Shaping Communities Together 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 of 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: o 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). o WRC requested inclusion of data on the need to restore the lakes. o Low flow conditions will be established in order to maintain downstream aquatic habitat within Allen Creek o NCHPO found no historic impacts.
Shaping Communities Together • 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: o Sanford spillway must provide overtopping protection up to ½ PMP storm (Probable Maximum Precipitation) o 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. o 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.
Shaping Communities Together o 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. o 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. o 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. o 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. o 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. o 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. o Dam Safety inquired if the encountered unsuitable material causes any concerns. Schnabel clarified that the findings do not raise significant concerns. o 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. o 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. o 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
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