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HomeMy WebLinkAboutINTERSTATE LAND PUD - PRELIMINARY - 34-88B - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT,]LIMIN CRY DRAINAGE AND tOSION CONTROL REPORT FOR :STATE LANDS DEVELOPMENT ART COLLINS, COLORADO PRELIMINARY DRAINAGE AND EROSION CONTROL REPORT FOR INTERSTATE LANDS DEVELOPMENT FORT COLLINS, COLORADO August 21, 1995 Prepared for: G.T. Land Stanford Plaza Suite 100 3555 Stanford Road Fort Collins, Colorado 80525 Prepared by: RBD, Inc. Engineering Consultants 209 South Meldrum Fort Collins, Colorado 80521 (970) 482-5922 RBD Job No. 226-042 N RMINC. Engineering Consultants 209 S. Meldrum Fort Collins, Colorado 80521 303/482-5922 FAX:303/482-6368 August 21, 1995 Mr. Glen Schlueter City of Fort Collins Utility Services Stormwater 235 Mathews Street Fort Collins, Colorado 80522 RE: Preliminary Drainage and Erosion Control Report for the Interstate Lands Development Dear Mr. Schlueter: We are pleased to submit to you, for your review and approval, this Preliminary Drainage and Erosion Control Report for the Interstate Lands Development. All computations within this report have been completed in compliance with the City of Fort Collins Storm Drainage Design Criteria. We appreciate your tune and consideration in reviewing this submittal. Please call if you have. any questions. Respectfully, RBD Inc. Engineering Consultants Prepared by: &av Donald W. Baker, E.I.T. Water Resources Engineer David K. Delaney, P.E. ' Project Manager,,r ■cc: File Reviewed by: 4), 77 Kevin W. Gingery, P.E. Water Resources Project Manager po REG/k AIR" 28324 Denver 303/458-5526 TABLE OF CONTENTS DESCRIPTION PAGE I. GENERAL LOCATION AND DESCRIPTION 1 II. DRAINAGE BASINS 1 III. DRAINAGE DESIGN CRITERIA 2 IV. DRAINAGE FACILITY DESIGN 3 V. STORM WATER QUALITY 10 VI. EROSION CONTROL 10 VII. CONCLUSIONS 10 REFERENCES 11 APPENDIX A VICINITY MAP A2 HYDROLOGY A3 DETENTION A10 DESIGN OF INLETS, STORM DRAIN, AND SWALES A13 CHARTS, TABLES, AND FIGURES A21 Total pages 30 APPENDIX B EXISTING BOXELDER CREEK FLOODPLAIN B2 FUTURE BOXELDER CREEK FLOODPLAIN B46 Total Pages 69 IL PRELIMINARY DRAINAGE STUDY FOR INTERSTATE LANDS DEVELOPMENT FORT COLLINS, COLORADO I. GENERAL LOCATION AND DESCRIPTION A. Location The Interstate Lands development is bounded by I-25 on the east, the I-25 Frontage Road on the west and north, and Prospect Road on the south. The site location can also be described as situated in the Southeast 1/4 of Section 16, Township 7 North, Range 68 West of the 6th P.M., City of Fort Collins, Larimer County, Colorado. The site location can be seen on page 2 of Appendix A. B. Description of Property The Interstate Lands development will be located on approximately 35.2 acres of land located at the northwest corner of I-25 and Prospect Road. Presently, the property is undeveloped. The area was previously farmland and currently sits in a fallow condition. The topography of the site generally slopes from the northeast eto the southwest at approximately 0.74 percent. II. DRAINAGE BASINS ■ A. Major Basin Description . The proposed development lies within the Boxelder Creek Basin. Natural drainageways run along the south and east boundaries of the development. Boxelder Creek roughly parallels the west boundary of the development. B. Sub -Basin Description The Interstate Lands development has been broken into 14 sub -basins. Sub -basins 1-12 will be developed with commercial buildings, hotels, street improvements, and drainage improvements. These sub -basins are shown on the Preliminary Grading and Drainage Plan in the back of this report. 1 III. DRAINAGE DESIGN CRITERIA A. Regulations The City of Fort Collins Storm Drainage Design Criteria is being used for the subject site. B. Development Criteria Reference and Constraints Runoff from the 10-year storm event is routed via curb and area inlets, and pipes to two water quality capture ponds located within the development. The runoff from the 100-year storm event that is not accommodated by the storm drain pipes flows via streets and overland flow to Boxelder Creek located to the west of the development. C. Hydrological Criteria The Rational Method was used to determine surface runoff for the project site. The 10-year and 100-year storm event criteria, obtained by the City of Fort Collins, were used in calculating runoff values. These calculations and criteria are included in the