Meeting Minutes Subject : Boiling Spring Lakes Dams - - PDF document

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Meeting Minutes

Subject: Boiling Spring Lakes Dams Construction/Reconstruction Project Preliminary Assessment – Interim Presentation Date: November 20, 2019 Location: NC Department of Environmental Quality Dam Safety (NC Dam Safety), Archdale Building, 512 N. Salisbury Street, Raleigh, NC 27604 Floor 5, Main Conference Room Attendees: See Sign in Sheet Purpose The purpose of the meeting was to discuss findings of the Preliminary Assessment conducted for the project as agreed upon during the meeting conducted on June 3, 2019 with the City of Boiling Spring Lakes (City), FEMA, NCDOT, and NC Dam Safety. The slides presented at the meeting are attached to these notes for reference and additional details. The primary tasks of this Preliminary Assessment

• f the four City-owned dams (i.e., Sanford Dam, North Lake Dam, Pine Lake Dam, and Upper Lake

Dam) include:

• Coordination with agencies to determine the most suitable permitting process
• Comprehensive hydrologic and hydraulic models
• Performance of subsurface exploration program to explore conditions potentially related to

uncontrolled seepage

• Utilizing East Boiling Spring Lake Road as an impounding structure for North Lake and Pine

Lake Discussion Outcome

• McGill discussed coordination efforts with related regulatory agencies, including NC Wildlife

Resources Commission (WRC), NC Department of Environmental Quality (NC DEQ), and US Army Corps of Engineers (USACE), as summarized by the following:

• USACE 401/404 Permit expected to be issued as NWP 3 for Maintenance Activities

and Water Quality General Certification 4132 based on conditions prior to Hurricane Florence (i.e., impacts to open water).

• WRC requested inclusion of data on the need to restore the lakes.
• Low flow conditions will be established in order to maintain downstream aquatic

habitat within Allen Creek

• NCHPO found no historic impacts.

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• Middle Dam (private) may be reconstructed under NRCS EWP grant.
• McGill stated that although all dams predated NC Dam Safety regulations and were

functional and impounding pre-Florence, they are now considered High Hazard Dams by NC Dam Safety. McGill requested confirmation by NC Dam Safety that the now breached dams must meet the current design codes and standard per 15A NCAC 02K.0204(e) when

• reconstructed. NC Dam Safety confirmed that the following design criteria, as stipulated in the

referenced code, will apply:

• Sanford spillway must provide overtopping protection up to ½ PMP storm (Probable

Maximum Precipitation)

• North Lake, Pine Lake, and Upper Lake spillways must provide overtopping

protection up to ⅓ PMP storm* Note *: NC Dam Safety added that the breached Middle Lake Dam (private dam, not part of this assessment) is also now considered High Hazard and must also meet the same standards.

• McGill noted that the Middle Lake Dam was being considered for reconstruction under the

City’s NRCS EWP grant. McGill has coordinated with the City’s selected engineer for this project regarding the status of the other dams and will share data as needed. The worst-case scenario between a reconstructed or removed dam will be considered in the design.

• McGill presented the hydrologic and hydraulic evaluation and initial design to meet the

current requirements. Various alternatives were presented for each dam. The PMP storm was developed using the methodology described in HMR51 / HMR52.

• NC Dam Safety clarified that 2 feet of freeboard is preferred for each dam based on the

design storm event. McGill clarified that the spillways will be sized to meet this requirement to the extent practicable on North Lake, Pine Lake, and Sanford Dams. McGill added that Upper Lake Dam might need to be an exception because only 3 feet are available between the normal water surface elevation in the lake and the overtopping elevation. NC Dam Safety agreed that a lesser freeboard depth may be used for Upper Lake Dam.

• Schnabel summarized relevant published regional geology information and available records.
• The following information was presented in regional geology publications reviewed

during this project phase:

• The soils in the vicinity of the project site consists of interbedded sands, clays,

and clay/sand mixtures of the Socastee Formation.

