HomeMy WebLinkAbout2023-01-17 Avila Ranch Basin Modifications (RR
MEMORANDUM
Date:
1-17-2023
To:
Hal Hannula
Organization:
City of San Luis Obispo – Supervising Civil
Engineer
From:
Robert Camacho
Title:
Principal
Project Name:
Avila Ranch
Project Number:
0661-02-LP17
Topic: Avila Ranch Basin A Update
Executive Summary:
Basin A, a bio-retention/detention basin, located in the southwest corner of Avila Ranch (Tract 3089)
Phase 1, near the corner of Buckley Road and Vachell lane, serves several stormwater management
functions:
• Retention – (95th percentile as defined in the State Post Construction Requirement Resolution
No. R3-2013-0032)
• Detention – (Peak Mitigation as defined in the Drainage Design Manual and outlined in the
approved Drainage Report)
Basin A was constructed as approved and is functioning as intended from a stormwater management
perspective. Although the stormwater retained in Basin A is not being absorbed into the soil
(“infiltrating”) at the pace anticipated, it only requires minor aesthetic modifications.
Background:
The location, size, function, and mitigation measures associated with Basin A were evaluated extensively
during the following planning review stages:
• EIR (Final Approved EIR June 2017)
• The Vesting Tentative Map for Tract 3089 (Prepared by Cannon – Approved by City of SLO)
• Preliminary Drainage Report (Prepared by Cannon – Approved by the City of SLO)
• Final Drainage Report (Prepared by RRM – Approved by City of SLO)
• Stormwater Control Plan for Phase 1 (Prepared by RRM – Approved by City of SLO)
• CLOMR and LOMR for Phase 1 (Prepared by RRM – Approved by City of SLO and FEMA)
• Improvement Plans for Phase 1 prepared by RRM / Oasis / Thoma / MKN – Approved by City of
SLO
Drainage Strategy:
The drainage strategy for Tract 3089, which includes Basin A, addressed five essential functions:
2
• Satisfy PCR 1, PCR 2 and a majority of PCR 3 (Site Design / Runoff Reduction, Water Quality
and Retention) requirements through the implementation of, reduced road widths, roadside
Bio Swales along Earthwood, Bio Filtration basins and the French drain collection system within
each motor court as shown in the approved Improvement Plans for Tract 3089 – Phase 1 and
the approved Stormwater Control Plan.
• Provide the PCR 3 retention volume for Drainage Management Areas (DMAs) as identified in
the approved Storm Water Control Plan.
• Provide PCR 4 Peak Management detention volume for the increased runoff generated from
the proposed development up to the 10yr – 24-hour storm event while discharging at the 10yr
pre-existing flowrate. The outlet flowrate is metered through a box outlet structure located in
Basin A.
• Meet the City of San Luis Obispo Drainage Design Manual criteria to provide detention volume
that mitigates (reduces) the site’s peak discharge flowrate at or below pre-existing condition
for all storm events up to the 100 year- 24-hour storm event.
• Provide over detention volume to accommodate the future development (Phases 2-6) as
identified in the approved VTM for Tract 3089 and in the approved Drainage Report for Phase
1 of Tract 3089. The design of Basin A’s discharge outlet provides for over detention allowing
for specific identified areas to discharge directly into Tank Farm Creek – after PCR 1, 2, and 3
have been achieved within each development area. This approach will ensure that the flows
encountered at the intersection of Buckley Road and Vachel Lane are at or below preexisting
condition.
Conclusion
It is the opinion of RRM, with concurrence of Wallace Group Engineering, that Basin A provides the
required onsite retention and flood control volumes (PCR 2, PCR 3, PCR 4, and DDM) to
accommodate the build out of Avila Ranch and is consistent with the EIR, VTTM, and Development Plan.
Recommendations
Inasmuch as the stormwater retained in Basin A does not infiltrate at the pace previously anticipated,
certain aesthetic modifications should be considered.
• Eliminate Biotreatment Soil Media (“BSM”) – the BSM layer in Basin A is no longer needed as
the water quality component is achieved upstream in a decentralized application via roadside
swales and Bio-retention basins throughout the site.
• Add Fencing – install fencing (with access gates for maintenance) to provide a safety barrier
given the proximity of homes and active children play area.
• Alter landscape - revise the landscape planting and irrigation in the lower portion of Basin A to
reflect the more frequent inundation and submergence (remove certain plant material, install
plant material that is more water hardy).
• Add a fountain to reduce the potential for stagnant water.
• Treat standing water as needed with Bacillus thuringiensis to reduce mosquitos and other
vectors.