HomeMy WebLinkAbout1994.04.00 - Campbell PUD Engineering ReportCt -6'1L
F. M.7
ENGINEERING
LAND SURVEYING
DEVELOPMENT CONSULTING
P.O Box 2199'935 North Fifth Avenue
Sequim, WA 98382
681-2161 . Fax 683-5310
April 1994
CA\,IPtsELL H.D EI{GINEERIT'IC REPCRT
PrePared for:
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CAT\,IPBELL R.D ET.ICINEERIT.|G REPCRT
PrePared for:
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This report was prePared to provide additional infornation to supPlenrent the
prans and other ;;i,.,s, "ur-iti"i-*ilh-ttre appiication for the carpbell Planned
Unit Developrnent. This intornaiion i" not intended to represeni final design of
the various utilities. plans ""i-"p""iiiotion" for conslruction of the various
systens wilt be-"uf*itt"a after';.;ii;i;;rv plan approval is obtained and final
design work is ccnPleted'
H{ASTT.IG
This proiect will be ccrpleted. in phases with the various phases bein8 as shown
on the nap labeled pHAslllc. rn"'r-.ii"r" designating the various Phases do not
represent proposed sequencing oi co.r"truction' - Actual seguence of construction
will depend on nrany factors to ;.;;i;;ined after preliminarv apProval'
WATER SfSTEilT
The urater syst€m for this proiect will consist of 8 inch dianeter PVC nains
installed in aII street rights oi *"y forminS an-interconnected network with
three points of connection to "ii"il; toi"".- Tt't"-tirst will be an extension of
an existi.s ,rain-J; Jaclsran Street. The second connection will be to an
existing Main down Hotccrb streei.--ihe-thira "onn.iion wirr be to an existing
nain in 43rd Stre"t to the east of this Project'
Provision has been rnade for further extension of the sater systern or connection
to rains on adiacent properties-tJ-the west and south of this proiect'
Hydrants, blourcffs, valves, fittings and service stubouts/neters will be
provided shere ;!fo on tfri pf*".--Construction aetails will be as Previously
approved by the City of Port Toilsend Water Department for other proiects'
SAI.IITARY SEVER SYSTETI
This systern will utilize gravity sorers with two offsite nains being utilized'
One will serve the upper portions oi-ttti" proiect and connect to an existing
sewer rain in Holconb Street. Tll.-;;inalt of tttu proiect will drain to a
previously "pprouJ sewer *in "*t";il running ""tb""- the south end of the
iairgrounds as shown on the Plans'
Sanitary sewer nains will generally be 8 inch- PVC pipe; laterals servir8 one or
two hcnres will be 6 inch. pirrar-a!"ign *ill be'ir,'"t"otdance with appropriate
Vashington State Departrnnt of. E";i;d "qui"'*ni"' Construction details will
be as previousty approved by th; aii;-of iort Townsend for other proiects'
S'IREET SYSTEIVIS
The standard street section to be used for this project will consist of 20 feetof asphalt pavement with an 8 foot graveled shoulder on one side for parking. Asignificant anpunt of the right of way will be taken up with a single grassy
swale used as a replacarent to typical roadside ditches or storm sewers in
keeping with the current trend away frqn asphalt and pipes.
Grassy drainage ways have been shown to be trernendously beneficial to thequality of stornp-ater runoff . Vith the exception of culverts where street
crossings cannot be avoided, the entire stornnater collection systen will also
provide biof i ltration.
The proposed structural section has been successfully used in other areas on the
Olynpic Pgninsula and nay be scnrewhat over designed for areas where good-
granular subgrade soils are encountered. This structural section will be usedfor the first phase of the proiect. If it proves to be overly thick, a reduced
structural section nay be submitted for approval.
STCRIilTATER MAT.IACEMENT
The proposed stornnater nanagsrent systen provides control of both quality and'-'quantity. An extensive network of grassy swales will be installed to providebiofiltration of all stornrater runoff before it leaves the project site.
Grassy swales that are not adjacent to streets will be constructed to the sane
width.and depth as those alongside streets.
The stornnrater collection systern will collect all runoff frqn the project site
as well as frcrn an estinated 8.3 acres above the site and convey it to anirregularly shaped, three cell detention pond as shown on the plans. The
detention pond will collect all runoff and release it at a rate less than thatof the existing runoff.
