TO: Mayor and Members of City Council
THRU: Adam Lindsay, Assistant City Manager
FROM: Sheila Thomas-Ambat, Public Services Director
DATE: August 5, 2024
RE:
Title
Arran Lake West Dam Overtopping Protection
end
COUNCIL DISTRICT(S):
Council District(s)
6
b
Relationship To Strategic Plan:
High Quality Built Environment
Executive Summary:
Staff seeks concurrence from City Council to move forward with project development and design of the Arran Lakes West Dam (ALWD) Overtopping Protection Project.
Background:
In December 2021, Council directed staff to begin design of two projects identified in the Beaver Creek 3 Watershed Study. The two projects were the Rhemish Drive Drainage Improvements and the Rockford Drive Drainage Improvement.
Both of these projects are currently under design. As part of the design process, staff use hydraulic models to predict the rise of stormwater elevations down gradient of the project site caused by the new infrastructure, under various storm conditions. Staff evaluates the downstream impacts as far downstream as the models show any significant increase in water surface elevations resulting from the proposed infrastructure.
There are several things from this analysis that may trigger a modification to the design of the proposed project infrastructure (possibly to include acquisition or condemnation). One is if the models show the “level of service” downstream would be reduced, meaning the increased volume or velocity of water projected downstream reduces the efficiency of downstream, existing stormwater infrastructure.
Both the Rockford Drive and Rhemish Drive projects discharge into Beaver Creek Tributary B (BCTB). Downstream of BCTB is Arran Lakes West Dam (CUMBE-067), a private “high” hazard dam with a City maintained roadway overtop of the dam.
Downstream impacts from the proposed improvements do not lower the existing level of service of the dam, however the depth and frequency of overtopping may slightly change based off the analyses conducted. Due to this, both an impact evaluation and detention evaluation were performed looking at both localized and regional detention options.
Issues/Analysis:
Localized Detention
Localized Detention was looked at for both the Rockford Drive and Rhemish Drive project locations, as well as the other 10 proposed solution locations identified in the Beaver Creek 3 Watershed Study upstream of ALWD. The evaluation included preliminary sizing of outlets and basins to mitigate 10-yr peak flow increases.
Challenges for sizing localized detention for each site included final grading, building setbacks and lack of adequate open space. Implementing localized detention facilities at all locations is likely to require demolition of existing homes as the amount of available open space is not sufficient for the estimated basin areas needed to limit the increases in peak flow.
Overall cost for implementation of local detention facilities ranged from $1,200,000 to $6,300,000, with an average of $2,700,000 and total of $33,000,000.
With localized detention, ALWD would not receive the increase in peak flows generated from the improvements upstream. However, ALWD would still remain a high hazard dam with limited freeboard for a 10-yr event.
Regional Detention Evaluation
Evaluation of regional detention initiated with confirming NC Dam Safety regulatory requirements such that any considered improvements were developed in compliance with applicable rules and regulations.
Numerous scenarios were assessed to evaluate the feasibility of mitigating peak flow increases by improving either the currently breached Bailey Lake Dam (BLD) or ALWD. Improvements at ALWD considered replacing the existing dual riser-barrel spillways with a new spillway, permanent lowering of normal pool, pre-storm drawdown of the pool, and leveling the dam top as well as providing overtopping protection. Rehabilitation of BLD was assumed to include repair of the embankment and existing breach, installation of a new combined spillway, and leveling of the dam top and installing overtopping protection.
The ALWD improvements discussed above each involve replacement of the spillway system. However, in all scenarios the Dam overtops frequently (50-year storm under existing conditions), thus any improvement also requires installation of overtopping protection to reduce the risk of erosion and associated embankment failure.
Given that flow mitigation isn’t required by Dam Safety if this protection is added, the most cost-effective regional detention alternative to protect ALWD is to install this protection and not rely upon flow mitigation via spillway improvements. Implementation of overtopping protection is likely to cause little change in hydraulic capacity, thereby not exacerbating or creating additional flood concerns.
Under this assumption, expected flows and WSEs will generally be the same as those previously presented for pre- and post-project conditions. As such, potential upstream flow increases from project implementation will be passed downstream of the Dam. However, limited development is present in this area no impacts from project implementation were identified in this study.
Cost Comparison
Localized Detention for Rockford Drive ($4.6M) and Rhemish Drive ($4.1M) total $8.7M. When considering localized detention for all improvements in the BV3 watershed, that total amount jumps to $33M
Overtopping Protection of ALWD totals $8.5M.
Conclusion and Recommendation
The overall findings when comparing Localized Detention vs. Regional Detention show that overtopping protection of ALWD likely presents the most direct, effective, and economical solution to protect the facility from potential flow changes. Implementation of local detention may require facilities at each project location and do not provide direct benefit to ALWD. Instead, protection of ALWD is reliant upon design, construction, and maintenance of each detention facility. Alternatively, overtopping protection directly protects the facility and is largely unaffected by changes to secondary system drainage systems.
Sequencing
It is recommended that no improvements from the Beaver Creek 3 Watershed Master Plan located upstream of ALWD be implemented until the overtopping protection has been constructed.
For more detailed cost and analysis options for both Localized Detention and Regional Detention, please read attached evaluation reports.
Budget Impact:
There is no impact to the General Fund. Project will be funded by Stormwater Enterprise Funds.
Options:
1. Direct staff to move forward with project development and design of the Arran Lakes West Dam Overtopping Protection project.
2. Do not direct staff to move forward with the Arran Lakes West Dam Overtopping Protection project and provide further guidance.
Recommended Action:
Direct staff to move forward with project development and design of the Arran Lakes West Dam Overtopping Protection project.
Attachments:
• ALW Dam Overtopping Protection presentation
• Beaver Creek Tributary B Detention Evaluation
• Beaver Creek Tributary B Detention Evaluation - Effects of PS01 and PS05 Memorandum