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IV. ENVIRONMENTAL ASSESSMENT

This chapter presents the environmental setting, impact and mitigation for five environmental disciplines: Geomorphology; Biology; Land Use; Public Health and Safety; and Cultural Resources. The discussion of mitigation refers to the list of Best Management Practices (BMPs) and other programmatic and compensatory mitigation in the Stream Maintenance Program.

A. GEOMORPHOLOGY

Creeks carry stormwater and waterborne sediment. Fluvial geomorphology is the study of sediment transport by flowing water and its effect on the size and shape of stream channels. When sediment transport is in equilibrium, sediment is neither deposited in the channel, nor removed from it (erosion). Sediment deposition in a channel is an indication that conditions are not in equilibrium with the existing balance of flow, sediment transport and natural channel forming processes so that sediment is deposited in a reach rather than transported downstream. Similarly, erosion and subsequent bank instability occur where the pattern of flow directs excess energy against the channel bottom or sides.

Most of District routine stream maintenance responds to changes in stream conditions that reflect geomorphic instability. Most of the instability is the result of channel changes caused by surrounding urbanization and by natural conditions, particularly sediment deposition due to the decrease in channel slope that occurs as creeks run from the foothills, across the alluvial plain to the Bay and from tidal influx of Bay mud.

Sediment removal and vegetation management maintenance are intended to maintain flood capacity by restoring channel design conditions to design flow cross section and hydraulic roughness. The geomorphic assessment here establishes the context for SMP activities. Because of the geomorphic conditions on the streams, there are few alternatives to maintenance that would meet the District's program objectives for protecting property.

The geomorphic assessment also considers how District activities in streams may affect the streams themselves. Sediment removal has the potential to cause erosion downstream in certain areas; vegetation die off following herbicide applications has the potential to release sediment trapped within the mass of vegetation causing deposition downstream; removal of woody debris may reduce channel bed diversity. The streams could be affected by bank protection, particularly with "hard" bank protection measures, that may result in a transfer of erosive energy downstream, resulting in new erosion and bank instability problems at locations downstream of the repair site. These potential stream impacts can be avoided through the maintenance project design process and by application of Best Management Practices (BMPs).

1. Introduction

Fluvial geomorphology is the study of sediment transport by flowing water and the consequential effect on the size and shape of stream channels. The geomorphology of the Santa Clara and Pajaro valleys reflects the fundamental physical environment of the creeks and directly defines the need for routine maintenance.

Change in the watershed or change in a channel's shape and alignment may cause a disequilibrium between sediment input and sediment output. The channel will react by retaining or exporting sediment and thus tends to change shape -- either filling in with sediment, or enlarging through erosion. Where these changes impair flood conveyance capacity or cause bank failure, the District undertakes to counter the changes through routine maintenance.

The EIR evaluates the geomorphology of District creeks, with an emphasis on maintenance work areas, to explain the relationship between natural and man made effects on channels and the ongoing need for maintenance. Geomorphic analysis of canals is not included since they are artificial waterways used for irrigation or transfer of water. This information is used in the analysis of the other environmental resources and in the assessment of alternatives to the proposed project.

Geomorphology is not an environmental resource like biology or cultural resources. Potential maintenance effects on fluvial geomorphic processes are not environmental impacts per se, but geomorphic effects may potentially lead to other environmental effects through further changes in channel conditions or cause a need for additional maintenance. Where applicable, the EIR also evaluates the potential geomorphic effects of routine stream maintenance. These potential effects are taken into account by the District maintenance design process and are addressed in BMPs.

a. Fluvial System Zones

The science of fluvial geomorphology draws on geology, geography, and hydrology to study the dynamics of channels and their form. The geographic area that drains to a common point is called a "watershed". A creek that drains a watershed discharges to a downstream water body, such as a lake or the ocean. This entire network is the "fluvial system". A fluvial system can generally be divided into three zones. The upper zone is the watershed where most of the water and sediment for the system originates, and is called the Erosion Zone. The middle zone is the reach where the stream/creek/river channel is the most stable and where its configuration is the best defined and sediment from Zone 1 moves through; this middle zone is called the Transport Zone. The lower zone is near the stream/creek/river mouth, where the alluvial river slope is reduced and may be under tidal influence. This zone is called the Deposition Zone.

b. Channel Dynamic Equilibrium

As indicated by these fluvial system zones, stream channels are dynamic. Stream channels generally attempt to evolve toward a state of quasi-equilibrium; that is, the channel adjusts its slope to provide, with the available discharge and the prevailing channel geometry, just the power required to transport the sediment load supplied from the drainage basin. This concept of channel equilibrium can be explained by the following relationship (Lane, 1955):

QS QsD50 where Q is the water discharge, S is the slope, Q is the bed material load, and D50 is the median size of bed material. Essentially, this equation shows that a change in any of the four variables will cause a change in the others such that equilibrium is restored. This relationship is useful to understand general trends in channel evolution.