The nagnitude of the existing runoff was determined by breaking the site into
the five basins shown on page I of the technical data. Tinres of concentrationfrcn each basin to the detention pond were calculated as shown on pages 2 - 7.
Hydrologic groupings of the soils for the five basins were taken frcrn U. S.Soil Conservation Service tables. Copies are included as pages 8 - l0 of thetechnical data. Runoff curve nrrrbers were determined frqn the above noted data
using the chart shown on page I l. Total precipitation was taken frqn theisopluvial charts included as pages 12 - 14. A snnrary of the pertinent datafor determining predevelopnent runoff is given on page 15.
This data was entered into King County Public Works Department's ccrputer
program FIYD for calculation of runoff hydrographs frcnr each basin. The
hydrographs were calculated for 2, 25 and 100 year events. The hydrographs frqnthe 5 basins were then cqr$ined into single hydrographs designated by the letterZ. Printouts frcrn the cqrputer run are included as pages 16 - 26 of thetechnical data. Ccrplete tabulated hydrographs for all basins and storm eventsare included as pages 27 - 44. Page 45 is a graph of the total predevelopnrent
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runof f frcm the project si te. Peak f lows are 0.23 cfs, 1.72 cfs, and 3.12 cfs
f.or 2 yc, 25 yr and 100 yr events respectively. In keeping with currentpractice, the naxinnm allowable offsite flow after developrnnt was taken to be
0.115 cfs, 1.72 cfs and 3.12 cfs for the 2 yr, 25 yr, and 100 yr events
respect i vely.
Postdevelopnnnt runoff was estinated in a similar nanner using projected
developrnnt paraneters. Tinres of concentration were recalculated on pages 45 -50. Runoff curve nrrrbers reflecting the changed land uses were determined as
shown on page 51. A cqrplete listing of postdeveloSnrent runoff paraneters is
included on page 52. Pages 53 - 63 contain printouts generated {uring the
ccrputer run. Hydrographs of postdevelopnnnt flows are included as pages 64 -81. A graph of the ccnbined postdeveloprrnnt hydrographs is shown on page 82.
l.lote that these flows are as yet undetained and greatly exceed the allowableoffsite release rates
These flows are then released into the detention pond as shown on the plans.
Various orifice and weir devices are used in cqrbination to produce the desired
release rates for given design events. Details of the control structures are
shown in the plans. A ccnplete listins of release rates, storage volunns, and
other data tabulated as a function of the depth of the pond is given in the
staging table shown on pages 83 - 85. Routing of the 2 year, 25 year and 100year design events through the ponds is shoun on the tables shown on pages 87 --,98.
The greatly irregular shape of the det€ntion pond uas designed to increase the
length of shorelines to increase the biofiltration effect and for aesthetic
purposes. The pond is designed to release water over an €ilrergency spillway when
the depth of water in the porxC exceeds 2 feet. The spillway is designed toprevent the depth of cater frcrn reachins 2.50 feet (depth of flow over the
sPillwaY = 0.50 feet)
After biofiltration, detention, settling, additional biofiltration and finally,
serving as an aesthetic annnity, the water will be released via a 12 inch pipe
as shown on the plan details.
To reduce the total volrrne of runoff and to preserve the existing subsurface
flow regine to the largest extent possible, roof drains will be routed into
drymells and/or infiltration trenches wherever the soils will allow. While afinal determination will depend of the soils encountered on each building site,it is believed that up to one third of the hsnes could be constructed withinfiltration systens for roof drains. .
IRAI{SPCRTATIc[.I
The prinary node of transportation for residents of this project will be
passenger vehicles. While the project is designed to encourage uses of other
rnrdes; such as transit and bicycle, calculations of traffic inpacts are radewith the assurption that nearly all transportation will be via private passengervehicles. This is done as a conservative measure to ensure inpacts are
adequately addressed.
Bicycle routes are designed throughout the project to provide continuous routing
where streets are dead ended. A bicycle/pedestrian path is planned to provide
access to the new middle school to be constructed innediately east of thisproject. The grades of surrounding streets are generally ccnpatible with
bicycle use and the populace is indeed utilizing this npde to a significant
degree. While nErny surrounding streets lack adequate shoulders, the aligrrnent
tends to reduce speeds of autcnrcbiles, lowering the intensity of conflicts to
scne degree. Personal experience verifies that this neighborhood is well suited
for bicycle transport.
