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HomeMy WebLinkAboutDrainage Reports - 03/12/1994HUNTINGTON HILLS CONCEPTUAL DRAINAGE PLAN Prepared for Huntington Hills 650 S. Cherry St., Suite 435 Denver, Colorado 80222 To Meet The ODP Submittal Requirements For The City of Fort Collins 256 West Mountain Avenue Fort Collins, Colorado 80521 February 1994 Revised March 1994 Prepared by Resource Consultants & Engineers, Inc. 3665 John F. Kennedy Parkway Building 2, Suite 300 Fort Collins, Colorado 80525 RCE Ref. No. 93-103 HUNT-ODP. REV RCERESOURCE CONSULTANTS & ENGINEERS. INC. A KLH Engineering Group Company HUNTINGTON HILLS CONCEPTUAL DRAINAGE PLAN Prepared for Huntington Hills 650 S. Cherry St., Suite 435 Denver, Colorado 80222 To Meet The ODP Submittal Requirements For The City of Fort Collins 256 West Mountain Avenue Fort Collins, Colorado 80521 Prepared by Resource Consultants & Engineers, Inc. 3665 John F. Kennedy Parkway Building 2, Suite 300 Fort Collins, Colorado 80525 RCE Ref. No. 93-103 HUNT-0DP.REV February 1994 Revised March 1994 ICK TABLE OF CONTENTS 1. INTRODUCTION.................................................... 1.1 2. EXISTING DRAINAGE PATTERNS ....................................... 2.1 3. CONCEPTUAL DRAINAGE PLAN ........................................ 3.1 4. SUMMARY........................................................ 4.1 LIST OF FIGURES Figure 1.1. Vicinity map ................................................. 1.2 Figure 2.1. Drainage basin map ........................................... 2.2 Figure 3.1. Overall Development Plan with existing drainage failure ................. 3.2 Figure 3.2. Conceptual drainage plan ....................................... 3.3 I Resource Consultants & Engineers, Inc. 1. INTRODUCTION This report presents the site drainage master plan developed for Huntington Hills Filing 3 in Fort Collins, Colorado. The master plan or conceptual drainage plan was prepared to meet the City's subrr !U requirements for Overall Development Plans. Figure 1.1, which uses the City's street zoning map as a base, shows the location of the site. The property is located in Section 12, Township 6 North, Range 69 West, and surrounds the Portner Reservoir property owned by the City of Fort Collins. The area is southwest of the intersection of Lemay Avenue and Southridge Greens Boulevard and within the Fossil Creek Drainage Basin. Some of the nearby land in this area is outside of the City limits and is within the jurisdiction of Larimer County. The site is to be developed in phases, and each phase will require its own drainage report, calculations, and plan to meet the City's submittal requirements for PUD Plans. Required preliminary reports are being prepared for Huntington Hills Filing 3 and Filing 4. Filing 4 is in the southeast part of the site and Filing 3 is in the west -central part. It should be noted that Huntington Hills Filing 2 is located just west of the site, as shown in Figure 1.1, and was the subject of a previously approved PUD. Previous drainage studies of the area that were reviewed to assist with the preparation of this report include (1) "Drainage Report for Huntington Mews, Phase 1 Final Plat" by Resource Consultants, Inc.-(RCI), March 5, 1981; (2) "Fossil Creek Drainage Basin Master Drainageway Planning Study" by Simons, Li & Associates, Inc., August 1982; (3) "Preliminary Engineering Report for the Rehabilitation of Portner Reservoir" by RCI, January 1987; and (4) "Final Drainage and Erosion Control Report for Huntington Hills Filing 2" by TST, Inc., June 1992. The Master Drainageway Planning Study for Fossil Creek only covered the immediate area of the creek itself. The report was not a master planning document for the overall Fossil Creek Basin; therefore, it did not make any recommendations for ways of routing storm runoff into Fossil Creek from various areas that would be developed in the future, such as the Huntington Hills Development. 1.1 Resource Consultants & Engineers, Inc. No Text 2. EXISTING DRAINAGE PATTERNS Figure 2.1, which uses the USGS maps of the area as a base, shows the existing drainage situation within the vicinity of Huntington Hills. Shown are various drainage subbasins in the area, direction of flow for surface runoff, and approximate 100-year floodplains. As noted in the figure, there are two drainage areas to the north of Huntington Hills, to the east and west of Mail Creek, that historically have drained onto the Huntington Hills property. Huntington Hills must accept the runoff which has occurred from these areas in their present undeveloped state, but does not have to accept the additional storm runoff which will occur after development, when much of the area will change from a pervious to impervious condition. The master drainage plan for the area must consider ways of transporting storm runoff from offsite areas either across or around the Huntington Hills property. Some of the Huntington Hills property is within the 100-year floodplain for Fossil