A channel that maintains a generally constant shape, with some movement within the river corridor, is considered "stable" and in dynamic equilibrium. On the other hand, a channel that has geomorphic processes driving an ever-changing geometry is considered geomorphically "unstable".

c. Geomorphology and the Stream Maintenance Program

It is largely channels adjusting to their basic state of equilibrium that results in the need for channel maintenance: aggrading channels are building-up as material is added to the channel bed and may require sediment removal to restore the channel capacity; degrading channels are cutting-down or eroding, and may require bank protection to check the erosive force of water or to shore up a steep and unstable creek bank. Urban development in areas that once was flood plain forces all stormwater flow through a restricted area. Portions of the channels have previously been significantly realigned and disturbed and these channels are in a perpetual state of dis-equilibrium. This requires a maintenance program to ensure the function and stability of the channels.

Sediment removal and vegetation management are intended to maintain flood capacity by restoring channel design conditions to maximum flow area and low hydraulic roughness. The deposition of sediment and hence the need to remove it is, in itself, an indication that the dimensions and profile of the flood protection channel are not in equilibrium with the existing balance of flow, sediment transport and natural channel forming processes.

Bank protection is prompted by a variety of conditions: increase in flow resulting from changes in channel configuration, maintaining flood protection, inadequate design of construction along the channel, and urban encroachment on the active margin of a channel.

2. Setting

a. Physiographic Setting

Santa Clara County lies at the southern end of San Francisco Bay, just inland from the Pacific coast, in the central Coast Range of California. Mountainous areas cover approximately two-thirds of the county. The valley areas where most of the District maintenance is done extend through the center of the county, south-easterly from San Francisco Bay (in the north) to the Hollister Basin in San Benito County (in the south). The valley areas comprise three interconnecting basins: the Santa Clara Valley, the Coyote Valley and the Llagas Basin.

Santa Clara County has four distinct physiographic regions or landscape units: 1) the Santa Cruz Mountain uplands, 2) Diablo Range uplands, 3) foothills, and 4) bay plains and alluvial valleys. These units reflect the relations of landscape evolution to dominant geomorphic processes, such as the erosion of uplifted mountainous areas and broad, flat plains of recent sediment deposition along San Francisco Bay.

1) Santa Cruz Mountains Uplands

The Santa Cruz Mountains Uplands occurs along the western boundary of the study area and includes rugged terrain characterized by steep slopes, and v-shaped canyons. Elevations range from about 500 to 3,000 feet above mean sea level, and slope steepness ranges from about 15 to 75 percent. Average rainfall in this region is the highest in the County, ranging from 30 to over 50 inches per year. Vegetation is predominantly forest, but includes some chaparral and grasslands. The underlying bedrock is complex, with deeply weathered sedimentary and metamorphic rocks. The area has been dissected by faulting and recent erosion is composed of landslide deposits and clayey colluvium of the Holocene and late Pleistocene ages. Surficial materials are made up of clay and clay loam containing occasionally large, angular to rounded blocks of sandstone, chert, limestone, greenstone, schist, and gneiss. This geologic unit comprises many individual landslides, some several square miles in area. Many surface landslide deposits coincide with bedrock distribution of the Central belt of the Franciscan Complex, much of which is highly sheared melange, highly susceptible to land sliding. Soil covering of the Santa Cruz Mountains is considerably deeper than that of the Diablo Range, primarily due to greater rainfall. Bedrock in this region crops out chiefly in stream bank exposures; along weathered ridge crests; and as large resistant blocks of chert or other resistant rock exhumed after erosion of the surrounding melange. The melange unit of the Franciscan Complex is highly susceptible to severe erosion in its natural state and has among the highest potentials for yielding sediments of units throughout western California.

Within the Santa Cruz Mountains, the principal geomorphic activity of streams consists of deepening canyons, since the average capacity of streams to transport sediment exceeds the quantity of sediment available for transport. The streams have little or no flood plain area, thus accommodate increased flow with increased velocity and depth. This adds to the natural instability of the steep slopes which form the stream channel banks; the downslope movement of rock and soil masses is a common occurrence. The primary resistance to erosion in this region is offered by a relatively dense, protective vegetation covering that diminishes rainfall impact and retards rapid runoff and bank erosion. Slopes and streambanks of the Santa Cruz Mountain uplands tend to erode rapidly where removed vegetation exposes soil and bedrock to heavy rainfall and storm flows.

