1990-571 G/R/J/V
Street Address
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Category
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Serial #
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Name
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Description
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Plan ck. #
Year
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HYDROLOGY EVALUATION
FOR
SUtJMIT AVENUE PROJECT'
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$eptember 20,1988
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DEC 2 6 1991 ~
CITY OF ENCINiTAS
DEPT. OF PUBLIC WORKS
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FOR
SHEro~J}-STEVENS. INC.
ENGINEERING DEPT.
by
June Applegate.
California RCE 30195
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2531 STATE STREET. CARLSBAO. CALIFORNIA 92008 (619) 729-7109 1-\ ~
4001 LITTLE APPLEGATE ~OAD, JACKSONVILLE. OREGON 97530 (503) 899-8933
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JUNE APPLEGATE & ASSOCIATES, CIVIL ENGINEERS
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HYDROLOGY EVALUATION FOR SUMMIT DRIVE
This project Is located on ~a East side of Su~lt ~enue. t~ lots
south of Summit place. The e.lstlng site Is cover~ wl~ ~.ndoned
greeMouses. These w111 all be removed, ~. land w111 b. re-graded
to aèc~odata nine new single f~l1y residences. an open space lot
and a private street. .
This report presents a h~rology comparison beueen ~e existing and
proposed condition of ~Is site. an Inurcepwr dlt~ design study,
and retentton basin design and maintenance reco~endations. .
CRITERIA:
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U.lng ~o Coun~of San Diogo'. Hydrology manual. ~e antlc1p~ted
hyd~loglc ~aracterlstlc. and response of ~e existing site ~hould
be compared with the hydrologic characteristics and response
.ntlclp.t~ fr~~. p\ans provided by Pasco Engineering. The
~ancurlsticS to be co~ared are su~asln overtand times and
travel times, runoff coefficients, and diversion.
To assure ~nl~m I~.ct to downstre~ properties. ~e su~.sln .
characteristiCS for ~e existing and proposed sites should be
compared. Ideally, ~e site can maintain similar characteristics,
If the time of concentration decreases or the runoff factor
increases the proposed site will contribute more runoff.
The county manual was also used to establish the 'CO-year storm
value for the in~rceptor ditch. Standard engineering h~raulic
calculations dete~lned the ditch capacitY. Although ~e ditch Is
design to better percolate runoff water than concrete ditches. ~e
capacl~ Is determined very conservativelY by p~vldlng l~year
capacl~ above the surface of the ditch. It Is not anticipated that
~e ditch will ever have to carry ~e full capacity ca1cu1ated.
To determine percolation time for the basin proposed at. the open
space 'at. the Sol1s Engln.er for the project. A. R. Barry &
Associates. perfo~d percolation tests at one foot bel~ ~e
anticipated botwm of tM basin. Since ~ese tests were run In~.
su~ertl~. ~e soils engineer added a 10~ margin of safety to
better reflect worst anticipated winter conditions.
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,!....E APPLEGATE & A8cIATES, ~IVIL ENGINEERS
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EVALUATION, ..
The neighborhood has no drainage outlet. It is a basin which was'
probably created when ancient sand dunes closed off the mouth of the
drainage course. The existing low point of the neighborhood basin
is on the project property. In other words, this site is downstream
from all the neighbors. The project proposes an excavation in the
opens pace. The volume of the excavation is equal to the volume of
the fill into the theoretical 10o-year inundation. (See Pasco'
report.) .
This openspace area will.continue to have clear drainage paths from
the neighborhood. Drainage will not be blocked. The drainage from
the south and east will continue to cross the property line directly
into the basin. Drainage from the north will travel in a proposed
...-......---'.- rock-l1ned,.ditch ,directly into the excavation. "
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The property slopes downward. to the east from Summit at an estimáted
seven percent. The bottom of the neighborhood basin is on the south
side of the property at the 'location of the proposed openspace
basin. This bottom of basin. is very flat. The flow line of which
falls at a grade less than ~ñe percent. The plans propose nine
relatively flat pads for the nine homes. Drainage around the houses
will be at a grade of one percent. The runoff water will be '
directed to,the p~ivate street, which slopes downhill to the
openspace. "
The anticipated time of concentration for the existing condition was
determined by use of the overland formula. The time of '
concentration for the exiting condition thus determined was 15
minutes.
