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HomeMy WebLinkAboutResponse to Staff CommentsNB 'I July 7, 2008 Ms. Erika Bridges McCLURE & BROWNE ENGINEERING/SURVEYING, INC. 1008 Woodcreek Drive, Suite 103 •College Station, Texas 77845 (979) 693-3838 •Fax (979) 693-2554 •Email: mcclure@tca.net Planning and Development Services City of College Station 1101 Texas Avenue College Station, Tx 77842 Re.: A&M Church of Christ Site Plan MBESI Proj No.: 1000-0044 Dear Ms. Bridges: Below is the summary of the revisions made per the staff's comments. PLANNING: Comment #1 : ACTION: LANDSCAPING: Comment #2: ACTION: Comment #3: ACTION: ENGINEERING: Comment #4: ACTION: Comment #5: Please be aware that we have not yet received the Mylar of the Final Plat for this property and that site plan approval can not be granted until the Mylar of the Final Plat is submitted. The Mylar has been delivered to the City. Provide Landscape Plan. Landscape Plan to be submitted under separate cover. Provide a general note that 100% coverage of groundcover, decorative paving, decorative rock, or a perennial grass is required in parking lot islands, swales and drainage areas, the parking lot setback, rights-of-way, and adjacent property disturbed during construction. See revised site plan. Plans -Please use Standard City Details for Erosion Control. See revised grading plan. In the drainage report you stated, "The existing pond will be ACTION: ACTION: ACTION: eliminated over time with the addition of the future buildings." This appears to be a "phasing" out of the existing pond rather than replacement of the existing pond with the proposed pond with this construction. Please clarify. The existing pond is to remain in place for now. The new pond has been sized to handle the all of the existing development along with the anticipated future buildings and parking. The existing pond has not been included in any of the routing calculations for the new pond. The proposed pond is being built now to accommodate all of the future development in order to eliminate the need to add to the storage of the pond with each subsequent site plan. FYI: There is a low area neardrainage area #10 that is being drained by a grate inlet that is labeled as "pond1" in the routing calculations due to its pond like characteristics. Please explain the proposed function and grading plans for the existing pond. As noted in the above comment, the existing pond has not been included in any of the new detention routing calculations. However, this pond is not being eliminated until needed in the future. The existing pond will continue to detain water for now. This will further reduce the flowrates of the water leaving the site. The finish floor elevation of the proposed building has been set 2. 5' above the spillway elevation of the existing pond to avoid flooding. Please provide flow data for the existing pond discharge vs. the proposed pond discharge. The comparison of the two pond discharges is not an "apples to apples" comparison because the new pond is located further down in the watershed and receives more inflow. However, the proposed pond will have more storage than the original pond which will allow for a slower release through the outlet. This is the desired effect due to the location of proposed outlet structure. The comparison of the two ponds: Storm Event Existing Pond Discharge Proposed Pond Discharge (from Original Report) 5yr. 15.1 cfs 17.0 cfs 10 yr. 15.5 cfs 17.3 cfs 25yr. 15.9 cfs 17.6 cfs 50yr. 16.5 cfs 17.9 cfs 100 yr. 17.1 cfs 18.3 cfs The proposed pond is located at the lowest portion of the site. Therefore the flowrate from the pond is equal to the peak runoff from the site. The comparison of the total discharge from the site: Storm Event Existing Site Discharge Proposed Site Discharge ACTION: ACTION: (from Original Report) 5yr. 22.7 cfs 17.0 cfs 10 yr. 25.3 cfs 17.3 cfs 25yr. 28.3 cfs 17.6 cfs 50 yr. 30.9 cfs 17.9 cfs 100 yr. 33.2 cfs 18.3 cfs As designed, the proposed pond will provide more detention than the original pond even with the future building additions. The pond has been designed in this manner due to the new location that will discharge directly into the concrete flume in the back of Lot 4. Please explain how the design has anticipated the future Appomattox Drive extension. The proposed pond's shape has been designed to approximately match the future ROW of Appomattox Drive. This shape can be altered slightly to match the exact location of the road without causing any problems with the storage/discharge of the proposed pond. The existing flume that will cross Appomattox Drive will have to be converted to a culvert during the construction of Appomattox. The drainage from the future road can be carried to the existing inlets located adjacent to the northwest property line (#15 and #16). This will allow the drainage to bypass the pond. The time of concentration of the roadway drainage (10 minutes) is much shorter than the time to peak of the pond (-50 minutes). Therefore the drainage from the road will not effect the peak runoff from the site. It appears the proposed emergency spillway would discharge into the single family lots and not to a designated existing channel. The emergency spillway has been located at the far north corner of the pond to provide the longest path to the existing channel. This will reduce the velocity of any drainage that does flow over the spillway. An existing swale is located along the property line that intercepts drainage from this portion of the site. This swale will take the drainage from the spillway, in the event that it is ever needed, to the concrete channel located on Lot 4. Comment #10: ACTION: Comment #11 : ACTION: Comment #12: ACTION: Comment #13: ACTION: ACTION: ACTION: SincerelY., rtson P.E. Please verify your Manning's n value. The calculations have been adjusted to use 0. 013 for concrete pipe. Please verify where the data you provided in summary table on Page 2 came from. It does not appear to match the data in the detention pond routing tables. The summary and routing tables have been adjusted to reflect the changes summarized in this letter. Based on the 100-yr WSE of 265.10 provided in the summary table, the 100-yr storm will not be contained within the pond with a spillway elevation of 265.00. Furthermore, 6 inches of freeboard above the WSE is required. Slightly more storage has been added to the pond in the upper elevations. The revised water surface elevation for the 100-yr storm is 264.96'. The spillway has been raised to 265.5'. Please provide the capacity of the emergency spillway outlet as it is required to carry the peak discharge of the 100-year storm per Section IV.E.2.f.3 of the design guidelines. The capacity of the spillway is: Weir equation: Flowrate (cfs) = 3.0*length*(head)/\(1.5) 3.0*25*0.501\(1 .5) = 26.5 cfs The capacity of the spillway is 26.5 cfs. This exceeds the proposed 100-yrdischarge of the pond (18.3 cfs). Please explain why the outfall utilizes an 18-inch pipe with a restrictor plat rather than using a smaller pipe without the plate. In our experience we have found that smaller pipes tend to clog much more frequently than an 18". If a clog occurs it will be located at the inlet to the pipe which is easy to maintain. Additionally the plate allows for adjustments to adjust the outlet if it ever becomes necessary in the future. A smaller pipe does not allow this flexibility. Do your post-development flows take into account the restrictor plate on the 18-inch pipe? The calculations for the discharge have been setup using the orifice and weir equation for the 18-inch pipe and the restrictor plate. See attached sketch of restrictor plate geometry. RE & BROWNE ENGINEERING/SURVEYING, INC.