HomeMy WebLinkAboutStormwater SIte Plan - BLD Engineering / Geo-tech Reports - 3/15/1999 76 - 66U,6
Stormwater Site Plan
Ed Busch Property - DCD WO#9905
P �
Union, Washington
Prepared for:
South Shore Construction
230 East Hillside Drive
Belfair, WA 99528
Prepared by:
Morta Engineering and Testing
P.O. Box 250
Ocean Shores, WA 98569-0250
(800) 590-0958
March 15, 1999
March 15, 1999
Report No. 001
Page 1 of 5
Morta Engineering and Testing
South Shore Construction
230 East Hillside Drive
Belfair, WA 98528
(360)275-0818, (360)405-8194 pager, (360)426-9673 fax
Attn: Richard Moore, Owner
Subject: Stormwater Site Plan for the Ed Busch Property- DCD WO#9905
Mason County,Washington
Introduction
On March 3, 1999, Steven P. Morta P.E. of Morta Engineering and Testing investigated the
above site to prepare a Stormwater Site Plan (SSP) to protect the neighboring properties and the
waters of Washington State, i.e., Hood Canal. Mr. Steve Morta met with Mr. Rich Moore onsite
to walk the property and in particular investigate the two installed culverts; the north and the south
culverts as well as the junction of the two streams. This hydraulic engineering report will
determine if the two installed culverts are adequately sized to accommodate the 100 year 24 hour
rainfall and to provide mitigation measures for reducing, if not eliminating, subsequent soil
erosion and sedimentation as a result of planned development of the area.
Hydraulic Analysis for North and South Culverts
To determine the adequacy of the 18" diameter north corrugated culvert an hydraulic
analysis was performed as shown on Sheet 1 of the appendix. Figure 2 was used as a guideline
for this analysis and shows the isopluvials, i.e., contour lines of constant rainfall intensities. For
the Hood Canal area this was shown to be about 7.0 inches per 24 hour period. However, rain
intensities on the order of 2.0 inches per hour have been recorded in the area. This was used in the
hydraulic analysis called the Rational Method (Ref. 1). A runoff coefficient of 0.20 takes into
account the combination of forest land and single family residences. And, finally, the area of
contribution was estimated at 4 acres. With this information the peak flow, Qp, can now be
computed as 1.60 cubic feet per second. The existing flow passing through the culvert was
somewhat lower than this value even though heavy rains had fallen a few days before this site
investigation was undertaken.
If the north culvert were flowing full at a grade of 20%its flow rate would be 34.59 cfs which
is over 20 times greater than the maximum anticipated. Therefore, this 18" diameter corrugated
culvert is more than adequate to handle the 100 year 24 hour rain intensity.
A similar hydraulic analysis was performed for the 18" dia. south corrugated culvert with a
maximum peak flow rate computed to be 2.5 cfs which is over 10 times less than it maximum
capacity of 34.59 cfs.
P.O. Box 250,Ocean Shores,WA 98569
(360)289-0958 Office,(800) 590-0958 Bus,(360)289-9682 Fax
(360)580-1379 Mobile,(360) 538-1736 Aberdeen Laboratory
March 15, 1999
Report No. 001
Page 2 of 5
Morta Engineering and Testing
Nearly 100 year 24 hour rainfall has recently occurred in the area and large landslides north of
Hoodsport have covered Highway 101 making it impassable. These two streams were flowing at
their near maximum capacities and still the water was no more than a couple of inches deep in the
18"diameter culverts as shown in Photo No. 4 for the north culvert and Photo No. 9 for the south
culvert.
Mitigation Measures at the North and South Culverts
Debris traps are recommended at the inlet of both culverts to help stop any loose debris from
flowing into the culverts and possibly clogging them up which in turn would cause the water to
flow over the roadway and potentially causing failure. Photos No. 1 and 2 show that a pile of
limbs and branches have been placed on the inlet side of the culvert to help catch organic debris
that may come flowing down the stream. These traps will require as a minimum weekly
monitoring during the heavy rainfall periods from mid to late November until the end of March.
Recompaction of the culvert embankment especially on the downslope side is required
preferably with a vibratory hoepack to prevent loose material from sloughing into the stream.
This is required at both the north and south culverts.
Rock armoring will be required at the inlet and especially at the outfall of both culvert to
minimize soil erosion and potential embankment failure. Photo No. 4 shows little to no rock
armoring on the north culvert. This is also required for the south culvert.
As a backup another 12" diameter black corrugated culvert can be placed above the
existing 18"diameter culverts to allow water to pass through the upper 12" culvert just in case the
lower 18" culvert gets plugged up with debris. This can be done at both the north and south sites
since there will be some additional fill placed in these areas to help level out the grade of the
access road.
