Peer Review comment ltr
August 23, 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
Tahoma Terra Fairway Multi-Family Housing
Dear Jim:
Per you request, Shea, Carr & Jewell, Inc. has performed a peer review of the Tahoma Terra Fairway Multi-Family Housing engineering construction plans and stormwater drainage report.
Our comments are as follows:
Storm Drainage Report:
Part I Storm Drainage Report – Section 1 indicates that the pervious asphalt alone will provide the required water quality treatment. However, according to Section III-3.6.7 of the
1992 DOE Manual, a pretreatment BMP must precede the pervious asphalt. Please review this section and revise the design to meet all of the requirements.
Part I Storm Drainage Report – Section 3 states that the soil report indicates that an infiltration rate of 6 to 20 inches is appropriate for the on-site soils. Although a rate of 2
inches per hour is used for design (with appropriate safety factor), the soil report rates indicate that the on-site soils have too high an infiltration rate to be used as a water quality
device. Excessively drained soils are not appropriate for use as a water quality treatment BMP (see Section III-3.3.3 of the 1992 DOE Manual). Typically, if an infiltration system
is appropriate for use as a water quality system, it is not appropriate for use as the streambank erosion control system. Per the Low Impact Development Manual, a separation and water
quality treatment layer could be added. According to this manual, it appears this layer would need to be approximately 18 inches thick. This could increase the total pavement section
to a point where there is no longer 3 feet of separation between the bottom of the pavement section and the high water table.
Part II Erosion Control Plan – Please provide direction on what should be done should sediment be tracked on to pervious asphalt.
Storm Drainage Report Appendix 1 – This entire section as it relates to the pervious asphalt design appears to mix the requirements of the 1992 DOE Manual with the requirements of the
Low Impact Design (LID) Manual. A mixture of requirements will likely result in a substandard design. If the 1992 DOE Manual is followed, the pervious asphalt system is not appropriate
for use as a water quality device. However, the use of StormShed modeling is appropriate for sizing computations. If the LID Manual is followed, a water quality layer could be added
to allow the use of this system as a water quality device. However, all sizing computations for both water quality and storage
should be based on WWHM or MSGS modeling. The design criteria specified for each manual is based on the modeling that is specified. If you use StormShed modeling but the requirements
from the LID manual, you will likely end up with a design that does not meet the design intent of either manual.
Storm Drainage Report Appendix 1 – There is a computational error in determining the value of f. One inch per hour equates to 0.083 ft per hour, not 0.042 ft per hour. Subsequent computations
are likely to be effected by this error.
Storm Drainage Report Appendix 1 – Please provide the StormShed input and output information. It is not possible to evaluate the storm drainage design of the porous asphalt without
this information.
Storm Drainage Report Appendix 1 – The computation for pervious pavement sizing is incomplete for its use as the stormwater treatment device. No computations have been shown indicating
the amount of treatment volume provided. Please note that this volume should be in addition to the volume required for storage.
Storm Drainage Report Appendix 1 – The computation for pervious pavement sizing is incomplete for its use as the storage reservoir. No computations have been shown indicating the volume
provided within the pervious asphalt base course. Further, no information has been provided on the required volume for the pervious asphalt base course. The only volume indicated is
45,138 cubic feet. This volume is not available within base course. A summary of these calculations including model information needs to be provided.
Storm Drainage Report Appendix 1 – Per Section III-3.6.8 of the 1992 DOE Manual, the porous asphalt system shall infiltrate the 10 year storm in 24 hours and the 100 year storm in 48
hours. Please provide the calculations for the 10 year storm.
Storm Drainage Report Appendix 1 – Please provide the WWHM model output for the sediment trap sizing computations.
Storm Drainage Report Appendix 1 – Sizing computations for the sediment traps shall be based on the criteria of the 1992 DOE Manual and require modeling to determine the required volume.
Please revise the computations to conform with Section II-5.8.6 of the 1992 DOE Manual.
Storm Drainage Report Appendix 2 – Per Figure III-3.18 of the 1992 DOE Manual, the typical section for porous asphalt paving includes a porous asphalt course, a filter course, and a
reservoir course. Please revise the section provided to include the filter course if the DOE Manual will be the source of design computations. Otherwise, follow the recommendations
of the section per the LID manual.
Storm Drainage Report Appendix 5 – The geotechnical investigation does not appear to make a recommendation regarding infiltration rates. The main storm report indicated that rates of
6 to 20 inches per hour were recommended. Please provide a revised geotech report indicating the recommended rates.
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 3 of 19:
The site plan conditions of approval require that the pathway around the multi-family units shall connect to the pathways provided throughout the open space. The pathways shown don’t
appear to meet this intent. There are a couple of pathways that appear to end abruptly just outside or inside the buffer area. In addition, one pathway appears to conflict with a proposed
retaining wall. It is unclear from the grading plan whether this pathway will be possible to construct from the grading (see comments for Sheet 8).
The conditions of approval also require a pocket park be located on the site plan. Please indicate where this pocket park is to be located.
Sheet 4 of 19:
The site plan conditions of approval indicate that the Oregon White Oak Trees located in the proposed open spaces shall have chain link fencing installed around the tree drip line.