Appendix A of this report. D. Hydraulic Criteria All hydraulic calculations within this report have been prepared in accordance with the City of Fort Collins Drainage Criteria, and are also included in the Appendix A. E. Variances from Criteria No variances are being sought for the proposed project site. 2 IV. DRAINAGE FACILITY DESIGN A. General Concept Each of the sub -basins within the development was graded to establish sump conditions at the stormdrain inlets. As a result, the storm sewer inlets were designed to collect the 10-year storm event runoff and route it through pipes to two water quality capture ponds located within the development. The 100-year storm event runoff will flow to the southwest via streets and overland flow to Boxelder Creek. The proposed drainage and grading plans are included in the back pocket of this report. The Interstate Lands development has been broken into 14 sub -basins. The 10- year storm event runoff from Sub -basins 7, 8, and 12 flows into a water quality capture pond located near Sub -basin 12, and then into Boxelder Creek. The 10- year storm event runoff from Sub -basins 1 through 6, 9, and 10 flows into a water quality pond located near the northwest corner of the I-25 Frontage Road and Prospect Road, and then into Boxelder Creek. Because the volume of the 10-year storm event runoff is greater than the volume of the water quality pond, the 10- year storm event runoff will overflow the water quality ponds and sheet flow directly into Boxelder Creek. The water quality capture ponds were designed using the Urban Drainage and Flood Control District of Denver and the City of Fort Collins Drainage guidelines in order to capture the first flush of stormwater and release it over a 40-hour drain time. Flow from Sub -basin 11 will flow directly into Boxelder Creek via a swale. Flow from Sub -basin 21 will flow west, cross the Frontage Road through a 24-inch culvert, then flow into Boxelder Creek. Flow from Sub -basin 22 will flow from I-25 through the existing irrigation ditch bordering the east portion of the development to Prospect Road. The portion of the flow that does not pass under Prospect Road through the existing 30-inch pipe will flow through Sub -basin 21 and to Boxelder Creek via the 24-inch pipe under the Frontage Road. Flows exceeding the capacity of the 24-inch pipe under the Frontage Road will weir -flow over the Frontage Road to Boxelder Creek. The parking lots in Sub -basins 4, 6, and 7 have been graded to collect runoff near the center of the parking lots using typical area inlets. The number of typical area inlets required for each sub -basin was determined using a 12-inch ponding depth over the inlet. The typical area inlet was defined as an inlet with a total inlet open area of 1.88 square -feet. Using a 20 percent reduction factor to account for clogging, the total flow per inlet was determined to be about 4.0 cubic feet per second per inlet. 3 tip,. Sub -basins 1, 2, 3, 5, 8, and 9 have been graded to route runoff from the individual pads into the roadway where the runoff will be collected by the 10-year storm sewer system using curb inlets. The curb inlets were sized using a 6-inch water depth over the inlet. B. Specific Details The 10-year storm event runoff from Sub -basin 7 was determined to be 3.5 cfs, and will be collected by two area inlets. One area inlet was determined to be adequate to drain the sub -basin, however two area inlets were required at the locations indicated on the drainage plan to maintain proper grading. The runoff will then flow in a 15-inch RCP to the storm sewer pipe in Sub -basin 8. The 100- year storm event runoff from Sub -basin 7 will flow through the parking lot and overland to Sub -basin 8. The runoff from the 10-year storm event in Sub -basin 8 was determined to be 9.4 cfs, and will be routed into the roadway where it will be collected by one 5-foot curb inlet located on each side of the roadway. The runoff will then flow to the water quality pond located near Sub -basin 12. A 21-inch RCP will be required to carry the runoff to the intersection with the 15-inch RCP carrying runoff from Sub - basin 7. A 24-inch RCP will be required to carry the runoff from Sub -basins 7 and 8 to the water quality pond. The 100-year storm event runoff will flow down the roadway into Sub -basin 3. The 10-year storm event runoff for Sub -basin 12 was determined to be 8.5 cfs. An area