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HomeMy WebLinkAboutSCENIC VIEWS PUD - FINAL - 3-96A - CORRESPONDENCE - STORMWATER-RELATED DOCUMENTSMr. Bob Blanchard February 5, 1997 Page 4 12. Cross sections are needed for the proposed swales. Please provide runoff and capacity calculation for cross-section A A of sheet 4. The swale must have capacity for the 100 year storm plus freeboard. Please show the flow depth on the cross-section. Cross-section added to plan. 13. The proposed grading for the lots shows areas of cross lot drainage. This type of grading is discouraged because cross lot drainage can cause localized drainage problems between neighbors. Please adjust the lot grading to minimize cross lot drainage. Grading revised to address concern. Please refer to the redlined report and plans for additional review comments. Comments noted and plans revised as needed. If you have any questions or need additional information, please do not hesitate to contact me. Sincerely, JR ENGINEERING, LTD. Roger L. Mieden, P.E. Project Manager RM/pjo x:\367600\wora\comments.doc Mr. Bob Blanchard February 5, 1997 Page 3 An evaluation of runoff from frequent rainfall events has been included in this report and defines the time that the pond areas will be submerged for different events. With the addition of a water quality volume that is below the main portion of the pond, frequent events will not spread outside of the water quality volume basin and a portion of the basin will remain unsubmerged. The water quality volume is intended to be pumped out automatically. A larger 500 gpm pump is also provided to draw down the main portion of the pond more rapidly. This pump would be turned on manually 24 hours after a large storm to meet concerns of the PV&L that the pumping system not add to peak flows within the canal during a storm. 7. The wetlands along the north property line are shown to be eliminated from the over lot grading and subdrain system. How will the elimination of the wetlands be mitigated? The bottom of the water quality volume area of the retention pond has been designed with a flat bottom to facilitate growth of wetland vegetation. A moderate amount of runoff should be available from the storm sewer inlets on each end of the water quality basin to provide a water supply to the area. 8. Stage -storage calculations are needed for the retention pond. The pond must be shown to have at least the volume required of 14 acre ft. Stage -storage calculations have been included with the current submittal. 9. The retention pond is shown with 2:1 side slopes. The maximum side slopes for ponds is 4:1, per City criteria. Please revise grading to 4:1 slopes. The pond grading was revised to include 4:1 slopes as requested. 10. The master plan model needs to include a rating curve for pond 202 that reflects the proposed grading. If the storage volume reaches the weir elevation, then the rating curve will need to reflect this. The SWMM model was revised to reflect current grading. '-No discharge occurs from the pond for storms up to and including the 100-year storm. 11. The simulation duration used for the model did not extend for the full length of the hydrograph for basin upstream from pond 202. Please extend the simulation period to include all upstream discharges. The simulation duration was extended to show 0 cfs at the end of the storm. Volume requirements increased from 14.0 to 14.2 acre feet but has no significant effect on grading. W. Bob Blanchard February 5, 1997 Page 2 the underlying bedrock was discussed as a control mechanism. In a recent telephone conversation with Mr. Lester Litton of EEC, Inc., he indicated that an excavated cutoff trench back -filled with select onsite clays could also be effective as an alternative. A detail of the cutoff trench is included on the construction drawings for reference. 3. If water is to be pumped into the ditch in order to drain the pond, then several issues regarding the pump system needs to be addressed. (1) The maintenance responsibility of the pump system needs to be addressed. Does a homeowner's association have the capabilities to maintain a pump system? (2) A pump system design is needed on the utility plans. (3) The quality of the water pumped into the canal needs to be considered. (4) Approval from the PV&L to pump water into the canal and to cross the canal with storm sewers is needed. (1) The maintenance responsibility of the pump system would remain with the homeowner's association. The pumping system is designed to be relatively simple in operation and routine maintenance of the system should not be extensive, although the association may want to consider subcontracting maintenance or repairs as a matter of convenience. (2) A pump system detail is included on the current plan set. (3) A water quality outlet structure has been added to the current set and is designed based on criteria developed by the Urban Drainage & Flood Control District for water quality enhancement of stormwater runoff. (4) The concept shown on the plans has been reviewed with the PV&L and a final agreement will be negotiated. 4. The subdrain system is shown to discharge into the retention pond. The additional inflow to the pond needs to be considered in the design of the pond. The subdrain system shown on previous plans has been deleted from the current plan set. S. Please show the existing property line for the project. Is the retention pond proposed on private property? If off -site grading is proposed, then executed easements are needed from involved property owners. The existing property line is indicated on the plans and is located within the project site. 6. The site and landscape plan shows the pond area as active open space. However, the pond will most likely have a wet bottom. How will the pond area be used as active open space with a wet bottom? JREngineering, Ltd. 6110 Greenwood Plaza Boulevard Englewood, Colorado 80111 (303) 740-9393 • FAX (303) 721-9019 February 5, 1997 Mr. Bob Blanchard CITY OF FORT COLLINS DEVELOPMENT REVIEW 281 North College Avenue Fort Collins, Colorado 80525 Re: Scenic Views P.U.D. - Stormwater Comments Dear Mr. Blanchard: 4935 North 30th Street Colorado Springs, Colorado 80919 (719) 593-2593 • FAX (719) 528-6613 4812 South College Avenue Ft. Collins, Colorado 80525 (970) 282-4335 • FAX (970) 282-4340 The following information is provided in response to comments from the Stormwater Utility Division dated January 3, 1997. The comments are noted below in italics and responses are noted following each comment. 1. The major area of concern with this project is the design of the retention pond. The retention pond issues include: groundwater levels in the vicinity of the pond, outfall for subdrain system, retention pond maintenance, volume provided, and pond grading. A11 of these issues need attention. Additional details are included with this submittal to clarify the functioning and design of the retention pond system. The subdrain system has been eliminated and is not reflected in the current design for the site. 2. The proposed pond design shows the bottom of the pond 8 to 10 feet below existing ground surface. With this excavation groundwater seepage into the pond is a concern. The soils report, by EEC, Inc. February 1996, discusses groundwater levels of 3 to 6 feet below ground in the vicinity of the proposed pond. The seepage of groundwater into the pond will cause ineffective retention volume and maintenance problems. This condition is not acceptable and needs resolution. The report discusses provisions to retain groundwater from the pond area. Please provide further details of the proposed provisions. An additional letter report prepared by EEC, Inc. on May 29, 1996 (copy enclosed for reference) addressed the issue of construction of a stormwater pond below the water table. At the time of the letter, a typical detention pond and a shallower depth was still under consideration, however, the concepts presented apply to the retention pond condition also. In the letter a slurry wall consisting of a soil/bentonite mixture placed in a trench ctivinto x:\367600\word\conunents.doc 1