• Limestones of the Comfort Member of the Castle Hayne Formation underlie

soils in the vicinity of the project sites. Comfort Member limestones are generally described as highly fossiliferous and susceptible to dissolution of carbonate contents.

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• Drawings from around 1960 were found for Sanford, North Lake, and Pine Lake
• Dams. These are considered design drawings, not as-builts, because there were not

sufficient information/dates on drawings to confirm otherwise.

• No records available for Upper Lake Dam.
• Schnabel presented the approach to and results of the geotechnical exploration conducted at

North Lake, Pine Lake, and Upper Lake Dams first.

• The presence of the North Lake and Pine Lake Dams embankment cores shown in

the 1960 Drawings could not be confirmed based on encountered conditions in borings.

• Fill soils consisted of poorly-graded sands with varying amounts of fines, and appear

to be compacted with compactive energy consistent with construction methods of the era in which the dams were constructed.

• Foundation soils are generally consistent with published regional geology information,

and generally consisted of loose poorly graded sands overlying loose clayey sands. Natural debris, such as organic soils and wood were encountered in some of the borings.

• Dam Safety inquired if the geotechnical borings reached the Castle Hayne formation

at these sites. Schnabel stated that rock was not encountered and clarified that determining depth to rock (if deeper than planned depth of borings) was not a goal of the exploration due to the relatively low hydraulic head of the dams, lack of surficial evidence of past sinkhole activity at the dam sites, and apparent lack of records of sinkhole activity at the dam sites. If by the completion of the preliminary assessment a need to conduct further investigation is identified, additional borings will be performed.

• Dam Safety inquired if the encountered unsuitable material causes any concerns.

Schnabel clarified that the findings do not raise significant concerns.

• Schnabel stated that based on preliminary findings, additional geotechnical

exploration at the sites does not seem warranted. McGill clarified that the goal is to proceed with the design with the data collected unless NC Dam Safety requires additional investigation.

• NC DEQ Dam Safety noted that they would prefer to see at least one deep boring at

each of the three upper dam sites to confirm depth to rock. Schnabel stated that conditions below the preliminary design phase borings will not likely affect the performance of the rehabilitated dams.

• Three conceptual remediation design alternatives were presented for each of the three

upstream dams.

• Summary of Alternatives:
• Alternative 1 consists of complete removal of the existing culverts and

appurtenances, construction of cast-in-place concrete (CIP) box culverts and downstream headwall, and construction of a CIP riser embedded in the embankment, and construction of a filter diaphragm around the CIP box

• culverts. A low-level drain and appropriate safety measures will be included in

Shaping Communities Together the design. NC Dam Safety has previously stated corrugated pipes are unacceptable for water conveyance for dams.

• Alternative 2 consists the same modifications as Alternative 1, except that the

riser is constructed at the upstream embankment toe instead of embedded in the embankment.

• Alternative 3 for North Lake and Pine Lake Dams consists of slip-lining the

existing corrugated metal pipes (CMP), removal of the existing upstream and downstream corrugated metal headwalls and a portion of the CMPs, grouting

• f the remaining gravel bedding below the existing CMPs, construction of a

CIP riser which is embedded in the embankment, and construction of a filter diaphragm around the sliplined pipes. The City emphasized that one of the goals of Alternative 3 is to avoid closing East Boiling Spring Road during construction.

• Alternative 3 for Upper Lake Dam consists of complete removal of the existing

spillway structures and construction of an open channel spillway with a bridge to maintain functionality of the road on the dam. McGill noted that this option may afford the most cost-effective potential to achieve more freeboard for Upper Lake.

• Dam Safety stated that anti-flotation design is required for all risers.
• Dam Safety stated that wave protection is required on the upstream side for all dams.
• Dam Safety stated that Alternative 3 for North Lake and Pine Lake Dams is the least

desirable and will require extensive documentation and demonstration that the gravel bedding grouting program under the CMPs will be sufficient to fill the voids to the full width and length of the bedding material to mitigate the seepage potential. A robust filtration system would also be required.

