Preliminary Drainage Analysis

Transcription

1 Preliminary Drainage Analysis Tanimura and Antle Employee Housing Town of Spreckels County of Monterey, California LIB May 29, 2015 Prepared For: Tanimura and Antle Produce Prepared By: 9699 Blue Larkspur Lane Suite 105 Monterey, CA Fax File No.: T:\Monterey Projects\3339\DOCS\REPORTS\HYDROLOGY\3339-Pre Hydrology Report.doc

2 9699 Blue Larkspur Lane Suite 105 Monterey, CA Fax Tanimura and Antle Employee Housing Preliminary Drainage Analysis Table of Contents 1.0 Introduction Design Criteria Proposed System Summary Conclusion... 5 Figures 1. Plate 25 of the Monterey County Design Guidelines Appendix I. Hydrology Exhibits II. Hydrology and Stormwater Basin Calculations III. Tanimura and Antle Cold Storage and Shipping Terminal Hydrology and Drainage Analysis Report by Engineering Department Associates, dated February 1987 IV. Monterey County Water Resources Agency Comment Letter T:\Monterey Projects\3339\DOCS\REPORTS\HYDROLOGY\3339-Pre Hydrology Report.doc

3 Tanimura and Antle Employee Housing Preliminary Drainage Analysis 1.0 Introduction The Tanimura and Antle Industrial Park is located in the town of Spreckels and encompasses approximately 155 acres south of Spreckels Boulevard. It currently is utilized for agriculture purposes and includes fields, storage buildings, and other structures. Of the 155 acre industrial park about 20% is impervious. Currently the site stormwater system drains to a collector sump and is pumped directly to the Salinas River during the winter rainy season. In the summer, the sump diverts dry weather flow to the Summer Pond for infiltration. A small portion of the site, Building 9 and the old bulk pulp warehouse (approximately 6 acres), appear to drain directly into the Summer Pond for percolation (there are 2 existing 12-inch pipes at the east side of the basin). (See Exhibit 3) The proposed Employee Housing project is situated on about 4 acres in the western portion of the subject parcel. The project includes 8 two story dormitory style buildings, inter-connecting pathways, a fire access roadway and parking around the perimeter of the buildings, and improved landscape throughout. The project is bound by existing buildings to the north and south, existing fire protection ponds to the east, and off-site agriculture fields to the west. Exhibit 1 illustrates the pre-project surface type condition of the entire parcel. Exhibit 2 represents the post-project condition. Whitson Engineers has prepared this Preliminary Drainage Analysis for the subject project to correspond with the storm drain facilities shown on the Preliminary Plans prepared for environmental review. Specifically, this report provides the methodology that will be used to calculate the size of the new on-site storm drain pipes, outline the required stormwater mitigations, and identify the facilities that will meet the design and regulatory requirements. T:\Monterey Projects\3339\DOCS\REPORTS\HYDROLOGY\3339-Pre Hydrology Report.doc

4 T&A Employee Housing Drainage Analysis May 29, 2015 Page 2 of 5 Job No.: Design Criteria STORMWATER REGULATIONS The subject property is close to a Monterey County Urbanized Area. Monterey County has elected to require the project to mitigate post-development water quality and peak flow discharges. Specifically, per the May 21, 2015 letter from the Monterey County Water Resources Agency (see Appendix IV), the project will be required to retain stormwater from the 85 th percentile 24-hour design storm event and limit discharge of the post-development peak flow to the pre-project peak flow for the 2- through 10-year storm events. OPPORTUNITIES AND CONSTRAINTS There are several constraints on the proposed site that affect the drainage design. The existing soil has a low infiltration rate, consisting of mostly clayey silt and the proposed layout is dense and bound by existing buildings which limit the placement of percolation facilities. However, about 500 feet to the south of the proposed housing project is an existing large underutilized percolation pond (the Summer Pond), where the infiltration rates are anticipated to be higher due to the fact it is excavated down to the sandy soils adjacent to the Salinas River. STORM DRAIN PIPE The storm drain pipe networks will be a private system and will be designed to convey at a minimum, a 10-year storm event with a minimum of 1.25 feet of freeboard. The rational method will be used for each individual watershed to determine the anticipated discharges and establish the specific pipe sizes required to meet the design standard. Pipe sizes are anticipated to range from 4 rain water leaders up to a 15 storm drain main. Pipe material is anticipated to be either HDPE or PVC. OVERLAND RELEASE On-site streets within the proposed project site will be designed to convey and overland release flood events up to the 100-year design storm without flooding new or adjacent buildings. The overland release direction will be to the west towards the existing farm fields. T:\Monterey Projects\3339\DOCS\REPORTS\HYDROLOGY\3339-Pre Hydrology Report.doc