2) Diablo Range Uplands

The Diablo Range region on the east, farther inland, differs from the Santa Cruz Mountains uplands because of a drier climate and greater land use intensity. This region is also topographically defined by narrow ridges and deep valleys with little or no flood plain area. Elevations range from about 500 to 2,800 feet above mean sea level, with slopes similar to those of the eastern Santa Cruz Mountains. The Diablo Range Uplands are also defined by narrow ridges and deep valleys with little or no flood plain area. Surface and bedrock deposits of the Diablo Range coincide with the Central belt of the Franciscan Complex, as do those of the Santa Cruz Mountains. However, because of the lower average rainfall (ranging from 20 to 32 inches), the Diablo Range uplands sustain less protective vegetation. The vegetation consists of a mix of annual grasslands, oak woodland and chaparral. Drier climate and vegetative differences limit the formation of deep soils such as those found in the Santa Cruz Mountains, despite similar geology. Active erosion in the Diablo Range most frequently occurs by downslope mass movement of fractured bedrock such as debris slides and rock falls.

The geomorphic cycle in this region consists of canyon cutting, the headward growth of stream valleys and the extension of alluvial fans along the eastern side the Santa Clara Valley. The pace of these processes vary with stratigraphy, structure of geologic units and episodic rainfall events (Crittenden, 1951). In many areas, human activities, such as reduction in vegetation and quarry activities and gravel mining, have increased the volumes of sediment carried into streams in a single storm event.

3) Foothills

Foothill areas have moderate slopes, rarely exceeding 20 percent. Rainfall averages about 20 inches per year, and characteristic vegetation with grazing is savannah, composed of annual grasslands, scattered oaks and chaparral. This region is underlain principally by older dissected and deformed alluvial fan deposits of the Quaternary period (less than 3.2 million years before present), and has well developed soil profiles (Helley and Brabb, 1971). These deposits consist of weakly to moderately consolidated gravels and sand, with interbedded silt and clay, that reflect the characteristics of the bedrock and surficial materials of the surrounding uplands. Alluvial fans grow during multiple episodes of sediment deposition, and have thicknesses of several hundred feet.

Older alluvial fans are moderately to deeply weathered, making available considerable amounts of easily erodible material. These deposits occur in areas of slight to moderately sloping topography, and are relatively stable, unless disturbed. In areas that have been modified by human activities, gullying and streambank failure are prevalent. Where slopes have been artificially steepened by construction activities, rotational slumps and other hillslope failures are common.

4) Bay Plains and Alluvial Valleys

The plains and alluvial valleys are situated between the base of the foothills and the shoreline of the San Francisco Bay, and reflect more recent geomorphic processes. This landscape unit is composed of flat- to nearly flat-bottomed valleys and plains with extensive areas of artificial fill and a broad alluvial apron of fan, floodplain and deltaic deposits derived from the adjacent mountain ranges. Slopes range from flat to slightly greater than 5 percent. Slopes as much as 25 percent and greater may occur where present streams have cut down through older fan deposits near the foothills. Rainfall ranges between 14 and 20 inches. Vegetation types include marshland, grassland and chaparral. More than any of the other landscape units discussed in this chapter, the landscape of this region has been altered by human activities: first primarily by agricultural and more recently by urban land development. Much of this area has been filled for development, drained for farming or diked for salt-evaporation ponds.

Valley areas are underlain by thick unconsolidated alluvium: gravel, sand, clay and silt that were deposited in the Holocene age (less than 12,000 years before present). The texture of these deposits range from cobble to clay, mixed or interbedded laterally and vertically in places. These poorly consolidated deposits are often saturated and have little or no stability where geologically deformed or artificially altered. Valley margins are composed of solitary or coalesced alluvial fans.

Along the southern margins of San Francisco Bay, the alluvial apron sediments interfinger with saline marsh deposits that consist of muds with interbedded layers of silt, fine sand, peaty mud and organic deposits of the bay. Marsh deposits are commonly less than 10 feet thick, and are underlain by much thicker bay mud deposits. The clayey soils in these deposits swell when wet and shrink when dry. These fine-grained materials are easily mobilized and may move upstream with tidal surges in the lowermost reaches of streams.