• McGill clarified that the purpose of presenting these alternatives is to understand the

requirements of NC Dam and then select the most cost-effective alternative. As such, the requirements of the grouting program might make Alternative 3 the least cost- effective one.

• Schnabel presented the approach to and results of the geotechnical exploration conducted at

Sanford Dam.

• An embankment core is visible on the sides of the breach at the general location

(within the embankment) that is shown on the 1960s Drawings. Core soils were not identified in any of the borings, though the borings were drilled downstream of the core location shown in the 1960s Drawings.

• Fill soils consisted of poorly-graded sands with varying amounts of fines, and appear

to be compacted with compaction energy consistent with construction methods of the era in which the dams were constructed.

• Foundation soils are generally consistent with published regional geology information,

and generally consisted of loose clayey sands.

• Rock was encountered at about elevation 0 ft, which is consistent with historical

records.

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• The first 5-10 feet is “shell-mold limestone” (calcirudite). Historical sinkhole

issues are reportedly attributed to this rock unit.

• Below calcirudite is mostly “shell-hash limestone” (calcarenite).
• In the borings advanced to about elevation -50, calcareous sand was

encountered approximately between elevations -42 and -50.

• Two alternatives, with two layout options for each, were presented for Sanford Dam.
• Alternative 1 consists of installing a labyrinth weir spillway.
• Alternative 2 consists of installing a riser structure and box culvert spillway.
• A positive cutoff wall that is embedded to about elevation -15 (about 15 feet below the

top of rock) will be installed with either alternative.

• For each Alternative, two options were considered:
• Option A - constructing the structure at the location of the existing riser
• Option B - constructing the structure at the location of the breach.
• Option A is more efficient hydraulically than Option B because it reduces the risk of

inducing scour at the embankment to the railroad bridge downstream of the dam. Option B was initially considered for potential cost savings. However, preliminary evaluation indicates comparable costs for both options.

• Dam Safety is in favor of the positive cutoff wall and indicated that it is preferred over

a grouting program, which might not completely fill the voids.

• Additional borings will be needed to sufficiently understand the variability of the dam

and its foundation.

• Similar to the upstream lakes, the most cost-effective solution will be recommended

to proceed with the design.

• NC Dam Safety noted that they preferred Alternate 1A – labyrinth at the current spillway

location (on the Allen Creek alignment).

• NC Dam Safety noted that a toe drain and permanent geotechnical instrumentation would be

required on Sanford Dam.

• FEMA stated that it is important to move to the next step (i.e., Project Scoping) since we are
• ver a year after the storm and the project funding is not yet obligated. McGill indicated that

the team will be moving with finalizing this assessment based on the input from NC Dam Safety at this meeting and submit a report in January. The City and design team will work with FEMA on the Project Scoping in parallel.

• FEMA stated that there is still uncertainty on the responsibility split between NCDOT and the

City for the reconstruction of North Lake and Pine Lake Dams. NC Dam Safety indicated that although the current understanding is that the City owns the land and NCDOT owns the road, NC Dam Safety considers NCDOT as co-owner of the dam.

• The City will work with NCDOT to get a Memorandum of Understanding (MOU) or

some other legal agreement in place. Other potential options include the City taking

• wnership of the road over the dams.

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• NC Dam Safety offered to be involved in the drafting of the MOU as needed.

Action Items

• City will work with NCDOT on resolving ownership/responsibility issues for Pine Lake and

North Lake Dams. NC Dam Safety and FEMA will assist as needed.

• McGill/Schnabel will finalize the Preliminary Assessment Report.

