PRELIMINARY ANALYSIS REPORT DAMS CONSTRUCTION/RECONSTRUCTION PROJECT CITY OF BOILING SPRING LAKES BOARD UPDATE December 3, 2019
DEQ Meeting 11/20/19 • Preliminary Analysis Report (PAR) • Agency Coordination • Codes and Standards • Preliminary H&H • Geologic Conditions • Design Considerations • Recommendations Alton Lennon Road (Sanford Dam) during Hurricane Florence
Preliminary Analysis Report 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 ³ McGill met onsite on 10.17.19 with: SPRING LAKE ALLEN CREEK • NC Wildlife Resources NORTH Commission, LAKE SANFORD DAM • NC Department of Environmental NORTH Quality and LAKE DAM E K A L • US Army Corps of Engineers on PINE G N LAKE I R P S G N site I L I O PINE B LAKE DAM 1. UPPER Permit based on pre Florence MIDDLE LAKE DAM LAKE DAM conditions - impacts to open water. MIDDLE LAKE UPPER SR 87 (anticipate NWP 3 for Maintenance LAKE Activities and Water Quality General 0 0.5 1 Miles Certification 4132). Boling Spring Lakes Site Map
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 5
Codes and Standards • All dams predated NC Dam Safety regulations Pre Florence - all dams were functional and impounding o Post Florence - all dams are now considered High Hazard o • 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 o North Lake, Pine Lake, and Upper Lake spillways must provide overtopping o protection up to ⅓ PMP storm • Geotechnical Evaluation and Embankments Design Sanford - Sink holes, Seepage, Stability o North Lake, Pine Lake, and Upper Lake – Seepage and Stability o
Preliminary Hydrologic/Hydraulic Evaluation • Hydrology – PMP analysis • Hydraulics – Combined modeling approach • Comparison to effective model • Initial spillway sizing Boiling Spring Lake Watershed 49.69 in • Preliminary breach conditions 28047 ac-ft
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 44 ⅓ PMP 335.9 35 38.6 E. Boiling Spring at Pine Lake Dam 5.4 40 ⅓ PMP 888.2 35 38.3 E. Boiling Spring at North Lake Dam 1.7 39 ½ PMP 6477 30 35.3 Alton Lennon Road at Sanford Dam 3.7 35.5 Upper Dam Breach 35.7 Upper and Middle Dam Breach
Overview of NLD/PLD/ULD Explorations • Intent of Exploration • Confirm approximate height of the dams • Characterize fill soils and foundation materials • Exploration Method • Auger Borings with Standard Penetration Tests (SPT) • General Findings • Dams are similar to design drawings. • Foundation soils: ‐ poorly ‐ graded sands overlying clayey sands ‐ N < 10 blows per foot (bpf) • Dam fill soils: ‐ poorly ‐ graded sands found in City vicinity ‐ 10 bpf < N < 30 bpf • No core soils identified • Pre ‐ construction natural debris, organic soils, and other materials
NLD/PLD/ULD Alternative 1 – Embedded Riser
NLD/PLD/ULD Alternative 1 – Embedded Riser • Benefits • Shorter spillway culvert • Convenient, land ‐ based access to low ‐ level drain • More visible to public boaters • Drawbacks • Larger riser plan area • More excavation • Large, robust headwall • Thin soil cover requires robust culverts (ULD) • Multiple ‐ part low ‐ level drain • Portion of low ‐ level drain is buried • Public has restricted access
NLD/PLD/ULD Alternative 2 – Riser in Lake
NLD/PLD/ULD Alternative 2 – Riser in Lake • Benefits • Smaller riser plan area • Less excavation • Smaller headwall • Public more challenged to access riser • Drawbacks • Longer spillway culvert • Thin soil cover requires robust culverts (ULD) • More challenging access to low ‐ level drain • Less visible for public boating safety
NLD/PLD Alternative 3 – Upgrade Existing
NLD/PLD Alternative 3 – Upgrade Existing • Benefits • Less excavation • Convenient, land ‐ based access to low ‐ level drain • More visible for public boating safety • Drawbacks • Retrofit vs new installation • Larger riser plan area • Large, robust headwall • Multiple ‐ part low ‐ level drain • Portion of low ‐ level drain is buried • Public has restricted access
ULD Alternative 3 – Open Channel Spillway
ULD Alternative 3 – Open Channel Spillway • Benefits • Convenient, land ‐ based access to low ‐ level drain • Smaller, more efficient spillway • Drawbacks • Requires bridge over spillway • Larger inlet plan area • Large, robust headwall • Robust spillway sidewalls • Multiple ‐ part low ‐ level drain • Portion of low ‐ level drain is buried • Public has restricted access
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
1986-1987 SD Seepage Event
Overview of SD Exploration
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
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)
SD Alt. 1A – Labyrinth in Existing Footprint
SD Alt. 1B – Labyrinth in Breach
SD Existing Spillway Removal
SD Alt. 1 – Labyrinth Spillway • Benefits • Spillway aligned with stream channel (Alt. 1A only) • Accessible low ‐ level drain • Spillway is open (i.e. not buried) • Additional seepage cutoff above spillway is not required • Drawbacks • Bridge is required • City less familiar with spillway operation
Example Labyrinth Spillway
SD Alt. 2A – Riser in Existing Spillway Footprint
SD Alt. 2B – Riser in Breach
SD Alt. 2 – Riser and Box Culvert Spillway • Benefits • Spillway aligned with stream channel (Alt. 1B only) • No bridge required • City familiar with spillway operation • Drawbacks • Buried culverts susceptible to joint issues • Additional seepage cutoff is required above culverts • Multiple ‐ part low ‐ level drain • Portion of low ‐ level drain is buried
Meeting Conclusions • Any objections to the presented approach/alternatives? • The most cost effective solution will be recommended for design. • FEMA stated that it is important to move to the next step • FEMA stated that there is still uncertainty on the responsibility split between NCDOT and the City on North Lake and Pine Lake Dams
Next Steps • Complete PAR – due 1/14/20 Complete FEMA Scope of Work for • each dam • Assist City in coordination with DPS and FEMA • HMGP funding • Separating out Sanford and Upper dams since there aren’t eligibility concerns • NCDOT MOU for EBSR at North Lake and Pine Lake Dams City approval to proceed with • Design Phase 33
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