inlet located near the loading dock of the proposed building will capture about 2.6 cfs. A 15-inch RCP will carry this runoff to the water quality pond located near the southwestern corner of Sub -basin 12. The remainder of the runoff will enter the water quality pond through a swale located at the southwestern corner of Sub -basin 12. The 100-year storm event runoff will also enter the water quality pond through the swale and by overland flow. Runoff volumes exceeding the capacity of the water quality pond will sheet flow onto Boxelder Creek. The runoff from Sub -basin 3 will be directed into the roadway running through the middle of the sub -basin. The 10-year storm event runoff, determined to be 12.5 cfs will be collected by 5-foot curb inlets located on each side of the roadway. The 10-year runoff will then flow into Sub -basin 10 through a 24-inch RCP. The 100- year storm event runoff will flow south down the roadway into Sub -basin 1. Similar to Sub -basin 3, runoff from Sub -basin 1 is routed into the roadway where the 10-year storm event runoff of 12.9 cfs will be collected by two 5-foot curb inlets, one on each side of the street. The runoff then flows through a 30-inch 4 RCP into Sub -basin 2. The 100-year storm event runoff will flow through the roadway into Sub -basin 2. Sub -basin 2 is the largest sub -basin in the development at 3.71 acres. The sub - basin contains a large grassy area that reduces the runoff coefficient to 0.74. The runoff will flow from the back of the sub -basin to the roadway. The 10-year storm event runoff was determined to be 13.4 cfs, and will be collected by 5-foot curb inlets located on each side of the street, and by area inlets located in the grassy area in the northern portion of the sub -basin. The area inlets were provided to more efficiently transport the runoff to the 10-year storm sewer system, and to maintain proper grading within the sub -basin. The 10-year runoff will flow from Sub -basin 2 to Sub -basin 9 through a 36-inch RCP. The runoff collected from the area inlets will flow through a 15-inch RCP to the 36-inch RCP. The 100-year storm event runoff will flow in the street to Sub -basin 9. The 10-year storm event runoff from Sub -basin 6 flows to the parking lot located in that basin. The 10-year storm event runoff is collected by four area inlets located in the parking lot. It was determined that two area inlets would be adequate to drain the sub -basin, however four area inlets were required to maintain proper grading. An 18-inch RCP will carry the 8.7 cfs 10-year runoff into Sub - basin 4. The three upstream area inlets are connected by a 15-inch RCP, while the two downstream inlets are connected by an 18-inch RCP. The 100-year storm event runoff flows through the parking lot and overland to Sub -basin 4. The 10-year storm event runoff from Sub -basin 4 also flows to the parking lot located within that sub -basin. Two area inlets will be used to collect the 5.1 cfs 10-year storm event runoff. The runoff flows from Sub -basin 4 into Sub -basin 5 through a 24-inch RCP. The 100-year storm event runoff will flow through the parking lot and overland into Sub -basin 5. The 10-year storm event runoff from Sub -basin 5 was determined to be 1.6 cfs. One area inlet will be used to collect this runoff. The runoff will flow to Sub -basin 9 through a 24-inch RCP. The 100-year storm event runoff will flow through the parking lot and overland to Sub -basin 9. Sub -basin 10 has been defined as the portion of the I-25 Frontage Road that will convey water to the intersection of the Frontage Road and the main roadway through the development. There are no inlets located in this sub -basin. The runoff from the 10-year and 100-year storm events will flow through the curb and gutter to Sub -basin 9. The 10-year storm runoff from this sub -basin was determined to be 6.5 cfs. 5 Sub -basin 9 acts as the collection point for the stormwater runoff before it crosses the I-25 Frontage Road and flows to the water quality pond and Boxelder Creek. Two 20-foot curb inlets located at the intersection of the Frontage Road and the development's main street collect runoff from both the 10-year and 100-year storm events. The 100-year storm event runoff from sub -basins 1 through 8, and 10 will flow to the low -point located at the intersection, where 48.5 cfs will be collected by the curb inlets. The runoff is then passed under the Frontage