TO: Mayor and Members of City Council
THRU: Adam Lindsay, Assistant City Manager
FROM: Sheila Thomas-Ambat, PE, Public Services Director
DATE: April 7, 2025
RE:
Title
Presentation on College Lake Dam and Proposed Solutions for Carvers Creek Watershed
end
COUNCIL DISTRICT(S):
Council District(s)
1l
b
Relationship To Strategic Plan:
Supports Goal 3. City Investment in Today and Tomorrow, through the Strategic Objective 3.1 Enhance city street connectivity, traffic flow, and stormwater systems
Executive Summary:
Staff seeks concurrence from City Council to approve staff to program the following two items into the annual CIP prioritization process and pursue grants as applicable: (A) the College Lake Dam redesign, and (B) the proposed solutions from the Carvers Creek Watershed Study.
Background:
A. College Lake Dam
College Lake is a City-maintained and currently drained facility. Its existing spillway has been modified to function as a culvert, preventing water from impounding with the facility. College Lake experienced damage from Hurricane Florence in 2018. Repairs were recommended to mitigate the immediate damages. According to the Post-Hurricane Florence Dam Assessments Memorandum (2018) by Hazen and Sawyer, even the undamaged spillway configuration of the College Lake Dam likely could not meet the NC Dam Safety requirements for a small high-hazard dam, which is College Lake Dam’s classification. The memorandum reported two options for the City moving forward:
1. Fully breach/remove the dam: This eliminates the operational costs and risks of continued service but also requires downstream mitigation improvements.
2. Redesign the dam: This requires additional costs to install a spillway configuration that meets the NC Dam Safety requirements. Namely, the dam must safely pass the 1/3 probable maximum precipitation (PMP) event.
The additional analysis conducted through the Carvers Creek watershed study indicates that the Redesigned Dam is the recommended solution, with an estimated planning-level cost of $15M - $20M.
B. Proposed Solutions for Carvers Creek Watershed
The Carvers Creek Watershed Study has identified 18 proposed solutions at a total estimated planning level cost of $54.2M. This cost does not include the College Lake dam proposed solution. The top proposed solutions range in severity score from 4 to 22 and in estimated costs from $0.7M to $4.7M. The majority of these solutions include pipe upsizing.
Issues/Analysis:
A. College Lake Dam
Fully breaching the dam would eliminate operational costs and risks of continued service. However, that option creates downstream impacts through flooding of downstream roads and properties that must be mitigated.
Multiple alternatives for downstream mitigation improvements were considered, including culvert upgrades and floodplain storage. The alternatives were analyzed through multiple metrics and costs were determined for each option.
For the Redesigned Dam, a conceptual spillway configuration for College Lake Dam was developed to meet the NC Dam Safety requirements. The downstream area was evaluated for the proposed dam through the same metrics as the fully breached dam alternatives. An estimated planning-level cost was also developed for the proposed dam based on preliminary geotechnical boring results.
B. Proposed Solutions for Carvers Creek Watershed
The top proposed solutions are represented as examples of the 18 proposed solutions. While these are the top proposed solutions for the Carvers Creek Watershed, all the proposed solutions from Carvers Creek Watershed will be programmed into the CIP along with proposed solutions from other completed watersheds if approved by Council.
Proposed solutions presented herein are not moving forward to project development at this time. However, approval enables staff to program them into the Stormwater CIP prioritization process and pursue grants as applicable.
The prioritization of proposed solutions takes into consideration many factors before recommending them to Council for project development. Solutions are sorted on Concern Area Score (“Worst First”) and efficacy. Staff has then targeted an equitable distribution between watersheds as well as strategic selections, such as a project’s ability to be considered for grant funding or a project’s proximity to other construction related activities (i.e. NCDOT improvements or Resurfacing).
The prioritization of proposed solutions should not be viewed as a static list to be done in sequential order. As study areas are completed, new proposed solutions are developed that adjust priority ranking and flexibility exists in those proposed solutions ranking the highest.
Budget Impact:
There is no impact to the General Fund.
Options:
1. College Lake Dam: Council provides consensus to approve redesigning College Lake Dam and pursuing applicable funding sources.
2. College Lake Dam: Council does not provide consensus and remands back to staff with additional guidance.
3. Proposed Solutions for Carvers Creek Watershed: Council provides consensus to approve the 18 proposed solutions to enable staff to program them into the annual CIP prioritization process and pursue grants as applicable.
4. Proposed Solutions for Carvers Creek Watershed: Council does not provide consensus and remands back to staff with additional guidance.
Recommended Action:
College Lake Dam: Council provides consensus to approve redesigning College Lake Dam and pursuing applicable funding sources.
Proposed Solutions for Carvers Creek Watershed: Council provides consensus to approve the 18 proposed solutions to enable staff to program them into the annual CIP prioritization process and pursue grants as applicable.
Attachments:CarversCreekWatershed.ppt