This fencing can serve as the permanent protection as well as the tree barricade during construction. These fences shall be installed as part of the erosion control preparation. Please
show these fences on the erosion control plan. Please also indicate the trees that shall be retained. A detail of the fence should also be provided.
Please show the grading associated with the construction of the northern most sediment trap. This trap is proposed along a slope and it is not readily apparent whether it can be constructed
in this area.
Please show the grading associated with the installation of the interceptor ditches. It appears that grading of these swales will cause a fair amount of disruption of the wetland buffers.
Construction within buffers will require the use of hand tools.
Sheet 5 of 19:
Please indicate the slope required for the interceptor ditch.
Sheet 6 of 19:
This sheet was not provided. Please provide for review.
Sheet 7 of 19:
See the drainage report comments regarding porous asphalt section.
Sheet 8 of 19:
What is the purpose of the grading between Building #3 and #4? It appears that a swale is being created that will direct drainage to the wall. Without a structure to collect the flow,
it appears that water will likely pond at the back of the wall.
Please label all proposed contours.
According to Section III-3.6.7 of the 1992 DOE Manual, pervious asphalt shall be 20 feet down slope from buildings. This separation is not provided.
The drainage report indicates that the roof drainage will be collected and directed to the base course of the pervious asphalt. Please provide details and/or notes indicating that the
disposition of roof drainage. Currently, roof drainage is not addressed.
The grading shown does not address the proposed pathways. There are a few areas where the proposed pathways would be at too steep of a slope to be traversable. Please show pathways
on the grading plan and show the associated grading.
The wall along the entrance drive will encroach in to the wetland buffer. Construction equipment will not be allowed within the boundary of the buffer. Any construction that occurs
within the buffer will require the use of hand tools.
Pervious asphalt can have a tendency to clog if not properly installed or maintained. The current drainage system has no backup or emergency overflow system in place in case of failure.
It is my suggestion that catch basins be placed within low areas. These catch basins could direct the flow to the base course under the asphalt. Without some type of overflow fail
safe, drainage would overflow in to the adjacent wetland in some areas. Overflow in to the adjacent wetland will not be allowed.
All Water Sheets:
The following comments apply to all of the water sheets:
Label the separation distance between water and sewer (all water and sewer sheets). A minimum of 10 feet of separation needs to be provided.
The minimum cover for water lines is 42 inches. Please revise all water plans.
It appears deflection will be required to construct the water lines as shown. The City of Yelm does not allow pipe deflection. Please revise design to eliminate all deflections shown.
The proposed fire hydrant locations adjacent to buildings 1 and 2 are unacceptable. Hydrants cannot be placed adjacent to a parking stall. Further, the hydrants are too close to the
building. Please check the fire code for appropriate locations.
The fire department connection locations for all three buildings are unacceptable. The locations shown are too close to the building and do not provide good access as they are adjacent
to parking stalls. Please check the fire code for appropriate locations.
Please provide the standard City of Yelm Detail for an appropriate back flow prevention device for the domestic services.
The domestic and fire services are unclear for all buildings. The 6 inch lines that connect to the 10 inch line have fire hydrant and domestic service connections. Therefore these
are water main lines that need to be dedicated to the City. These water main lines cannot also serve as the fire line in to the building. Please clarify which is main line and where
the fire line begins for all buildings.
Please indicate where the post indicator valve will be for the fire service for all buildings.
Please indicate the size of the fire lines in and out of the buildings.
Please indicate the size of the domestic water line, water meter, and backflow preventer for the domestic service for all buildings.
Please label the proposed easement for the water line.
Please indicate the connection type for all fittings – FL, FLxMJ, etc.
Sheet 10 of 19:
Note #2 is labeled as a 90 degree bend but appears to be a 22 ½ degree bend. Please revise.
Given the necessary revisions to accommodate minimum cover needs and removal of deflections, it appears that the high point in the line can be eliminated which would remove the need
for an air release valve. Where possible, water lines shall be installed to minimize the need for air release valves.
Tees on the mainline require 2 gate valves. Please add a valve for note #6. Also, please show all valves indicated on the plan view.
Sheet 11 of 19:
Tees on the mainline require 2 gate valves. Please add a valve for notes #12 and #17. Also, please show all valves indicated on the plan view.
Sheets 12 and 13 of 19:
The water connection to the north appears to require excavation within a wetland buffer according to the Thurston County GeoData website. Has the wetland and buffer for this parcel
been mapped? It appears that the encroachment in to the buffer for water line construction will require tree removal and related buffer impacts. Please provide documentation of the
impacts to the effected buffer.
Sheet 14 or 15 of 19:
Please provide a water system map.
Sheet 16 of 19:
It appears pipe deflection will be required for pipe installation. The City of Yelm does not allow pipe deflection. Please revise layout to eliminate deflection.
The water crossing shown at approximately station 3+30 does not appear to have the required 18 inches of separation. Please revise or provide adequate encasement per Section 6.130 of
the City of Yelm Development Guidelines.
Sheet 17 of 19:
The water crossing shown at approximately station 4+46 does not appear to have the required 18 inches of separation. Please revise or provide adequate encasement per Section 6.130 of
the City of Yelm Development Guidelines.
The plan view and profile view show different numbers for the length of the STEP main. Please correct.
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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