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HomeMy WebLinkAboutDrainage Letter10497 Town and Country Way, Suite 600 • Houston, Texas 77024 • 713-600-6800 • fax 713-600-6801 www.freese.com TO: Alan Gibbs, P.E. CC: Project File FROM: Hector Olmos, P.E., CFM; Leslie Munoz, EIT, CFM SUBJECT: A&M Church of Christ Drainage Letter Review DATE: 06/16/2017 PROJECT: CCL14552 -Update of Drainage Manual Introduction Freese and Nichols, Inc (FNI) has been asked to review the drainage submittal for the A&M Church of Christ proposed improvements based on the submission dated May 3, 2016 by McClure & Browne, Engineering/Surveying Co. The submitted report included a short description of the methodology and findings, elevation-storage-discharge curves, summary of results, rational method table, storage routing tables, and related exhibits. Additionally, the City provided the drainage report for the original submittal from 2002. The original report did not include any calculation tables, but it included project methodology and related exhibits. Project Description According to project report, the A&M Church of Christ was built in 2005 and was designed to include a detention pond of 2.97 acres to mitigate the increase in impervious area. The project site drains partly onto the northeast and partly to the southwest into unnamed tributaries to Wolf Penn Creek. The site was constructed and designed to carry flow from the site through inlets in the parking lot and through a storm sewer system onto the detention pond. The detention pond was designed with an 18" outlet pipe at elevation 262.82' with a steel plate and an overflow weir at elevation 267.5'. Since the original construction of the church in 2005, the pond was expanded to 4.2 acres for reasons unknown and the steel plate was removed. In 2016, the church wanted to expand their facilities and the submitted report was meant to evaluate the effect of the proposed additions to the detention system . The report does not suggest any increase in the pond size . Report Findings The report states "Based on the topographic survey of the site, and evaluation of the existing outlet structures, I conclude that the existing detention pond at A&M Church of Christ should adequately provide stormwater detention for the existing and proposed development." A&M Church of Christ Drainage Review 06/16/2017 Page 2 of 5 FNI Findings FNI received the following items: 1. Drainage Letter for the addition to A&M Church of Christ, dated May 3, 2016 by McClure & Browne a. Narrative of analysis objectives and summary of results b. Elevation-storage curve for the pond based on as-built survey c. Rational method calculations table d. Exhibit B-1: Existing Conditions Drainage Area Map e. Exhibit B-2: Proposed Condition Drainage Area Map f. As-Built topographic survey of existing detention pond g. Proposed detention pond routing tables h. Stage-discharge tables i. Elevation-storage graphs j. Storage-discharge graphs 2. Drainage Report for A&M Church of Christ, dated December 2002 by McClure Engineering, Inc. a. Narrative of analysis objective, methodology, and summary of results b. Exhibit A: Flood Insurance Rate Map Excerpt c. Exhibit B-1: Existing Conditions Drainage Area Map d. Exhibit B-2: Proposed Condition Drainage Area Map 3. Photos a. April gth_ rock dam b. June 9th -detention pond outfall with plate 4. Doodle on as-built detention pond survey After reviewing the submitted reports and supporting documentation as well as the available Google Earth aerial imagery for the site from 1995 to 2017, FNI offers the following: General: 1. Submitted report {2016) includes exhibits B-1 and B-2 which are identical to exhibits B-1 and B-2 from the original submittal in 2002. It was not clear what the proposed improvements were. Based on aerial imagery, FNI found that a building was added between 2014 and 2016. This building footprint seems to relatively match an outline within the submitted exhibits. Additionally, it was found that the church building was constructed in 2004 and at that point closely resembled the exhibits from the original submittal in 2002. However, between March 2004 and October 2005, the parking lot adjacent to the pond was doubled in size. The addition to the parking lot in 2005 removed approximately an acre of land from the pond area and added impervious cover of about 0.5 acres to two areas: Bypass 1 and an unnamed area to the south of the pond . This is not reflected either in the original submittal nor in the 2016 report. Figure 1 shows the changes within the site from 2004 to 2017. 2. The report mentions that an as-built topographic survey was performed in order to develop a new elevation-storage curve for the pond as it existed on the site. The topographic survey and narrative indicate that the high bank of the pond is at elevation 267'. This would imply that the pond includes some of the parking lot addition. A&M Church of Christ Drainage Review 06/16/2017 Page 3 of 5 3. Report uses an existing condition which represents the undeveloped state of the tract before the building was constructed. While this was appropriate for the original submittal of 2002, the existing conditions for the purposes of evaluating impacts and detention needs for the proposed additions/ improvements should be the conditions representing the church layout before the planned expansion. Hydrology 4. Submitted report mentions that the proposed improvements would increase the impervious cover of subbasins 5 and 6. Our review revealed that it would also affect subbasin 7. It is unknown if this was omitted from the report only or if the flow for subbasin 7 was not updated accordingly. It is also unknown if the addition of the building was correctly reflected in the submitted rational method calculations as there were no originals {2002) to compare against. 