For the proposed project, the anticipated time of concentration
determined, using the urban overland 'nomograph will be approximately
18 minutes. The reason for the increased amount of time is because
the relatively flat (1%) building pads require a longer overland
flow time.
It is anticipated that the ~unoff factors will be reduced. Past use
of the site has been commercial. Proposed use is single family
residential. The soil hydrologic type is "A". The runoff factor
will reduce from 0.70 to 0.49 in the proposed condition. .
The County's manual does not provide runoff coefficients for the
existing ~ondition of abandoned greenhouses. It does however have
soil conservation service curve numbers for commercial, medium
density residential and barren land. For "A" type soil, commercial
curve number is 89, medium 'density residential is 73 and barren is
78. These 'numbers indicate that medium density residential will
. provide better infiltration of the rainwater into the ground.
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fL APPLEGATE (_OCIATES, CIVIL ENGINEERS
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PROPOSED INTERCEPTOR DITCH
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Sherman-Stevens, Inc. proposes to create an interceptor ditch which
will run easterly along the northerly property line and southerly
along the easterly property line.
The proposed width of the ditch is 2 feet ,and the proposed depth is
1 foot. There will be 2:1 slopes rising from both sides of the
ditch. At a depth of 2.3 feet, falling at 1%, the ditch will carry
28 cubic feet per second at a velocity of 3.8 feet per second. This
is the value of the anticipated 100-year flow for thi,s ditch.
Even though this low velocity does not dictate protection of the
ditch, lining is still recommended. Vegetation or rock-lining to
,retard the transportation of, colloidal materialto,the'openspace"is' ,..,
recommended. (Colloidal material will clog and hamper the
percolation capacity of the openspace excavation.)
A sketch of the geometry.of the channel is included. Vegetative
lining is one option presented.
A sketch for optional rock lining is also ,included in this report.
The Frenchdrain'under the interceptor ditch should be lined with a
filter fabric. The filter fabric should be between the gravel and
the so11. .
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The gravel of the French drain is for the purpose of slowing runoff
and allowing it to percolate into the ground, as it would in a ,
natural channel. The usual perforated pipe seen in French drains
would only speed the drainage and be counter-productive for our
purposes. As ~entioned above, capacity is not a problem here.
The filter cloth will be necessary to prevent piping of the soil
around the French drain. Without the cloth, the smaller soil
particles could be carried by water into the French drain and then
carried away.
During construction, the gravel of the French drain should not be
placed against soil. The gravel should be gently (so to not tear
the fabric) placed against the filter fabric.
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.ONE APPLEGATE & ¡.IATES:~IV1L ENGINEERS
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Using San Di~o Coun~ p~cedu~s. percolation rates of 20 minutes.
per inch were determined at a depth of 5 feet ('foot lower than the'
anticipated basin boU~ by ~e So11s Engineer. A. R. Barry &
Associates. ~un~ procedures requt~ a v~tat1on be~een tests w
be less ~an 10 pe~ent. This assures a reason~le srluration of
~e area test~. H~~er. l~er rrles are antlclpat~ due ~ small
quantities of colloidal material delivered by the runoff water.
Projected r"es are ~ minutes per inch. (If c10gging increases ~e
percolrlion time w unacceptable levels. maintenance as o~l1ned
below ~s recommended.) ,
~, . Using on1y the openspace, the area for perco1~tion is 11,000 square
..--"'..' .-' ..--feet.. . (ObvIously. . perco lot Ion. wIll .1 so occur outsIde .of. the" - .. .-~- ...- .-
f e.c~rllonln addition to this calculated a"a;) Pasco Engineering
J . detemlned. total volume for ~e 10~year swm w be 1.24 .cre-
.. feet. .
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OPENSPACE EXCAVATION
1.24 acr~feet perco1attng at the rate of 30 minu~' per inch over
en ereeof 11.000 square feet wi11 take 29 hoursw soak into ~e
. ground..~ '.
