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HomeMy WebLinkAboutHEWLETT-PACKARD EXPANSION OF BUILDING 6 - I-L SITE PLAN REVIEW - 54-88C - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTNo Text INC Engineering Consultants 2900 South College Avenue Fort Collins, Colorado 80525 303/226-4955 January 11, 1989 Mr. Glen D. Schlueter City of Fort Collins Stormwater Utility P.O. Box 580 Fort Collins, Colorado 80522 RE: DRAINAGE ANALYSIS FOR THE HEWLETT PACKARD BUILDING 6 - PRESTON-KELLEY SUBDIVISION Dear Glen: Hewlett Packard of Fort Collins is located on the northeast cor- ner of the intersection of County Road Number 9 and Harmony Road (Colorado State Highway 68). More specifically this project is located in the southwest quarter of Section 33, Township 7 North, Range 68 West of the 6th Principal Meridian. This report provides an analysis of the affect on the existing site drainage system caused by the proposed Building 6 project. Building 6 is to be located just to the northwest of existing Building 3 as shown on the HP Master Site Plan (Exhibit A). The Building 6 project consists of the construction of Building 6, a Cafeteria, Energy Center, Building 6 Annex, and additional parking areas. These new features are located within the shaded area of Exhibit A. The new buildings will cover approximately 5.1 acres while new pavement and walkways will cover ap- proximately 8.3 acres. However, approximately 2;.1 acres of new building will be constructed over existing pavement while new grass will be built over approximately 0.25 acres of existing pavement. This results in a net increase of 11.105 acres of im- pervious area to the HP site. ' This drainage analysis consists of three main parts. First, since the Building 6 project will result in a relatively large increase in impervious area at the HP site, the 100-year dis- charge from Dam Pond (located within the southeast corner of the site) to Fossil Creek Reservoir Inlet Ditch (located off -site) was analyzed. Second, a hydrology and hydraulic profile analysis was conducted for the new storm sewer lines associated with Building 6 (Lines BB and CC). Finally, the impact of Building 6 on the magnitude and route of the upgradient, off -site 100-year flow as it enters and passes through the HP site was analyzed. Other Offices: Vail, Colorado 303/476-6340 • Colorado Springs, Colorado (719) 598-4107 Several previous drainage studies have been conducted at the HP site and have been reviewed and incorporated, where appropriate, into the analysis. In 1976 and 1977, Anderson & Hastings Con- sulting Engineers, Inc. prepared an overall master site drainage plan and performed storm sewer design calculations for the south- west section of the HP site, including Building 1 and storm sewer line A. In 1979, Stewart and Associates prepared a drainage study of the northern half of the HP site, including the presently undeveloped area at and to the north of future build- ings 4,5,and 7. The Fox Meadows Basin (Basin H) Drainage Master Plan was prepared by Resource Consultants Inc. in 1981 and in- cludes the HP site. It outlines the peak flows that can be dis- charged from the HP site to Fossil Creek Reservoir Ditch Inlet when the Fox Meadows Basin has been fully developed. It is believed that another drainage report exists that addresses Dam Pond. However attempts to locate this report were unsuccessful. Finally, in 1988, RBD conducted drainage studies of the HP Tem- porary Building and the CICD Receiving Area Expansion and analyzed storm sewer routing through the existing K-line. Exhibit AA shows the general areas covered by these previous drainage studies. Dam Pond Analysis The performance of Dam Pond before and after construction of the Building 6 project was evaluated for the 100-year storm event. Table 4 of the Fox Meadows Basin Drainage Master Plan indicates that the peak outflow from Dam Pond for the 100-year event when the Fox Meadows Basin is fully developed should be 69 cfs. This discharge was used as a basis for evaluating the results of the Dam Pond analysis. The storage capacity of Dam Pond was determined from survey notes and a topographic map generated after a July 1988 site survey. The normal water level in Dam Pond prior to storm events is 4886.55 feet, with water occurring above this depth discharging through an 18 inch outlet pipe. Dam Pond is provided with a spillway at an elevation of 4891.57 feet to handle flood flows during the 100-year or higher event. Between these two elevations, 12.5 acre-feet is available for detention of on -site stormwater runoff. Discharges from Dam Pond go int§ Fossil Creek Reservoir Inlet Ditch and flow south. The Rational Method was used to calculate peak runoff rates for the 100-year storm event. The drainage basin bou6daries before and after construction of the Building 6 project rare shown on fold -out exhibits B and C, respectively. It should be noted that flows occurring within the southwest section of the site to man- hole A-9 were calculated as part of the Anderson & Hastings analysis in 1976 and 1977. The cumulative C coefficient, cumula- tive area, and cumulative time of concentration determined by An- derson & Hastings at A-9 were used as initial conditions for Basin Al. In addition, drainage basins indicated by letters R, G and CY on fold -out Exhibits B