3.8 HYDROLOGY AND FLOODING This section summarizes existing hydrologic conditions in the study area, presents the regulatory guidance for hydrologic resources, and evaluates potential adverse environmental effects of project implementation on hydrology. The examination of hydrology is based primarily on (1) previous hydrologic and hydraulic analyses prepared for the project, including Upper Truckee River and Marsh Restoration Project: Processes and Functions of the Upper Truckee Marsh (Conservancy and DGS 2003) and Upper Truckee River and Marsh Restoration Project Alternatives Evaluation Report (Conservancy and DGS 2005); and (2) the preliminary engineering schematic conceptual design prepared for the alternatives. Additional important information is referenced from other relevant published and unpublished academic studies and reports and other documents issued by federal, state, and local agencies. For a discussion of geomorphology and water quality issues, please refer to Section 3.9, “Geomorphology and Water Quality.” Cumulative hydrology and flooding impacts are addressed in Section 3.18, “Cumulative Impacts.” Consistency with TRPA goals and policies is presented in Section 3.10, “Land Use.” 3.8.1 AFFECTED ENVIRONMENT REGULATORY SETTING Federal The following federal laws and regulations related to hydrology and flooding are relevant to the proposed alternatives and described in detail in Chapter 5, “Compliance, Consultation, and Coordination”: ► Clean Water Act ► Floodplain regulations ► Truckee River Operating Agreement ► Executive Order 11988 State The following state law related to hydrology and flooding is relevant to the proposed alternatives and described in detail in Chapter 5, “Compliance, Consultation, and Coordination”: ► Porter-Cologne Water Quality Control Act TRPA 1987 Regional Plan Goals and Policies The Goals and Policies document of the 1987 Regional Plan presents the overall approach to meeting the environmental thresholds. A key component is the Land Use Element, which identifies fundamental philosophies directing land use and development in the Tahoe Basin. It addresses topics such as suitable development locations and maintenance of the region’s environmental, social, physical, and economic well-being. Code of Ordinances Proposed project construction activities are regulated by Section 60.4, “Best Management Practice Requirements,” of the TRPA Code of Ordinances (TRPA Code), specifically in relation to best management UTR and Marsh Restoration Project DEIR/DEIS/DEIS AECOM and Cardno ENTRIX California Tahoe Conservancy/DGS, Reclamation, and TRPA 3.8-1 Hydrology and Flooding practices (BMPs), especially temporary, permanent, and standard BMPs. In accordance with the Handbook of Best Management Practices, and as required in Section 33.5, “Grading and Construction Schedules,” of the TRPA Code, temporary BMPs shall be implemented on construction sites and maintained throughout the construction period (TRPA 2003:25-1). Permanent BMPs must be applied within the parcel or all project area boundaries, unless the project is exempt. TRPA states that a project that involves Stream Environment Zone (SEZ) restoration may be exempt from the aforementioned requirements. SEZ is defined by TRPA as the major and minor streams, intermittent streams, drainage ways, meadows and marshes, primary and secondary riparian vegetation, and other water influence zones areas within the Lake Tahoe Region that provide natural treatment and conveyance of surface runoff (TRPA 2004:28). Standard BMP requirements that are applicable to the project deal mainly with drainage conveyance. Drainage conveyances through a parcel shall be designed for at least a 10-year, 24-hour storm. Drainage conveyances through an SEZ shall be designed for a 50-year storm, at a minimum. Chapter 35, “Natural Hazard Standards,” of the TRPA Code also addresses floodplain management. TRPA defines a floodplain as that portion of a river valley, adjacent to the channel, that is built of sediments deposited during the present geological/climatic regime. Hydrologic statistics are used to estimate the magnitude of peak streamflow expected on a 100-year recurrence interval, and the area that could be inundated from that event is defined as the 100-year floodplain. TRPA reviews all additional development in 100-year floodplains and regulates any necessary public uses located in the floodplains. The 100-year floodplain is determined by the limits delineated by the Flood Insurance Rate Maps (FIRMs) issued by the Federal Emergency Management Agency (FEMA). TRPA prohibits development, grading, and filling of lands within the 100-year floodplain, with certain exceptions, including specific public outdoor recreation facilities and water quality control facilities. TRPA