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HomeMy WebLinkAboutHEWLETT-PACKARD, BUILDING FOUR - PDP - 54-88G - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTREVISION: FINAL DRAINAGE AND EROSION CONTROL STUDY FOR HEWLETT-PACKARD COMPANY BUILDING 4 FORT COLLINS, COLORADO July 31, 1998 f _ THE SEAR -BROWN _ GROUP Standards in Excellence i REVISION: FINAL DRAINAGE AND EROSION CONTROL STUDY FOR HEWLETT-PACKARD BUILDING 4 FORT COLLINS, COLORADO July 31, 1998 Prepared for: IDC Portland Office 2020 S.W. Fourth Avenue, 3rd Floor Portland, Oregon 97201 Prepared by: The Sear -Brown Group 209 S. Meldrum Fort Collins, Colorado 80521 (970) 482-5922 RBD Job No. 799-001 TABLE OF CONTENTS DESCRIPTION PAGE I. GENERAL LOCATION AND DESCRIPTION 1I A. Location 1 B. Description of Project II. DRAINAGE BASINS 1 A. Major Drainage Basin Description 2 B. Sub -Basin Descriptions 2 C. SWMM Model Compliance III. DRAINAGE DESIGN CRITERIA 3 A. Regulations B. Development Criteria Reference and Constraints 3 3 C. Hydrological Criteria 3 D. Hydraulic Criteria 3 E. Variances from Criteria IV. DRAINAGE FACILITY DESIGN 4 A. General Concept 4 B. Specific Details V. STORM WATER QUALITY 7 A. General Concept VI. EROSION CONTROL 7 A. General Concept 7 B. Specific Details VII. CONCLUSIONS g A. Compliance with Standards 8 B. Drainage Concept 8 C. Storm Water Quality 9 D. Erosion Control Concept 9 REFERENCES APPENDIX 1 VICINITY MAP 3 HYDROLOGY } DESIGN OF STORM SEWERS DESIGN OF INLETS RIPRAP DESIGN EROSION CONTROL CHARTS, FIGURES AND TABLES EXCERPTS FROM BUILDING 5 REPORT, 100 YEAR SWMM MODEL 11 40 72 77 REVISION TO THE FINAL DRAINAGE AND EROSION CONTROL STUDY FOR HEWLETT-PACKARD BUILDING 4 FORT COLLINS, COLORADO I. GENERAL LOCATION AND DESCRIPTION A. Location The Hewlett-Packard Building 4 site is located in southeast Fort Collins (see vicinity map in the Appendix), north of Harmony Road and east of County Road 9 within the southwest 1/4 of section 33, township 7 north, range 68 west of the 6th P.M., City of Fort Collins, Larimer County, Colorado. B. Description of Property The Hewlett-Packard Building 4 site is north of the existing Fort Collins Hewlett- Packard office and industrial park. The site encompasses approximately 32 acres. The existing topography generally slopes from west to east at approximately 0.7 percent. Native grasses overlaying lean clay with sand or sandy lean clay cover the majority of the site. In the southern portion of the site there are existing parking lots with modular buildings. There are newly constructed parking areas and an office building, Building 5, are located east of the site. The existing Building 2 is southeast of the construction area. II. DRAINAGE BASINS A. Major Basin Drainage Description The Hewlett-Packard site lies entirely within the Fox Meadows Drainage Basin which is approximately bounded by Horsetooth Road on the north, Harmony road on the south, the Cache La Poudre River and I-25 on the east and Lemay Avenue on the west(refer to the general location Map in the Appendix). A Master Plan for the Fox Meadows Drainage Basin (Basin H) was Prepared by Resource Consultants, Inc., in 1981. This area was studied again by Nolte and Associates in 1990 when a master drainage plan was prepared for the Hewlett-Packard site. Nolte's master drainage report did not alter any of the assumption or conclusions which were made in the Fox Meadows Master Drainage Plan. The Regional Channel located along County Road 9 and the north boundary of the Hewlett-Packard property which was recommended in the Nolte report was designed and built with the Hewlett-Packard Building 5 project. The channel was designed for the 100 year storm and discharges to the north detention pond. The north detention pond discharges to a series of detention ponds to the south through an existing 30 inch diameter pipe. Nolte's report recommended that the 30 inch diameter outlet pipe of this north pond be enlarged to 48 inch diameter at the full development of the entire Hewlett Packard site. This increased diameter is required to avoid surcharging the north detention pond which would result in uncontrolled overland flow. This enlargement is part of this project but is not included in this report. It is intended that an addendum to this report will be submitted and construction of this improvement be completed before the certificate of occupancy is granted. B. Sub -Basin Drainage Description Historically the site receives flows from the west at two discharge points along County Road 9. Flows discharge from the Symbios site through 2-30 inch diameter concrete pipes under County road 9 at the top of the regional drainage channel. Flow from a second discharge point at the northwest corner of the Hewlett-Packard property will ultimately come from three sources (1) Symbios Logic (2) English Ranch (3) local drainage from 2 residential properties. The off -site and on -site storm runoff then flows east where it is intercepted by local roadside ditches or the ditch adjacent