20181120 Ver 1_No Rise Letter Report_20180810

April 30, 2018
Mr. John H. Larch III, PE, MPA
City of Fayetteville
Engineering and Infrastructure
433 Hay Street
Fayetteville, NC 28301
Subject: Project 17C21025.00, No-Rise Certification Analysis for Mirror Lake Dam
Hurricane Repairs, Cumberland County, North Carolina.
Dear Mr. Larch,
SCHNABEL ENGINEERING SOUTH, P.C. (Schnabel) is pleased to present this No-Rise Certification for
the Repair of Mirror Lake Dam.
PROJECT DESCRIPTION
Overview
Mirror Lake Dam is an earthen embankment dam located in central Fayetteville, North Carolina. Mirror
Lake Drive, a two-lane, asphalt paved road (Mirror Lake Drive) maintained by the City of Fayetteville, runs
along the crest of the dam.
The dam was built in 1959 by Tom Mclean for recreational purposes and was reportedly modified in 2002.
Currently, the dam is 16 feet high and approximately 300 feet in length. The dam has upstream and
downstream slopes of 4H;1V and 2H:1V respectively. The Mirror Lake Dam spillway system consists of a
two stage concrete drop box structure with four outlet pipes. The first stage of the concrete drop box
consists of a 21-foot wide weir with a crest at the normal pool elevation of the lake (El 135.79). The
second stage of the concrete drop box consist of the back side of the box 21.2-foot and the two sides of
the box that are 4-feet at El 139 (total second stage weir length is 29.2 feet. Spillway flows are conveyed
downstream through the embankment via four, 42-inch diameter reinforced concrete pipes (RCPs).
Mirror Lake Dam is in series with Evans Lake Dam and Branson Lake Dam. Evans Lake Dam discharges
into Hybarts Branch which flows south and into Mirror Lake. Mirror Lake Dam discharges in Hybarts
Branch approximately 700 feet upstream from its confluence with Branson Creek. From here, Branson
Creek flows south-east towards Branson Lake.
City of Fayetteville
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Mirror Lake Dam is regulated by the North Carolina Department of Environmental Quality Office of Dam
Safety (NC Dam Safety) as a small-size, high-hazard dam. The spillway design storm for a small-size,
high-hazard dam in NC is one-third of the Probable Maximum Precipitation (1/3 PMP).

Mirror Lake Dam overtopped as a result of Hurricane Matthew in October 2016. This resulted in
significant erosion of the downstream slope of the embankment. Other damaged areas included the
upstream slope adjacent to the spillway inlet, several utilities, a section of Mirror Lake Drive, and portions
of the spillway outlet system. Mirror Lake Drive remains closed as a result of this damage. On August 1,
2017, the City of Fayetteville retained Schnabel to perform dam evaluation and design to repair Mirror
Lake Dam.
Mirror Lake Dam
Mirror Lake Dam is located within FEMA designated zone AE on Panel 0427 of the current Flood
Insurance Rate Maps (FIRM) Cumberland County, North Carolina dated January 05, 2017. The area
adjacent to Mirror Lake Dam within the Zone AE is residential.
Hybart’s Branch
Mirror Lake Dam was constructed across Hybart’s Branch approximately 700 feet upstream of the
confluence between Hybart’s Branch and Branson Creek. The floodplain along Hybart’s Branch from the
confluence with Branson Creek to the headwaters of Hybart’s Branch is in Zone AE 100-year floodplain
on Panels 0418, 0427 and 0428 of the current Flood Insurance Rate Maps (FIRM) Cumberland County,
North Carolina dated January 05, 2017. The floodplain along Hybart’s Branch from the upstream end of
Mirror Lake to Evans Dam is wooded with residential area adjacent to the woods. The floodplain along
Hybart’s Branch from Mirror Lake Dam to the confluence with Branson Creek is wooded.
DATA SOURCE
A site topographic survey was performed by CH Engineering in September 2017. The site survey
included the principal spillway, the crest centerline of the dam, location of utilities, sewer lines, first floor of
the house located on the left downstream abutment of the dam, and other additional information needed
for design.