RECOMMENDATIONS FOR OPENSPACE
The depth of the openspace basin should be maintained as shown .on
we plans. If filling occurs, We area should be re-excavrted to
the indicated elevations.
Should clogging occur which decreases the percolation rate to'an
unaccept~le level, ~e first step for maintenance is ~ n~ve ~e
sod in the bottom of thè openspace.
If additional restoration is required, seepage pits can be
exc~.ted, It ~uld be prefer~le to only extend the pits 3 feet
past the bottom of the basin. At the 1eve1 4 feet be1ow the bottom
of the basin. a v.~ porous possible aquifer has been found.
Bank erosion due W water entering ~e openspace f~ the south and
from the east should be prevented. Flexible vegetated protection
su~ as will~ waddling m~ be possible. A biologist e"perieneed in
such fo~s of erosion protection should be consul~d. H.roer
solutions are abo possible. such as cu~off walls. crib walls and
small steps. The total drop is only four feet, so in no case are
massive d~p structures needed. It is feasible ~ attain ~e drop
with four sma", one-foot drops.
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)flie APPLEGATE & AS'?iATES, èlV\L ENGINEERS
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INTERCEPTOR DITCH
FOR
S Ut+111
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OPTIONAL ROCK LINING
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,.HE APPLEGATE & AjPIATES. C1VIL ENGINEERS
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. INTERCEPTOR DITCH
FOR
S\Jtllt1IT
WITH
OPTIONAL VEGETATION LINING
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.;.NE APPLEGATE & ASSOCIATES, CIVIL ENGINEERS
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CONCLUSIONS
Changes to the hydrology of this site need no mitigation. There is
no diversion, no decrease in time of concentration nor increase in
runoff factors. So, it is anticipated that runoff will not in-
crease. .
The proposed rock-lined'interceptor ditch/French drain has more thän
enough capacity for the 10o-year storm. Velocities are low enough
so no bank erosion protection is required.
The open space excavation will provide the volum~ of water storage
taken up by the project's fill during the 100-year inundation. The
.....,-_. -. -.-... excavation should increase the neighborhood basin absorbtion rate. -
slightly.
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EXISTING CONDITION
- OVERLAND FLOW:
l .. 380 FT .
H eff . 10 FT
- 3 .385
Tc" (11.9 x L I H) . 0.05 HRS.. 3.1 MIN.
COUNTY HYDROLOGY MANUAL'ALLOWS FOR AN ADDITION
10 MINUTES FOR NATURAL WATERSHEDS 13 MIN.
NATURAL FLOW LINE 230' AT 2 FPS.. 2.0 ,MIN
0.07 MILES
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URBAN OVERLAND FLQW
. '. L . 170 1.0%
I L .. 300 1.8%.',3.5 FPS 0
L .. 80 12.0% ~,. 7 FPS
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15 MIN., BASIN TIME
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16.5 MIN.. Tc OVERLAND
1.5 MIN., Tt GUTTER 0
0.0 MIN., Tt GUTTER
18 MIN., BASIN TIME
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..IIe APPLEGATE Ie A CIATES. CIVIL ENGINEERS
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DITCH
100 YEAR FLOW VALUE:
URBAN, SOIL TYPE A, RUNOFF COEF. C'.
. OVERLAND FLOW:
L. 1600 FT.
H eff - 27 FT
0.40
0.30 MILES
3 .385
Te - (11.9 x II H) - 0.18 HRS. 11.0 MIN.
10o-YR RAIN INTENSITY, I 100. 3.9 "/HOUR
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TRIBUTARY AREA, A-
6.0 ACRES
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10o-YR QUANTITY OF FLOW, Q,~OO. CI~-
9.4 CFS
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DITCH:
D . 2.27.'
d - 1'.
s - 0.01
SIDE SLOPES.
D-l a
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.2 : 1
A - 3.14/2 + 2(D-1) + 2(D-1)1 -
P - 3.14 + SQRT(5(D-1» -
2.67 0.5
. 1.486 A s
Q . , .67- a
n P
V - Q/A - 3.4 FPS
5.6 SF
7.6 I
19 CFS
WHICH IS LESS THAN 6 FPS, NO LINING REQUIRED
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APPENDIX XI
IV-A-14
Revised 1/85
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