The bay plains and alluvial valleys are primarily regions of sediment deposition, although localized areas of erosion occur along unprotected banks of laterally eroding streams. Sediment transported by streams from the adjacent mountains form alluvial fans, sand and gravel bars on stream floodplains and in artificial channels, and deltaic deposits in sloughs and in the bay.

b. Geomorphic and Hydrologic Setting

The principal drainage in the Santa Clara and Coyote Valleys is Coyote Creek. Coyote Creek originates in the Diablo Range and enters Coyote Valley at its topographic divide with Llagas Basin. The creek flows northwesterly through Coyote Valley and Santa Clara Valley before entering San Francisco Bay. Other major drainages that pass through Santa Clara Valley include Guadalupe River, Los Gatos Creek, San Tomas Aquino Creek, Saratoga Creek, Calabazas Creek, Stevens Creek and Permanente Creek, all of which originate in the Santa Cruz Mountains. Drainages entering Santa Clara Valley from the Diablo Range are smaller, reflecting the steeper terrain generally drier climatic conditions. Of these, the larger streams are Penitencia Creek and Berryessa Creek. The drainages of the Llagas basin are tributaries to the Pajaro River, which eventually enters the Pacific Ocean at Monterey Bay. Principal drainages of the Llagas Basin are Llagas Creek and Uvas Creek, both of which originate in the Santa Cruz Mountains.

Table IV-A-1 presents the watershed characteristics of a selection of streams within the Santa Clara Valley Water District jurisdiction. The column "watershed (origin)" identifies which upland physiographic unit is the origin of the watershed. The last three columns present the peak discharge rates for the 100-, 10-, and 2.3-year recurrence interval storms. The 2.3-year flows approximate the bank-full discharge.

The concept of channel forming or bankfull discharge is key to the application of fluvial geomorphology because it represents channel forming processes involving sediment transport. When identified and measured in the field, it provides a basis to determine what stable, bankfull channel forms are suitable to a particular site given the geomorphic setting. The bankfull discharge is that flow which moves the most sediment through time, and as a result, does the most work in forming the channel. The bankfull discharge is one capable of moving the sediment sizes found on the channel bed and occurs fairly often. While larger flows can carry greater volumes of sediment, they do not occur very often; the bankfull discharge moves sediment and occurs fairly often between 1 and 3 years for peak flood flow recurrence. Using the bankfull channel dimensions, one can develop a regional database relating drainage area to stable bankfull channel dimensions or a Regional Hydraulic Geometry Survey.

Table IV-A-1

Watershed Characteristics of Typical Creeks
Streams Subject to
Sediment Removal 		Watershed
(Mountain Range Origin) 		Drainage
Area
(sq. mi) 		Relief
Ratio 		Discharge (cfs)
100
Year 		10 Year 2.3 Year
San Francisquito Creek d/s Hwy. 101
 San Francisquito (Santa Cruz) 42.04 .02 9,300 4,900 2,000
Lower Penitencia u/s Berryessa Creek
 Berryessa (Diablo) 5.16 .04 1,200 770 420
Lower Penitencia d/s Berryessa Creek
 Berryessa (Diablo) 27.21 .003 6,700 3,900 1,900
San Tomas Aquino u/s Hwy. 237
 San Tomas Aquino

(Santa Cruz)

40.00 .04 9,000 5,900 3,300
Saratoga u/s San Tomas Aquino Ck.
 Saratoga (Santa Cruz) 16.56 .05 4,100 2,700 1,500
Los Coches u/s Hwy. 680
 Berryessa (Diablo) 4.02 .05 1,400 620 200
Calera d/s Escuela Parkway
 Berryessa (Diablo) 2.93 .17 920 430 150
Stevens Creek d/s L'Avenida
 Stevens Creek

(Santa Cruz)

38.04 .02 7,900 5,000 2,700
Sunnyvale East Channel u/s Guadalupe Slough
 Sunnyvale East 6.91 .004 1,200 510 430
Berryessa Creek,

confluence with Penitencia to Hwy. 237

Berryessa (Diablo) 22.05 .04 5,600 3,200 1,500

d/s = downstream of; u/s = upstream of

Note: The relief and slope characteristics of drainage basins can be expressed in a variety of ways, but probably the simplest is the relief ratio, defined as the difference in elevation between the highest and lowest points of the basin, divided by the length of the basin in a line roughly parallel to the major drainage. Within a region of roughly uniform climate and geology, this parameter is a useful index of sediment production and flood peaks (Dunne and Leopold, 1978).

Source: Mitchell Swanson

Geomorphic Setting for Streams Subject to Routine Maintenance

Foothill Canyon refers to the lower headwater area where the canyon floors periodically widen to create sediment deposition areas. These sites are often utilized as "debris basins" which are designed to trap coarse sediments and debris that may otherwise obstruct the channel in lower gradient reaches downstream.

