HomeMy WebLinkAboutEarthwood-Ball Trust Memo-09-27-2022
MEMORANDUM
Date: 09-26-2022
To: Hal Hannula (Supervising Civil Engineer)
Organization: City of SLO
From: Robert Camacho
Title: Principal
Project Name: Avila Ranch
Project Number: 0661-02-LP17
Topic: Ball Trust’s property overland escape route for the 100 yr storm event @ Earthwood
R.O.W.
Based on the Approved plans and the Approved Drainage Report for (Tract 3089) Avila Ranch Phase One,
the existing peak runoff from the Ball Trust property (Tract 2943) and the portion of Earthwood Right
of Way that is just north of the Avila Ranch Subdivision, is to be routed to Tank Farm Creek.
Per the approved plans there is a constructed low point in Earthwood near the shared tract boundary
for Tract 3089 and Tract 2943. At this low point there is a storm drain point of connection for the Ball
Trust Property (Tract 2943) and ensures 1% min street grades in Earthwood.
(See Appendix I) for the approved plan and profile of Earthwood Lane demonstrating the low spot near
the northern tract boundary of Tract 3089, the catch basins and storm drain stubs are located near
station 38+53.91.
Based on the approved drainage report for both tracts the estimated runoff from the Ball Trust
Property is 54 cfs during the 100-yr storm event. Per the approved plans prepared by GTA the
proposed basin for Tract 2943 is designed to detain up to the 100-yr post development flows while
metering the discharge flowrate at or below preexisting conditions through the use of a concrete
structure (see Appendix II)
If the basin concrete headwall and the inlets in Earthwood near the northern boundary for Tract 3089
were to fail, then the 13’ wide drainage easement will provide an overland escape route for the
estimated 100-yr flow rate. As shown in the grading exhibit (see Appendix III) there is a drainage
channel along the north side of lot 186 and the west side of lots 186 & 190 of Tract 3089 that will allow
for the safe overland flow of the 100-yr flow rate.
The basin in Tract 2943, prepared by GTA, is equipped with a 69’ +/- wide concrete weir in the event
the primary outlet fails that will convey the stormwater into the 13’ drainage channel. The
owner/developer of Tract 2943 will need install “Light” RSP to ensure soil stability and energy
dissipation prior to discharging into the swale located on Tract 3089.
See Appendix IV for cross sectional data of the proposed drainage channel. As the water flows in a
southerly direction the water surface elevation will need to be at approximately 114.98 prior to flowing
into Venture Right of Way and turning into a sheet flow condition.
As discussed in our site meeting with city staff and both developer/owner representatives the water
surface elevation in the swale and “Ball Trust” basin will need to reach an equilibrium at elevation 114.98
prior to discharging into Venture, if the primary outlet fails. As shown in the plans prepared by GTA
the Top of the basin is 116 and the Pad elevations of Lot 186 and 190 of Tract 3089 are 115.2 and
116.40 respectively. This will ensure that the FF will be a min of 1’ above the estimated 100-yr water
surface elevation.
Based on the calculations outlined above the anticipated overland escape route is consistent with the
parameters outline in the City of San Luis Obispo Drainage Design Manual.
Sincerely,
Robert Camacho P.E.
Appendix I
Approved Earthwood Street Profile
Appendix II
Approved for Tract 2943 prepared by GTA
Appendix III
Grading and Drainage Exhibit for Drainage Channel
Appendix IV
Drainage Channel cross sectional design
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Tuesday, Sep 13 2022
Avila Ranch By-Pass
Trapezoidal
Bottom Width (ft) = 2.50
Side Slopes (z:1) = 2.00, 2.00
Total Depth (ft) = 3.00
Invert Elev (ft) = 10.00
Slope (%) = 0.65
N-Value = 0.030
Calculations
Compute by: Known Q
Known Q (cfs) = 54.00
Highlighted
Depth (ft) = 1.96
Q (cfs) = 54.00
Area (sqft) = 12.58
Velocity (ft/s) = 4.29
Wetted Perim (ft) = 11.27
Crit Depth, Yc (ft) = 1.63
Top Width (ft) = 10.34
EGL (ft) = 2.25
0 2 4 6 8 10 12 14 16 18 20
Elev (ft)Depth (ft)Section
9.00 -1.00
10.00 0.00
11.00 1.00
12.00 2.00
13.00 3.00
14.00 4.00
Reach (ft)