HYDROLOGY STUDY LA MIRADA BOULEVARD La Mirada, California

Transcription

1 HYDROLOGY STUDY LA MIRADA BOULEVARD La Mirada, California TTM Prepared for: The Olson Company 3010 Old Ranch Parkway, Suite 100 Seal Beach, CA Contact: Mr. Aaron Orenstein (562) Property: La Mirada Boulevard La Mirada, California APN: Prepared by: C&V Consulting, Inc Burbank Foothill Ranch, California (949) Contact: Mr. Ryan Bittner, P.E. & Mr. Gerardo Garduno, Project Manager March 2015

2 TABLE OF CONTENTS I. Acknowledgement and Signature Page...1 II. Introduction & Methodology Site Location Map...2 III. Design Criteria IV. Conclusion..4 V. References.. 5 Supporting Calculations Existing Condition Area A: 10-Year Storm Event...7 Existing Condition Area A: 25-Year Storm Event...8 Proposed Condition Area A: 10-Year Storm Event...9 Proposed Condition Area A: 25-Year Storm Event...10 Proposed Condition Area B: 10-Year Storm Event...11 Proposed Condition Area B: 25-Year Storm Event...12 Proposed Condition Area A: 85 th Percentile, 24-hr Storm...13 Proposed Condition Area B: 85 th Percentile, 24-hr Storm...14 Street Capacity Analysis...15 Exhibits Year 24 Hour Isohyet Map 1-H1.20 Showing Site Location 2. Hydrology Map (1) Existing Development 3. Hydrology Map (1) Proposed Development

3 HYDROLOGY STUDY La Mirada Boulevard C&V Consulting, Inc. Hydrology Study For La Mirada Boulevard and Granada Avenue La Mirada, California TTM ACKNOWLEDGEMENT AND SIGNATURE PAGE This Hydrology Study has been prepared by C&V Consulting, Inc. under the supervision of Ryan Bittner, P.E. Ryan Bittner, P.E. C&V Consulting, Inc Date 1

4 HYDROLOGY STUDY La Mirada Boulevard C&V Consulting, Inc. Introduction / Methodology: The project site consists of approximately 2.72 acres of generally flat land which is currently zoned R (A)(F) Single Family Residential with two overlay districts, (A) Farm Animal and (F) Flood Hazard. The project site falls under the Low Density Residential category of the City of La Mirada s General Plan Land Use Policy Map, Figure LU-2 which allows for a maximum density of 6.0 units per acre. We propose the development of 33 residential condominiums, a density of 12.1 homes per acre. Through entitlement, a zone change and general plan amendment will be processed. The subject project is located at La Mirada Boulevard, in the City of La Mirada. North of the project site is the Imperial Convalescent Hospital. La Mirada Boulevard borders the site on the west. There are Single-Family Residential homes which border the site to the east and south. Figure 1 Site Location Map 2

5 HYDROLOGY STUDY La Mirada Boulevard C&V Consulting, Inc. Pre-Development Condition: Under the existing condition, the onsite runoff generally sheet flows in a south and southwesterly direction towards La Mirada Boulevard, then south along La Mirada Boulevard into an existing off site catch basin which diverts the runoff to the existing storm drain infrastructure in La Mirada Boulevard. The existing 27 RCP storm drain system in La Mirada Boulevard is owned and maintained by the City of La Mirada. Ultimately, runoff entering this storm drain system within La Mirada Boulevard is discharged into Milan Creek which connects to Coyote Creek- North Fork, then converges with Coyote Creek which connects to the San Gabriel River Reach 1 (Estuary to Firestone) then the San Gabriel River Estuary prior to discharging to the Pacific Ocean. Currently the site consists of a two-story building (the La Mirada Friends Christian Pre-School) on the west side and a one story building, temporary building and shed on the east portion of the site. The remainder of the site contains parking areas, drive aisles, landscaped areas and playground facilities. Access can be made from La Mirada Boulevard. The existing site is approximately 57% impervious surface. The site is projected to have 80% impervious surface. Proposed Condition: The proposed development will consist of 33 condominium units consisting of 18 buildings and associated improvements (internal roadways, walkway and landscaping). Overall, the site is estimated to be 80% impervious. The onsite runoff will flow towards the curb and gutter located on each side of the main drive aisle. The flow will be conveyed in a southwesterly direction towards the two on-site catch basins which will connect to the existing storm drain system in La Mirada Boulevard. Based on a site visit and the plans provided by the City of La Mirada, there is an existing catch basin located south of the southwest corner of the project site that connects to an existing 18 Reinforced Concrete Pipe (RCP) storm drain main within La Mirada Boulevard. The 18 RCP main flows south in La Mirada Boulevard and connects to an existing 27 RCP storm drain main. Additionally, per plan P.C. 3658, provided by the Los Angeles Department of Public Works (LACDPW), there is an existing 6 storm drain easement that is located south of the southerly property line. For water quality treatment requirements two Biofiltration Systems are proposed to intercept the first 0.9 of rainfall, the 85 th percentile 24-hr storm event. A Standard Urban Stormwater Mitigation Plan (SUSMP) has been prepared according to the City of La Mirada standards to address water quality calculations. 3