Foothill Valleys are narrow valleys occurring below canyons where streams are relatively steep, straight and bounded by bedrock and terraces. Slight changes in gradient can cause deposition. A flat valley floor indicates long term sediment depositional processes.

Alluvial Fans are gently sloping fan-shaped masses of loose sediments deposited by a stream where it issues from a narrow mountain valley onto a plain or broad valley, or where a tributary stream is at or near its junction with the main stream. Alluvial fans occur along the edges of the Santa Clara and Coyote Valleys, and are natural areas of sediment deposition and laterally migrating channels. Alluvial fans are also highly indicative of long term sediment deposition.

Alluvial flood plain valleys are broad stream-formed valley floors created by the modern channel meandering between foothill or terrace valley sides. Streams in this setting are generally free to adjust their boundaries and change locations on the valley floor. Broad flood plain valleys are areas of sediment storage where net sediment outflow approximates sediment inflows. Flood plain valleys have experienced sediment deposition naturally for a considerable period of recent geologic time in the process of building a valley floor.

Deltas are low, flat lobes of recently deposited sediments at or near the mouth of the stream (including the upper tidal reaches) where the accumulation of sediments supplied by the stream exceeds the sediment transport capacity and competence of flow. Deltas are natural sinks of fine alluvial sediments where hydraulic conditions have low energy. Deltas in the District have been extensively modified by narrowing the stream into a single channel. This can have the effect of inducing sediment deposition in the natural delta zone, or shifting coarse sediment transport further toward the Bay into the tidal zone.

Tidal Channels: are formed by tidal flood and ebb currents within a salt marsh/estuary system below the delta zone of alluvial sedimentation. Flood tide cycles bring bay muds into channels; ebb tides flush channels in proportion to the tidal prism (or the volume of incoming tide).

Artificial Channels: Artificial channels are excavated channels, sometimes with levees, constructed to divert runoff around structures (such as bridges, developments, roads and freeways) or to provide drainage to reclaimed marsh plain.

c. Regional Factors Affecting Channel Conditions

1) Faulting

Santa Clara County is located in a seismically active region. Santa Clara County is transected by the San Andreas and Calaveras Fault zones. The San Andreas Fault Zone is located near the west edge of the county in the Santa Cruz Mountains. The Calaveras Fault zone bisects the county along the northwest-southeast trend through he Diablo Range. In addition, the southerly extension of the Hayward Fault zone is located within the county, a few miles southwest of the Calaveras Fault zone. These faults have been the source of several large historic earthquakes that have subjected Santa Clara County to strong shaking and are considered sources of future large earthquakes. Along the San Andreas Fault, a magnitude 8+ earthquake is possible with associated horizontal displacement of a few tens of feet. An earthquake of magnitude 7+ is possible along the Calaveras Fault with lateral displacements of several feet. Other potentially active faults cross the Santa Clara county area including the Silver Creek, Crosley, Monte Vista, Coyote Creek, and Sargent-Berrocal.

2) Tectonic Movements

Tectonic movements include both submergence (subsidence) and uplift. Movements of large land masses occur as a result of displacement along faults during earthquakes. The extent of these movements could affect local features such as water levels in lakes and coastline, and stream gradients. Horizontal displacements generally have little effect on sea or coastline levels; however, vertical movements could impact areas of uplift with the secondary effects of increased erosion and areas of submergence with increased sedimentation. In Santa Clara County, the predominant sense of tectonic movement is horizontal, dominated by strike-slip faulting, although some vertical movement have been documented. Future ground displacement will probably be predominantly horizontal with associated small amounts of vertical displacement. The direct effects of vertical movement will probably be noticeable along the San Francisco Bay shoreline where submergence or uplift of the shoreline could occur. Historical comparisons of first order geodetic leveling during the period 1912 to 1989 along west-to-east crossings in the San Francisco Bay region reflect active regional uplift of the central California Coast Ranges and differential vertical movement localized across the Hayward, Calaveras and other active fault zones (Gilmore, 1992). Average rate of measured historical uplift appear comparable with long term regional uplift rates that probably have averaged 1-2 millimeters per year since the middle Pleistocene (past 500,000 years).

3) Changes in Sea level

During Pleistocene time (about 2.5 million years to 10,000 years ago) the sea level alternatively rose and fell several times in response to world-wide glaciation and interglacial periods. Sea level during glaciation was 150 feet to more than 300 feet below present sea level (Schlocker, 1969). Changes in sea level can be attributed to tectonic uplift or submergence as discussed previously.