Creek. In developing and regrading the site, it may be desirable to use fill in some areas near the creek which would result in an encroachment into the floodplain. Any such encroachments should be noted as part of the PUD submittals but should not extend beyond the limits of the 100-year floodway. In Fort Collins, the 100-year floodway is defined as the flow area where calculated water surface elevations in the floodplain would not rise more than 0.5 feet due to encroachment. Portner Reservoir and its inlet ditch for diverting water from Fossil Creek are in poor repair. Because of this, the Colorado State Engineer has placed storage restrictions on the reservoir. In recent years the reservoir has not been used to store water. The 100-year floodplain elevation, shown in Figure 2.1, is 4915.2 feet. This elevation is from the 1987 rehabilitation study for the reservoir by RCI. It assumes that improvements will be made to the dam, outlet works, and spillway and that water would be normally stored ijthe reservoir at an elevation of 4914.6 feet. Both the normal and 100-year water surface elevations would result in water extending a short distance onto Huntington Hills property (Figure 2.1). If Portner Reservoir is not rehabilitated, as planned in the 1987 rehabilitation study, the maximum 100-year water surface elevation would be less than 4915.2 feet. Currently, negotiations are ongoing between the developer and the Parks and Recreation Department to rehabilitate the reservoir. The revised plan that is being considered is to create a 20-acre reservoir with a highwater line of 4912.0. The Huntington Hills area that presently drains directly into Portner Reservoir is shown in Figure 2.1. When this area is developed, runoff entering the lake from the urbanized areas may be of a poorer quality than the present situation. It would be desirable, when the area is developed, to have storm drainage facilities that would direct at least some of this storm runoff away from the lake. Water quality concerns for the lake and its potential for a fishery were discussed in the Portner Reservoir Rehabilitation Study (RCI, 1987). 2.1 Resource Consultants & Engineers, Inc. 1 Runoff from offsite areas flows onto Huntington Hills property ORunoff drains directly into Fossil Creek ODrainage divide for area directly contributing to Portner Reservoir Runoff from Huntington Hills drains into Portner Reservoir Drainage divide for area tributary to Stone Creek Dalong south side of Huntington Hills property �Runoff from Huntington Hills drains into 100-year floodplain for unnamed draw ORunoff from Huntington Hills to offsite areas Louden Ditch transition from open ditch to closed conduit O9 Drainage area tributary to existing detention pond 10 Runoff intercepted by inlet ditch and subject to spilling into Huntington Hills project LEGEND: Subdrainage Divide .............. Approximate 100-year Flood Plain Huntington Hills Phase 3 Property Line a".> Direction of Runoff ® Detention Pond TSCALE 1'=1000' No ,000' Figure 2.1. Drainage basin map. The Portner Reservoir Inlet Ditch extends approximately 4,000 feet from its headgate on Fossil Creek to Portner Reservoir. Most of the first 2,000 feet of ditch is located in Larimer County, outside ' of the City limits. The ditch presently intercepts storm runoff from upslope areas as shown in Figure 2.1. Some of this storm runoff, especially in the first 2,000 feet of ditch, is from areas that are presently developed. The ditch along its entire length is in poor repair and partly filled with vegetation. During a 100-year flood, the ditch would undoubtedly fill to capacity and then spill at various low spots along its embankment. It should be noted that the Louden Ditch (Figure 2.1), can be used to deliver water to the Portner Reservoir Inlet Ditch. During severe floods, a small amount of water imported into the ' area by the Louden Ditch would also contribute to floodwaters intercepted by the Inlet Ditch. As currently planned, the inlet ditch will be abandoned for diversion purposes. A new ' underground diversion structure will be constructed to fill the reservoir. The existing ditch will be breached and any stormwater intercepted by the current ditch will be spilled and diverted back to Fossil ' Creek at the west edge of the Huntington Hills site. ' There is an existing detention pond near the west part of the site that does not currently have a spillway. Water from the pond is presently discharged through a 30-inch diameter pipe to wetlands ' and from here into Stone Creek that extends along the south side of the Huntington Hills property. Construction drawings for Huntington Hills Filing 2 have included the construction of the spillway with the other storm drainage improvements. The pond capacity is 2.75 ac/ft and the Filing 2 drainage plan ' included a proposed spillway of 224 ofs capacity. The City has indicated that this pond is not necessary since adequate channel capacity exists downstream. The spillway is being constructed