and C (the northern part of Build- ings 2 and 3 and areas north of these buildings) were analyzed as part of the CICD Receiving Area Expansion Drainage Analysis (RBD, September 29, 1988). Flow calculated during that analysis at manhole K-2 was used in the "before" building 6 calculations while flow calculated at manhole K-3 was used in the "after" analysis. Finally, the area north of the existing HP site fence all drains to the east and not into Dam Pond and is therefore not analyzed as part of the Dam Pond evaluation. The drainage basins shown on fold -out Exhibits B and C and their associated storm in- lets and manholes are shown conceptually on Exhibits D and E for the "before" and "after" Building 6 cases. The hydrology calculations were carried downstream along the major storm sewer lines until they emptied into a detention pond. The runoff coefficient, C, in the Rational Formula was assumed to be 0.20 for grass areas and 0.95 for roofs, pavement, and walkways. However, the value of C assigned to each basin was weighted according to land use. The rainfall intensity was ob- tained from the City of Fort Collins Rainfall Intensity Duration Curve (November 1975) as presented in the City of Fort Collins Drainage Design Criteria Manual. The time of concentration (tc)between manholes or inlets was calculated from the storm sewer pipe flow velocity. It is recognized that during the 100- year event, the lines are generally surcharged and may not con- tain all the flow. However, due to the complexity of accounting for this condition, it was assumed that all flow occurred within the pipes. This is a conservative approach since pipe flow results in a lower tc calculation, which results in a higher rainfall intensity. The total peak discharge at the downstream end of each storm sewer line was used to construct a hydrograph for that line. Triangular hydrographs were constructed with the peak equal to the calculated peak discharge and the time to peak equal to the time of concentration. The area under the hydrograph equals the volume of runoff and was determined by multiplying the rainfall intensity at the time of concentration by the area in square feet and by the area's weighted runoff coefficient. The 100-year hydrographs constructed for the "before" case for storm lines T,A,K (see Exhibit D) were added together to form the inflow hydrograph to South Ponds 1 and 2. This hydrograph is shown as Exhibit F. South Ponds 1 and 2 were assumed to act together as one pond since they are connected by a larger pipe (48-inch) than the South Pond 2 outlet pipe (30-inch). The com- puter program POND-2 (from Haestad Methods) was;used to route this hydrograph through a pond that had the disc arge charac- teristics of South Pond 2 and a detention volume t at considered both South Ponds 1 and 2. The resulting outflow hydrograph was added to a hydrograph that was constructed for Basin F (overland flow to Dam Pond). This summed hydrograph then became the inflow hydrograph to Dam Pond and was routed through Dam Pond using the computer program POND-2. The output from the program is provided as Exhibit G. The peak outflow from Dam Pond during the 100-year event without Building 6 is shown on the output to be 37.5 cfs. For the "after" Building 6 case, South Ponds 1 and 2 and North Pond 2 were all assumed to act together as one pond. This as- sumption is possible because flow between North Pond 2 and South Pond 2 is more a function of water surface elevation in the two ponds than it is of pipe size. The 100-year hydrographs con- structed for the "after" case for storm lines A, K, T, AA, BB, 1 -3- and CC and for Basin E (overland flow to North Pond 2) were added together to form the inflow hydrograph to a pond made up of South Ponds 1 and 2 plus North Pond 2. This inflow hydrograph is shown as Exhibit H. The computer program POND-2 was used to route this hydrograph through a pond that had the discharge characteristics of South Pond 2 and a detention volume that considered both South Ponds 1 and 2 and North Pond 2. The resulting outflow hydrograph was added to a hydrograph constructed for Basin F (overland flow to Dam Pond) and the summed hydrograph became the inflow hydrograph to Dam Pond. The hydrograph was routed through Dam Pond using the computer program POND-2. The output from the program is provided as Exhibit I. The peak outflow from Dam Pond during the 100-year event with Building 6 is 42.6 cfs compared to 37.5 cfs without Building 6. Although approximately 11 acres of impervious area will be added to the HP site with the Building 6 project, additional detention storage is provided through the use of North Pond 2. Because of this, the peak outflow from Dam Pond during the 100-year event does not increase significantly after Building 6 is constructed. Both the "before" and "after" cases have peak outflows less than the 69 cfs outlined in the Fox Meadows Basin Drainage Master Plan. It is felt that the calculated values are conservative since the assumptions made throughout the analysis were conservative. Analysis of Building; 6 Proposed Storm Sewer Facilities The Building 6 storm sewer system consists of the BB -line, and one lateral, the CC -line. The BB -line will drain the new Cafeteria, the courtyard west of Building 6, the southern half of the Building 6 roof, the