Road through a 3'x4' concrete box culvert into the water quality pond. The portion of the runoff from the 100-year storm event that does not pass through the box culvert will weir flow over the Frontage Road into the water quality pond and Boxelder Creek. C. Boxelder Creek Floodplain Study Boxelder Creek borders the west side of the Interstate Land First Filing. It was therefore necessary to determine the extent of the 100 year floodplain through the site. A study performed by Simons, Li & Associates, Inc., 1981, (SLA 1981) was used as the basis for the current Federal Emergency Management Agency Flood Insurance Rate Maps (FEMA FIR Maps) for this reach of Boxelder Creek. 01 C.1 Existing Floodplain/Floodway The original study by SLA 1981 did not determine the bounds of the 100 year floodway along this section of Boxelder Creek. It appears the floodway was not determined downstream of the Boxelder Creek/Interstate 25 crossing, which is just upstream from this site, because flow patterns are complicated by inflow from irrigation ditches immediately west of I-25 and split flows and cross -over flows from the Cooper Slough are encountered south of Prospect Road. However; the portion of Boxelder Creek which borders the proposed project is below the irrigation ditches and north of Prospect Road. It was determined, in a meeting with the City of Fort Collins Stormwater Utility, the floodway was definable for this portion of Boxelder Creek. Since no computer files of the original study were known to exist, the model had to be recreated and verified against a hard copy secured by RBD, Inc., before floodway computations could be performed. A HEC-II model which consisted of cross -sections 11 through 18 of the SLA 1981 study was generated and analysis results checked against the original study. Once consistent results were obtained, Method 4 of the floodway encroachment routines was used with a target maximum increase in water surface elevation of 0.5 ft. Included in the appendix of this report is a copy of the original SLA 1981 analysis results and the results from the regenerated model of sections 11- 6 7 18 including encroachment computations. Also included are two exhibits: one shows the re -computed existing floodplain and floodway, and the other is a copy of a portion of the current FEMA FIR Map for this region. The analysis results concur with the FEMA map very well with the exception of a backwater area along the north side of Prospect Road. The FEMA map shows this area inundated, but the analysis does not consider this area as part of the conveyance mechanism. This area within the floodplain is an interpolation of the SLA 1981 results based upon the topography. The base flood elevation at Section 14, just upstream from this inundated area, is 4901.62. Although the frontage road as constructed would contain the 100 year flood waters, the minimum opening elevation for all structures within this area will be set at 4903.12. It should be noted, neither of the above models consider the current frontage road alignment in the analysis. C.2 Future Condition Floodplain As shown in the SLA 1981 study, the existing structure which carries Boxelder Creek under I-25 does not have the capacity to pass the entire peak 100 year flow reaching the upstream side of the interstate of 3830 cfs. As required by the City Stormwater Utility, the next part of the flood study involved an investigation of flooding potential should the crossing at I-25 be improved to pass the entire 100 year discharge. The purpose of this second study is to establish minimum finished floor or minimum opening elevations which will avoid damage to structures should the future improvements occur. Since the analysis was in essence Master Planning of future drainage improvements, it was also necessary to consider future improvements to the Prospect Road crossing. This crossing controls the impact of flooding over the subject tract which lies immediately upstream. A preliminary design of required improvements to the culverts under Prospect Road was included in the second HEC-II model. The frontage road was also included in the model of the second study. C.2.1 HEC-II Model Generation Cross -sections used in the second model were surveyed in the field at approximately same locations as those in the original study. Cross -sections were extended to include the frontage road. In order to avoid the complications