Structural fill shall be placed on both embankments with a loose lift thickness not to exceed
12" and then compacted to 95% of maximum dry density per ASTM D-1557 which is the
modified proctor at both culvert sites.
Hydraulic Analysis at the Confluence of both North and South Streams
As shown in Figure I both streams mentioned above converge together near the base of the
hillside as shown in Photo No. 13. The combined stream then continues to flow into an existing
channel on the northeast side of a residence down at the base of the hillside located next to SR
106 as shown in Photo No. 12.
As shown in Sheet 3, the stream meets a man-made rock and concrete dam with only a 6"
diameter opening for the stream to flow into. During the recent heavy rainfall this 6" plugged up
with debris causing the stream to actually flow around this small dam as shown in the drawing on
Sheet 3. There is evidence that the some water had actually flowed overland toward the house.
P.O.Box 250,Ocean Shores,WA 98569
(360)289-0958 Office,(800)590-0958 Bus,(360)289-9682 Fax
(360)580-1379 Mobile,(360)538-1736 Aberdeen Laboratory
March 15, 1999
Report No. 001
Page 3 of 5
Morta Engineering and Testing
This is why its very important to replace the 6" pipe with a 24" pipe as shown in Sheet 3 so that
the entire stream will flow in the pre-existing channel and eventually into Hood Canal. At the
time of the site visit the stream was flowing clear.
Before constructing a 24" diameter hole in the existing rock and concrete dam it is important
that it is on Ed Busch's property instead of the new owners of the home just downstream of the
dam. If it does belong to the new home owners then permission will be required from them to
actually perform this work which would prevent them from getting flooded.
Most of the hydraulic analysis has already been completed on page 1 above. Essentially, the
maximum flow rate passing through the dam will be the convergent flow of the two individual
culverts resulting in 4.1 cubic feet per second. However, this combined flow will need to be
increased due to the additional collection area from the culvert outlets to the dam itself which
results in about a 50%increase in the flow or 6.2 cfs. A 24" diameter pipe with a 10% grade can
accommodate a flow of 46.52 cfs or nearly 8 times the expected maximum flow. There's
conservatism in the 24" diameter opening in the dam to ensure that it can accommodate flows that
may originate from the construction of single family residences above the existing residence.
Mitigation measures will now be addressed to prevent overflow of the concrete dam with its 24"
diameter opening.
Mitigation Measures for the area above the 24" diameter pipe
A trash rack will need to be placed in front of the 24" diameter opening to prevent debris
from clogging it up which will require periodic maintenance to remove and clean the trash rack
especially during the winter months when the rainfall and the potential of excess organic debris is
prevalent.
To ensure that this pipe and the existing channel and 18" diameter culvert downstream of the
dam does overflow is to ensure that no residences or appurtenances are constructed within 150
feet on either side of the stream valley edges all along the length of both streams. This also
applies to any other streams within the proposed development.
Each proposed residential owner is to develop their own Stormwater Site Plan for their own 5
plus acre site to ensure that no surface water runoff from impermeable surfaces such as roof
down spouts, driveways, and sidewalks is directed into a stream channel whether it is flowing or
not since some of these streams only appear during the rainy season. This is to prevent increased
flows down below where a residence has already been established along State Route 106.
To prevent additional streams from being created on the hillside due to new construction
which could jeopardize existing residences down below it is important that each new homeowner
contain their surface water runoff in the form of French drains flowing into drywells or equivalent.
By preserving the trees and vegetation on their land this will help to absorb most of the water that
falls on their property.
P.O.Box 250,Ocean Shores,WA 98569
(360)289-0958 Office,(800) 590-0958 Bus,(360)289-9682 Fax
(360)580-1379 Mobile,(360) 538-1736 Aberdeen Laboratory
March 15, 1999
Report No. 001
Page 4 of 5
Morta Engineering and Testing
Existing constructed roads in the proposed develop need to be fully utilized so as not to create
additional impermeable roadway surfaces if it is not required. This geotechnical engineer will
need to be contacted prior to the construction of any new roads to ensure that it will be stable and
that it will not create additional surface water runoff into the existing streams.
Preserve the existing trees and ground cover since they help to reduce soil erosion and
sedimentation of the streams by absorbing most of the rainfall that falls on them. Trees have the
capability of"pumping" water up out of the ground through the process of evapotranspiration
whereby their root systems pull in water and through the process of capillary action draw this
water out through their leaves and needles. This is why it is important to leave as many trees and
vegetation in their natural state.
The reason people live on hill sides is to take advantage of the panoramic views they get of
Hood Canal below and also of the Olympics to the north. The first tendency is cut down trees in
front of their homes to get this view which leaves vulnerable the homesites below it due to erosion
and potential slides. However, rather than cutting down these trees and vegetation which help to
prevent soil erosion and stream sedimentation it is better to trim back the branches to more than
25 to 35 feet up from the base of the tree. This provides more of a natural view and at the same
time it also preserves the trees and keeps them alive to help hold the soils inplace.
