reservoir dam (i.e., pipe fully open), the outflow is 28 cfs,
meaning that a TSS concentration of 8.1 mg/L would mimic that found in Addie Branch at 28 cfs during
the recession limb of a storm hydrograph.
To estimate the maximum potential annual sediment release if the bottom discharge pipe were to
consistently discharge 28 cfs throughout an entire year, the expected TSS concentration of 8.1 mg/L was
multiplied by the total water volume discharged during the year to produce the maximum sediment
discharge rate of 223 tons per year. This maximum annual sediment discharge is considerably less than
the average of 570 tons per year that have historically been deposited in the reservoir from the
upstream watershed. Even with these conservative assumptions, the annual volume of sediment
released downstream of the dam through a fully open bottom discharge would be considerably lower
than the volume of sediment transported by the watershed into the reservoir.
Based on these hydrologic and sediment transport analyses, the City intends to follow the Management
Plan for Sediment Release described in Section 2 in order to release cold water and sediment
throughout the year with de minimis impacts downstream. The Monitoring Plan described in Section 2
will be followed to document any downstream impacts associated with the sediment release program.
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2. Management Plan for Sediment Release from the North Fork Reservoir
The sediment release management plan will be implemented to discharge bottom cold water and
sediment from the lower outlet pipe while achieving de minimis sediment impacts downstream. The
bottom release valve will be operated manually following the City of Hendersonville Standard Operating
Procedures for North Fork Reservoir Bottom Release Valve. During drought conditions, the City will
follow its Water Shortage Response Plan and adhere to its Operational Guidelines for Raw Water
Withdrawals within Table 4 of section 7.6 of the approved Environmental Assessment for Raw Water
Intake and Water Treatment Plant Improvements dated March 2020 for managing the North Fork
Reservoir intake as needed.
2.1. Continuous Bottom Release During Baseflow Conditions:
• During baseflow, maintain the outlet pipe gate valve open at least 5.2 inches to discharge at
least 8 cfs in compliance with the required minimum downstream flow rate condition. This
opening occurs when the valve handle is turned 15 1/2 times from its closed position.
• If watershed inflow to the reservoir during baseflow conditions exceeds 8 cfs, the valve
opening may be Increased to match watershed inflow to the reservoir while maintaining full
reservoir pool conditions.
2.2. Storm Event Flushing Releases:
• During rainfall events of greater than one-half inch during a running 24-hour period, open
the outlet pipe gate valve fully to maximize the discharge of bottom water and sediment.
• After the storm event when flow over the dam weir ceases, partially close the gate valve to
maintain full reservoir pool conditions while continuing to release at least 8 cfs.
2.3. Monitoring Plan for Downstream Impacts of Sediment Release
• Following storm events of greater than one-half inch, visually inspect the outlet pipe to
ensure no blockage and inspect the stream bed within 1,000 ft downstream of the dam to
observe any sediment accumulation conditions.
• Establish six permanent monumented stream cross -sections at the locations of three riffles
and three pools downstream of the dam for repeated topographic surveys.
• Survey the cross -sections before initiation of the sediment release program to establish
baseline morphology.
• Repeat the cross-section surveys annually for three years and compare data in order to
determine changes overtime.
• Visually inspect the cross -sections quarterly for three years. If any cross-section is observed
to have more than 2 inches of sand accumulation on the stream bottom, survey the cross-
section to compare with previous measurements.
• Photograph cross -sections during surveys.
• Report monitoring results including graphs, tables, and photographs, to regulatory agencies
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to determine if any changes are needed in the sediment release plan.
North Fork Reservoir Sediment Release