to the newly constructed parking lot for Building 5 which takes the runoff north to the regional drainage channel. C. SWMM Model compliance The Original Master Drainage Report for the Hewlett Packard Site, Preston Kelly Subdivision, Fort Collins, Colorado created a SWIVIM computer model for the site. During the development of the Hewlett Packard Building 5 project the Final Drainage Study for Hewlett Packard Company Building 5 report this model was modified to reflect the new conditions as well as update the SWMM parameters to those the City of Fort Collins criteria. Please see the Appendix for a copy of the model with the revisions. The purpose of this modified SWMM model was to size the regional drainage channel which is located along the north property line of the Hewlett Packard site. The Hewlett Packard Building 4 project is located primarily in subcatchment area 32. The original input file to the modified SWMM anticipated a catchment area to contain 31.3 acres with a percent impervious area of 60 percent. The proposed 2 drainage catchment area has an area of 32.1 acres with a percent impervious area of 32 percent. Due to the nearly equivalent subcatchment area and the much smaller percent of impervious area proposed the site, conditions will be well within the parameters set by the Building 5 SWMM model. III. DRAINAGE DESIGN CRITERIA A. Regulations The City of Fort Collins Storm Drainage Design Criteria as established by the Fox Meadows Basin Drainage Master Plan will be used in the preparation of this report. B. Development Criteria Reference and Constraints The Hewlett-Packard Building 4 site drainage is designed in conformance with the Fox Meadows Basin Drainage Master Plan (Basin H) and the Hewlett-Packard Master Drainage Plan by Nolte. C. Hydrological Criteria The runoff to the regional drainage channel was previously calculated for Building 5 and excerpts from that are contained in the Appendix. The most recent 100-year storm event hyetograph and infiltration parameters determined by the City of Fort Collins were used in the calculation of the runoff values. The rational method is used to determine the peak flow from the developed runoff for design of the hydraulic structures and interior roadway drainage facilities. These are designed using the 10 and 100 year rainfall criteria from the City of Fort Collins. D. Hydraulic Criteria All calculations within this study have been prepared in accordance with the City of Fort Collins Drainage Criteria and are included in the appendix. E. Variances from Criteria No variances are requested for the proposed site. 3 IV. DRAINAGE FACILITY DESIGN A. B. General Concept The site will be designed to conform with the Master Drainage Report For Hewlett Packard, with the majority of the storm water flows being conveyed to the regional channel to the north. Offsite flows from the west and north are intercepted by the regional channel and are accounted for in the SVVMM modeling. Offsite flows to the south and west are conveyed away from the site in the existing Hewlett Packard storm drainage system. West of Building 4, the storm water from all of the parking area flows to curb inlets and then is piped to the regional channel. The area south of the parking and building 4 drains south, as it has historically, to the local storm drainage system. . The perimeter road drains to curb inlets, then to pipes and then to the regional channel. Building 4 has four main roof drain pipes which outfall to the north and east, these outfall to a collection system which empties into the regional channel. East of Building 4 is a complicated system of existing and proposed storm drains. The existing storm sewers generally take storm water to the southeast and are sized for the minor storm. The proposed system will take flows to the regional channel and be sized for the 100 year storm. Specific Details County Road 9, Basins OS-1, OS-2 and OS-3 lie to the west of the site and drain to the regional channel through existing inlets. The storm flows from County Road 9 are accounted for in the regional channel SWMM model as are the storm flows which enter the site from Symbios and English Ranch. The storm runoff which is generated in Basin 1 is also accounted for by the SWIVIM model. The rest of the hydrology calculations for the site are based on the rational method. Basins 20, 21, 19, 18, 17, 14, and 11 all drain to the existing storm drain system to the south. A new area inlet has been added to Basin 14 and Basin 17 to facilitate the new grading. This storm drainage system, generally 12" diameter pipes at 0.3 % slope, has a limited capacity of approximately 2 cfs. Evidently the storm drainage system has worked adequately up until now. In order to not worsen 0 downstream conditions we have reduced the amount of impervious surface area. Basins 21,19,18 and 17 will have a reduced impervious area with the removal of the existing modular buildings and associated parking and their replacement with landscaped