HomeMy WebLinkAboutPD 2024-0012; CARRILLO ES PLAYFIELD REPAIR; CARRILLO ELEMENTARY SCHOOL PLAYFIELD - 24' SQUARE CATCH BASIN AT TOE OF EXISTING SLOPES; 2025-06-09SWS ENGINEERING, INC.
Civil Engineering I Lan d Surveying I Land Planning I Construction Staking
June 9, 2025
PN: Carlsbad# PD2024-0012, SWS#24-172
David Rick
City of Carlsbad
1635 Faraday Avenue
Carlsbad, CA 92008
RE: Carrillo Elementary School Playfield Repair -24" Square Catch basin at toe of existing slopes.
Dear David,
This letter will describe the sizing methodology and calculations used to design the 24" square catch
basin inlets in the brow ditch along the northeast and northwest play field perimeter at Carrillo
Elementary School.
The Hydrology Report by SWS Engineering, and approved by the City of Carlsbad, was created using the
Rational Method as outlined in the San Diego County Hydrology Manual dated June 2003 to determine
the runoff flow rates.
Per the approved Hydrology Report the existing slope runoff is collected in a county standard brow ditch
and conveyed to three 24" square catch basins. The "U" shaped section of the brow ditch transitions to
the flat 24" wide catch basin where the brow ditch flow terminates (images 1, 2, and 3).
The calculated discharge volumes at the catch basins are taken from the Hydrology Report (images 4, 5
and 6) and used for the sizing of the catch basin inlets. As the basin 800 has two catch basins the
discharge is divided by two (1.75cfs/2 = 0.875cfs). I am evaluating the worst-case situation at discharge
node 903 with a CFS of 1.399 and an allowable pond depth of 0.67ft (8" curb).
As stated in section 2.3.2.2 (Grated Inlets), of the County of San Diego Hydraulic Design Manual,
specifically Grated Inlets in Sag, "A grated inlet in a sag location operates as a weir at shallower depths,
and as an orifice at larger depths." Given these catch basins occur at the termination of the brow ditch
and surrounded by concrete barriers at a minimum depth of 8 inches, "splash over" cannot occur.
Therefore, the orifice condition will occur.
Weir Equation: Q = CwPed312 (Hydro Manual Equation (2-16))
Q = 3 x Length of opening with 50% clog x (ponding depth)312
Q = 3 X (2ft ~ 10/12 X 0.50) X .67ft3/2
Q= 1.36 CFS< 1.39 of calculated flow
1635 Lake San Marcos Dr. I Ste. 200 I San Marcos I CA I 92078
760-7 44-0011
SAN DIEGO NASHVILLE PHOENIX ORLANDO
• sws ENGINEERING, INC.
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Orifice Equation: Q=CA(2gd)112. Area of inlet openings per Image 7 and assuming 50% clog)
Q= flow ( CFS)
C= Orifice Coefficient of 0.67
A= Open Area of grate (FT2)
g= gravitational acceleration 32.2 FT /5/5
d= Depth of water over grate (FT)
Q = 0.67(1.61FT2)(2*32.2*0.67)112
Q= 7.09 CFS
Q(S0% CLOG)= 7.09CFS * 0.5
Q(S0% CLOG)= 3.55 CFS
Q = 3.55 CFS> 1.39 CFS therefore the inlet has sufficient capacity for calculated flow.
Image 1 -brow ditch termination where "U" shape transitions to match cath basin width
1635 Lake San Marcos Dr. I Ste. 200 I San Marcos I CA 192078
760-744-0011
SAN DIEGO NASHVILLE PHOENIX ORLANDO
--.~~ SWS ENGINEERING, INC.
Civil Engineering I Land Surveying I Land Planning I Construction Staking
Image 2 -Curb contained catch basin. 8" on the low side transitions to 12" deep brow ditch depth.
Image 3 -Curb contained catch basin. 12" deep in brow ditch. Transition from "U" shape to 2' wide
catch basin.
1635 Lake San Marcos Dr, I Ste. 200 I San Marcos I CA I 92078
760-7 44-0011
SAN DIEGO NASHVILLE PHOENIX ORLANDO
• SWS ENGINEERING, INC.
Civil Engineering I Land Surveying I Land Planning I Construction Staking
~
Image 4 -Post Development Map from Hydrology Report
SJ2. COO to Point/St«t:.:in
lMl?ROVED CHANNEL !AAV!:L TIME • • * •
Upstream point devation = 275.19:l(Fe.)
Down.stream point elevation = 214. 99C (Ft.)
thannel length thr:.i si.barea = 116.COC(Ft.)
Channel i:>ase width O. SOC (f't.)
Slope or 'z • of left channel ba..~k = l. ace
s:¢pe or 'z' oE right cha:md 'banlc: = l . 3c,
1'.ann:.ng's 'N' = C.015
Man::-d p-:.i-f ' . 0
o· q) .r ,1 J:. ::e.-: •
. cp-h of" ow= '.1.654("-::.), Average ve-oc::.ty = l.98~!Ft./.s)
Channel f l'.ow -eop w:. 't. • 2C.!.{!"t.)
F!.ow ve:cc1t.y = l. 9S (Ft/sl
Travel ti:r,e = :>. 98 min.
'!in:e of concent:i:ation = 5.61 :e:.n.
::r:.tica: depth = 0. 48', (!"t.)
Image 5 -CivilD calculation at node 803 from Hydrology Report
1635 Lake San Marcos Dr. I Ste. 200 I San Marcos I CA 192078
760-744-0011
SAN DIEGO NASHVILLE
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PHOENIX
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• sws ENGINEERING, INc.
Civil Engineering I Land Surveying I Land Planning I Construction Staking
P:c.ceas from Pcint/St&tJ.c:·n 902. 000 tc Point/Station
••• • ;.-aROVEO Cl!Al\"NEL TRAVEL TIME •u•
Up1tr«.1L":: point elevation = 277. 83<l (Ft.)
Ocn,-natre&.T. pc!.nt elevation = 270. 710 (!'t.)
Channel length thr:i ai:barea = 3€2. 000 (Ft.)
Channel l:aee width = 0 . 500 {Ft. )
Slope or 'Z' cf left er.&nnel ba.nit = l,300
Slope or 'Z' cf r:.ght channel ba.nk = l. !CO
V.&!".::.i::g'.r. 'l,' = C.015
xim .) = . :·;;; :r:,
,& II" verage velocity= 4.E2B(Ft/a)
Cha.nr.el flow top width. = l . SC C Ft . }
Flow Velocity= ,Q,63{Ft/a}
Travel time = 1.30 min,
Tir.:.e of coiicentrati.on = S. 94 rt.in.
Criti.cal depth= C.4l4(f't.l
Image 6 -CivilD calculation at node 903 from Hydrology Report
A 24-inch square catch basin inlet area typically has an open
grated area designed for water entry and to prevent debris
from entering the drainage system. The open area of a stan-
dard 24-inch square.grate is around 231.69 square inches,
according to some manufacturers. The flow rate of water that
can pass through this area can vary depending on the spe-
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cific grate design and material, but some manufacturers estimate a flow rate
of around 708 .. 77 GPM. @
Image 7 -google search of a standard "open area" of a 24" square catch basin.
1635 Lake San Marcos Dr. I Ste. 200 I San Marcos I CA I 92078
760-744-0011
SAN DIEGO NASHVILLE PHOENIX ORLANDO