5 T&A Employee Housing Drainage Analysis May 29, 2015 Page 3 of 5 Job No.: STORM INTENSITY The design storm intensity (i) for the project area is 0.36 inches/hour for a 2-year 1-hour storm per Plate 25 of the Monterey County Design Guidelines (see Figure 1). A conversion factor of 1.48 is provided to determine the 10-year, 1-hour storm intensity of 0.53 inches/hour (1.48 x 0.36 = 0.53). To adjust the 10-year storm intensity for varying times the following formula is used: I t 7.75 i t c i10=0.53 inches/hour Where It is the intensity of a 10-year storm of time tc minutes. This formula becomes the intensity curve for the 10-year storm: I t t c RUNOFF COEFFICIENTS The C-value of undeveloped land in the project area will be designated as 0.30 while impervious surfaces will carry a C-value of A composite C-value for a given watershed will be calculated based on the amount of each surface type present to confirm the sizes of the proposed storm drain pipe. It should be noted that based upon the soil type, a pre-development coefficient of 0.30 is considered conservative, but it is understood to be the County standard for predevelopment conditions. TIME OF CONCENTRATION In establishing the flow rate entering an upstream node of a pipe network, a set concentration period is required to represent the elapsed time that rainwater travels to the catch basin. For this project a 6 minute time of concentration will be used, based on an assumed 5 minute interval for the water to travel from a rooftop to the gutter and 1 minute from the gutter to a catch basin or inlet. T:\Monterey Projects\3339\DOCS\REPORTS\HYDROLOGY\3339-Pre Hydrology Report.doc

6 T&A Employee Housing Drainage Analysis May 29, 2015 Page 4 of 5 Job No.: Proposed System Summary OVERVIEW The Tanimura and Antle Employee Housing project proposes a system of on-site storm drain pipes to collect and convey all post-project stormwater runoff to a large existing percolation pond (Summer Pond) about 500 feet south of the development area. Impervious surfaces will be minimized and downspouts will discharge to pervious surfaces via a splash block (downspout disconnect) where feasible. OFF-SITE WATERSHEDS There are no known off-site watersheds that are tributary to the existing percolation pond or the Employee Housing Site. Apart from incidental local runoff, it is our understanding that the Summer Pond is primarily utilized for diverted summertime runoff and is not significantly utilized for stormwater collection during the rainy season. Exhibit 3 represents the limits of the development area watershed that will be conveyed to the existing pond. STORM WATER PONDS The proposed mitigation for runoff retention and management of peak flows will be accomplished using the Summer Pond, an existing stormwater percolation pond as described above. This pond would provide, at a minimum, retention volume during the 85 th percentile design storm events for the new development area, and would limit discharge of the post-development peak flows to the preproject peak flows for the 2- through 10-year storm events. Although not anticipated to be utilized, in the event of a large storm event, an overflow riser and pipe would allow stormwater to overflow to an adjacent basin to the south before overtopping its banks. Beyond the southern basin lies the Salinas River. The existing, underutilized percolation pond (Summer Pond) has a surface area of about 3.9 acres, an average depth of about 15 feet, and a volume of about 28.5 acre-feet (assuming 2 feet of freeboard). WATER QUALITY The pre-project use of the proposed development area is agricultural fields and is mostly pervious. As shown on Exhibit 2, the proposed project consists of impervious surfaces such as roofs, asphalt, and pathways. This change could affect the quality of stormwater runoff. Even though the use of agriculture T:\Monterey Projects\3339\DOCS\REPORTS\HYDROLOGY\3339-Pre Hydrology Report.doc