Building 6 Annex roof, and the shipping and receiving area. The CC -line will drain the northern half of the Building 6 roof, a small area north of Building 6, and the Energy Center roof. The CC -line connects to the BB -line at man- hole BB-4 as shown in Exhibit J. Flow at the downstream end of the BB -line will either flow into North Pond 2 or flow south to South Pond 2, depending on hydraulic conditions. The upstream end of the BB -line will connect to the existing K-line between manholes K-3 and K-2, thereby diverting flow in the K-line to the BB -line after it reaches manhole K-3. The Rational Method was applied to the CC -line a�d BB -line for the 10-year and 100-year storm events. The calculations are in- cluded as Exhibit K. The calculated discharge caus6s the pipe to surcharge during the 100-year event. A storm drainage analysis computer program, STORM (by CIVILSOFT), was run for the 100-year event to calculate the hydraulic grade line and determine if it will go above the ground surface. The numbering of the pipe net- work is shown in Exhibit J. The water surface elevation at the downstream end of the BB -line was taken to be 4906.85 feet. This elevation was determined during the Dam Pond analysis and is the maximum pond water elevation when runoff from the 100-year event for the A,K,T,AA,BB and CC lines and Basin E is routed through a pond made up of North Pond 2 and South Ponds 1 and 2. The output from the program STORM is provided as Exhibit L. It indicates that the hydraulic grade line remains below the ground surface during the 100-year event. This was done because the courtyard i -h- between buildings two and six will be completely enclosed by buildings. Because the hydraulic grade line remains below the ground surface, it means that the surrounding buildings are protected from the 100-year storm. Off -Site Flow Analysis Up -gradient, off -site flow enters the HP site from the west via an 18-inch CMP under County Road 9, as shown on Exhibit M. The Fox Meadows Basin Drainage Master Plan indicates that the peak flow entering the HP site at this location during a 100-year event is 28 cfs. The pipe directs this flow to combine with on - site flows that develop on 35 acres of undeveloped land north of the existing HP buildings. The 100-year peak flow that occurs from these 35 acres of undeveloped, on -site land was calculated to be 27.9 cfs using the Rational Method. This is the same both before and after the construction of building 6 because the 35 acres north of the existing HP buildings is unaffected by the building 6 construction. The 27.9 cfs on -site flow combines with the 28 cfs off -site flow for a total of 55.9 cfs. Six irrigation laterals exist within the northern, on -site undeveloped area and direct this flow east to the Fossil Creek Reservoir Inlet Ditch. The magnitude and route of flow that occurs within this part of the HP Site will not be changed by construction of the Building 6 project. Summary The drainage analysis presented in this report indicates three main points. First, flow from Dam Pond during the 100-year event after the construction of Building 6 will not exceed the 69 cfs peak flow identified in the Fox Meadows Basin Drainage Master Plan. A conservative estimate of the peak discharge from Dam Pond during the 100-year event with Building 6 was calculated to be 47.6 cfs, while without Building 6 it was calculated to be 37.5 cfs. Second, the storm sewer system designed to accommodate the Building 6 project will not be at capacity during the 10-year event. During the 100-year event, the system will be surcharged but the hydraulic grade line will not exceed the ground surface elevation. Third, off -site flows that enter the HP site from the west combine with on -site flows that develop on 3'5 undeveloped acres north of the existing buildings. The magnitude and route of flow that occurs within this part of the HP site will not be changed by construction of the Building 6 Project. This analysis was prepared to satisfy current City cif Fort Col- lins criteria in storm sewer design and drainage calculation methodology. Sincerely, 'v m v\\ RBD, Inc. 24512 Joseph C. Goldbach, P.E.NA '%,��e°'•••••••°•�i,�"�� l i l l l l 11l\\\\\� cc: 282-001 r i EXHIBIT LETTER A AA B C D E F G H I J K L M LIST OF EXHIBITS TITLE HP Master Site Plan Locations of Previous HP Site Drainage Studies Drainage Basin Boundaries Without Building 6 (fold out) Drainage Basin Boundaries With Building 6 (fold out) Conceptual Layout of Storm Sewer Inlets and As- sociated Drainage Basins - Without Building 6 Conceptual Layout of Storm Sewer Inlets and As- sociated Drainage Basins - With Building 6 100-Year Event Inflow Hydrograph to South Ponds 1 and 2 - Without Building 6 POND-2 Computer Program Output for Dam Pond Without Building 6 100-Year Event Inflow Hydrograph to South Ponds 1 and 2 and North Pond 2 - With Building 6 POND-2 Computer Program Output for Dam Pond With Building 6 Building 6 Storm Sewer System Hydrology Calculations for the 10-Year and 100- Year Events for the BB -line and CC -Line STORM Computer Program Output - Hydraulic Grade Line Calculation for the BB -Line and CC -Line i Off -Site Flow Analysis „I i 0 U 0 LL- lal co co cr) D 0 coz co < oco 3 Z CN 1 G7 cr O .� I cr LL. m Z LL- C:> 0 eq Z, \.o 0- ��-Iqc�--.xiaocco n r'co =1 L.coL I