City of Fayetteville
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HYDRAULIC MODEL UPSTREAM OF THE DAM
The HEC-RAS version 5.0.3 was used to analyze the proposed Mirror Lake Dam impact on the 100-Year
flood elevations. The Effective Model obtained was in National Geodetic Vertical Datum of 1929
(NGVD29). The model was converted into North American Vertical Datum of 1988 (NAVD 88) by
subtracting 0.87 feet based on the average conversion in the 2007 Flood Insurance Study (FIS) for
Cumberland County. The elevation used in the HEC-RAS model and the tables presented here reference
NAVD 88.
Hybart’s Branch Effective 100-year Elevation Model
As described previously, Mirror Lake is within the FEMA designated zone AE on Panel 0427 of the
current Flood Insurance Rate Maps (FIRM) Cumberland County, North Carolina dated January 05, 2017.
The elevations within the FIRM Panel are presented in the vertical datum NAVD 88. According to the
FIRM panel, the Base Flood Elevation (BFE) just upstream of the dam is El 143.0 feet and one
downstream of the dam, along Branson Creek is El 128.7 feet.
EFFECTIVE MODEL
The Effective and floodway models for Hybart’s Branch were obtained from North Carolina Floodplain
Mapping Program (NCFMP) in August 2017. The model was obtained electronically as a HEC-2 file in
PDF format. The floodway model includes the 100-year flood with and without encroachments for
Hybart’s Branch. Mirror Lake Dam is modeled as a bridge in the HEC-2 file with a weir coefficient of 2.5.
DUPLICATE EFFECTIVE MODEL
The Effective Model was imported into HEC-RAS version 5.0.3 to create the Duplicate Effective Model.
The model was converted to NAVD88. Mirror Lake Dam, modeled as a road in the effective model was
converted to an inline structure in the duplicate effective model. No other changes were made.
EXISTING CONDITIONS MODEL
The Existing Conditions Model was revised to reflect the current dimensions and spillway capacity of
Mirror Lake Dam. The top of dam stations and elevations were revised using the 2017 survey. The
spillway rating curve of the drop box spillway was added to the inline structure as a gate with a spillway
rating curve.
POST-PROJECT MODEL
The Existing Conditions Model was then revised to include the proposed spillway design to create the
Post-Project Conditions Model. The proposed spillway system will consist of a two-stage, 40-foot by 28-
foot concrete riser box and four precast concrete cylinder outlet pipes (PCCP). The first stage of the
concrete riser box will consist of a 22-foot wide sharp crested weir at the normal pool elevation of the
lake, El 135.8. The second stage will consist of two 9-foot sharp crested weirs located on each side of
the first stage and the 28-foot long riser side walls. The second stage will have a total weir length of 74
feet with the crest at El 139. The back wall of concrete box riser will be 40-foot long with the crest at El
140.The spillway flows will be conveyed downstream through the dam via four PCCP pipes. The PCCP
City of Fayetteville
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pipes will consist of two 60-inch diameter, one 48-inch diameter and one 42-inch diameter pipes. The
pipes will be 98 feet long with inlet invert elevations at El 127.7 and outlet invert elevation at El 126.8. The
downstream slope of the Dam was armored with overtopping protection designed for 1/3PMP. The top of
the dam stations, elevations and the gate spillway rating curve were revised within the HEC-RAS model
to represent the proposed conditions.
Results
Effective versus Duplicate Conditions
When compared to the Effective Model, the Duplicate model has a decrease in both the 100-year
floodplain and floodway models. In the floodplain, there is a decrease of elevation upstream of Mirror
Lake Dam of 0.43 feet while the floodway decreases 1.09 feet upstream of the dam. The area
downstream of the dam has a decrease of 0.03 feet in elevation as well. The differences of the effective
model and the duplicate model are due to the conversion of a HEC-2 model into a HEC-RAS model, and
Mirror Lake Dam being modeled as an inline structure instead of a bridge.

Table 1: Floodplain and Floodway Water Surface Comparison for the Effective and Duplicate
Models