introduced by the irrigation ditch crossings at the north boundary of the site, this model ended at the downstream side of the ditch crossings (approximately section 17.1 of SLA 1981 study). This was considered conservative since any 7 constriction of flows by the ditch crossings would only lessen the impact downstream, thus reducing the floodplain across the proposed development. Cross -sections 11-13 of the SLA 1981 study were extended to investigate the effect of the increased flows. The total discharge reaching the upstream side of I-25 in the SLA 1981 study is 3830 cfs. An additional 440 cfs is shown to flow to the south in the median of I-25 which is assumed will remain in the median after improvements are made. Since the SLA 1981 study increased the discharge from 1740 cfs to 1940 cfs between sections 12.1 and 12.2, the downstream and upstream sides of Prospect Road respectively, the discharge for this second study was also increased by 200 cfs from 3830 cfs to 4030 cfs at the same location. This became the discharge required to pass under Prospect Road. C.2.2 Prospect Road Improvements Using the HEC-II Special Culvert option, an iterative process was used to determine size and number of box culverts required to pass the entire 100 year discharge (4030 cfs) beneath Prospect Road. A profile of the centerline of Prospect Road indicated a minimum elevation of 4901.12 towards the east end of the surveyed cross - sections. Cross-section 112 is approximately 110 feet downstream of Prospect (corresponding to SLA 1981 Section 12). This section showed the potential for a split flow condition. For the entire width of the cross-section to be effective in conveying the discharge, flow would have to leave the channel east bank upstream of the section but downstream of the road since the Prospect crossing is designed to pass the entire discharge without overtopping. When flow is contained within the channel banks at Section 112, the computed water surface elevation is above the minimum road elevation, thus, overtopping of Prospect occurred regardless of the culvert design. But, with overtopping of the road, the entire Section 112 then would become effective for conveying flow. Leaving the entire cross-section 112 effective for flow, a split flow condition occurs at the section and the water surface elevation is below the roadway with the entire discharge passing beneath Prospect Road. The conclusion is, as the water surface elevation rises in the channel at Section 112, the flood waters overflow the east bank between Prospect Road and Section 112 before the E backwater can overtop the road. Thus, creating the split flow situation and Section 112 is completely effective for conveying the discharge. For this situation, eight (8) - 5.5' x 9.5' box culverts, or a structure of equivalent conveyance, will be required to pass the entire 4030 cfs beneath Prospect Road. An approximate sketch of the crossing modeled in the HEC-II run is included in the appendix. Also included, is the HEC-H results including plots of the cross -sections along with an exhibit delineating the extent of the future floodplain. C.3 Conclusions and Recommendations The extent of the existing 100 year floodplain and floodway are shown on the Drainage Plan in the folder at the back of this report. All proposed structures situated within the existing floodplain will have minimum finished floor and minimum opening elevations 18 inches or greater above the base flood elevation. Because of this site's proximity to Boxelder Creek, the peak stormwater runoff generated from this site will reach Boxelder Creek and pass downstream long before the peak of the 100 year hydrograph for Boxelder Creek reaches the site (see rational calculations in Appendix A and copy of SLA 1981 hydrograph contained in Appendix B of this report). In addition, no development is proposed within the floodplain except for the backwater area which was not considered part of the discharge conveyance mechanism. Therefore, development of this area will not impact the existing 100 year water surface elevation of Boxelder Creek, and onsite detention of localized stormwater runoff is not required. The proposed drainage system for the development will outlet just above the flowline of Boxelder Creek channel. Because of the lag between peak runoff from the site and peak discharge in Boxelder Creek, tailwater effects on the drainage