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HomeMy WebLinkAbout2022-12-23 Avila Ranch Basin Modi MEMORANDUM Date: 12-10-2022 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 The intent of this memo is to confirm that Basin A, a Bio-retention/detention basin, located in the southwest corner of Avila Ranch Phase 1 (Tract 3089), near the corner of Buckley Road and Vachell Lane was constructed as approved and functioning as intended. Executive Summary: Basin A 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) As detailed herein, Basin A was constructed as approved and is functioning as intended. The fact that the water retained in the basin is not infiltrating at the pace anticipated, only requires some minor aesthetic modifications, including, but not limited to, relocating landscaping to an area above the high- water mark, the installation of fencing for safety purposes, and the inclusion of water circulation improvements to protect against stagnant water. Background: The location, size, function, and final 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) · Phase One Stormwater Control Plan (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 prepared by RRM / Oasis / Thoma / MKN – Approved by City of SLO) 2 Drainage Strategy: The drainage strategy for Tract 3089, which includes Basin A (see Exhibit A below for Basin A cross section), addresses (5) essential functions: 1) 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 plans for Tract 3089 – Phase 1 and the approved Stormwater Control Plan. 2) Provide the PCR 3 retention volume for Drainage Management Areas (DMAs) as identified in the approved Storm Water Control Plan 3) 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 the box outlet structure that is located in Basin A (See Exhibit B below for Basin outlet detail) 4) Meets 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. 5) 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. Exhibit A 3 Exhibit B Phase 1 Improvement Plans, including Landscape Plans, were developed and approved as a byproduct of the Drainage Strategy. The Landscape Plans were also designed to be consistent with the approved conceptual landscape plan shown in the Specific Plan, which provided a Bio-retention area, hydroseed turf area, and play area. (See the approved plans prepared by Oasis Associates) These approved Improvement Plans included detailed planting, irrigation and site improvement plans for the play area which is east of Basin A, including a children’s play equipment area for 5-12 year old’s, a swing set, ½ basketball court area, tables for family gathering space and 2 points of connection to the bike path that adjacent to the existing creek. Phase 1 Grading and Improvements During the 2020 mass grading operations ground water levels were discovered to be significantly higher than the ground water levels noted in Geosolution’s 2015 soils investigations. The soil borings from the 2015 soil investigations indicated that ground water near Basin A was found at elevation 88-89. 18 ft below the existing ground surface (prior to grading activity) or at elevation 88-89 (see image below for boring log from GeoSolutions) in contrast to the 2020 grading operations which noted ground water at elevation 90+. Ground water was higher in other locations as grading operations moved in a northeasterly direction. 4 PCR 3/4 & DDM Compliance Subsequently in 2021 RRM and Wallace Group worked together at the request of Wathen Castanos to evaluate the functionality of Basin A, including confirmation that the basin provides PCR 4 and the City of San Luis Obispo’s DDM compliance. The following (3) conditions were evaluated to verify functionality of Basin A Condition I · Basin A has a maximum discharge of 77 cfs while over-detaining during the 100 storm – post development condition · Elevation at the bottom of Basin A of Bio Soil Media = 93 · Bottom of Rock section in Basin A = 89 · With no significant increase in flowate (Qcfs) near culvert @ Buckley Road or increase in water surface elevation downstream of the Avila Ranch Development Condition II · Basin A will be over-detaining during the 100 storm – post development condition · Elevation at the bottom of Basin A becomes 98.5 · Tail water surface elevation of 102 · With no significant increase in flowate (Qcfs) near culvert @ Buckley Road or increase in water surface elevation downstream of the Avila Ranch Development Condition III · Basin A will be over-detaining during the 100 storm – post development condition · Elevation at the bottom of Basin A becomes 96.5 · No tail water at outlet · With no significant increase in flowate (Qcfs) near culvert @ Buckley Road or increase in water surface elevation downstream of the Avila Ranch Development 5 During a 100 yr 24-hour storm event (assuming no volume in the roadside basins or motorcourts) the water surface elevation in Basin A would raise to 103.8 which is still below the Basin Overflow weir elevation of 104. After inputting these values into the Hec-Ras model the results just downstream of the outlet and along Buckley Road indicated that there is less than a 0.005’ increase to the water surface elevation in Tank Farm Creek downstream of the outlet and therefore meets the intent of the design shown in the approved plans of having no impact directly down stream of Avila Ranch. (see results comparison below) Results comparison Results for Condition 1 - Original Model indicated water surface elevation is still 102.03 (Q-100 @ station 266.60 (near the existing culvert @ Buckley Road = 879 CFS) Results for Condition II - Overly conservative Model indicated water surface elevation is still 102.03 (Q – Station 266.60 = 911.77 CFS) RRM also ran another version assuming a Basin A water surface elevation of 96.5 as described in Wallace Group’s summary. Setting the bottom of the basin elevation at 96.5, to mimic the standing water condition, the peak discharge reduces to 85.90 and the water surface elevation is at 103.47 which is below the bike path weir of 104. (See below) This does not take into consideration the addition volume recaptured in the motor courts or roadside basin, which would further reduce the peak discharge and the water surface elevation. Results for Condition III - Results from Hydraflow Hydrographs Extension for AutoCAD Civil 3D assuming Basin bottom 96.5 6 As shown in the Hydraflow Extension results above, the peak discharge flowrate from Basin A would be 85.90 cfs during the 100yr 24-hour storm event. This is less than a 9 cfs increase from Condition I or 1% increase seen at Station 266.60 which is less than significant. Conclusion Based on these studies performed by RRM and evaluation by Wallace Group, the design in the approved plans prepared by RRM provides the necessary 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. By adding the items listed in the recommendation section below to Basin A, Avila Ranch Development will still be providing a decentralized approach by incorporating multiple Source Control Measures throughout the site for PCR 2 and PCR 3 compliance, which has been reviewed and approved by the city. It is our opinion that these modifications are defensible to the Regional Board. The proposed modification will still allow for direct infiltration to occur once the surface water levels have lowered, especially in the motorcourts and in the upstream basins. Recommendations Although Basin A meets the applicable design standards and over detention for Tract 3089, the following modifications to address aesthetics should be implemented in Basin A: 1) Eliminate BSM – the grading contractor installed the 24” drain gravel layer in the bottom of Basin A during the grading phase, but the 24” BSM layer is no longer needed inasmuch as the water quality component is achieved upstream in a decentralized application via roadside swales and Bio-retention basins throughout the site. 2) Add Fencing – installation of fencing (with access gates for maintenance) to provide a safety barrier given the close proximity of homes and active children play area 3) Altering the Landscaping – revise the landscape planting and irrigation in the lower portion of Basin A to reflect the more frequent inundation and submergence. 4) Adding a fountain – to help keep water moving to assist with maintenance of the basin and water quality during the time that there is standing water in the bottom of the basin. This small system will increase the level of oxygen within the pond which will support microbial population growth. These microbes break down nitrates and phosphates which significantly helps reduce algae growth. 5) Treat standing water with Bacillus thuringiensis (“Bt”) – to avoid mosquitos and other vectors, standing water should be treated with Bt on an as needed basis. 6) Treat standing water with Barley Straw (or approved equal) as the straw bales slowly decay, the straw releases enzymes that in maintaining water clarity without harmful chemicals. This will be done on a as needed basis.