4) Land Subsidence due to Groundwater Withdrawal

Intensive withdrawal of groundwater from the alluvial aquifers in the San Jose area had caused a decline in the artesian head and resulted in land subsidence. A total of 12.7 feet was measured from 1916 to 1969 and was caused by the compaction of fine grained confining beds and interbeds as their pore pressure decreased (Poland, 1969, and Poland and Ireland, 1988). Subsidence resulted in flooding of lands in the southern part of San Francisco Bay (northern Santa Clara valley). Since 1967, recovery of artesian head has been substantial because of increase in surface water imports, favorable local water supply, decreased withdrawal and increased recharge.

5) Erosion and Landsliding

Landslides are common in the Santa Clara County area and are one of the chief agents of erosion. Major factors that control erosion and landsliding in the county include hilly terrain underlain by unconsolidated surficial deposits and weak sheared, fractured bedrock, occasional periods of prolonged rainfall, occasional large earthquakes, and man-made activities. Soil materials derived from erosion and landsliding contribute significantly to increased sedimentation in creeks and streams.

6) Human Activity

Most sediment removal sites are situated within geomorphic landforms that are naturally susceptible to sediment deposition, such as deltas or alluvial fans. The tendency of human development is to occupy these flat areas and modify waterways to accommodate the greatest volume of flow (water only, not sediment) in a channel with the least amount of width, regardless of sediment transport. In contrast, alluvial fans and deltas are areas where streams naturally deposit sediment by extensive lateral migration of channels over broad areas. Human occupancy of sedimentation-prone landforms represents an exposure to a geologic hazard such as that encountered in building in a flood plain, on an active landslide, or in a fault zone. This use has inherent risk, including a potential reduction in flood protection, higher costs for maintaining channel capacity, and addressing geomorphically unstable conditions.

7) Changes in Watershed Cover and Drainage Network

Grazing, vegetation clearing and urban development in the contributing drainage basins can simultaneously reduce the resistance of soils and non-cohesive sediments to erosion while increasing the amount of runoff. Replacement of natural vegetation with urban cover and highly efficient drainage systems increases the volume of runoff and the peak flow rate for frequent events. These changes often cause channel incision, bank erosion and channel expansion processes that produce significant quantities of sediment that can further disrupt downstream reaches and cause sedimentation problems.

Observations made along foothill valleys in the southwest and southeast portions of the District show that new developments have severe negative impacts on District streams with respect to geomorphic stability and sediment generation. Active channel incision and widening have been observed on streams below developments constructed within the past few years. Peak flow increases caused by additional runoff from subdivisions can cause significant widening and erosion quickly. These conditions may cause additional sediment removal problems downstream.

8) Channelization

Channelization projects designed to increase hydraulic capacity often expand channel dimensions and straighten channel pattern. The construction of channels to unnatural dimensions leads to sediment deposition as the stream attempts to re-create smaller, equilibrium dimensions. Channelization often leads to vegetation clearing and changes in the hydraulic conditions. Channel construction results in the reduction of shade that allows the proliferation of dense stands of tules and/or scrub willow. With some of the older designed district channels this then necessitates the need for further maintenance (i.e. removal) to sustain the design hydraulic roughness.

When human development and channelization removes width from a channel/flood plain system, natural sediment transport may continue to deposit sediment at the site, resulting in the loss of flow area and flood protection. In some cases, transport may become more efficient and send sediment loads further downstream where deposition would ultimately occur.

9) Effect of Hydraulic Structures

Hydraulic structures are features such as bridges, pipelines, bank protection, concrete channel lining, hydraulic transition structures (e.g. grade control and energy dissipation devices) that affect the hydraulics of flow in a stream and induce changes in sediment transport. For example, bridges with obstructing piers or deck structures can limit passage of flow and cause a "backwater" upstream. Backwater can reduce the sediment transport capacity to the point where deposition occurs upstream of the bridge. Backwater can also reduce flow capacity, which may induce flooding of the area. Levees, grade control structures and culvert crossings may also have hydraulic effects significant enough to alter sediment transport, sediment removal requirements, and impact aquatic habitat and migratory routes. Hydraulic structures on steeper channel slopes may also provide concentrated flows that create riffles and improve aquatic habitat.

d. Maintenance Issues in District Channels

1) Sedimentation

Sedimentation of District streams is a common and reoccurring maintenance requirement. Sediment generation and deposition is a natural, ongoing process that can be altered by human activities, especially efforts to control drainage and flooding and to reclaim flood plain areas for urban uses and agriculture. District sediment sources are either 1) stream deposited sediments that originated from the streambanks and surrounding watersheds or 2) marine muds deposited by tidal inflows from San Francisco Bay.

