TO: Mayor and Members of City Council
THRU: Adam Lindsay, Assistant City Manager
FROM: Sheila Thomas-Ambat, PE, Public Services Director
DATE: September 25, 2023
RE:
Title
Cross Creek Watershed Study Downtown Area Update
end
COUNCIL DISTRICT(S):
Council District(s)
1,2,3, and 4l
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 pursue funding sources to further develop advanced conceptual design for the Channel Improvement Option.
Staff also seeks concurrence from City Council to approve staff to pursue funding sources to continue to evaluate Roses Lake as a dry detention option for potential flooding mitigation.
An update on the Channel Improvement option and the Roses Lake dry detention option was presented in detail to the City Council at the September 5, 2023 Work Session under agenda item 4.03. City Council approved to move forward on consent at a City Council Regular Meeting.
Background:
In FY 2019, City Council authorized a fee increase and directed staff to accelerate its watershed study program and related capital project program to help control stormwater runoff, reduce localized minor flooding, and improve our resiliency to major flooding. At the December 6, 2021, Council Work Session, City Council approved the first tranche of projects from the first four studied watersheds to move forward to development and detailed design. Staff also presented results in June 2022 from a subset of proposed solutions in the Blounts Watershed Study and requested that the Russell and Person Street Bridge Replacements with Blounts Creek Stream Enhancements Proposed Solution (Russell and Person Streets Proposed Solution) be approved to move forward to project development and design.
The downtown area has been subject to recurrent flooding, particularly over the last 10 years, with high economic damages.
With climate change uncertainty, a long-range plan to address flooding concerns has the potential to not only avoid future damages but also support future development within the downtown area. The Roses Lake dry detention option has the potential to mitigate flooding in the upstream portion of the watershed, and the Channel Improvements option has the potential to mitigate flooding in the downstream area of the watershed.
Existing flooding was modeled for the 5-yr, 10-yr, 25-yr, 50-yr, and 100-yr design storms, as well as a November 2020 storm and June 27, 2023 storm, both with notable flooding downtown. Gage data combined with flooding reports have been used to evaluate model performance and incorporation of recent projects, like the Rowan Street culvert, incorporated into analysis efforts. Additionally, our modeling of the 100-yr flood extents is similar to analysis provided in FEMA mapping. A key area of flooding concerns, both from modeling results and past observations, is the section of Cross Creek from Hillsboro Street to Green Street, including the Festival Park area. Review of site conditions and historical records show this area of flooding concerns coincides with a past millpond.
Proposed solutions were then developed and modeled for the modeled storms. Downtown flooding concerns are significant with no simple solution, in part due to the high variability of the Cross Creek channel geometry. Solutions evaluated to mitigate the flooding in the downtown area include detention, diversion, floodplain buyout, and increased capacity. The flood mitigation toolbox specifically included the following options:
• Detention: Roses Lake, Glenville Lake, Storage at CAT 1 Site
• Diversion: Single Tunnel, Multi-Inlet Tunnel, Pump Station
• Floodplain Buyout: Floodplain buyout
• Increased Capacity (Channel Modification Concept): Floodplain widening, floodplain berms
Roses Lake:
The Roses Lake dry detention option provides the greatest benefits not directly to the downtown area but upstream of downtown, primarily to single-family homes. Based on the preliminary concept development, this option has the lowest cost of options considered (~$10M). While it provides incremental flood reduction along Cross Creek, particularly for single-family homes throughout the central part of the watershed, analyses indicate it does not fundamentally address flooding concerns in the downtown area.
Diversion Tunnel:
The tunnel diversion concept consists of 5500 linear feet of 10-foot diameter tunnel, with a primary intake by Murchison and Bragg, running through Moore St, and discharging by gravity into Cross Creek. The objective of this solution is to divert flow around the worst flooding concerns downtown; however, capacity limitations for larger storm events combined with runoff from downtown downstream of the tunnel limit the ability of this solution to fully address downtown flooding concerns. Based on preliminary concept development, this option has the highest cost of options considered (~$100M).
Multi-inlet Tunnel Concept:
The multi-inlet tunnel concept consists of the main tunnel diversion and additionally 2,600 linear feet of tunnel at Hillsboro and Festival Park. The objective of this modified concept is to divert runoff from within the downtown area; however, it faces similar limitations as the other tunnel concept.
Pump Station:
The pump station concept is intended to target those areas where flooding occurs, like the Tunnel option but with the addition of pumps. A large pump station faces similar limitations as a tunnel concept, with the added burden of pump O&M. While a large pump station is not expected to be a feasible solution for downtown flooding, smaller pump stations could be combined with flood walls to provide localized flood relief.