100-year Floodplain Floodway
River Effective Duplicate Difference Effective Duplicate Difference
Station Effective Effective
4550 151.62 151.62 0 151.62 151.62 0
4050 148.18 148.18 0 149.17 149.19 -0.02
3550 147.29 147.29 0 148.22 148.25 -0.03
3150 146.95 146.95 0 147.76 147.78 -0.02
2700 145.44 145.43 0.01 145.6 145.51 0.09
2400 143.44 143.73 -0.29 144.35 144.85 -0.5
1900 143.08 143.48 -0.4 143.4 144.29 -0.89
1400 142.98 143.41 -0.43 142.95 144.04 -1.09
1100 142.98 143.41 -0.43 142.98 144.05 -1.07
560 142.98 143.41 -0.43 142.98 144.06 -1.08
550 142.98 143.41 -0.43 142.97 144.04 -1.07
525 Mirror Lake Dam Mirror Lake Dam
500 133.89 133.92 -0.03 133.92 133.93 -0.01
495 131.03 131.03 0 132.09 132.09 0
300 130.25 130.25 0 131.3 131.3 0
10 129.36 129.36 0 130.17 130.17 0
5 129.34 129.34 0 130.15 130.15 0
1 129.33 129.33 0 130.13 130.13 0
City of Fayetteville
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Duplicate Effective versus Existing Conditions
When compared to the Duplicate Effective Model, the Existing Conditions Model has a decrease of up to
0.94 feet upstream of the dam for the 100-year floodplain and 0.52 feet for the floodway. The decrease in
the water elevation when compared with the Existing Model is due to more accurate modeling the
spillway capacity using the surveyed dam and spillway dimensions compared to the modeling approach in
the duplicate effective model.

Table 2: Floodplain and Floodway Water Surface Comparison for the Duplicate Effective and
Existing Conditions Models

100-year Floodplain Floodway
River Duplicate Existing Difference Duplicate Existing Difference
Station Effective Conditions Effective Conditions
4550 151.62 151.62 0 151.62 151.62 0
4050 148.18 148.18 0 149.19 149.19 0
3550 147.29 147.29 0 148.25 148.24 -0.01
3150 146.95 146.95 0 147.78 147.78 0
2700 145.43 145.43 0 145.51 145.51 0
2400 143.73 143.18 -0.55 144.85 144.57 -0.28
1900 143.48 142.62 -0.86 144.29 143.85 -0.44
1400 143.41 142.47 -0.94 144.04 143.52 -0.52
1100 143.41 142.47 -0.94 144.05 143.54 -0.51
560 143.41 142.47 -0.94 144.06 143.54 -0.52
550 143.41 142.47 -0.94 144.04 143.53 -0.51
525 Mirror Lake Dam Mirror Lake Dam
500 133.92 133.92 0 133.93 133.93 0
495 131.03 131.03 0 132.09 132.09 0
300 130.25 130.25 0 131.3 131.3 0
10 129.36 129.36 0 130.17 130.17 0
5 129.34 129.34 0 130.15 130.15 0
1 129.33 129.33 0 130.13 130.13 0

Existing Conditions versus Post-Project Conditions
When compared to the Existing Model, the Post-Project Model shows a decrease in water surface
elevation. Upstream of the dam the 100-year floodplain decreases by 2.03 feet and the floodway
decreased 0.32 feet. The Post-Project Model shows a decrease in the water elevation when compared
with the Existing Condition Model. The proposed spillway design has slightly more capacity than the
current spillway.

City of Fayetteville
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Project 17C21025.00/ April 30, 2018 Page 6 Schnabel Engineering South, P.C.
Table 3: Floodplain and Floodway Water Surface Comparison for the Existing Conditions and
Post-Project Models

100-year Floodplain Floodway
River Existing Post-Project Difference Existing Post-Project Difference
Station Conditions Conditions Conditions Conditions
4550 151.62 151.62 0 151.62 151.62 0
4050 148.18 148.18 0 149.19 149.18 -0.01
3550 147.29 147.29 0 148.24 148.24 0
3150 146.95 146.95 0 147.78 147.78 0
2700 145.43 145.43 0 145.51 145.51 0
2400 143.18 142.94 -0.24 144.57 144.43 -0.14
1900 142.62 141.63 -0.99 143.85 143.59 -0.26
1400 142.47 141.1 -1.37 143.52 143.2 -0.32
1100 142.47 141.11 -1.36 143.54 143.22 -0.32
560 142.47 141.11 -1.36 143.54 143.22 -0.32
550 142.47 140.44 -2.03 143.53 143.21 -0.32
525 Mirror Lake Dam Mirror Lake Dam
500 133.92 133.92 0 133.93 133.93 0
495 131.03 131.03 0 132.09 132.09 0
300 130.25 130.25 0 131.3 131.3 0
10 129.36 129.36 0 130.17 130.17 0
5 129.34 129.34 0 130.15 130.15 0
1 129.33 129.33 0 130.13 130.13 0