Agenda Date: 05/05/2026
Subject:
Title
Resolution 21-2026: Approving the first amendment to the Intergovernmental Agreement between the City of Littleton and Mile High Flood District regarding funding of Major Drainageway Planning and Flood Hazard Area Delineation for Slaughterhouse Gulch
Body
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From: |
James L. Becklenberg, City Manager |
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Prepared by: |
Brent Soderlin, Director of Public Works and Utilities |
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Sarah White, Water Resources Manager |
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Presentations: |
N/A |
PURPOSE:
Approving the first amendment to the Intergovernmental Agreement (IGA) with Mile High Flood District for funding of Major Drainageway Plan and Flood Hazard Area Delineation for Slaughterhouse Gulch.
LONG-TERM OUTCOME(S) SERVED:
Sustainable Community with Natural Beauty; High-Quality Governance
DISCUSSION:
The City of Littleton entered into an Intergovernmental Agreement with Mile High Flood District (MHFD) on August 20, 2024 to undertake master planning and floodplain delineation on Slaughterhouse Gulch. As work has progressed on this Master Planning Study and the associated hydrology, additional funding needs are needed to complete the studies. Project costs are slightly higher than originally anticipated due to 4 main components:
1. Rising Study Costs - These increases are in line with those that we are seeing across the industry with design and construction costs as well. Moving forward we will be starting projects with a higher beginning cost to address these increases. This project and associated assumed funding was from the 2020 5-year Plan and labor costs have increased significantly since then.
2. Detailed Hydrologic Modeling - The Slaughterhouse Gulch study includes a more robust hydrologic analysis than assumed when originally funding the study. Because Slaughterhouse Gulch consists of both above and below ground (piped) segments, it was necessary to model the area with both a traditional analysis for the open channel flow and also to do a more robust model for the piped sections of the gulch. The project team agreed that this additional, more detailed, modeling was the correct approach for this watershed. Had we not done this analysis, we would not have a good understanding of the capacity challenges along the gulch and would not be able to provide useful recommendations for future capital improvement projects to address those problems. In addition, there is the added complexity of surface flows interacting with the High Line Canal, which crossed the watershed and has its own hydrologic model. Coordination with that model and the High Line Canal Master Plan was necessary for developing the hydrology for this study. Without both of these analyses we would not have a good baseline understanding for flooding concerns that will be used as the starting point for the upcoming Flood Hazard Area Delineation (FHAD).
3. Field Conditions Inventory - more data was collected as part of the field conditions inventory than assumed when funding the study. This data is important to develop the concerns and problems for the watershed and will allow for the consultant to provide realistic alternatives to address the problems. This will be useful for determining both future CIP projects and smaller maintenance needs.
4. Comprehensive Planning Study - MHFD has recently moved away from a traditional master plan towards an Urban Stream Study, which is a more comprehensive analysis of the watershed. This ensures the stormwater infrastructure is integrated with other infrastructure (transportation, parks, open space, utilities) so that the system can provide multiple benefits to the urban community. This study will include a Stream Stability Analysis, and it will look for problems and solutions centered around Vegetation and Community Values, which weren't necessarily included in previous studies.
BACKGROUND:
Slaughterhouse Gulch drains approximately 2.2 square miles and is comprised of three main drainage basins, Lower Slaughterhouse Gulch, Upper Slaughterhouse Gulch and the South Tributary. The Lower Slaughterhouse Gulch drainage basin is approximately 450 acres just east of Broadway to its outfall at the South Platte River. Upper Slaughterhouse Gulch is approximately 955 acres upstream of the lower basin to its origin near University Boulevard and Arapahoe Road. The Slaughterhouse Gulch South Tributary is less than 100 acres, confluence west of Windermere at Berry Ave. The drainage basin is entirely within Arapahoe County and within City of Littleton and City of Centennial jurisdictions.
The Slaughterhouse Gulch drainage basin is mostly comprised of residential and commercial developments. The previous studies reflected a higher single family residential composition than what exists today and predicted future use and infill development.
Slaughterhouse Gulch is piped through most of the City until it outfalls at South Windermere Street. Powers Park is an inline regional detention facility, meaning the main channel runs through the detention basin. The majority of the storm infrastructure is undersized which can lead to localized street flooding.
Watershed planning is an important way to help protect people and property against flooding, erosion, and stormwater quality problems. These plans contain a strategy and work plan to identify flood risk management projects, guide new land development projects and identify high risk areas.
The Slaughterhouse Gulch drainageway was originally studied in 1974, and subsequently updated in 1982, 1983, 1992 and 1993. The South Tributary had a limited study in 2000 and is not included in the limits of these proposed studies.
Mile High Flood District (MHFD) along with project sponsors, Southeast Metro Stormwater Authority (SEMSWA) and the City of Littleton, are proposing updates to the Major Drainageway Plan (MDP) and Floodplain Hazard Area Delineation (FHAD). The studies will evaluate the hydrology and floodplain limits of Slaughterhouse Gulch in consideration of new rainfall data and development that has occurred in the basin since the previous study in 1993. The MDP will provide a conceptual design of recommended improvements in the watershed, including estimated costs. The MDP will be used as a guide for implementing drainage improvements. The FHAD study will identify flood risk information and boundaries. The FHAD will then be used to update the FEMA floodplain maps as a physical map revision, updating the regulatory boundaries of the floodplain.
Prior Actions or Discussions
The original IGA was approved by City Council as Resolution 94-2024 on August 20, 2024.
FISCAL IMPACTS:
The total revised project costs is estimated to be $530,000. The City’s share of the costs of the project, as set forth in the Agreement, shall be a contribution of 50% of project costs split with SEMSWA for the master plan preparation, and 0% for the floodplain study costs.
The revised costs for the study are shared with Mile High Flood District as follows:
City of Littleton: $82,500 (MDP 25%)
Southeast Metro Stormwater Authority: $82,500 (MDP 25%)
Mile High Flood District: $165,000 (MDP 50%)
Mile High Flood District: $200,000 (FHAD 100%)
The first amendment to the IGA identifies an addition City of Littleton cost of $32,500. This will funded from the Storm Drainage Enterprise Fund.
STAFF RECOMMENDATION:
Staff recommends approval of Amendment to the Intergovernmental Agreement.
ALTERNATIVES:
If not approved, future design of improvements will be based upon the 1993 study which may have inaccurate hydrology that would lead to either over-designing infrastructure (greater cost), or under-designing infrastructure (increasing risk to public). Not updating floodplain maps as the watershed develops and rainfall data updates lead to under and over enforced floodplain regulations.
Not entering into a partnership agreement with MHFD and SEMSWA would require the City to fully finance the studies at $530,000 instead of the $82,500 proposed.
PROPOSED MOTION:
Proposed Motion
I move to approve Resolution 21-2026 approving the first amendment to the Intergovernmental Agreement between the City of Littleton and Mile High Flood District regarding funding of Major Drainageway Planning and Flood Hazard Area Delineation for Slaughterhouse Gulch.