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HomeMy WebLinkAboutCorrespondence - Applicant Communication - 11/19/2024 R:\1900\1953-LWIC\09_SondersEast\11-19-24 TM Responses.docx November 19, 2024 To: John Gooch, P.E. Aspen Engineering From: Hayden Strickland, P.E. W. W. Wheeler & Associates, Inc. Re: Sonders East ____________________________________________________________________________ John, W.W. Wheeler & Associates, Inc. (Wheeler), has prepared this technical memorandum to provide responses to questions raised during the October 22, 2024 meeting between the Sonders East team and WRCC, Inc. (WRCC). Ditch Design Details – In discussions since our October meeting, WRCC has expressed additional questions regarding the proposed ditch design including:  Need for armoring of the ditch, particularly on the outside edge of the proposed bend. This may include right after construction but before vegetation has been established, or under final conditions assuming vegetation has been established.  Potential for sedimentation deposition, including on the inside of the proposed bend and at the drop structures.  Side spill location and velocities and depths of flow. Please provide additional analysis addressing these issues. This may include a HEC-RAS 2d model, or other analysis as appropriate. To: John Gooch, Aspen Engineering November 19, 2024 Page 2 Drop Structures – We understand that Sonders will construct access ramps into the canal above and below each drop, and therefore, WRCC would prefer that the access road next to drop structures be routed out and away from the bank so that the slope of the road is less if it were routed adjacent to the bank. Additionally, the drop structures should include concrete aprons that stretch at least 10 feet upstream and downstream of each drop structure. All concrete conduits shall conform to the more stringent of the requirements found in AASHTO Specifications and ACI 350, “Code Requirements for Environmental Engineering for Concrete Structures.” All concrete shall have a 28-day compressive strength of not less than 4,500 psi. All joints must be watertight. Canal Lining – We discussed lining the canal with clay at the meeting, but did not provide specific requirements. If Sonders chooses to line the canal with clay, the ditch banks and ditch bottom must be constructed with a minimum of two feet thick compacted clay material with a plasticity index of at least 15% and 40% passing the # 200 screen. Additionally, WRCC will require a separate seepage agreement. The sides of the bank but not the canal bottom must be seeded with a low growth seed mix. Sonders should evaluate flow velocities to determine if erosion blankets or other materials are needed to achieve established growth. Please provide this analysis to WRCC for review. Access Road Details – WRCC understands that Sonders is proposing to construct a 25 ft access with an additional 5-foot shoulder on the outside of the road. This is acceptable to WRCC. WRCC requires Sonders to construct a 20-foot-wide access road located 3 feet from the top of the ditch bank. The access road should be designed with a maximum cross slope of 3%, sloping away from the ditch, a maximum longitudinal slope of 12:1 (8.33%), a minimum turning radius of 45 feet, and be designed to withstand HS-20 loading. The road should be covered with 6 inches of CDOT Class 6 road base material. Additionally, all curbs shall have curb cuts to allow access. Finally, access from a public road should be gated with a minimum 18’ wide heavy-duty hinged tube gate with locking hasp/latch. The gate should be located a minimum of 25 feet from the public road to allow a maintenance truck to pull off the road and park in front of the gate. To: John Gooch, Aspen Engineering November 19, 2024 Page 3 East Side Access Road – While WRCC requires access roads on both sides of the ditch, they are willing to accept a formal access road with 6-inches of Class 6 material located on the east side of the ditch, and a less formal access road on the west side. Borrow Ditches - Sonders has proposed constructing a borrow ditch along the outside of the access road to capture local runoff. Sonders provided a drawing on November 13, 2024 showing these borrow ditches. On the west side of the No. 8 OD, these borrow ditches convey water either to drainage facilities Sonders has within their development or to and through the 43-acre green space, where runoff is expected to infiltrate. On the east side of the No. 8 OD runoff from the borrow ditches is conveyed to the Sod Farm detention in most areas, but there are localized areas where the ditches become localized low points, and stormwater would collect and infiltrate in these areas. While Wheeler has not yet reviewed the drawing in detail, we require that Sonders provide an analysis showing the capacity of these borrow ditches and where water will pool under various flows. If possible, please provide a comparison of the local runoff flows with storm event exceedances. At what flows will the borrow ditches overtop, and what happens to any water spilling out of them? Wet Utility Crossing – Sonders is planning on two wet utility crossings including a sanitary sewer crossing and a non-potable water line crossing. WRCC has the following requirements and guidelines. Spacing All utilities crossing under the Ditch shall