system design was not considered. All top of inlet elevations for the drainage system are set above the 100 year base flood elevation to avoid any potential impacts of flood water backing up in the stormdrain pipes. An investigation was performed to determine the impact on the 100 year floodplain across this development should the structure carrying Boxelder Creek beneath Interstate 25 be improved. With the improvements to the culverts under Prospect Road, the frontage road acts as a levee containing E the flood waters. All finish floor elevations of structures in the site are set above the future 100 year base flood elevation adjacent to the structure. The recommended improvement to the culvert under Prospect Road is to install eight (8) - 5.5' x 9.5' box culverts. This arrangement gives a base flood elevation of 4901.07 at the upstream end of the culverts which is below the minimum roadway elevation. The channel will require regrading upstream and downstream of the culverts to form the flowline to the width of the culvert system. Concrete aprons and wingwalls upstream and downstream of the culverts are also recommended. The recommended box culverts will not be constructed by this development, but rather they are anticipated to be designed and constructed by the City of Fort Collins at some future date. V. STORM WATER QUALITY A. General Concept Beginning in October 1992, the water quality of storm water runoff was required to be addressed on all final design utility plans. Since the construction of the Interstate Lands development will begin after October 1992, we have sought to find various Best Management Practices for the treatment of storm water runoff. The Interstate Lands development will be provide two water quality ponds to help improve the runoff water quality before it flows into Boxelder Creek. VI. EROSION CONTROL A. General Concept During final design for this site, appropriate measures will be designed to control erosion from the site. The first of these measures to limit erosion is to properly reseed all disturbed areas which will not be paved. All construction activities for this site must comply with the State of Colorado permitting process for Stormwater Discharges Associated with Construction Activity. A Colorado Department of Health NPDES Permit will be required before any construction grading can begin. VII. CONCLUSIONS A. Compliance with Standards N1 10 N1 All computations that have been completed within this report are in compliance with the City of Fort Collins Erosion Control Reference Manual for Construction Sites and the Storm Drainage Design Criteria Manual, B. Drainage Concept The proposed drainage concepts presented in this report and on the construction plans adequately provide for the transmission of the developed on -site runoff to Boxelder Creek located on the west edge of the development. The combination of on -site street capacities in the curb and gutter, and the on -site storm sewer system will provide for the 10-year, and the 100-year developed flows to reach the water quality ponds and Boxelder Creek. Due to localized stormwater runoff reaching Boxelder Creek long before the peak basin runoff in Boxelder Creek, no onsite detention of localized stormwater runoff is required for this development. C. Storm Water Quality Concept Because storm water quality has become a requirement, the site has addressed this storm water aspect. Two water quality ponds will be constructed to help improve the stormwater runoff quality before it reaches Boxelder Creek. D. Erosion Control Concept Appropriate erosion control measures will be designed during the final design of this site. All disturbed areas which will not be paved, will be reseeded to help limit erosion from the site. All construction activities will comply with the State of Colorado permitting process for Stormwater Discharges Associated with Construction Activity. A Colorado Department of Health NPDES Permit will be required before any construction grading begins. REFERENCES 1. Storm Drainage Design Criteria and Construction Standards by the City of Fort Collins, Colorado, May 1984, Revised January 1991. 2. Erosion Control Reference Manual for Construction Sites by the City of Fort Collins, Colorado, January 1991. 3. Simons, Li, & Associates, Inc., Cooper Slough Boxelder Creek Master Drainageway Planning Study, August 1981. 11 a M CAp1E \F. RESERVOIR INLET SCALE 1 "=2000'