5. Subbasin Bypass 1 is modeled as "undeveloped" when there is an existing parking lot (expansion from 2005). The area of the expanded parking lot (about 0.5 acres of more impervious cover) seems to have been re-routed to drain onto the detention pond, but the additional flow was not accounted for in the rational method calculations. This underestimates the flow incoming to the detention pond . This means that the submitted analysis, may underestimate the water surface elevations and discharges out of the pond. 6. The area adjacent to, and east of, the pond appears to drain in the southeast direction and is not named or used in the analysis. The drainage boundary between this area and Bypassl is different between the "existing" and "proposed" conditions. Since there was no development nearby the drainage divide, the boundary should have remained the same. The submitted as-built topo supports the delineation in "existing" conditions. This means that the proposed condition underestimates the flow produced by subbasin Bypass 1 and the resulting flow at Study Point A. 7. Additionally, the parking lot was expanded into this unnamed area adjacent to and east of the pond. This area was graded to drain into the detention pond. This addition of parking (0.4 ac more of impervious cover) increases the area, impervious cover and flow produced by subbasin 13A. No mention of this was made in the report and the additional flow was not accounted for in the rational method calculations. This underestimates the flow incoming to the detention pond, and the submitted analysis may underestimate the water surface elevations and discharges out of the pond. 8. The rational method table shows that the subbasin times of concentration do not account for travel time across the storm sewer system . This means that the flow computed must represent the flow at the outlet of the sub basin (inlet or swale). It is unclear how the flow was routed through the storm sewer system to lag the flow from these subbasins to the detention pond. The submitted pond routing only shows a combined flow coming from subbasins 7, 13 and 14. It is unknown whether this combined flow represents the flow that occurs after routing the subbasin flow through the pipe system. Subbasin hydrographs or models were not submitted to verify this. 9. The rational method table shows that the intensities calculated for the 100-year event do not match the intensities expected using the IDF equation from the Unified Stormwater Design Guidelines. However, the A&M Church of Christ Drainage Review 06/16/2017 Page 4 of 5 intensities used are larger than the required and sothey represent more conservative 100-year peak flows for the subbasins. Hydraulics 10. Submitted report states that the steel plate which was originally restricting the inflow into the 18" pipe was removed at some time in the past and the report suggested for the plate to be reinstalled. Based on our analysis, it appears that the outflow curve for the outlet structures (18" pipe) is not correct. Using the same assumption as the report that the plate would cover half of the pipe opening, our results show that the flow from the pipe at elevation 266' is approximately 11.8 cfs . in contrast to the 7.09 cfs, which would increase discharges out of the pond. 11. It is unclear if the submitted report considered tailwater conditions in the development of the discharge curve or not. However, even with tailwater below the flowline of the pipe, our analysis shows that the discharges from the pipe and steel plate do not match those included in the discharge curve in the report. 12. Submitted report uses the outflow curve from the outlet structures in routing the flows from the development through the detention pond. Based on our analysis, it seems that there may be errors with the routing calculations. Using the rating curve and inflows as provided in the report, our analysis shows that the pond water surface elevation for the 100-year with the steel plate should be 265.7' instead of 266.29'. A summary of pond elevations and outlet flows can be found in the Table 1. 13. Because of the discrepancies in the outflow curve and storage routing, the total proposed flows after the addition of the flows from Bypass 1 and subbasin 16, as calculated in our analysis, are larger than stated in the report (19 cfs vs. 23 .1 cfs for the 100-year and 13 cfs vs. 15.4 cfs for the 5-year). A summary of pond elevations and outlet flows can be found in Table 1. 14. Based on our analysis, the pond can contain the flows as represented with the submitted report. However, since the analysis neglects part of the area draining to Bypass 1 and the parking additions (totaling about an acre and a half of additional impervious cover, of which about an acre is unaccounted for), it is uncertain if the detention pond would adequately mitigate impacts from the church expansion. 15. Based on received photos, it appears that the installed plate does not cover half the opening as was assumed during the analysis by Brown & McClure. Pictures show that about a third of the opening is covered by the plate. This would increase the outflow through the outlet pipe . A&M Church of Christ Drainage Review 06/16/2017 Page 5 of 5 Using XPSTORM 2016.1 Pond Pond flow at Scenario Stage Outflow Sutdy Point ft cfs A cfs 5-year with plate 265.61 100-year with plate 266.36 100-year with RC 265. 7 9.5 12 6.7 From submitted report (2016) Pond Pond flow at 15.2 23.3 13.6 Scenario Stage (ft) Outflow Sutdy Point (cfs) A (ds) 5-year with RC 265.48 6.4 13 7.4 100-year with RC 266.29 7.4 19 Where RC stands for rating curve. The rating curve was taken from Exhibit D-3 (with plate) from submitted report. Table 1-Summary of Pertinent Water Elevations and Flows •• FREESE •1 ~NICHOLS Exhibit 1 Page 1 •• FREESE I ~NICHOLS Exhibit 1 Page 2 •• FREESE I ~NICHOLS Exhibit 1 Page 3 •• FREESE I ~NICHOLS Exhibit 1 Page 4