7 T&A Employee Housing Drainage Analysis May 29, 2015 Page 5 of 5 Job No.: pesticides would be eliminated, the addition of roads and rooftops could introduce new pollutants. However, since all of the stormwater from the development area would be conveyed to the existing percolation pond and infiltrated, all potential pollutants from the development area would be contained and mitigated at the pond through biotreatment. HYDROLOGY CALCULATIONS Hydrology for the 10- and 100-year design storm events was analyzed using the method outlined in NRCS Technical Release 55, commonly known as the TR-55 Method. Since the existing pond is under-utilized, the 100-year flow from the project site and the area of the pond itself was determined to confirm that the existing storage volume would be enough to retain the flows from the proposed development. The required volume for the proposed Employee Housing development is 1.9 acre-feet. As stated earlier, the total storage volume of the existing pond was found to be approximately 28.5 acre-feet. Therefore, the new project minimally affects the total storage of the pond and would be able to retain the 100-year storm event. This is also considered conservative, as the proposed conveyance facilities will only be designed to convey the 10-year flow. Peak flow from the development area for the 10-year design storm to be routed to the percolation pond (Summer Pond) was calculated as 3.59 cfs. 4.0 Conclusion The proposed Tanimura and Antle Employee Housing project will safely and effectively convey stormwater runoff from a variety of storm events. The project would control erosion, roadway runoff, infiltrate stormwater, and prevent flooding of existing and proposed new buildings via a network of pipes, overland release and an existing stormwater percolation pond. The storm drain pipe networks would be designed to convey a 10-year storm event and direct it to an existing percolation pond. The existing percolation pond volume has ample capacity to provide full on-site retention (infiltration) of stormwater runoff for the 85 th percentile design storm events and maintain peak discharge rates at preproject levels for the 2- through 10-year design storms. T:\Monterey Projects\3339\DOCS\REPORTS\HYDROLOGY\3339-Pre Hydrology Report.doc

8 FIGURES

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10 APPENDIX I HYDROLOGY EXHIBITS

11 LEGEND EX IMPERVIOUS AREA (32.8 AC) EX PERVIOUS AREA (122.7 AC) Whitson Engineers 9699 Blue Larkspur Lane Suite 105 Monterey, CA F CIVIL ENGINEERING L AND SURVEYING PROJECT MANAGEMENT Project No.:

12 LEGEND EX. PERVIOUS TO REMAIN PERVIOUS (1.69 AC) EX. IMPERVIOUS TO PROPOSED PERVIOUS (0.05 AC) EX. PERVIOUS TO PROPOSED IMPERVIOUS (2.74 AC) EX. IMPERVIOUS REPLACED AS IMPERVIOUS (0.45 AC) EX. IMPERVIOUS TO REMAIN IMPERVIOUS (1.01 AC) TOTALS TOTAL SITE AREA: TOTAL WORK AREA: TOTAL PRE-PROJECT IMPERVIOUS AREA: AC 5.94 AC 1.51 AC TOTAL POST-PROJECT IMPERVIOUS AREA: 4.20 AC Whitson Engineers 9699 Blue Larkspur Lane Suite 105 Monterey, CA F CIVIL ENGINEERING L AND SURVEYING PROJECT MANAGEMENT Project No.:

13 Whitson Engineers 9699 Blue Larkspur Lane Suite 105 Monterey, CA F CIVIL ENGINEERING L AND SURVEYING PROJECT MANAGEMENT Project No.:

