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HomeMy WebLinkAbout1241 G street - Drainage study - Bakersfield High School DRAINAGE ANALYSIS FOR KERN HIGH SCHOOL DISTRICT’S BAKERSFIELD HIGH SCHOOL 1241 ‘G’ STREET GRIFFITH STADIUM REMODEL PROJECT BAKERSFIELD, CA March 2016 Swanson Engineering, Inc. 5500 Ming Ave., Suite 265 Bakersfield, CA 93309 Bob Swanson Tel. 661.831.4919 Fax 661.831.4929 DESCRIPTION Kern High School District (KHSD) is planning to remodel Griffith Stadium at Bakersfield High School at 1241 ‘G’ Street in Bakersfield, California. As part of the remodel to the stadium, there will be two new concession buildings, a new ticket booth, a new road and parking lot, and an updated track and football field. Along with these improvements there will be a new drainage system installed for the area of construction. EXISTING CONDITION The majority of the high school campus surface drains to California Avenue. In general the campus surface drains south to California Avenue and then west along the north side of California Avenue to the catch basins at California Avenue and A Street. The directions of slope for the gutters at the campus are shown in the exhibits. There is an existing on-site storm drain system in the west playfield between the stadium and the tennis courts. This storm drain line currently takes on runoff primarily from the west playfield. There are City of Bakersfield storm drain catch basins on the north side of California Avenue near the intersection of A Street and California Avenue. These catch basins are connected to the City storm drain system that runs through the Oleander neighborhood on the south side of California Avenue and runs over to the west side of Highway 99 to the drainage basin at the north end of Saunders Park. The current high school storm drain system connects to the City system at the southeast corner of the tennis courts. The City system main storm drain line is an 18” storm drain line at this location and the existing pipe that connects the high school line to the City system is an 18” reinforced concrete pipe which goes directly into the street manhole. These catch basins and the adjacent manholes are the collection point for the entire 44 acre campus drainage area into the City storm drain system. PROPOSED DESIGN As part of this project, the campus will be adding storm drain lines to help drain the new facilities around the stadium. These storm drain lines will either go to the existing City connection at the southeast corner of the tennis courts along California Avenue or to a shallow on-site sump on the north side of the tennis courts along Campus Way. The existing 18” connection pipe from the high school system to the City system will remain. A new 18” line on the high school side will drain to this existing City 18” line. We are proposing to add a shallow sump with this project on the north side of the tennis courts. This sump will have a maximum water depth of 18.” Due to the minimal water depth, the shallow sump will not be fenced. The shallow sump will have 4:1 side slopes and a 2’ wide level top around the perimeter. This shallow sump is on site and will be maintained by the KHSD. The sump will take on the flow from a portion of the abandoned Campus Way runoff and the runoff from the north tennis courts. The lowest drain inlet connected to this shallow sump is on site along the south end of the vacated Campus Way. This inlet has a grate elevation of 397.86. The bottom of the sump is set at 396.4 with a design water elevation of 397.9. This lowest drain inlet will also serve as an overflow to insure the depth of water does not exceed 18”. If the water depth gets up to the grate inlet elevation, it will flow out of the drain and down A Street to the catch basin at the northeast corner of California Avenue and A Street. The sump is designed with a water capacity of 6,070 cubic feet (see calculations). The required volume for the sump for the area draining to it is approximately 5,915 cubic feet (see exhibit). ANALYSIS OF EXISTING AND PROPOSED FLOWS The campus drainage was reviewed in terms of the whole campus drainage area since the City catch basins at California Avenue and A Street have been taking on the flow from the entire campus. The drainage area for these catch basins is approximately 44 acres. The new playfield north of the abandoned 14th Street toward the west end was previously impervious when it consisted of a large building and pavement. It was converted to a playfield a few years ago. Therefore, this change was reflected in the pre and post construction analysis. The current stadium remodel project adds impervious surface with the artificial track surface, additional site concrete walkways, and asphalt for basketball courts. This new impervious surface is balanced out by the addition of the shallow sump and inclusion of the recent conversion of the old structure to a playfield. The existing campus surface drains to California Avenue in gutters along various streets. The farthest point from the inlet to the City system is in the new northeast parking lot. For purposes of this analysis, we took the path of the runoff from the farthest corner of the northeast parking lot to the inlet for a culvert at the northwest corner of abandoned streets 14th Street and G Street. From there, the system flows in a curb and gutter, a culvert pipe across 14th Street and 13th Street, and a syphon drain across G Street at California Avenue. After the syphon, the flow continues along the curb and gutter on the north side of California Avenue. Using the time of concentration for the initial reach across the northeast parking lot as a basis, the additional time was determined by the gutter time from the northeast parking lot to the catch basin along California Avenue near the southeast end of the tennis courts. The gutter velocity was estimated using the Kern County Hydrology Manual Figures D-1, D-2b, and Appendix 6.2. The time of concentration for the whole campus was calculated at 40 minutes through this method (see calculations). For a time of concentration of 40 minutes, there is an intensity of 0.76. This intensity was used for the drainage analysis. The estimated flow for the drainage area at California Avenue and A Street was calculated as 23.5 cfs for the design versus 23.9 cfs for the existing condition (see exhibits). IMPACT OF PROPOSED DESIGN ON CITY STORM SYSTEM & CONCLUSIONS Since the design flow is less than the existing flow and the impervious area draining to the City system has been reduced, there is no need to modify the City storm drain pipes or ultimate sump. There is a reduction in impervious area flowing to the City system when the site is analyzed as a whole. This is due to the recent conversion of an old structure to a playfield as well as the addition of a shallow on-site sump used to capture some of the runoff. These factors combine to decrease the impervious area flowing to the City system and therefore reduce any potential negative impact on the City system. EXHIBITS AND CALCULATIONS Attached are brief calculations and exhibits showing how the above information was derived. The exhibits attached are as follows: Calculations 1 – Shallow Sump Capacity Calculation Calculations 2 – Drain Inlet Capacities for Largest Drainage Areas Calculations 3 – Time of Concentration and Intensity for Entire Drainage Area Sheet 1 – Existing Flow at the Northeast Corner of California Ave. and A Street Sheet 2 – Post Construction Flow at the Northeast Corner of California Ave. and A Street Sheet 3 – Preconstruction Drainage Flow Directions for the Existing Campus Sheet 4 – 11x17 Plan showing the Drainage to the On-Site Shallow Sump Aerial Showing the City Storm Drain Path from BHS to the Saunders Park Drainage Basin Aerial for Saunders Park Drainage Basin Shallow Sump - Exhibit - 1 Shallow Sump – Cross Sections - 2 CALCULATIONS EXHIBIT MAPS BgiDkP sreg drn guDgaPrSgsuDrr e e lggeoG N ➤➤ N 1e2g©letff05 1e2g©letff05 1e2g©letff05 elllgoG5gel1©gtff 02 5gel1©gtff 02 5gel1©gtff 02 2 0 0 0 3 0 4 0 4 0 3 0 2 1 1 0 3 0 4 0 5 4 0 3 0 0 1 0 G 0 G 1 G 5 0 0 2 0 2