may permit erosion control, habitat restoration, wetland rehabilitation, SEZ restoration, and similar projects within a 100-year floodplain. To receive a permit from TRPA, the proponent of a restoration project within the floodplain must show that the project is necessary for environmental protection, must demonstrate that there is no reasonable alternative to reduce the extent of encroachment, and must fully mitigate all impacts (TRPA 1980:28-3). TRPA’s development restrictions and exemption findings for 100-year floodplains do not apply to the shorezone of Lake Tahoe, except where the study area is determined to be within the 100-year floodplain of a tributary stream, which is where this study area lies. Therefore, the restrictions and exemptions for shorezone protection in Chapter 81, “Permissible Uses and Accessory Structures in the Shorezone and Lakezone,” of the TRPA Code apply in addition to those for floodplain protection mentioned above. (See Section 3.9, “Geomorphology and Water Quality,” for further discussion.) Plan Area Statements TRPA’s plan area statements (PASs) outline land use classifications, special policies, planning considerations, permissible uses, and maximum allowances for the Tahoe Basin. The PASs in which the study area is located are PAS 099 (Al Tahoe), PAS 100 (Truckee Marsh), PAS 102 (Tahoe Keys), PAS 103 (Sierra Tract Commercial), PAS 104 (Highland Woods), and PAS 111 (Tahoe Island). The criteria and permissible uses for each PAS covering the study area are described in Section 3.10, “Land Use,” of this DEIR/DEIS/DEIS. Considerations include commercial and residential uses that infringe on the SEZ and restrictive highway crossings over the Upper Truckee River and Trout Creek that alter the natural functioning capacity of the SEZ. The relevant special policies outlined in this PAS are: (1) Stream zones should be restored where U.S. Highway 50 (U.S. 50) crosses the Upper Truckee River and Trout Creek. (2) SEZs should be restored in the vicinity of the crossings of Trout Creek at U.S. 50 and at Black Bart Road. (3) New roadway alignments through SEZs are to be discouraged. AECOM and Cardno ENTRIX UTR and Marsh Restoration Project DEIR/DEIS/DEIS Hydrology and Flooding 3.8-2 California Tahoe Conservancy/DGS, Reclamation, and TRPA El Dorado County Land under the jurisdiction of El Dorado County is located upstream of the portion of the Upper Truckee River located within the study area, so the El Dorado County Code’s provisions on grading (Chapter 15.14) do not directly apply to the project. However, the project’s compliance with TRPA and City of South Lake Tahoe (CSLT) grading requirements and water quality and flooding obligations required by the Clean Water Act will also provide consistency with El Dorado County requirements. City of South Lake Tahoe The study area is located entirely within CSLT’s jurisdictional limits. Chapter 8, “Building Regulations,” of the CSLT Ordinance requires that projects prepare a grading plan for review and grading-permit approval by the CSLT building official. The grading plan must include the present contours of the land and the proposed final grade and location of improvements. The purpose of the review is for CSLT to safeguard adjoining properties and public streets and ways from damage by unnatural flows of surface waters, and to prevent construction of homes and other buildings in areas likely to become flooded. CSLT is the floodplain administrator for FEMA in the project area, responsible for implementing the federal policies and regulations for floodplain management. ENVIRONMENTAL SETTING Hydrology The study area’s hydrology is the result of numerous physical environmental parameters. Among the important parameters are watershed characteristics and climatic conditions affecting the Upper Truckee River and Trout Creek, streamflow magnitudes and regimes, runoff from adjoining urban drainage areas, direct precipitation in the study area, Lake Tahoe surface-water elevations, and groundwater elevations and gradients. Surface Water Watersheds The study area is situated at the downstream end of the Upper Truckee River and Trout Creek watersheds (Exhibit 3.8-1), the two largest watersheds that drain to Lake Tahoe. The watershed areas of the river and creek are 56.5 and 41.2 square miles, respectively; these areas comprise 18 percent and 13 percent of the total area tributary to Lake Tahoe (Rowe and Allander 2000:7–8). The general characteristics of these two watershed areas provide a background for understanding specific hydrologic events and patterns observed in the study area. The watersheds of the Upper Truckee River and Trout Creek share several important characteristics related to surface-water and groundwater hydrology. The geology of both basins