Attachments

• Sign in Sheet
• Presentation Slides

11/20/2019 1

PRELIMINARY ANALYSIS REPORT DAMS CONSTRUCTION/RECONSTRUCTION PROJECT CITY OF BOILING SPRING LAKES

November 2019 Outline

• Preliminary Analysis Report

(PAR) Overview

• Agency Coordination
• Codes and Standards
• Preliminary H&H
• Geologic Conditions
• Design Considerations
• Recommendations

Alton Lennon Road (Sanford Dam) during Hurricane Florence

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PAR Overview

A meeting on June 3, 2019 with FEMA, NCDOT and NC Dam Safety Program revealed that additional tasks were necessary to fully define FEMA’s Disaster Recovery Scope of Work related to the BSL dams. These tasks are the primary focus of this Preliminary Analysis Report. They include:

• Coordination with agencies to determine the most suitable permitting

process

• Comprehensive hydrologic and hydraulic models
• Subsurface exploration program to fully address potential issues related to

sinkhole formation

• Utilizing East Boiling Spring Lake Road as an impounding structure for

North Lake and Pine Lake

Agency Coordination

McGill met onsite on 10.17.19 with:

• NC Wildlife Resources

Commission,

• NC Department of Environmental

Quality and

• US Army Corps of Engineers on

site

1.

Permit based on pre Florence conditions - impacts to open water. (anticipate NWP 3 for Maintenance Activities and Water Quality General Certification 4132).

UPPER LAKE MIDDLE LAKE PINE LAKE NORTH LAKE SPRING LAKE BOILING SPRING LAKE A L L E N C R E E K

³

1 0.5 Miles SANFORD DAM NORTH LAKE DAM PINE LAKE DAM MIDDLE LAKE DAM UPPER LAKE DAM SR 87

Boling Spring Lakes Site Map

11/20/2019 3

5

Agency Coordination (cont.)

2.

WRC requested inclusion of data on the need to restore the lakes.

3.

Low flow conditions will be established in order to maintain downstream aquatic habitat within Allen Creek

4.

NCHPO found no historic impacts.

5.

Middle Dam (private) may be reconstructed under NRCS EWP grant.

Middle Dam post Hurricane Florence

Codes and Standards

• All dams predated NC Dam Safety regulations
• Pre Florence - all dams were functional and impounding
• Post Florence - all dams are now considered High Hazard
• NC Regulations require reconstructed all dams meet current design

standards per 15A NCAC 02K.0204(e)

• Hydrologic and Hydraulic Evaluation and Spillways Design
• Sanford spillway must provide overtopping protection up to ½ PMP storm
• North Lake, Pine Lake, and Upper Lake spillways must provide overtopping

protection up to ⅓ PMP storm

• Geotechnical Evaluation and Embankments Design
• Sanford - Sink holes, Seepage, Stability
• North Lake, Pine Lake, and Upper Lake – Seepage and Stability

11/20/2019 4

Preliminary Hydrologic/Hydraulic Evaluation

• Hydrology – PMP analysis
• Hydraulics – Combined modeling approach
• Comparison to effective model
• Initial spillway sizing
• Preliminary breach conditions

Hydrology

GEORGE II HWY E BOILING SPRING RD ALTON LENNON RD Allen Creek

DA-6 2.93 sq.mi. DA-2 2 sq.mi. DA-7 2.57 sq.mi. DA-8 1.84 sq.mi. DA-1 1.83 sq.mi. DA-5 A 1.36 sq.mi. DA-4 1.14 sq.mi. DA-3 0.81 sq.mi. DA-5 B 0.53 sq.mi.

1.5 0.75 Miles

³

Legend

Middle LakeDam North Lake Dam Pine Lake Dam Sanford Dam Upper Lake Dam Drainage Basin QL2 Digital Elevation Model

Elevation (ft NAVD88)

<66 >-2.5

11/20/2019 5

Rainfall - PMP

Boiling Spring Lake Watershed 49.69 in 28047 ac-ft

Rainfall - PMP

9.531 (30 min) 49.69 10 20 30 40 50 60 2 4 6 8 10 12 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 Cumulative Precipitation (Inches) Incremental Precipitation (Inches) Time (Hours) Hyetograph Cumulative

