HomeMy WebLinkAbout2h_Froom Drainage Master Plan_ MEMO
MEMORANDUM
Date: February 26, 2015
To: Victor Montgomery Organization: RRM Design Group
From: William Strand Title: Manager of Engineering
Project Name:
Froom Ranch Specific Plan (Il Villaggio)
Project Number: 1014012
Topic: Froom Ranch Specific Plan (Il Villaggio) Stormwater
Location and Existing Conditions
The Froom Ranch Specific Plan (Il Villaggio) project is located in the city of San Luis Obispo,
California on the west side of Los Osos Valley Road and approximately 600 feet northwest of
Highway 101. The project boundary is approximately 111 acres with approximately 76 acres
proposed to be developed. The site is underlain by soils with high clay content which are poorly
suited for infiltration (5.). The site is mainly undeveloped and used as range land with
approximately 4 existing buildings and dirt roads at the northern end of the site. An existing
drainage channel (Q100 = 102 cfs) runs south along Los Osos Valley Road conveying runoff from
upstream development through the property (1.). Froom Creek also flows through the site along
the northwestern and southeastern boundaries and has a 100-year flow rate of 1,066 cfs (2.).
Approximately 28 acres (37%) of the project area lies within the FEMA 100-year floodplain (Zone
A). The site has two existing stormwater basins that receive runoff from the adjacent Home
Depot and Irish Hills commercial sites. The basin serving the Home Depot development was sized
to retain a water quality volume equal to the 95th percentile capture volume of 1.27 ac-ft. The
detention basin serving the Irish Hills shopping center has a capacity of 2.03 ac-ft. (sized per
County of San Luis Obispo Standards to detain the 50-year storm while discharging at the 2-year
storm flow rate).
Proposed Development
Proposed improvements include the construction of approximately 55 acres of single family,
multi-family, and assisted living and 21 acres of commercial developments. Existing drainage
patterns along Los Osos Valley Road, and Froom Creek will be maintained. Froom Creek will be
slightly re-aligned and modified to contain the 100-year storm event; however, the channel will
still release at the historical point of discharge. A channel with a capacity of 102 cfs will be
constructed to convey flows parallel to Los Osos Valley Road. Above and below ground
stormwater basins will be constructed to provide storage for the existing 1.27 ac-ft. Home Depot
basin and 2.03 ac-ft. Irish Hills basin. Culverts and channels will be designed to provide capacity
for flows from the 100-year storm event. No upgrades to culverts at Highway 101 are proposed.
Stormwater Requirements
Stormwater requirements are listed in the Post-Construction Stormwater Management
Requirements for Development Projects in the Central Coast Region. The proposed
improvements to the Froom Ranch property will require compliance with Runoff Retention and
Peak Management. The design of this project will be driven by the Runoff Retention requirement
since this volume is greater than the Peak Management Volume.
Runoff Retention
Most of the proposed project area lies within Watershed Management Zone 1—requiring
capture of the 95th percentile storm event without runoff for water quality (3.). The 95th percentile
storm depth is 2.0 inches. Since harvesting/reuse and infiltration are infeasible, retention volumes
shall be multiplied by 1.2. The total required runoff retention volume is 10.91 acre-ft.
Sub-Basin
#
Depth
(inches)
Landuse Post-Dev.
Runoff
Coeff.
Dev. Area
(acres)
Capture
Vol. (ac-
ft.)
1 2.0 Commercial 0.75 20.95 3.14
2 2.0 CCRC 0.75 11.16 1.67
3 2.0 Residential 0.70 27.04 3.79
4 2.0 Residential 0.70 16.51 2.31
75.66 10.91 Total
Peak Management
Post-development peak flows discharged from the site shall not exceed pre-project peak flows
for the 2- through 10-year storm events (respective depths of 2.00” and 4.51”) (4.). The required
Peak Management volume is 7.71 acre-ft.
Existing On-site Retention Ponds
Sub-
Basin
Pre-
dev.
Runoff
Coeff.
Landuse Post-
Dev.
Runoff
Coeff.
Dev.
Area
(acres)
95th
Percentil
e Depth
(inches)
Detention
Basin
(Q-50 – Q-
2) ac-ft.
95th
Percentile
Capture
Vol. (ac-ft)
Home
Depot
0.45 Commerci
al
0.75 10.14 2.0 -- 1.27
Irish Hills 0.45 Commerci
al
0.75 18.62 2.0 2.03 2.33
Notes:
1. Per pg. V-6, Eagle Hardware EIR 10-1-2014
Sub-
Basin
#
Pre-
Dev.
Runof
f
Coeff
.
Proposed
Landuse
Post-
Dev.
Runoff
Coeff.
Dev.
Area
(acre
s)
2-Year,
24 Hr
Depth
(in.)
10-
Year,
24 Hr
Depth
(in.)
2-Year Peak
Manageme
nt Volume
(ac-ft)
10-Year
Peak
Manageme
nt Volume
(ac-ft)
1 0.45 Commerci
al
0.75 20.95 2.00 4.51 1.05 2.36
2 0.45 CCRC 0.75 11.16 2.00 4.51 0.56 1.26
3 0.45 Residential 0.70 27.04 2.00 4.51 1.13 2.54
4 0.45 Residential 0.70 16.51 2.00 4.51 0.69 1.55
75.66 7.71 Total
2. Per pg. 22, Revised Hydraulic Study Report Los Osos Valley Road/US 101 Interchange
Improvement Project. November 2010. San Luis Obispo County
3. Per Section B.4 Post Construction Stormwater Management Requirements for
Development Projects in the Central Coast Region
4. Per Section B.5 Post Construction Stormwater Management Requirements for
Development Projects in the Central Coast Region
5. Per USGS Web Soil Survey