2) Stream Deposited Sediment

The headwaters in the uplands are steep and contain erosive terrain that produce relatively high volumes of sediment. The combination of steep terrain, generally fine grained and deeply weathered bedrock and the occurrence of moderate to extreme rainfall events favors episodic mass wasting or landsliding of hillsides into steep mountain streams, often during major floods.

The mountain and foothill streams emerge onto alluvial fans that straddle the edges of the valleys where rapid reductions in channel slope and increases in flood plain width cause sediment deposition. The alluvial fans merge with alluvial flood plains of larger streams on the valley floor (e.g. Coyote Creek, Guadalupe River, Llagas Creek) and eventually merge with the Bay alluvial plains and tidal marshes.

The ability of a given stream or flood protection channel to carry sediment is a function of flow, hydraulics, channel slope, sediment sizes and sediment volume. Coarse sediments (boulders and cobbles) are often easily transported in steep, confined mountain streams. Coarse sediments generally settle immediately below the mouths of upland canyons, foothills, valleys and at the heads of the alluvial fans that straddle the mountains (e.g., Penitencia Creek at Piedmont Avenue). Foothills (generally older, uplifted alluvial fans) and modern alluvial fans extend into the Santa Clara Valley streams such as the Guadalupe River and Coyote Creek. Alluvial streams of the valley floor transport sediment through processes of channel bed scour and fill in straighter, steep channels and point bar building and lateral erosion in meandering channels. The dimensions (channel width and depth) and the patterns (straight, meandering or braided) of alluvial streams reflect the sensitive balance between sediment, flow and human induced activities.

Most streams in the sediment removal program experience a progressive loss of sediment transport capacity (the potential volume of sediment moved by the hydraulics of a given flow) and competence (the maximum size of sediment particles moved by the hydraulics of a given flow) as they flow from steep headwater reaches into the low energy zone of the San Francisco Bay. As sediment transport energy decreases, sediment particles are laid down in progressively smaller, "graded" deposits. San Tomas Aquino Creek contains prime examples of graded deposits where gravel deposits below Scott Avenue grade into sand and mud over the 3 mile distance to the Highway 237 crossing. Often, alluvial sediments settle in the upper to mid tide zone (+8.0 feet to -0.2 feet MSL), an area that coincides with the modern delta and many of the District's sediment removal activities. The size and location of graded deposits depends upon available hydraulic force along the stream: for example, under certain conditions, gravel sized material transported by San Francisquito Creek in East Palo Alto can reach the tidal mudflats of San Francisco Bay, because it has a deep and narrow channel.

3) Bay Muds

Bay muds originated from alluvial sources that were subsequently deposited to form the broad mudflats of San Francisco Bay. Bay Muds enter District streams through a combination of wind-driven entrainment and flood (incoming) tide transport. Twice each day, a rising tide brings mud up into the channels (e.g. Lower Penitencia, San Tomas Aquino Creek), and as the tide turns to the ebb (outgoing) portion of the cycle, the mud settles and remains. Bay muds are a chronic source of sediment, regardless of rainfall and stream flow

e. Classification of Sedimentation Problems

Table IV-A-2 shows a system for classifying causes of sediment problems at individual sediment removal sites. The causal factors are a combination of the natural setting and human-induced sedimentation causes that underlay the problem at specific sites.

The classification scheme incorporates channel classification concepts developed by Rosgen. Rosgen (1994) merged the concepts of bankfull discharge and geomorphic stability into a stream classification system based upon bankfull channel dimensions, pattern and profile (slope gradient). The Rosgen stream classification system distinguishes different stream types as being the dominant stable channel forms by quantified measures of dimension, pattern and profile.

Table IV-A-2

Sedimentation Causal Factors

Site Type Setting Factors Sediment

Sources

 Rosgen Stream Type Representative

Streams

1 Alluvial Fan

Foothill Valley

Alluvial Flood Plain

 Reduced sediment transport energy due to reduction in slope, increased width and/or backwater Watershed

(headwater slopes and low order channels);

Fluvial (channel bed and bank erosion)

