05-0498 Peer Review No 1 033007
March 30, 2007
Mr. Jim Gibson, P.E.
Development Review Engineer
City of Yelm Community Development Department
P.O. Box 479
Yelm, WA 98597
Re: Peer Review – Site Construction Documents
Twin County Credit Union
Dear Jim:
Per you request, Shea, Carr & Jewell, Inc. has performed a peer review of the Twin County Credit Union engineering construction plans and stormwater drainage report. Our comments are
as follows:
Storm Drainage Report:
Storm Drainage Report Section 9 – The design impervious area indicated does not match the information provided in appendix B. The amount of impervious area is approximately 1.24 acres
according to Appendix B. The pervious are will also require revision.
Erosion Control Report Section 1– The sequence should include the installation of the erosion control elements.
Storm Drainage Report Appendix B – There is a math error on page #1 of the drainage calculations. Please fix the area and all subsequent calculations based on the updated information.
Storm Drainage Report Appendix B – Page 2 of the drainage calculations indicates that there is information provided from a previous drainage report for the existing site. I do not see
these calculations. Please provide this information.
Storm Drainage Report Appendix B – Page 2 of the drainage calculations shows a volume provided of 577 cubic feet for the infiltration gallery in Basin #1. It is compared to a volume
of 457 cubic feet. Where does the 457 cubic feet come from? Is this the modeled required volume? Please indicate.
Storm Drainage Report Appendix B – Page 3 and 4 of the drainage calculations needs to be modified to reflect the corrected area per comment #2. Further, at the top of page 3, it indicates
that a Hydra computation was performed indicating a required wetpond volume for Basin #1 of 288 cubic feet. I do not see this Hydra information. Please provide.
Storm Drainage Report Appendix B – Page 4 of the drainage calculations indicates that the existing infiltration pond has a bottom area of 1700 square feet. The plans provided show a
pond of much smaller size. From scaling of the bottom dimensions, it appears to have a bottom area closer to 1000 square feet or less. Please check the calculation of pond areas and
volume. Once revised, it may be necessary to increase the pond volume to handle the expected flows.
Storm Drainage Report Appendix B – Page 3 of the drainage calculations indicates that the wet pond has a bottom area of 741 square feet. The plans provided show a pond that seems slightly
smaller. From scaling of the bottom dimensions, it appears to have a bottom area closer to 600 square feet or less. Please check the calculation of pond areas and volume. Once revised,
it may be necessary to increase the pond volume to handle the expected flows.
Storm Drainage Report Appendix B – Page 4 of the drainage calculations has an error in the comparison of required wet pond volumes. Please revise.
Storm Drainage Report Appendix B – Page 5 of the calculations has several inconsistencies. There are 3 different flow values given and 2 different slope values. Further, it is indicated
that the pipe size is 8 inches but all of the calculations are based on a 4 inch pipe. Please revise this page to correct the inconsistencies.
Storm Drainage Report Appendix B – Please provide the Hydra input information for all basins analyzed. Specifically, I am looking to see where the basin areas including total impervious
and pervious area and infiltration rate assumed. If this information is already present in the files provided, can you please call attention to it in the report?
Storm Drainage Report Appendix B – Please provide pipe sizing calculations for existing pipes to ensure that they are of sufficient size to handle increased flow.
Please indicate whether a maintenance plan has already been submitted to the City with the original pond construction approval. If a maintenance plan does not exist, please provide
one.
Construction Plans:
All Sheets:
The following comments apply to all sheets:
Some minor redline comments have not been called out in the letter but need to be addressed in the revised plans.
Sheet 1 of 4:
Please include the conditions of approval on this sheet or provide a separate cover sheet which includes this information.
The plans as currently shown are not stakable. Location information for structures needs to be provided. In addition, this location information needs to be tied to something such as
a benchmark.
Condition of Approval #2 indicates that a refuse collection area shall be designated. Please provide.
It is unclear what will happen to the drainage along the widened drive aisle to the drive up area. It appears that drainage will runoff in to the buffer area along the south property
line and possibly to the adjacent parcel. All runoff from the site needs to be collected and conveyed to the stormwater management system on-site. Please address drainage collection
in this area. This may require adding curb and gutter along the south side of the drive aisle to collect and convey the drainage to CB-4.
Please provide additional grading information for the drive aisle between the building and the drive up area. According to the spot grades that are provided, it appears that the area
is almost flat with several low areas. How will drainage get out of this area?
Please provide invert information for the proposed cleanouts for the roof drain system. Please also provide invert information for the cleanout of the existing roof drain tightline
wherever connections are made.
Please provide slope information on the proposed roof drain tightline.
Sheet 2 of 4:
Note #9 is not referenced anywhere on the sheet. Is there a well that requires decommissioning?
Extend the silt fence on the south property line as indicated on the plan.
Sheet 4 of 4:
Replace the sidewalk detail provided with the standard City of Yelm Detail 4-11.
Should you have any questions or comments, please contact me at (360) 352-1465 at your earliest convenience.
Sincerely,
Shea, Carr & Jewell, Inc.
Amy M. Head, P.E.
Project Manager
N:\project\605-05\Phase 5 – Twin County Credit Unione\Correspondence\030607firstreviewcommentltr.doc