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4. Environmental Impact Analysis 4.9 - Hydrology and Water Quality

The Water Quality Technical Report (WQTR) (Appendix 8-1 to this EIR) assesses the potential
impacts of the proposed Sand Canyon Plaza Mixed-Use Project (the Project) on water quality and
stream channel hydromodification in the Project’s receiving waters. To evaluate potential impacts
of the Project on water quality, pollutants of concern are identified based on regulatory and other
considerations. Potential changes in water quality are addressed for pollutants of concern based on
literature information and professional judgment. Impacts consider Best Management Practices
(BMPs) selected to be consistent with the Los Angeles County Municipal Separate Storm Sewer
System (MS4) National Pollutant Discharge Elimination System (NPDES) Permit. The level of
significance of impacts is evaluated considering significance criteria that include predicted runoff
quality for Project conditions versus existing conditions; MS4 Permit and Construction General
Permit requirements; and reference to receiving water quality benchmarks, including Total

Maximum Daily Load (TMDL) waste load allocations and water quality standards from the Basin
Plan and California Toxics Rule.

Potential hydrologic impacts related to storm water runoff volumes and peak flow rates from the
50-year storm event are addressed in the Drainage Concept/Hydrology, City of Santa Clarita, Tract
No. 053074, MTD No. 1697 report (Appendix 8-3 to this EIR) and the Sand Canyon Plaza
Hydrology Technical Memorandum (Appendix 8-2 to this EIR). Water quality discharge flow
rates, volumes, and BMP sizing are addressed in the LID/Water Quality, City of Santa Clarita, Sand
Canyon Plaza report (Appendix 8-4 to this EIR).

4.9-3 Existing Conditions

The Project location is shown in Figure 4.9-1, Sand Canyon Plaza Vicinity Map. The Project site is
located on the northeast corner of the intersection of Sand Canyon Road at Soledad Canyon Road
in the City of Santa Clarita (approximately 87 acres). Highway 14 (Antelope Valley Freeway) is
located immediately to the south of the Project area.

A portion of the site is currently developed with 123 mobile homes that will be removed as part of
the proposed development. Remaining portions of the site are currently undeveloped.

1. Topography and Drainage

The Project site elevation ranges from approximately 1,600 feet above mean sea level (AMSL) to
approximately 1,820 feet AMSL.

A ridge runs across the Project site from northeast to southwest, separating the site into two
primary drainage basins, Drainage Basin A and Drainage Basin B (Figure 4.9-2). The flow direction
is generally north to south in both drainage basins.

Tebo Environmental Consulting, Inc. Sand Canyon Plaza Mixed-Use Project Draft EIR
March 2017 4.9-2