HomeMy WebLinkAboutMemo - Mail Packet - 07/01/2025 - Memorandum from Tawnya Ernst re Response to Council Questions Regarding South Fort Collins Sanitation District’s Drainage Easement Request (Ordinance No. 101 2025)
Natural Areas Department
970-416-2753 /P
ternst@fcgov.com /E
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MEMORANDUM
Date: June 24, 2025
To: Mayor and City Councilmembers
Through: Kelly DiMartino, City Manager
Tyler Marr, Deputy City Manager
Dean Klingner, Community Services Director
From: Tawnya Ernst, Land Conservation Lead Specialist
Subject: Response to Council Questions Regarding South Fort Collins Sanitation District’s Drainage
Easement Request (Ordinance No. 101 2025)
The purpose of this memo is to address questions raised about the South Fort Collins Sanitation District
drainage easement request on Fossil Creek Reservoir Natural Area.
Question 1: Stormwater Discharge
(Original AIS text): Larimer County Planning is requiring SFCSD to construct storm drainage and water
quality improvements as part of the site plan approval process for the new SFCSD headquarters
improvements. The SFCSD drainage system being proposed would capture and direct runoff towards an
onsite detention pond. The SFCSD site will feature a full spectrum detention pond (as defined in the Urban
Drainage and Flood Control District manual) designed to capture stormwater and release it slowly with
discharges that approximate pre-developed conditions – reducing pollutant loading and channel erosion.
(Councilmember Ohlson’s question): What does “approximate” mean? It is better or worse?
Could you explain why it is not a lot better?
(Response): The highlighted statement was intended to indicate stormwater discharging into
through the new culvert will not substantially differ from current conditions in which no culvert
exists, rather than differing from before development of the SFCSD facility. Full spectrum
detention ponds reduce pollutant loading and channel erosion by mimicking natural watershed
hydrology, releasing stored water slowly to mirror pre-development conditions. This approach
reduces peak runoff rates, allowing for more effective settling of pollutants and minimizing the
erosive force of stormwater discharge. The full spectrum detention pond manages runoff from a
wider range of storm events, including smaller, more frequent ones. Traditional detention ponds,
on the other hand, primarily focused on controlling peak flow rates during large storm events to
prevent flooding.
Question 2: Riprap
(Original AIS text): Alternative D (Preferred) Similar to Alternative A, this alternative proposes a new
culvert on the east side of the SFCSD property that would take stormwater underneath Highway 392 and
route it directly to Duck Lake. The total area that would drain to Duck Lake is approximately 30-acres
(including 6 acres of the SFCSD site and 24 acres of Fossil Creek Reservoir).
• The proposed culvert underneath Highway 392 is a 24”x38” elliptical concrete pipe capable of
conveying the entire 100-year storm event of 43 cfs. The 10-year, 5-year, and 2-year storm events
are approximately 10 cfs, 5 cfs, and 2 cfs respectively.
Docusign Envelope ID: 469508E9-87F1-47A9-B51A-CED7020EA76D
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• Construction work would involve installation of a concrete flared end section on the end of the
concrete pipe and placement of rip rap that will cover an area approximately 15’x8’. This will involve
excavation of roughly two feet of soil and installation of 6 inches of bedding material and then
approximately 18 inches of 3 inch to 5 inch-riprap. SFCSD has offered to bury the riprap to eliminate
the visual intrusion.
(Councilmember Ohlson’s question): Could you clarify the language regarding “offered to
bury…”? Is this actually written into the agreement that they will bury it? Will it be done? If not, he
requests that it be put in as an option for when Council considers this on 2nd reading.
(Response): South Fort Collins Sanitation District staff have agreed to bury the riprap. This
requirement will be documented in the final recorded easement and staff monitoring project
construction will ensure it occurs.
Question 3: Eagle roost site
(Original AIS text): An ecological characteristics study was conducted. The project area contains Duck Lake,
a pond waterfowl used as a migration resting and mating area. The proposed project area does not support
an extensive population of native vegetation, although several native species are present. The CDOT right-
of-way areas are characterized by upland grassland species, including western wheatgrass (Pascopyrum
smithii), pigweed (Amaranthus spp.), and various annual weeds. In the wetland zone along the boundary
of Duck Lake, cattails (Typha spp.) and assorted sedges (Eleocharis spp.) predominate. Additionally, a
solitary rubber rabbitbrush (Ericameria nauseosa) was observed near the proposed outflow location for the
new stormflow path. A CODEX report indicates that documented occurrences of protected species have
occurred within 1 mile of the Project Area. According to the High Priority Habitat dataset and field
observations, the project is within a buffer for an eagle roost site associated with Fossil Creek Reservoir.
(Councilmember Ohlson’s question): Can you please explain if this is a problem or not? What is
the impact on the eagle roost site (that this project is within 1 mile of the roost site)? Apparently
raptors have the broadest buffers due to their endangered species designation.
(Response): Natural Areas’ wildlife biologist reviewed the most recent raptor nest dashboard data
(collected in 2025) for active nests around Fossil Creek Reservoir. There are several active nests
(shown with red dots below). The closest nest is a Red-tail hawk. The required 1/3-mile buffer for
the hawk’s nest slightly overlaps SFCSD property but does not encompass the project site. An
active eagle’s nest exists on the northeast corner of the reservoir. The project is not anticipated
to have an impact on nesting raptors. Historic winter roost sites are not within the ½-mile
recommended buffer by CPW. No endangered raptor species are known to occur at Fossil Creek
Reservoir. The bald eagle was delisted in 2007. In addition, SFCSD intends to time construction
to minimize disturbance during the breeding and nesting season.
Docusign Envelope ID: 469508E9-87F1-47A9-B51A-CED7020EA76D
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Docusign Envelope ID: 469508E9-87F1-47A9-B51A-CED7020EA76D