as an interim measure. When Skyway Drive is extended, culverts under the road will be installed to cant' the 100-year flows through the pond and into Stone Creek. The drainage area tributary to the existing pond is shown in Figure 2.1. Previous studies used the area west of State Highway 287 (College Avenue) as part of the area tributary to the pond. A field reconnaissance showed that the grading along Highway 287 allows most of the storm runoff to be directed to the south along the west side of the highway and does ' not cross over the highway to the east (Figure 2.1). The area west of the highway was considered, in this study, as not being tributary to the existing detention pond. The Fossil Creek Drainage Master Plan does not provide guidance for onsite detention requirements. Discussions with city personnel indicate that the existing storm detention pond is not needed for the area. Channels are capable of carrying the 100-year discharge and the detention pond ' created downstream by the Union Pacific Railroad embankment, as shown in Figure 2.1, provides a significant amount of detention storage for the overall area. 2.3 Resource Consultants & Engineers, Inc. There are three areas in the west part of the site (Figure 2.1) where storm runoff presently drains from Huntington Hills to offsite areas. The master drainage plan, and plans eventually submitted for the PUD, should provide for acceptable ways of handling this storm runoff. Storm runoff along most of the property's south side enters the 100-year floodplain for Stone Creek in this area. Here, storm runoff from Huntington Hills flowing onto offsite areas is not a problem. Storm runoff from the extreme southeast comer of the site would flow onto a small triangular piece of ground, as shown in Figure 2.1, that is offsite and outside of the 100-year floodplain. This triangular area was recently regraded as part of the Brittany Knolls Development, located south of Stone Creek and west of Lemay Avenue. The plan will be to divert runoff from the back of lots in this area to the west into Stone Creek. A brief analysis was made of existing drainage conditions. The stormwater management model was utilized to simulate existing conditions runoff. Appendix A contains a model schematic and output for the 2- and 100-year storm runs. Flows are summarized in Section 3. 2.4 Resource Consultants & Engineers, Inc. 3. CONCEPTUAL DRAINAGE PLAN The proposed Overall Development Plan is shown in Figure 3.1. This shows the existing drainage features and general drainage patterns as it relates to the proposed development. The conceptual drainage plan developed for the Huntington Hills area is shown in Figure 3.2. This figure shows existing 2- and 100-year flows at concentration points. The plan essentially handles the various drainage situations that were noted previously in Section 2 and to a large extent Figure 3.2 speaks for Itself. Notes 1 to 4 are in the upper right part of Figure 3.2. Note 1 calls for drainage easements where storm runoff, from offsite areas to the north of Huntington Hills, can be collected and routed around or through the property. Note 2 shows some likely locations where encroachments could be made into the 100-year floodplain. The encroachments should remain outside of the limits of the 100-year floodway unless there are special provisions. Note 3 indicates those areas where storm runoff from the Huntington Hills Development will be routed directly to Fossil Creek. Note 4 indicates that various streets, such as the extension of the Fossil Creek Parkway, should be used to carry storm runoff away from Portner Reservoir and into Fossil Creek. This will help to reduce the amount of urban runoff that enters Portner Reservoir and would help with water quality in the reservoir. Notes 5 and 6 are in the upper left part of Figure 3.2. Note 5 calls for abandoning the Inlet Ditch so that storm runoff intercepted by the ditch upstream of Huntington Hills Development will be spilled to Fossil Creek. Note 6 indicates that some urban runoff from the west -central part of the Huntington Hills Development will also be directed to the north into Fossil Creek, in addition to that intercepted from offsite areas. Drainage easements and storm sewers will transport runoff through a proposed development to Fossil Creek Notes 7 to 9 are in the lower left part of Figure 3.2. Note 7 indicates that runoff from a small part of the development along the west part of the site will be directed into the storm drainage system for the previous Huntington Hills Filing 2. Engineering analyses indicate that there is sufficient excess capacity in the Filing 2 system to handle the small amount of additional runoff; calculations to support this will be included with the PUD drainage report for Filing 3, Phase 2. Note 8 indicates that as part of extending Skyway Drive for the development, culverts will be installed in the road to pass flows through the existing detention pond into Stone Creek. It is understood that the City may acquire this area so that It can be maintained as a wetland. Skyway Drive will be a collector status street so the calculated depth of water flowing over the road for a 100-year storm must be less than 6 inches. Note 9 indicates that storm runoff in the southwest part of Huntington Hills Filing 3 will be directed into the existing detention pond. This will avoid discharging storm runoff offsite to the south and onto the small piece of triangular ground that is above the elevation of the 100-year floodplain of Stone Creek. 