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  • City of Los Angeles Boyle Heights Mixed-Use Community Project SCH. No. 2008061123 October 2011

    Page IV.F-1 WORKING DRAFT – Not for Public Review

    IV. Environmental Impact Analysis F. Hydrology and Water Quality

    1. Introduction This section provides an analysis of the project’s potential impacts associated with

    hydrology and water quality. The analysis is based in part on the Storm Water Hydrology Study prepared for the project site in April 2011 and the Surface Water Quality Study prepared for the project site in January 2011, both of which were prepared by Stantec. These reports are provided in Appendices G-1 and G-2 of this Draft EIR, respectively.

    2. Environmental Setting a. Existing Conditions

    (1) Surface Water Hydrology

    The project site includes approximately 68.8 acres located within the Boyle Heights community of the City of Los Angeles. In a hydrologic context, the project site lies within the Los Angeles River Watershed, a drainage area that is shaped by the path of the Los Angeles River and covers approximately 871 square miles. The Los Angeles River Watershed encompasses the Santa Susanna Mountains to the west, the San Gabriel Mountains to the north and east, the Santa Monica Mountains and Los Angeles coastal plain to the south, and ends at the Long Beach Harbor. Within this watershed, stormwater that does not percolate through the ground into the groundwater table is directed into storm drains that eventually discharge into the Los Angeles River, which in turn discharges to the Pacific Ocean.

    The topography of the project site presents a westward increase in elevation of approximately 65 feet over a distance of nearly 3,000 feet. The highest elevation of the project site is at the northwest corner, with the lowest point at the northwest corner at the intersection of S. Grande Vista Avenue and Lydia Drive. Approximately 54 percent of the project site is currently developed with impervious surfaces, including several apartment buildings and accessory structures as well as surface parking lots and public streets/sidewalks. The remaining 46 percent of the project site consists of pervious surfaces including lawn and landscaped areas interspersed around buildings as well as “The Mall,” a landscaped open space/drainage channel running from east to west, with a

  • IV.F Hydrology and Water Quality

    City of Los Angeles Boyle Heights Mixed-Use Community Project SCH. No. 2008061123 October 2011

    Page IV.F-2 WORKING DRAFT – Not for Public Review

    perpendicular section bisecting the channel from north to south. The project site is located in the southern portion of an approximately 1,000 acre regional drainage area that is bounded by the Santa Monica Freeway (I-10) to the north, the I-10/Golden State (I-5) Freeway connector to the west, Grande Vista Avenue to the east, and Olympic Boulevard to the south.1

    The project site is underlain by fill soils to a maximum depth of 18 feet. The fill soils consist primarily of moist, stiff to medium density, fine-grained silty to sandy clays and clayey to silty sands which are yellowish brown to brownish grey, mixed with some gravel. The native soils underlying the fill on-site consist predominantly of sands, silty sands, clayey sands, and sandy silts. The native soils are generally fine-grained with some gravel and cobbles, range from orange brown to yellowish brown to brownish grey in color, and are dense to very dense and hard. The geologic materials consist predominantly of alluvial sediments deposited by rivers and streams, typical of the Los Angeles County area.

    The project site is part of an approximately 80.9 acre drainage area and is served by two distinct storm drain networks. The first is a major public storm drain owned by the City of Los Angeles that traverses the site from west to east. This 75- to 84-inch concrete pipe has a capacity of approximately 410 cubic feet per second (cfs) where the pipe is 84 inches in diameter and enters the project site from the northwest at the intersection of Glenn Avenue and 8th Avenue and exits the project site to the east at Grande Vista Avenue before draining into the Los Angeles River to the southeast. The second storm drain network that serves the project site is a major County of Los Angeles storm drain comprised of an 11-foot by 6.5-foot concrete box with a capacity of approximately 588 cfs. This County storm drain is located immediately adjacent to the east boundary of the project site in Grande Vista Avenue and flows south to the Los Angeles River.