Floodplain Buyout:
This option involves addressing flooding risks by acquiring properties within the floodplain and removing the associated buildings. Floodplain buyouts are an established mitigation strategy and grant funding opportunities exist; however, buyouts would not fundamentally address the capacity limitations of Cross Creek and have some negative implications, like loss of tax base. Floodplain buyouts could be conducted in combination with or as a precursor to other improvement options, like channel improvements.
Increased Capacity (Channel Improvements Option):
The Channel Improvements option involves re-grading the stream channel from MLK Freeway to Green Street to provide a defined channel and flood corridor and increase the capacity of Cross Creek. Replacement of bridges and culverts through this corridor would be part of this project. Based on preliminary concept development, this option has a cost of $50M plus necessary property acquisition, the specifics of which would be established through the development of a more detailed concept. Of the options considered, the channel improvement provides the largest reduction in flood depths and extents within the downtown area.
Issues/Analysis:
Downtown flooding concerns from Cross Creek are significant, with flooding beyond the banks of Cross Creek during a 5-yr event and many road crossings downtown having a level of service of 10-yrs or less. The geometry of Cross Creek through the downtown area, contributing runoff from upstream and nearby areas, and development within the effective floodplain are key contributing factors to flooding concerns.
By reducing the flow of contributing runoff downstream, dry detention at Roses Lake is expected to reduce downstream flood elevations generally by 1-2-ft during a 10-yr storm event. The flood reduction benefits of Roses Lake diminish as Cross Creek approaches downtown such that Roses Lake does not fundamentally solve downtown flooding concerns. The Roses Lake option could likely be implemented within a general timeframe of the next 5 years with further benefit-cost-ratio (BCR) analysis and adequate funding.
By providing more capacity for storm flows and a defined floodplain, the channel improvement concept is expected to substantially reduce flood depths and extents in the downtown area. During a 10-yr storm event, flood depths were modeled to be lowered by 3-4-ft, and flood extents during a 100-yr storm with improvements in place are similar to a 10-yr storm under existing conditions. Due to the project's extent, cost, and complexity, implementation is likely to occur over a longer period of time (10+ years) with options for implementing distinct phases of the conceptual improvement.
By proceeding with further evaluations of the Roses Lake Option and the Channel Improvement Option, the City will be better poised to create an implementation plan and acquire properties as opportunities are presented. This evaluation will also allow for more informed decisions regarding future development plans within the downtown area. This combination option provides benefits both upstream and in the downtown area, with substantial benefits related to roadway flooding, disconnected structures, and traverse road crossings, as well as many co-benefits. The Channel improvements option will provide environmental and public benefits including recreational opportunities, and water quality benefits. The Channel Improvements option is scalable, incremental, and could be implemented in phases.
The Roses Lake option has a BCR that is based on conceptual-level design and is 0.75. The current BCR is likely too low to support grant applications. However, with a more detailed conceptual-level design, the BCR could be developed using the robust BRIC-level BCA guidelines and potentially would increase. Additional funding is needed for this level of design and applying the BRIC-level BCA guidelines.
The Channel Improvement option BCR is 0.17, primarily due to the high cost (~$50M plus property acquisitions) and benefits of the project that are not captured by the preliminary BCR. The scalable approach coupled with a more detailed evaluation and concept design could raise the BCR substantially by accounting for economic disruption avoided and other damage reduction benefits beyond the simple structure and content damage estimates included in the current BCR. Additional funding is needed for advanced concept development and more detailed evaluations of costs and benefits.
Budget Impact:
No budget impact.
Options:
1. Approve staff to pursue funding sources to further develop advanced conceptual design for the Channel Improvement Option.
2. Approve staff to pursue funding sources to continue to evaluate Roses Lake as a dry detention option for potential flooding mitigation.
3. Do not approve staff to pursue funding sources to further develop an advanced conceptual design for the Channel Improvement option and remand back to staff with additional guidance.
4. Do not approve staff to pursue funding sources to continue to evaluate Roses Lake as a dry detention option for potential flooding mitigation and remand back to staff with additional guidance.
Recommended Action:
Approve staff to pursue funding sources to further develop an advanced conceptual design for the Channel Improvement Option. Approve staff to pursue funding sources to continue to evaluate Roses Lake as a dry detention option for potential flooding mitigation.
Attachments:
CRO September Council Meeting.pdf