With the exception of foot traffic to the middle school, pedestrian traffic will
not be a significant rncde of transportation for this project.
Traffic inpacts on nearby streets were quantified by calculating the levels of
service o_f the various npvenrents of the intersection of Jaclsran Street ard 49th
Street, which is expected to be the npst heavily inpacted intersection.
Calculation of the nurber of trips per day to be generated by this project isfairly straightforward. The use ofilO trips per day per residential unit is
, well docurnnted by the tnstitute for Transportation Engineers (ITE). This trip,i 'iate was applied to an estinated,lOO total resldential unitd to arrive at an
expected traffic increase of 1000 trips per dayi This nnthod does not take into
account the probable reduction in trip generation rates frcn apartrents. Thea reduction wa! deliberately left out as a !:onservatidb neasure. Average daily.trip rates were converted ittleipeak hourly rates by use of ;a-K coefficient of I
q.gj12, wtrich has been historically used on the Olfrpic Peninsula.
Estination of the existing traffic rates uas nuch less straightfoirard. The
intersection under consideration will face peak traffic volures during the
sumEr rrcnths when tourist traffic is high and the fairgrounds are being nDre
extensively used. For this reason, no counts could be taken at the tinre this
report was prepared. C.ounts were available frcrn 1993 at the intersection of
Adniralty and San Juan. Those counts were taken during a period of high use as
the fairgrounds, wtrich possibly skerwed the results.
To estinate existing traffic volunes, those counts were used and cqrbined with
experience and professional judenent to arrive at the estinated tripdistribution shown on page 99 of the technical data. Directional distributions
of. 60/40 were used to reflect the residential character of'nuch of the area.
Appendix I of the technical data contains ccrplete calculations of the levels of
service of the intersection using the estinrated existing traffic volunes. Dtreto the uncertainty in estirnating existing volrrnes, the data presented in the
Appendices should be viewed with caution. They should be considered as
indicative of overall levels of service only.
To estinate the inpacts of this project, the estinated 1000 trips per day was
reduced to an hourly volune by applying a coefficient as described above. The
additional trips were then distributed annng the various npvements, which were
then rounded and rebalanced. Conplete calculations of the levels of service at
the estinated future volurnes is contained in Appendix I I of the technical data.
ln sunrary, the only rDvernent showing less than level of service A is npvennnt
l0 (left turn onto 49th, southbound frqn Jaclaran), which presently has a level
of service of C (average). The estinated level of service of this nrcvernent is
not decreased by the addition of the project generated traffic. Other nrcvernents
renain at level of service A.
TEMPCRARY EROSICn.I GNIROL
Protection against erosion during construction will be provided,by a layered
approach utilizins 4 levels of defense. The first level will be the prevention
of soil uptake by running water by protecting exposed soils, limiting the length
of tinre the soils is exposed and following the guidelines given in the DE,
Technical Manual. The second level will be provided by snall' on-site silt
fencing, _check dars and possibly siltation ponds provided at each lot as
required by the Technical Manual.
The next level of defense will be provided by the grassy swale collection systenr
(or check dans prior to growth of grass) and a siltation pond to be provided as
each phase is constructed. These are necessary to ensure that the raxim"rn area
that can be served by a single siltation pond is not exceeded. The systern for
each phase wilt be designed in accordance with the Technical lilanual and
sutmitted along with construction plans and specifications for the streets and1 utilities. Check darm will be extensively utilized on steeper grades to prevent
harmful velocities frcrn developing. Where quarry spalls or hand placed riprap
is necessary, a thin layer of soil rnay be hand spread over the stones to provide
for eventual growth of grass for biofiltration purposes.
The final level of defense will be a snaller siltation basin provided
innpdiately prior to the detention pond. This pond is not required by currentpractice, but should be installed to ensure that flows entering the detention
pond are of the highest quality.
l,lo offsite runoff will be allosed except through the stornnrater collection
systar and detention pond. This is necessary to avoid short circuiting the
erosion control features.
Details of the erosion control devices will be in confornance with those
previousty approved by the City of Port Townsend.
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