    The site consists of two separate primary drainage areas. The first drainage area connects to the City of Los Angeles storm drain that traverses the site. Four public storm drain inlets on-site connect to the City storm drains and are located at Glenn Avenue and Camulos Place; Glenn Avenue and Hostetter Street; Glenn Avenue and Camulos Street; and Rosalind Place near Lydia Drive. The tributary areas associated with these inlets comprise an area of 34.2 acres within the project boundary as well as 7.3 acres beyond the project boundary. The second drainage area within the project site flows to the County drainage system and includes the central open space area in Wyvernwood known as The Mall. The Mall does not have an underground storm drain but is surface drained by a grassy swale. This open space area is comprised of 34.5 acres. Two additional small portions of the site drain to 8th Street and Olympic Boulevards. These streets are served by other City of Los Angeles inlets and storm drains that drain to the Los Angeles River. 1 Storm Water Hydrology Study, Stantec. Prepared April 2011.

  • IV.F Hydrology and Water Quality

    City of Los Angeles Boyle Heights Mixed-Use Community Project SCH. No. 2008061123 October 2011

    Page IV.F-3 WORKING DRAFT – Not for Public Review

    Drainage of the project site can be further divided into seven subareas as shown in Figure IV.F-1 on page IV.F-4. Subarea 1 is located within the northwestern portion of the project site and is comprised of approximately 16.2 acres. Subarea 2 is located immediately adjacent to the project site to the southwest and is approximately 7.3 acres. Subarea 3, which is partially outside of the project site, is located to the south and consists of approximately 10.6 acres. Subarea 4 is within the southeastern portion of the project site and is comprised of approximately 7.4 acres. Subareas 1, 2, 3, and 4 drain to on-site streets and storm drain inlets which are connected to the City storm drain that runs through the project site. Thus, the City storm drain currently provides drainage for approximately 34.2 acres within the project site and approximately 7.3 acres outside of the project site, or a total of approximately 41.5 acres. Subarea 5 within the central and eastern portion of the project site consists of approximately 34.5 acres and drains to The Mall where a surface swale conveys runoff to the east and eventually to the County storm drain in Grande Vista Avenue. Subarea 6, which is located in the southeastern portion of the project site, consists of approximately 2 acres and drains off-site to additional City storm drains in Olympic Boulevard. Subarea 7, which is located in the northern portion of the project site, consists of approximately 2.9 acres and drains off-site to additional City storm drains in 8th Street. Existing storm drain facilities are shown in Figure IV.F-2 on page IV.F-5. The existing City and County storm drain systems that serve the project site accommodate existing runoff and have no deficiencies.2 While The Mall may have local areas that pond stormwater during storm events, no incidents of on-site flooding have been noted that endanger structures or public streets. Furthermore, the project site is not located within a 100-year flood plain.3

    To estimate the existing stormwater runoff rates at the project site, hydrology calculations were performed using the Los Angeles County of Public Works 2006 Hydrology Manual. Runoff rates were calculated for 50-year storm events. As shown in Table IV.F-1 on page IV.F-6, the existing stormwater runoff rate at the project site is approximately 152.5 cfs during a 50-year storm event. Of this amount, approximately 78.2 cfs drains to the City storm drain that runs through the project site, 65 cfs drains to the County storm drain, 3.8 cfs drains to the off-site City storm drain located in Olympic Boulevard, and 5.5 cfs drains to the off-site City storm drain located in 8th Street.4 The Los Angeles County of Public Works has indicated that a maximum of 2.9 cfs per acre may be directed to the County storm drain from the project site. As discussed above, 34.5 acres of the project site drain to the County storm drain system, which provides for a maximum discharge of 100 cfs into that system. The project site currently discharges approximately

    2 Ibid. 3 Ibid. 4 Ibid.

  • Source: Stantec Consulting Inc., April 30, 2009.

    Figure IV.F-1 Existing Hydrological Conditions

    Boyle Heights Mixed-Use Community Project

    P age IV.F-4

  • Source: Stantec Consulting Inc., June 8, 2011.

    Figure IV.F-2 Existing Storm Drain Facilities

    Boyle Heights Mixed-Use Community Project

    P age IV.F-5

  • IV.F Hydrology and Water Quality

    City of Los Angeles Boyle Heights Mixed-Use Community Project SCH. No. 2008061123 October 2011

    Page IV.F-6 WORKING DRAFT – Not for Public Re

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