Best Management Practices (BMPs) shall be implemented as specified in the Storm Water
Management Manual for the Puget Sound Basin - The Technical Manual, (Ref. 2). This manual
provides the guidelines and the analysis for proposed site development. A proposed homeowner
can contact this geotechnical engineer prior to the construction of their home to ensure that the
BMP's in the Manual are properly implemented for their specific applications. This will depend
upon the location of their residence, the amount of impermeable surface that will be created, and
other key factors that are called out in the Manual.
It is important that the proposed general locations of the homes be specified in advance to
ensure that the proposed 5+acre homesites are distributed far enough apart to minimize the effects
of additional impermeable surfaces and the potential for increased soil erosion and the resultant
sedimentation and potential overflow of the existing streams down below.
Conclusions and Recommendations
1. The 18" diameter culverts located on the north and south streams as shown in Figure 1 are
sized adequately for the anticipated 100 year 24 hour rainfall. However, in case of blockage
by debris that may plug up these culverts it is recommended that a 12" diameter culvert be
place above the existing 18"culverts to act as backups. This is feasible since the road at these
stream crossings will be raised for a smoother grade.
2. Both the inlet and outlet ends of the culverts shall be sufficiently rock armored to protect the
culverts from potential washout due to soil erosion around and under the culverts.
P.O.Box 250,Ocean Shores,WA 98569
(360)289-0958 Office,(800)590-0958 Bus,(360)289-9682 Fax
(360)580-1379 Mobile,(360)538-1736 Aberdeen Laboratory
March 15, 1999
Report No. 001
Page 5 of 5
Morta Engineering and Testing
3. The embankments and the additional fill shall be compacted to 95% of maximum dry density
as determined by the modified proctor method per ASTM D-1557.
4. There shall be no construction activity within 150 feet of either side of the stream valley crests
to ensure that there will be no additional surface runoff directed into existing streams.
5. Adequately sized trash racks shall be secured to the inlet side of the north and south culverts
as well as the 24" diameter opening at the man-made dam. These racks will need to be
cleaned out approximately once or every other week during the winter months to ensure that
the streams continue to flow freely. During the rest of the year these racks need to be cleaned
out on a monthly basis if there is still flowing water.
6. It will be the responsibility of each new homeowner to prepare a Stormwater Site Plan prior to
P tY P eP
construction to ensure that their additional surface water runoff does not adversely impact the
with the use of Department of Ecology's
natural dram age stems ahead in lace. Also, ep gy
� systems Y P
BMPs the amount of soil erosion and sedimentation will be minimal.
7. The natural state of the hillside is to be preserved as much as possible including trees and
existing ground cover since they absorb large amounts of rainfall and help to keep the soils
from over saturating by pumping out water through their root-trunk-leaves transpiration
system.
8. No clear cutting will be allowed. To provide a view, branches will be trimmed 25 feet to 35
feet from the base of the tree rather than cutting down the trees themselves which will help to
preserve the health of the trees.
9. Bare areas especially on the inlet side of the culverts within the stream valleys are to be
replanted with indigenous plants such as ferns, salal, alder and fir saplings to reestablish the
natural treed setting of these areas. This again will help to reduce soil erosion and stream
sedimentation.
10. After the addition of the 12" diameter culverts and subsequent embankment compaction as
well as the construction of the 24" diameter pipe in the manmade dam and the installation of
the trash racks, this professional civil engineer is to be called at(800) 590-0958 to review the
work performed and prepare a final report to be submitted to the Mason County Public Works
Director and the County Road Engineer.
P.O.Boa 250,Ocean Shores,WA 98569
(360)289-0958 Office,(800)590-0958 Bus,(360)289-9682 Fax
(360)580-1379 Mobile,(360)538-1736 Aberdeen Laboratory
March 15, 1999
Report No. 001
Page 6 of 5
Morta Engineering and Testing
If you have any questions regarding this engineering report do not hesitate to call me at(800)
590-0958 or on my cellular phone at(360) 580-1379.
P. 1t10
Sincerely
�09 Morta Engineering and Testing,
x
SIDNAL ti-t1 Steven P. Morta, P.E.
Professional Civil Engineer and Owner
rEXPIRES:1/14/ p2 O 0 I�
Technical references:
1. Civil Engineering Reference Manual by Michael R. Lindeburg,P.E., dated 1986.
2. Stormwater Management Manual for the Puget Sound Basin (The Technical Manual),
prepared by the Washington State Department of Ecology, dated February 1992.
3. Soil and Rock (n - Volume 04.08, prepared by the American Society of Testing Materials,
dated 1998.