areas. This reduces the amount of storm water historically flows through this system. The modular buildings and the associated parking which will be removed have a total area of 3.2 acres. See pages 1 la and 1 lb of the appendix for an exhibit. The rest of the storm drainage system which conveys the majority of the storm water generated by the site consists primarily of three main storm drain outfalls. The west outfall conveys flows from Basins 22, 22a and 23. These basins are primarily parking lots where each area drains to its own type `R' inlet. These Basins then drain to a pipe (Profile A) which flows north toward the regional channel. These storm flows are then joined by flows from the perimeter road, Basins 2,3,4 and 5 (Profile D). The floor drain of the water meter building also outlets into this pipe system, however, the floor drain pipe will not receive any stormwater runoff. These combined flows then discharge into the regional channel. Because of a conflict with the electric and telephone duct a series of eliptical pipes with headwall are used to discharge stormwater to the regional channel. A Type `R' inlet box is being used as vault for the multiple pipes. We did not account for this Type `R' inlet in our drainage calculations as it will make little difference to the overall calculations. This entire length of pipe is designed for the 100 year storm. Also, due to a conflict with the same duct bank, another vault with a series of elliptical pipes is used to discharge flows to the regional channel. As a matter of economy, the three incoming pipes, which were separate, are now combined and tie into a large Type `R' Inlet. In the middle of the site, the storm drain system (Profile B) conveys flows from the parking area, which includes Basins 24, 24a, 24b, 25, 25a,26a, 40, a landscape area west of the building, Basin 26, north towards the regional channel. These storm flows are joined by roof drainage from Basins 9 and 10, and groundwater flows from Basins 37, and 38, which accept flow through triple area inlets. The storm drain pipe then connects to a type `R' inlets which receive flows from Basins 7 and 8. The flows then go to the large typ `R' Inlet which combines the flows of the three pipe runs. There is a large discrepancy between the time of concentration for Basin 6 and that for the parking and roof drainage. We compared the results for two scenarios. (1) including Basin 6 with the long time of concentration and (2) excluding Basin 6 with a short time of concentration. We chose the worst case for our design of the pipe system. Please refer to the Hydrology tables in the appendix for details. The pipe discharges the storm flows to the regional channel. This entire pipe system is designed for the 100 year storm. 5 f The storm drainage for the eastern portion of the site is by far the most complicated. This consists of a mixture of existing and proposed storm drains. Starting from the south, Basin 14 accepts flows through a new area inlet and discharges storm water to the exsisting system. There are two main storm drainage systems which serve the east side of the building. One, Profile C, serves to pick up the 10 year flows from the roofs of the building, and the 100 year flows, including overflow from the roof drains, from basins 30 and 31, and 36 through single and triple area inlets. Because of the complexity of the system it was necassary to have a low level pipe system. The only way to drain this system was through an existing 18 inch pipe which took flows through the HP campus to one of the southern detention ponds. Unfortunately, there is not enough capacity in this pipe to accept even the 10 year storms. Therefore, an overflow system needed to be designed to take the excess flows. This was done on Profile E where the next pipe south of the existing 18 inch pipe has a reverse grade. The reverse grade is set at 1% so that sedimentation is minimized. An orifice was placed on the entrance to the pipe which takes flows to the existing 18 inch pipe. This orifice was sized to pass 3 cubic feet per second(cfs) in order to avoid surcharging the the existing system. This is half of the existing system's capacity. Basin 12 and 13 generate storm water from the main roof and discharge the flows to the east to the storm drain system, Profile C. This system discharges to the Regional Drainage Channel and is designed for the 100 year storm. During the 100 year storm the excess flows from the roof are discharged to the east into basins 14,30, 31. Basins 16, 29 and 32 are small service areas to the east of the building which are ' served by the storm drainage system, Profile E. Basin 32 has an area inlet which accepts the 100 year flows as well as the overflow from Basin 12 roof drain. Flow from Basin 16 and 29 enter the system through Type "R" inlets. Stormwater from the service road, Basin 32 and 35, are also served by Profile E. Basin 33, which is the truck loading area is served by a strip drain (Profile F) which then outfalls