6 HYDROLOGY STUDY La Mirada Boulevard C&V Consulting, Inc. Design Criteria: 1. According to the Los Angeles County Department of Public Works Hydrology Manual 50- Year 24-Hour Isohyet Map 1-H1.10, the drainage area is located in Soil Group 009, the site receives 5.8 inches of rainfall over a 24-Hr storm (Q 50 ). 2. The LACDPW Time of Concentration Calculator Program was used to determine the Tc, run-off flow, run-off volume for site, and the first 0.9 run-off from site (85 th Percentile 24- hr Isohyet). 3. The site was analyzed for a 10 & 25-year storm event per the requirements of the January 2006 Los Angeles County Department of Public Works Hydrology Manual. A Rational Method Analysis was performed and the appropriate calculations are provided herein. 4. The drainage areas for the proposed development were determined based on the proposed street/land flow areas and the existing offsite topography. Based on the available information, off-site flows to the project are not anticipated at this time. 5. The length of the flow path used to calculate the time of concentration was the longest runoff distance to the point of discharge for that sub area. 6. The slope of the flow path used to calculate the time of concentration was determined based on the total elevation change across that flow path per the proposed grades. 7. The percent impervious area for the existing condition was calculated based on the existing topographic map and photos from the site. 8. The percent impervious area for the proposed condition was calculated based on the proposed site plan. 9. The Hydrology Map attached to this study is made part of this study. 10. The rainfall frequency multiplication factors used to estimate peak runoff for additional storm events were taken from Table of LACDPW Hydrology Manual. Conclusions: The results from this hydrology and hydraulic analysis demonstrate the following: The drainage design for this site meets or exceeds the level of urban flood protection as described in the Los Angeles County Department of Public Works Hydrology Manual. The estimated 10-year, 24-hour peak flow of the proposed development is 4.42 cfs. The estimated 25-year, 24-hour peak flow of the proposed development is 5.88 cfs. 4

7 HYDROLOGY STUDY La Mirada Boulevard C&V Consulting, Inc. Flow rates increased, approximately 3.5% in the proposed conditions due to an increase in impervious area in the proposed condition. Existing Flow Rates (cfs) 10 Yr Storm Event 25 Yr Storm Event Area A Total Proposed Area A Area B Total Existing & Proposed Impervious vs. Pervious Table Size % % Size Impervious Pervious Pre-Development 1.56 ac 57% 1.16 ac 43% Post-Development 2.18 ac 80% 0.54 ac 20% Refer to the Exhibit 2 and 3 for Existing and Proposed Hydrology Maps in this report for information pertaining to land use and imperviousness coverage. Regarding design capture volume and post-development water quality information, refer to separately prepared SUSMP report by C&V Consulting, Inc. dated March 2015 for additional information. REFERENCES 1. Los Angeles County Department of Public Works, Hydrology Manual, January Los Angeles County Department of Public Works, Tc Calculator Excel Spreadsheet. 3. Hydraflow Express Extensions for Civil 3D Preliminary Grading Plan for this project by C&V consulting. 5

8 HYDROLOGY STUDY La Mirada Boulevard C&V Consulting, Inc. Supporting Calculations 6

9 C&V Consulting, Inc. Hydrology Calculation Sheet Existing Condition Area A 10-Year Storm Event 7

10 C&V Consulting, Inc. Hydrology Calculation Sheet Existing Condition Area A 25-Year Storm Event 8

11 C&V Consulting, Inc. Hydrology Calculation Sheet Proposed Condition Area A 10-Year Storm Event 9

12 C&V Consulting, Inc. Hydrology Calculation Sheet Proposed Condition Area A 25-Year Storm Event 10

13 C&V Consulting, Inc. Hydrology Calculation Sheet Proposed Condition Area B 10-Year Storm Event 11

14 C&V Consulting, Inc. Hydrology Calculation Sheet Proposed Condition Area B 25-Year Storm Event 12

15 C&V Consulting, Inc. Hydrology Calculation Sheet Proposed Condition Area A Volume for first 0.9 inch run-off 85 th Percentile 24-hr Isohyet ( 13

16 C&V Consulting, Inc. Hydrology Calculation Sheet Proposed Condition Area B Volume for first 0.9 inch run-off 85 th Percentile 24-hr Isohyet ( 14

17 C&V Consulting, Inc. Hydraulic Calculation Sheet Street Flow: Analysis was performed to inspect the surface flow capacity of the proposed internal roadway before flooding would occur outside of the roadway improvement. The following calculation depicts the water surface elevation conveying the 25-year storm event along the typical street section. A flow rate of 5.88 was used in this calculation consistent with the largest flow rate being conveyed within the site. 15

18 Exhibits

19 Exhibit 1 50-Year 24 Hour Isohyet Map 1-H1.20 Showing Site Location

20 Exhibit 2 Hydrology Map (1) Existing Development

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22 Exhibit 3 Hydrology Map (1) Proposed Development

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