3.1 Resource Consultants & Engineers, Inc. IFoazill (Greek Meadowa Fort cod . r-lr\HIN \\\� o� LOUDEN DITCH L EXTENSION FILING 21 8.I PIJV,4C, �__11-mvlwv�W1i gr � cc L g EXISTING DETENTION POND — 1141 GRIM OrILLWAT r u Turn crr�r OP5-I\I SPAcr am o Im 7W 4m SCALE :1" = 2W PARCEL ' L' MIRAMO 1.05 DU/AC- RP \il IQ A W(-Al-r• Brittany! I nefle -1 L P orr M lLTIPLe A%miL.Y RL P Cathoft Church w. g �l E>� ppb�p � NN NJ PARCEL 'E' 2.3 DWAC. c g a ® D; 4Yxe ..ax. b Q� wu Ii3 19x0 General Flow Direction See Drainage Report for details and estimates of Peak Flows. Figure 3.1. Overall Development Plan with existing drainage features. J I 1 u 1 r, to N y a N e os°.FOSsi/ Creek0 di 4.° o DITCH TO BE ABANDONED / AND DIVERTED TO FOSSIL CREEK \ t POTENTIAL DIVERSION TO •. \ 47 4 PORTNER RESERVOIR ( PIPED) Smokey treet Louder, Oitcn Oy4 I Dunne Dri— II O J L�aturn —_ �/Ve I r--------7:jl EXISTING SPILLWA' TO BE RELOCATED 1 � I 1 I I I ~ 1 P7581 PROJECT BOUNDARY •• 2 PORTNER RESERVOIR r2_71 L� . • I PROJECT BOUNDARY / L • �10 2 `coo\ e ° eon ORoute storm runoff from offsite areas around or through property along drainage easements. Encroach (fill) into 100-year floodplain; limit Oextent of encroachment to 100-year floodway and protect banks as needed. Construct storm drainage facilities (storm 0 drainage, gutters, etc.) to direct storm runoff to Fossil Creek. OUse streets to divert water into Fossil Creek. Abandon inlet ditch and construct a spillway Oto safely handle storm runoff intercepted by ditch upstream of Huntington Hills. Construct storm drainage facilities (storm drains, g gutters, etc.) to direct storm runoff into Fossil Creek through proposed development. Route storm runoff from front lots into storm O7 drainage system for Filing 2. Construct culvert on Skyway Drive to serve as Og overflow for existing detention pond. Construct storm drainage facilities to direct storm 0 runoff into existing detention pond. Construct storm drainage facilities to direct 10 storm runoff into Stone Creek along south side of property. Route storm runoff into 100-year floodplain for 1 1 Stone Creek. Construct storm drainage facilities to direct 12 runoff into Portner Reservoir. 2N 0 Existing flows TSCALE 1•=500' N 51 o' Figure 3.2. Conceptual drainage plan. I 11 11 r Note 10 indicates that storm runoff along the extension of Fossil Creek Parkway and south of the street will be directed into Stone Creek along the south side of Huntington Hills. Stone Creek basically serves as the dividing line in this area between the Huntington Hills property and the properties to the south. Discharge from Huntington Hills will be directly into the 100-year floodplain for the creek. In the extreme southeast part of the site, the creek's 100-year floodplain is no longer directly adjacent to the Huntington Hills property line. Note 11 indicates that a drainage easement will be necessary along the property line at this location so that storm runoff can be collected and routed to the west back into the 100-year floodplain. This will avoid discharging storm runoff offsite onto another small piece of triangular ground that is above the elevation of the 100-year floodplain. Note 12 calls for urban runoff from that portion of the development east and north of the extension for Fossil Creek Parkway to be discharged into Portner Reservoir. With the conceptual drainage plan, the amount of area tributary to Portner Reservoir will be reduced, compared to what it is for present conditions. Along Fossil Creek there are no improvements required as part of the Fossil Creek Master Drainage Study. Where local runoff is discharged into the creek erosion control will be required. In some locations where unstable banks exist along the creek, some bank stabilization may be required. 3.4 Resource Consultants & Engineers, Inc. 4. SUMMARY This report was prepared to document the reasoning behind the proposed conceptual drainage plan for Huntington Hills; all the components of the master plan are shown in Figure 3.2. A master plan was needed to meet the City of Fort Collins' submittal requirements for Overall Development Plans and because the City does not have a document for the Fossil Creek Basin for a planned development of drainage facilities in this area, except for the creek itself. The conceptual drainage plan report will supplement and serve as a reference for future submittals that will be made for specific PUD plans. ' 4.1 Resource Consultants & Engineers, Inc.