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Home My WebLink AboutFORT COLLINS-LOVELAND WATER DISTRICT EXPANSION - ODP210001 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT 1218 Ash Street | Suite A Windsor, CO 80550 970.674.3300 March 3, 2021 Mr. Matt Simpson, PE City of Fort Collins Stormwater 700 Wood Street Fort Collins, CO 80521 Re: Fort Collins-Loveland Water District Overall Development Plan Drainage Report Dear Matt, The Fort Collins-Loveland Water District (FCLWD) developed their existing office building and maintenance facility in 1999-2000 at 5150 Snead Drive. A portion of the property was platted and zoned Commercial to support the development. The District owns the undeveloped land north and east of the existing building and is in the process of master planning additional outdoor storage and an additional maintenance building on the vacant portions of the site. This remainder of land is not zoned for this use, so the site is going to go through an Overall Development Plan process with a request for “Additional Permitted Uses”. This was determined to be a simpler process than trying to do a rezoning to Commercial since the desired uses are very limited compared to the uses allowed in Commercial zoning. The entire ODP area was previously studied with the original 1999 project. Pertinent portions of this report are attached. Prior to development, the 5-acre site owned by FCLWD as well as 4.3 acres to the north all drained southeast and through the single-family homes in Fossil Creek Meadows and ultimately to Fossil Creek. With the development of the site, it was decided to redirect the flows west to Snead Drive and then south through an upsized pipe under Fossil Creek Parkway. The site was designed with a water quality pond only, detention was not required because of the proximity to Fossil Creek. In the 1999 report, all of the site, the adjacent portion of Snead Drive, and the off-site flows were calculated to release 24.7 CFS. All of this flow was designed to go into the proposed storm sewer and swale to the creek. The portion of this flow coming from the water quality pond was 20.8 CFS (from the majority of the site and the off site lots to the north). The spillway for the water quality pond was designed to drain south and into Fossil Ridge Drive. The proposed site layout for this ODP is rough and fluid in nature. It will be modified and likely phased into multiple PDP submittals. Today it is assumed that the ultimate site will have about 3.5 acres of impervious surface (buildings, parking, and storage). This will very likely change as the project moves forward. Following is a high-level look at the original design “C” value of the acreage draining to the pond and the proposed design “C” value with added impervious area: 2 Adding imperviousness to the site will require converting the water quality pond to water quality and detention. We are currently assuming that the pond will be allowed to drain the original report’s flow of 20.8 CFS and that the City will continue to want all off site flows routed through the pond and to Snead Drive and not through the neighborhood to the south. The required detention to take the site hardscape area up to 3.5 acres and release at 20.8 CFS is approximately 0.33 acre-feet of storage: LOCATION:FCLWD - ODP PROJECT NO:1410-048-00 COMPUTATIONS BY:mpo DATE:3/3/2021 Runoff % coefficient Impervious C HARDSCAPE 0.95 100 LANDSCAPE 0.20 2 SUBBASIN TOTAL TOTAL HARD LANDSCAPE RUNOFF % DESIGNATION AREA AREA AREA AREA COEFF. Impervious (ac.) (sq.ft) (sq.ft) (sq.ft) (C) EXISTING OS1 (NORTH)4.32 188,179 16,117 172,062 0.26 10 SITE 4.41 192,100 53,579 138,521 0.41 29 EXISTING TOTAL 8.73 380,279 69,696 310,583 0.34 20 PROPOSED OS1 (NORTH)4.32 188,179 16,117 172,062 0.26 10 FUTURE SITE 4.41 192,100 152,460 39,640 0.80 80 FUTURE TOTAL 8.73 380,279 168,577 211,702 0.53 45 RUNOFF COEFFICIENTS & % IMPERVIOUS - CONCEPTUAL DETENTION VOLUME CALCULATIONS Rational Volumetric (FAA) Method 100-Year Event LOCATION:FCLWD - ODP PROJECT NO:1410-048-00 COMPUTATIONS BY:mpo DATE:3/3/2021 Equations:Area trib. to pond =8.73 acre Developed flow = QD = CIA C (100) =0.66 Vol. In = Vi = T C I A = T QD Developed C A = 5.8 acre Vol. Out = Vo =K QPO T PROP Release rate, QPO =20.8 cfs storage = S = Vi - Vo K =1 (from fig 2.1) Rainfall intensity from City of Fort Collins IDF Curve with updated (3.67") rainfall Storm Rainfall QD Vol. In Vol. Out Storage Storage Duration, T Intensity, I (cfs) Vi Vo S S (min) (in/hr) (ft3) (ft 3) (ft 3) (ac-ft) 5 9.95 57.5 17264 6240 11024 0.25 10 7.72 44.6 26790 12480 14310 0.33 20 5.60 32.4 38866 24960 13906 0.32 30 4.52 26.1 47056 37440 9616 0.22 40 3.74 21.6 51914 49920 1994 0.05 50 3.23 18.7 56043 62400 -6357 -0.15 3 It is currently assumed that the existing water quality pond will be completely redesigned and rebuilt to add the required detention. The outlet structure no longer meets City standards, and the pond will need to be reconfigured to increase the volume. Area in the southeast portion of the site has been reserved for additional stormwater detention, and if this area is not enough, underground detention with StormTech chambers (or some similar solution) may be required. All new impervious area will need to meet current City Water Quality and Low Impact Development criteria. There is a neighboring parcel going through the City development review process at this time (Guardian Self Storage). This project has created a SWMM model to assess much more area than is involved with the FCLWD ODP. It appears that their modeling is matching closely with the existing flow off of the FCLWD site. When the FCLWD project moves to a PDP level, the drainage and how it interacts with the adjacent parcel will be studied further. A “beat the peak” analysis will also be required to verify that the new impervious areas and detention do not harm downstream conditions. We assume it will be best to use the adjacent site’s model (once it is approved) to similarly show that the FCLWD site beats the peak. Attached is the ODP Utility and Drainage Plan. Proposed contours have not been designed at this time, but will be with subsequent PDP submittals. The 1999 Drainage Plan and excerpts from that report are also included for your reference. We appreciate your time and consideration in reviewing this document. If you have any questions or need additional information, please feel free to call at any time (970) 460-8471. Sincerely, Michael Oberlander, P.E. Colorado Professional Engineer 34288 Attachments: 1. ODP Master Utility and Drainage Plan 2. Fort Collins Loveland Water District (Subdivision) Approved Drainage Report (Excerpts)