05-0657 Shea Peer Review No 1 GMS 030907
March 9, 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
Yelm Town Homes
Dear Jim:
Per you request, Shea & Carr, Inc. has performed a peer review of the Yelm Town Homes construction plans and stormwater drainage report. Our comments are as follows:
Storm Drainage Report:
Storm Drainage Report Part 1 – Drainage Plan – Conveyance of roof drainage is not addressed. According to the grading and drainage plan, it will be tightlined to the storm drainage
system. Please specify how roof runoff will be handled and revise calculations as necessary. Currently, the drainage calculations do not include the total amount of impervious area
if roof drains are included.
Storm Drainage Report Appendix 1 – Storm Drainage Calculations – The SCS curve number assigned to the lawn areas is high for the type of on-site soils. Given the high infiltration rate
recommended by the geotechnical engineer, it is more appropriate to use a Type A soil for Table III-1.3. This would yield a curve number of 68.
Storm Drainage Report Appendix 1 – Storm Drainage Calculations – The StormShed input data does not match the post developed basin area summary. The total amount of impervious area is
0.54 acres (including roof area) not 0.30 acres. If roof runoff is also to be directed to the infiltration trench, it needs to be sized for the entire 0.54 acres of impervious area.
Therefore, the StormShed computations need to reflect this total impervious area. Please note, roof area drainage need not be included in computations of required treatment volume
or flow as long as roof flow does not mix with dirty flow. Therefore, separate StormShed computations may be required to assess treatment requirements and storage requirements. Please
revise StormShed computations and related facility sizing computations.
Storm Drainage Report Appendix 1 – Storm Drainage Calculations – The calculations for the infiltration trench are hard to follow and don’t seem to follow a logical flow. For instance,
in one step, the hydraulic gradient is assumed to be 1. With a hydraulic gradient of 1, the distance L is not longer required. However, the value of L is subsequently given and used
for calculation of the flow. Please be consistent between assumptions and calculations provided.
Storm Drainage Report Appendix 1 – Storm Drainage Calculations – Please provide a calculation of the storage volume provided by the proposed infiltration trench.
Storm Drainage Report Appendix 1 – Storm Drainage Calculations – Please provide drawdown calculations for the 10 year, 24 hour storm. According to Section III-3.6.5 of the Department
of Ecology (DOE) Stormwater Manual, infiltration trenches shall be designed to completely drain stored runoff within one day following the occurrence of the 10 year, 24 hour storm.
Currently, drawdown calculations are only provided for the 100 year event.
Storm Drainage Report Appendix 5 – Soil Evaluation Report – Please provide a map of the site showing the location of the test pits used for establishing the proposed infiltration rate.
In addition, although groundwater was not encountered in any of the test pits, an assessment of the high groundwater elevation needs to be provided. Infiltration facilities need to
have a 3 foot separation from the high groundwater elevation.
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.
When using keynotes, you need to provide a number for each note. Since location information is given in the keynote, a typical notation number cannot be used. Please revise all keynotes
to have individual call out numbers.
Sheet 2 of 10:
Please provide filter fabric fence along the southern property boundary as shown on the plans.
Inlet protection should be indicated for proposed catch basins. Please provide a notation on this sheet and a detail on Sheet 3 of 10.
Will the existing curb be removed to install the construction entrance? If the curb is to remain, it will likely be damaged during construction and require repair. Please note what
will occur with this curb.
Sheet 3 of 10:
Please provide details for inlet protection and the proposed level spreader.
Please ensure notations and details match. The size of the quarry spalls called out for the construction entrance does not match between the details and notes. The depth of post bury
for the filter fabric fence also does not match between the details and notes.
Sheet 4 of 10:
As a separate drawing for frontage improvements has not been provided, comments related to frontage improvements on Longmire Street will be made on this sheet.
A cross section showing the existing conditions and proposed improvements for Longmire Street shall be provided. It appears that the curb is existing and planter and sidewalk are the
only items to be added, but it is not clear based on the drawings provided.
Please include City of Yelm Standard Detail 4-11 – Cement Concrete Sidewalk.
Sheet 5 of 10:
.Backflow prevention is required for the irrigation and fire lines. A backflow prevention detail has been provided on Sheet 6 but backflow prevention is not indicated on this sheet.
If the backflow prevention for the fire line is within the building, please note this on the plans.
Where is the fire line going in to each building? A separate fire service is needed for each building. Please show the required fire lines and indicate their size.
Please indicate the location of the post indicator valve for each building. If it will be located on the building, please make a note of it.
The FDC for the building on the north side of the drive aisle (units 1-6) needs to be located in the vicinity of a fire hydrant. The fire hydrant must be located on the same side of
the drive aisle as the FDC.
Please indicate size of existing water mains.
Please indicate type of pipe material for the water main.
Please provide a profile of the proposed water main. The profile should provide the information indicated in the plan checklist provided in Chapter 3 of the City of Yelm Development
Guidelines.
Please show an easement over the water main per the requirements of the City of Yelm Development Guidelines.
Sheet 6 of 10:
If the backflow prevention for the fire line will be on the exterior, please provide a detail.
Sheet 7 of 10:
Please indicate size of existing sewer mains.
Please indicate type of pipe material for the sewer mains.
Please provide a profile of the proposed sewer mains (both the gravity and STEP main). The profile should provide the information indicated in the plan checklist provided in Chapter
3 of the City of Yelm Development Guidelines.
Each gravity service shall have a cleanout. Please indicate these on the plans, including invert information.
Please provide an easement over the sewer force main and STEP tank per the requirements of the City of Yelm Development Guidelines. No easement is required for the gravity main as this
will remain private.
Sheet 8 of 10:
Provide a detail for the gravity service connections to the gravity main.
Sheet 9 of 10:
A notation indicates that roof drainage will be tightlined to the storm drain system. Roof drainage was not included in storm drainage sizing calculations. Please revise the storm
drainage report to reflect the roof drainage tightline.
Please show the proposed roof drain tightlines. It is not clear how they would connect to the storm system. If they connect upstream of the AquaSwirl, the AquaSwirl will need to be
sized for the roof drainage as well. If they connect downstream, how will this occur?
Please show the sewer and water systems in the background to ensure no conflicts occur. This will be especially critical once the roof drain tightline system is shown.
According to the infiltration trench detail on Sheet 10, the trench shall have a minimum cover of 2 feet. With a pipe invert of 336 feet, the top of the trench will be at elevation
340 feet. Therefore, the first 40 feet of the trench will not have minimum cover. Please revise the grading or lower the trench to achieve minimum cover.
Sheet 10 of 10:
Please revise the infiltration trench detail as shown.
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
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