4. SEPAXPA Site Visit by Alan A. Tahja, P/W - County Hydraulic Engineer dated
February 8, 1999
5. Slope Stability Investigation for Pebble Beach Tracts and Vicinity Located near Union,
Mason County,Washington,Report No.002,dated Sept. 16, 1997
P.O.Boa 250,Ocean Shores,WA 98569
(360)289-0958 Office,(800)590-0958 Bus,(360)289-9682 Fax
(360)580-1379 Mobile,(360)538-1736 Aberdeen Laboratory
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MASON COUNTY
PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER
$Helton,Washinpon 96564
DATE:_ Febru= 8, 1999
INTER-t)EPARTMENTAL COMMUNICATIONS
TO: Randy Neff, DCD - Planner
FROM: Alan A. Tahja, P/W- Co. Hydr. Engr. DCD WO-## 9905 „
SUBJ: SEPAXPA Site Visit NAME: Ed Busch
Randy,'
Following our visit to the Ed Busch property on Thursday February 4, 1999,I would like to offer
the following comments regarding the development of the property;
1) A Stormwater Site Plan (SSP)needs to be developed for the property which provides for
the protection of neighboring properties, and waters of Washington State. Conversion of
the temporary logging roads and development of home sites has a potential of creating in
gcess of 5,000 square feet of impervious surface,which requires a stormwater plan to be
developed by a licensed engineer.
2) Several creeks or surface drainages were observed traversing the property. An element of
the SSP should be the hydraulic sizing of the road crossing culverts. Without an analysis
supporting the sizes of the culverts installed, a potential for a washout quid delivery of mud
and silt to adjacent properties and Hood Canal from the roads is present. The basins
feeding the surface drainages need to be delineated and flows anticipated from a 100 year
24 hour event developed to determine what minimum pipe size is needed at each crossing.
3) During the rainy season(Oct. 1 thru April 30),all exposed and un-,vorked soils with erosion
lJoteutial are required by Mason County's Surfaoe Water Ordinance to be stabilized by
suitable application of BMPs. A temporary Erosion and Sediment (E&S) Plan is an
element of the SSP that should be developed for this site.
Please!feel free to contact me at County extension 461 if you have any question regarding these
comments, or if you feel there are other issues that need to be addressed as engineering issues
pertaining to this development.
$incerely,
T„
A n A. Tahja
File: H_\WP\SZRMWTR\REVIEWS\BUSCH.IDC
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Storm Drainage Plan 3 -3 -99
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Title:Photo No. 1 Title:Photo No.2 Title:Photo No.3
Date:March 3, 1999 Date:March 3, 1999 Date:March 3, 1999
Description: East stream crossing with Description:East stream with heavily Description:East stream flowing into 18"
natural debris barrier vegetated valley dia.culvert
Title:Photo No.4 Title:Photo No.5z Title:Photo No.6
Date:March 3, 1999 Date: March 3, 1999 Date:March 3, 1999
Description:East stream outlet Description:East stream looking down Description:West stream looking up from
stream from culvert roadway
t -
Title:Photo No.7 Title:Photo No.8 Title:Photo No.9
Date:March 3, 1999 Date:March 3, 1999 Date:March 3, 1999
Description:West stream rock armored Description:West stream looking down Description:West stream rock armored
inlet from roadway outlet
Morta Engineering and Testing
I'au I
Storm Drainage Plan 3 -3 -99
Title:Photo No. 10 Title:Photo No.11 Title: Photo No. 12
Date:March 3,1999 Date:March 3, 1999 Date:March 3, 1999
Description:West stream road,view to Description:West stream looking down Description:View of home down stream-
Hood Canal from road evidence of some flooding
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Title:Photo No. 13
Date:March 3, 1999
Description:Convergence of east and west
streams
Morta Engineering and Testing
Page 2
M. ■T DESIGNED DATE_2LZ-Z/9 5 -2 JOB N 4%
MORTA ENGINEERING & TESTING CHECKED DATE
P.O. Box 250 Ocean Shores, WA 98569
Business: (360) 289-0958 Toll Free: 1-800-590-0958 SHEET OF
Fax (360) 289-9682
Testing Laboratory, Aberdeen, Washington (360) 538-1736 PROJECT EL
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MORTA ENGINEERING & TESTING CHECKED DATE
P.O. Box 250 Ocean Shores, WA 98569
Business: (360) 289-0958 Toll Free: 1-800-590-0958 SHEET 2 OF 3
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M.E.T.
MORTA ENGINEERING & TESTING CHECKED DATE
P.O. Box 250 Ocean Shores, WA 98569
Business: (360) 289-0958 Toll Free: 1-800-590-0958 SHEET, OF
Fax (360) 289-9682
Testing Laboratory, Aberdeen, Washington (360) 538-1736 PROJECT
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III-1-46 FEBRUARY, 1992
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