to the new proposed sewer. Both Profile C and Profile E combine with Profile B in the type `R' Inlet on the north side of the access road. The Chemical storage Building has overflow piping to take any chemical spill to a concrete containment vault. This vault is shown near to the east side of building 4 on the erosion control plans at the back of the report. Basin 15, the roof area of a new chemical storage building joins the existing system and flows North. The li R 11 ii roof drainage from this building flows to a storm sewer to the east of the building. This pipe conveys storm water to the north where it connects to the existing system downstream of the orifice plateo Profile E. Basin 16 and 28 are existing basins which discharge to an existing inlet and 12" line which ties into the proposed Profile E. Basin 14 flows are accepted through an area inlet and taken via rerouted piping to the south. The historic conditions remain valid and the area drains south as it has historically. Basin OS-4 is an existing basin which is the present chemical storage area which is set below the surrounding grade. It is served by it own area inlets. V. STORM WATER QUALITY A. General Concept Beginning in October of 1992, the water quality of storm water runoff was required to be addressed on all final design utility plans. Hewlett-Packard is anticipating construction beginning in the Summer of 1998. Therefore for this study, we have sought to find various Best Management Practices for the treatment of storm water quality runoff. Runoff is conveyed by a grassed lined regional channel through existing detention ponds in order to provide an opportunity for pollutants to be removed. The detention pond releases at a very slow rate therefore many of the pollutants will settle out. Vl. EROSION CONTROL A. General Concept The Hewlett-Packard Building 4 site is in the moderate wind and moderate rain erodability Zones per the City of Fort Collins zone maps. The potential exists for erosion problems during construction, and after construction until the disturbed ground is again vegetated. In accordance with the City of Fort Collins Erosion Control Reference Manual for Construction Sites, the erosion control performance standard is 76.30 % during construction and 89.8% after construction. The erosion control specified on the Final Drainage and Erosion Control Plan will result in a performance standard during construction of 79.09% and 97.08% after construction. 7 B. SpecifiC Detail Prior to overlot grading all silt fence must be in place and gravel inlet filter must be placed on all existing area inlets. The areas covered by road and parking lot must have a 1 inch layer of gravel mulch (1/4" to 1 '/2" gravel) applied at the of at least 135 tons/acre. All other disturbed areas except Basin 6 should have a roughened surface. Disturbed areas of Basin 6 will have straw mulch applied and crimped. After installation of the concrete sidewalk, culvert and curb inlets, the inlets shall be filtered with a combination of concrete blocks, %a" wire screen and 3/4" coarse gravel. After installation of the storm drains, riprap protection and gravel inlet filters shall be installed. If the disturbed areas will not be built within one growing season, a permanent seed shall be applied. The estimate of probable costs for erosion control is ($31,939) x (1.5) _ $47,909 for an escrow amount. All construction activities must also 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 All computations within this report have been completed in compliance with the City of Fort Collins Storm Drainage Design Criteria. B. Drainage Concept The proposed drainage concepts adequately provide for the developed runoff from the site and are within the parameters set out in the Master Drainage Plan for Hewlett Packard. C. Water Ouaft Control Because storm water quality has become a requirement, the site has addressed this storm water aspect. Swales and detention ponds have been used to improve the water quality. D. Erosion Control Concept The proposed erosion control concepts adequately provide for the control of wind and rainfall erosion from the Hewlett-Packard Building 4 Expansion. Through the construction of the proposed erosion control concepts, the City of Fort Collins performance standards will be met. The proposed erosion control concepts presented in this report and shown on the erosion control plan are in compliance with the City of Fort Collins erosion control criteria. REFERENCES 1. Storm Drainage Design Criteria and Construction Standards by the City of Fort Collins, Colorado, May 1984, Revised January 1992. 2. Erosion Control Reference Manual for Construction Sites by the City of Fort Collins. Colorado, January 1991. 3. Fox Meadows Basin(basin H) Drainage Master Plan, Fort Collins, Colorado, by Resource Consultants Inc., February 25, 1981. 4. Master Drainage Report, Hewlett Packard Site, Preston Kelly Subdivision, Fort Collins, Colorado, by Nolte and Associates, revised October 1990.