A. Purpose and Scope
Increasing growth in Placer County has lead to increasing problems associated with stormwater runoff.
Flooding is a primary problem. Much of the growth has occurred adjacent to streams which drain the region, resulting in significant damages to property, losses from the disruption of commercial activities, and potential loss of life when the streams overflow. Further, development in the watersheds of these streams affects both the frequency and duration of damaging floods.
Other problems connected with stormwater runoff include erosion, sedimentation, degradation of water quality and losses of environmental resources.
Every development project within the county has some effect on the stormwater runoff both on-site (within the boundaries of the project) and off-site (downstream areas outside the boundaries of the project). Furthermore, although project-specific effects may be small, the cumulative impact of several similar projects may be significant.
Local government is responding with a greater commitment to alleviate current problems and plan for growth to minimize future ones. This manual stems from that commitment.
The purpose of this manual is to provide consistent, specific guidance and requirements for stormwater management, including regulation of the development process, to achieve stormwater management objectives.
The manual is intentionally an evolutionary document. Its initial focus is on flooding problems. Over time the scope will expand to include more on sedimentation, erosion, water quality, and environmental effects. It will also be updated periodically to reflect new information and technology.
B. Overview - Organization and Use of Manual
This manual presents policy, guidelines, and specific criteria for the development and management of natural resources, facilities and infrastructure for stormwater management.
Policy is presented to help clarify specific criteria and to aid in their interpretation or application. General goals and policies are presented in Chapter II and specific policies are stated in appropriate chapters.
The chapters of the manual are organized as follows:
I. Introduction - States the purpose of the manual, provides an overview and a brief description of the region, and briefly describes the Placer County Flood Control District.
II. Policy - States general goals, principles and policies.
III. Master Plan - Describes the role of the master plan in stormwater management.
IV. Regulatory Requirements - Identifies the various regulatory agencies which have jurisdiction over aspects of stormwater management.
V. Hydrology - Provides policies, guidelines, and criteria for determining flows and volumes of runoff.
VI. Drainage Systems - Provides policies, guidelines and criteria for the design of drainage systems and related facilities, including streets and gutters, pipes and culverts.
VII. Storage - Provides policies, guidelines and criteria for the planning and design of storage facilities.
VIII. Streams and Channels - Provides policies, guidelines and criteria for planning, designing, and maintaining open channels, including both artificial and natural channels.
IX. Erosion and Sedimentation- Provides policies, guidelines and criteria for addressing erosion and sedimentation concerns in the development of drainage systems.
Supplements, including a glossary, are provided following the chapters identified above. Since this manual will be periodically revised, the table of contents contains information on the current version of each chapter, and page headers indicate a date as well.
C. Description of Region
This manual applies primarily to the developing areas of Placer County which extend westward from Colfax to Sutter County. This western area contains most of the County's population and is rapidly growing.
Elevations in Western Placer County range from about 30 feet above mean sea level at the western boundary to about 3000 feet near Colfax. The topography ranges from nearly flat in the far western areas to rolling Sierra Foothills with moderate to steep slopes in the east.
The climate is characterized by a hot, dry summers and winters with moderate to heavy precipitation. Precipitation falls as rain resulting from extensive storms which originate in the Pacific Ocean. Normal annual precipitation varies with elevation and ranges from 19 inches to 42 inches.
Flood flows are generally confined to well-defined stream channels in the foothill portions of the region. Farther west, the flat landscape results in extensive overflow of flood waters into the adjacent floodplains. In both areas, the channels are often dense with riparian vegetation.
Vacant, undeveloped land is typically covered with annual grasses with oak and brush woodlands occurring in many locations. Cottonwoods, willows, oaks and dense brush occur in riparian areas. Pasture and orchards are the predominant agricultural developments.
Soils in the watershed are generally shallow and permit little infiltration, especially when previous precipitation has saturated the soil mantle.
D. Flood Control District
The Placer County Flood Control and Water Conservation District was formed by legislative resolution on Senate Bill 1312 (Johnson), effective August 23, 1984.
The Flood Control District is supported through a cooperative effort by the County and the Cities of Auburn, Colfax, Lincoln, Rocklin and Roseville and the Town of Loomis. District policies and activities are largely guided by the consensus of participating members. Other governmental agencies, particularly the Placer Resource Conservation District and the Soil Conservation Service, have played instrumental roles in the formation and guidance of the District.
The objectives of the Flood Control District for reducing the effects of flooding are:
- Maintain major drainage facilities, primarily stream channels, and detention and retention basins.
- Provide technical support to local governments, as exemplified by this manual.
- Perform regional drainage studies, including master drainage plans, and implement the regional projects and programs delineated therein.
- Provide flood potential advisories to local governments.
- Gather information and data on flooding events.
- Coordinate flood reduction activities with adjacent jurisdictions.
This manual was developed with contributions from many sources. Dave Dawdy, hydrologic consultant, provided the first draft. Thereafter, a subcommittee of the Flood Control District Technical Advisory Committee was generally responsible for producing the manual, and subsequent drafts were prepared by the District Staff. Other contributors included general members of the Technical Advisory Committee, local agencies, the Stream Management Task Force, and reviewers of the various drafts.