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Hurricane Florence

Location Rainfall Total Location Rainfall Total 6.2 miles NW Elizabethtown, NC 35.93" Bennettsville, SC 18.05" 3.7 miles S Wilmington, NC 30.10" Watha, NC 17.49" 4.1 miles WNW Hampstead, NC 29.52" Lumberton, NC 17.49" Sunny Point Military Terminal, NC 27.44" Gallivants Ferry, SC 17.01" Green Swamp/Nature Conservancy, NC 27.40" 3 miles N Lumberton, NC 16.11" 0.8 miles E Smith Creek, NC 27.20" Marion, SC 15.79" 0.9 miles ENE Oak Island, NC 26.98" 2.6 miles N Pawleys Island, SC 15.75" 7.3 miles NE Wilmington, NC 26.58" 8.4 miles WNW Myrtle Beach, SC 15.41" 6.1 miles NW Whiteville, NC 25.91" Whiteville, NC 15.38" Turnbull Creek/Bladen Lakes State Park, NC 23.67" 7 miles SE Hemingway, SC 15.26" 2.9 miles WSW Loris, SC 23.63" 2 miles SW Pawleys Island, SC 14.41" 7.8 miles SW Bolivia, NC 23.33" Blenheim, SC 14.38" 1.4 miles ENE Loris, SC 23.18" Bald Head Island, NC 12.17" Wilmington International Airport, NC 23.02" Soules Swamp/Whiteville, NC 11.50" 2.3 miles NE Lumberton, NC 22.76" Conway, SC 11.43" Whiteville, NC 22.58" 3 miles NE Socastee, SC 11.11" Caswell Beach, NC 22.57" 6.0 miles S Georgetown, SC 11.08" Cape Fear Lock & Dam #1 near E. Arcadia, NC 22.21" Hartsville, SC 11.02" 0.2 miles SSW Yaupon Beach, NC 22.07" Darlington, SC 11.02" 4 miles E Burgaw, NC 22.06" 6.2 miles E Conway, SC 9.63" Back Island/Holly Shelter Swamp, NC 20.87" Myrtle Beach International Airport, SC 9.50" 3.0 miles SE Wilmington, NC 20.11" Hemingway, SC 9.32" Marion, SC 19.56" Bennettsville, SC 9.08" 8.6 miles SSE Lumberton, NC 18.51" Georgetown, SC 8.55" 3.8 miles NW Dillon, SC 18.38" Florence, SC 7.62"

* A newspaper report claimed 36" fell in Southport, NC at a fire department. Although radar data suggests exceptional rain fell across this area, insufficient evidence has been collected so far to verify this report.

https://www.weather.gov/ilm/HurricaneFlorence

Runoff Hydrographs

100-year FEMA 100-year Revised Upper Lake Dam 2360 847.6 Middle Lake Dam 2360 1099.5 SR 87 over Boiling Spring Lake 2360 1099.5

• E. Boiling Spring at Pine Lake Dam
• 302

Pine Road Between Spring Lake and North Lake 259 202

• E. Boiling Spring at North Lake Dam

552 773 Alton Lennon Road at Sanford Dam 3560 3509

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Hydraulics Initial Spillway Sizing

Overtopping Storm Design Flow (SDF) Water Surface Elevation Elevation Event Flow Normal SDF Freeboard Upper Lake Dam 41.3 ⅓ PMP 943.1 38 40.4 0.9

• E. Boiling Spring at Pine Lake Dam

44 ⅓ PMP 335.9 35 38.6 5.4

• E. Boiling Spring at North Lake Dam

40 ⅓ PMP 888.2 35 38.3 1.7 Alton Lennon Road at Sanford Dam 39 ½ PMP 6477 30 35.3 3.7 Upper Dam Breach 35.5 Upper and Middle Dam Breach 35.7

11/20/2019 8

Overview of Geologic Conditions

• Soils
• Castle Hayne Formation

(NCAC, 2011)

Project Sites

Pertinent Available Records

• Original Design Drawings (1960)

Henry M. von Oesen Consulting Engineers (von Oesen) Sanford Dam / North Lake Dam (NLD) / Pine Lake Dam (PLD)

• Additional Sanford Dam Records
• 1977 Subsurface Investigation Report

S&ME

• 1979 Sinkhole Repair Record Drawings

von Oesen

• 1980 Phase 1 Inspection Report

USACE

• 1983 Sinkhole Hazard Reduction Plan

Planning & Design Associates, P.A.