14 APPENDIX II HYDROLOGY AND STORMWATER BASIN CALCULATIONS

15 9699 Blue Larkspur Lane Suite 105 Monterey, CA Fax Preliminary Drainage Analysis Tanimura and Antle Employee Housing Peak Flow Hydrology Calculations Site TRIBUTARY AREA (acres) [1] Impervious Area (C=0.90) 2.30 Pervious Area (C=0.30) 2.26 Total Area 4.56 RUNOFF COEFFICIENT Composite Coefficient, C 0.60 Coefficient C(g) Coefficient C(g) Coefficient C(g) Coefficient C(g) Coefficient C(g) AVERAGE INTENSITY RAINFALL (it) Time of Concentration, t c (min.) [2] 10 Maximum 2-year, 1-Hour Intensity, i2 (in./hr) [3] 0.36 Maximum 2-year Intensity, it2 (in./hr) [3] 0.88 it2=7.75 i/sqrt(t c ) Intensity Conversion Factors [3] f f f f RUNOFF (cfs) Q2 = C * C(g)2 * it2 * A 2.42 Q10 = C * C(g)10 * it2 * f10 * A 3.59 Q25 = C * C(g)25 * it2 * f25 * A 4.61 Q50 = C * C(g)50 * it2 * f50 * A 5.59 Q100 = C * C(g)100 * it2 * f100 * A 6.73 Notes: [1] See Exhibit 2 for details. [2] Utilizes a minimum time of concentration of 6 minutes to an inlet and 4 minutes for pipe conveyance. [3] Rainfall Intensities Chart, Monterey County Department of Public Works - Standard Details, Plate No. 25 5/29/2015 9:11 AM T:\Monterey Projects\3339\DOCS\REPORTS\HYDROLOGY\3339-Hydrology_01.XLS

16 Peak Site Volume from 100-year Storm= 1.9 ac-ft Hydrology for the 10- and 100-year design storm events was analyzed using the method outlined in NRCS Technical Release 55, commonly known as the TR-55 Method. Input Data Area of Watershed: 4.56 acres Area of Existing Pond: 3.01 acres Percent of Impervious surfaces: 50.4% SCS curve number: 85 Lag time: 10 minutes Amount of Rainfall of 10 year storm: 2.49 Amount of Rainfall of 100 year storm: 4

17 APPENDIX III Tanimura and Antle Cold Storage and Shipping Terminal Hydrology and Drainage Analysis Report by Engineering Department Associates, dated February 1987

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22 Appendix IV Monterey County Water Resources Agency Comment Letter

23 MONTEREY COUNTY WATER RESOURCES AGENCY PO BOX 930 SALINAS, CA (831 ) FAX (831) DAVID E. CHARDAVOYNE GENERAL MANAGER STREET ADDRESS 893 BLANCO CIRCLE SALINAS, CA DATE: APPLICANT: APN: ADDRESS: PROJECT: 5/12/2015 T & A Housing Spreckels General Development Plan/Administrative Permit to allow an 800-person fa rm worker housing development on 5 acres REVIEWED BY: COMMENTS: The project is in close proximity to an Urbanized Area (as defined by the Monterey County Stormwater Ordinance, MCC 16.14} and is subject to providing a drainage impact analysis as described in Policies S- 3.1 through 3.9 of the 2010 Monterey County General Plan Update (GPU}. MCC and the 2010 GPU require the proposed project to include post-construction stormwater faci lities designed to protect water quality and mitigate post-development peak flow discharge. The Monterey County Water Resources Agency has determined the project shall be designed to retain the g5th percentile rainfall event and limit peak flow discharge from the site. If on-site retention is not feasible, stormwater facilities shall be designed to include oil/grit separators and detention facilities to limit the 100-year post-development runoff rate to the 10-year pre-development rate. Prior to deeming the application complete, the applicant shall provide the following information: A preliminary drainage analysis, prepared by a registered civil engineer, shall be provided to the Water Resources Agency for review and approval. The analysis shall include the following: o Drainage calculations certifying the retention facilities will have adequate volume to treat stormwater discharge from events up to the 85th percentile 24-hour rainfall event as determined from local rainfall data. Additionally, the project shall include facilities to limit discharge of the post-development peak flow to the pre-project peak flow for the 2- through 10-year storm events. o A geologic investigation shall be performed at the site of any proposed retention facility to determine the suitability of subsurface materials for stormwater retention. The potential impact of stormwater retention, with respect to geologic hazards, shall also be analyzed. The geologic report shall be provided to the Water Resources Agency for review and approval. H :\DEV _RVW\DOCS\2015\t&ahousingRE. doc