be at least 5 feet below the invert of the Ditch. Sonders is responsible for survey of the crossing and shall provide a drawing showing the proposed crossing, ditch bottom (not water surface) for review. Marker Posts Sonders shall install marker posts approximately 30 feet from the top of the ditch bank. Additional measures should be included to assist the WRCC’s maintenance personnel in locating the pipe in the field. This includes as-built survey information (such as surveyed coordinates and distance/depth to top of pipe from marker post or edge of right-of-way) and may To: John Gooch, Aspen Engineering November 19, 2024 Page 4 include tracer wire. If Sonders uses tracer wire, it shall place the tracer wire in the trench above the casing pipe and concrete collars. Casing Requirements Both the sanitary sewer and the non-potable water line must be carried in 12-gauge welded steel casing pipe for 35 feet from the top of the ditch bank on each side of any open ditch, including the following criteria: a. The casings shall be liquid tight, and the casings sealed to the carrier pipe at each end of the casing. b. The carrier pipe shall be separated from the casing pipe using insulated casing spacers properly dimensioned and spaced to safely accommodate the utility or pipeline and prevent shorting of the carrier pipe’s cathodic protection system. Where applicable, the carrier pipe shall include restrained joints for the entire length within the casing pipe. c. The end of the casing pipes shall be sealed with Capico end seals or an approved alternative with stainless steel straps. d. The minimum specifications for the casing pipes are shown in the table below: Utility Pipe Diameter Material Wall Thickness 12-inch-diameter or smaller Steel 3/16-inch-thick wall Over 12-inch and less than 24-inch-diameter Steel 1/4-inch-thick wall 24-inch to less than 60-inch-diameter Steel 3/8-inch-thick wall 60-inch and larger diameter Special design by a Professional Engineer registered in the State of Colorado. e. Casing pipes should be a minimum of six inches larger in diameter than the carrier pipe to provide a minimum of three inches of clearance around the carrier pipe. Carrier pipe shall be centered inside the casing pipe. f. WRCC requires seepage cutoff walls be installed at the end of each utility crossing to prevent piping from the ditch along the utility pipe. Depending on the soil conditions where the utility crossing is located, a filter collar may be needed to ensure material cannot erode. Reinforced concrete cutoff walls should be installed no farther than 25 feet from the top of the ditch bank. The concrete cutoff wall should extend a minimum of 24 inches above the casing pipe, should be a minimum of 12 inches thick, and all To: John Gooch, Aspen Engineering November 19, 2024 Page 5 reinforcement should have a minimum of four inches clearance from the pipe or edges of the cutoff wall. Finally, the wall should be keyed a minimum of six inches into the natural ground. All backfill above the cutoff wall should be compacted to at least 95% of maximum standard proctor density. Please provide a detailed drawing and description of the proposed cutoff wall and confirmation that a filter collar is not necessary. g. All pipe joints located within 25 feet of the top of the ditch bank shall be radiographically or ultrasonically tested. The test results shall be provided to the Companies. h. A pressure test shall be performed on the pipeline after installation. The hydrostatic testing shall be conducted at 125% of the maximum operating pressure for a minimum of eight hours. The Companies shall be invited to the test and given at least 48 hours’ notice prior to the test. All test results shall be provided to the Companies. Design Flow and Side Spills – There was significant discussion on the required design flow that the relocated canal needed to carry. Wheeler understands that the current design carries approximately 1,000 cfs before reducing the flow capacity to approximately 285 cfs. The excess water currently spills out of the canal on the west side, flows overland and back into the canal, before spilling out on the east side of the canal into the sod farm inadvertent detention area. Wheeler reviewed the ICON study and discussed the flows with the City of Fort Collins. They indicated that the modeled flows assume existing (i.e. non developed) conditions for the lands upgradient of Elder Reservoir but included the developed flows from Sonders and Sonders east. WRCC is concerned that the current approach may cause damage to the No. 8 Canal where water spills back into the canal and then out, with the potential to deposit debris in this area. Therefore, we request that Sonders convey the 1,000 cfs far enough down ditch so that there is only a single side spill to the east. Sonders should provide more details on where water that spills to the east ends up. Any water spilled from the ditch may not return to the ditch, either as surface or groundwater flow. The water spilled to the inadvertent detention area must not return to the ditch. At the meeting there was discussion about several tile drains. Please provide additional details including the start and end location of the drains, the size of each, for what purpose they were originally intended, and which drains will be removed as part of this development.