• 1988 Remedial Repair Investigation and Design Report

S&ME

• 1991 Spillway Repair Record Drawings

Robert K. Russel, P.E.

• 2000 Remedial Repair Record Drawings
• D. Miller & Associates, P.A.

11/20/2019 9

Original Design Drawings (1960) Original Design Drawings (1960)

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Overview of NLD Exploration Overview of PLD Exploration

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Overview of ULD Exploration NLD/PLD/ULD Design Goals

• Restore the lakes to pre-breach condition
• Safely pass design floods
• Extend design life
• Facilitate ability to drain lakes
• Promote public safety
• Meet additional current codes and standards

11/20/2019 12

NLD/PLD/ULD Alternative 1 – Embedded Riser NLD/PLD/ULD Alternative 2 – Riser in Lake

11/20/2019 13

NLD/PLD Alternative 3 – Upgrade Existing ULD Alternative 3 – Open Channel Spillway

11/20/2019 14

Seepage Events at Sanford Dam (SD)

• Records available for four seepage events
• 1962
• 1976-1978
• 1986-1987
• 2001-2002
• Average of one event per decade in first 40 years

1962 SD Seepage Event

• Description in 1983 Hazard Reduction Plan
• Sinkholes formed adjacent to MOTSU railroad.
• Owner was required to perform grouting in 1964 and 1969.

11/20/2019 15

1976 SD Seepage Event 1976 SD Seepage Event

11/20/2019 16

1986-1987 SD Seepage Event 1986-1987 SD Seepage Event

11/20/2019 17

2001-2002 SD Seepage Event

Schnabel Photograph, February 2002

Overview of SD Exploration

• Intent of Exploration
• Confirm approximate geometry of the dam
• Characterize fill soils and foundation soils
• Characterize foundation rock
• Measure permeability in rock and foundation soils.
• Evaluate variability in foundation rock
• Exploration Methods
• Records review
• Electrical Resistivity Imaging (ERI) geophysical survey
• Auger borings with SPTs
• Hydraulic conductivity (slug) testing
• HQ-size rock coring
• Hydraulic conductivity (Packer) testing in rock

11/20/2019 18

Overview of SD Exploration

SD Foundation Rock

Calcirudite Limestone (“shell-mold”) Calcarenite Limestone (“shell-hash”)

Castle Hayne Formation

11/20/2019 19

Conclusions from SD Exploration

• Location of concentrated seepage
• Variability of foundation rock
• Existing spillway structure
• Soils at existing spillway

SD Design Goals

• Reduce risk to Dam Safety due to uncontrolled seepage
• Safely pass design flood
• Restore the lake to pre-breach condition
• Extend design life
• Facilitate ability to drain lake
• Promote public safety
• Meet additional current codes and standards

11/20/2019 20

SD Common Design Modifications

• New Spillway
• Repair / regrade embankment
• Install positive seepage cutoff
• Remove existing spillway

Seepage Failure Mode

(taken from Best Practices in Dam and Levee Safety Risk Analysis, USBR / USACE, 2015)

11/20/2019 21

SD Alt. 1A – Labyrinth in Existing Footprint SD Alt. 1B – Labyrinth in Breach

11/20/2019 22

SD Existing Spillway Removal Example Labyrinth Spillway

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SD Alt. 2A – Riser in Existing Spillway Footprint SD Alt. 2B – Riser in Breach

11/20/2019 24

Conclusion

• Any objections to the presented approach/alternatives?
• The most cost effective solution will be recommended for design.

Discussion

?