 A, B, C, F, G Saratoga,

Upper Guadalupe River,

Calera

2 Delta Decrease in slope, increase in flood prone width, tidal backwater Watershed and Fluvial Sources; Marine

Bay Muds

 C, D, and F San Francisquito,

Lower Penitencia,

Lower Guadalupe River,

Lower San Tomas Aquino,

Lower Stevens

3 Tidal Channel Marine Bay Muds deposited on incoming tide and lack of tidal prism to flush channel on ebb tide. Periodic extension of delta deposits in tidal backwater zone (<10.0 feet MSL) Marine Bay Muds and finer watershed sediments. F to tidal-formed channels (Rosgen system not applicable) Sunnyvale East,

Lower Penitencia,

Lower San Tomas Aquino

4

Channelized

Stream

Channels in Foothill Valley, Alluvial Flood Plain, Alluvial Fan, Delta and Tidal Settings

 Overly wide channels are out of equilibrium with natural regional channel morphology; insufficient sediment transport capacity Watershed, Fluvial, and Marine Bay Muds Not stable forms. Lower Stevens,

San Tomas Aquino,

Upper Berryessa,

Lower Penitencia,

Lower Llagas

5

Streams influenced by in stream Hydraulic Structures Disrupted sediment transport caused by backwater at bridges, grade control, or hydraulic transition structures Watershed, Fluvial and Marine Bay Muds Not applicable Los Coches,

San Francisquito,

Guadalupe River,

Lower Saratoga,

Source: Swanson, 1998

Type 1: Alluvial Channels at Slope Transition in Non-tidal Setting

Type 1 sedimentation settings involve a significant reduction in channel gradient such as what occurs on an alluvial fan or at the mouth of a mountain canyon. Type 1 streams are typically found along the edges of the Santa Clara and Coyote Valleys on alluvial fans or within Foothill Canyons and Valleys.


Sediment transport, both volume of sediment and size of material transported, is proportional to flow depth times channel slope. As a stream emerges from a steep and narrow canyon onto a broad valley, both depth and slope decrease abruptly as the confining walls of the canyon give way to a broad alluvial fan surface. Since both factors decrease, the size and volume of sediment transported decreases. Channel slope gradually decreases as a stream descends an alluvial fan towards the valley floor. The stream deposits sediment along its path, depending upon the size of the flood event.

Type 2: Delta Channels at Slope Transition in Tidal Setting


Type 2 channels occur in delta geomorphic settings where the effects of tidal sedimentation and hydraulics merge with that of the inflowing stream. Prior to human activity, these areas were broad, enabling the fluvial sediment load to deposit and merge with the tidal marsh plains. In this transitional area, channel slope often reduces dramatically at the head of tidal influence.

Type 3: Tidal Channels

Tidal channels are those that receive tidal inflow from San Francisco Bay, generally channels that have average elevations of +10.0 feet above Mean Sea Level or less. Within the county, tidal channels range from remnants of natural sloughs that were once part of large salt marsh/slough complexes, such as Guadalupe Slough, to artificial channels constructed behind the south bay levees in reclaimed marsh, such as Sunnyvale East Channel.

As described in the setting section, tidal channels are subject to sedimentation through a combination of wind entrainment of Bay Muds in San Francisco Bay and tidal inflows (flood tide). Sedimentation occurs regularly each day regardless of rainfall, stream flow and sediment delivery from terrestrial sources. The primary reason for tidal sedimentation in flood protection channels is a lack of tidal prism, or a sufficient volume of tidal ebb flow to generate scour. As discussed above, a clear relationship between channel cross sectional area and tidal prism has been established in the technical literature as well as in data collected in South San Francisco Bay. Most of the District's flood protection channels in tidal zones have been isolated from the vast marsh plains (now reclaimed and developed land) that contained the greatest proportion of tidal prism.

Type 4: Channelized Streams

Channelized streams are subject to sedimentation in a variety of geomorphic settings ranging from tidal channels to channels in alluvial fan areas. In an attempt to confine a large flow in minimal amount of flood plain width, flood protection channels are designed to maximize flow area and minimize hydraulic roughness. These designs often only consider "clearwater" hydraulic conditions and do not factor in sediment transport and geomorphic tendencies toward a channel configuration that reflects the flow /sediment supply balance. In other words, there is often a naturally formed channel configuration that is stable for the flow and sediment loads imposed upon it (further discussion below). Flood protection channels diverge from this natural tendency with exaggerated dimensions and flow areas. This results in sedimentation or erosion as streams move to a more stable form. An example of this is found on San Tomas Aquino Creek, which was channelized in 1962. San Tomas Creek also experiences tidal sedimentation in its lower reaches, however reclamation of adjacent marshlands has significantly reduced tidal prism and ebb flow scour.

Type 5: Streams Influenced by In-channel Hydraulic Structures

Hydraulic structures such as bridges, culvert crossings, bank protection structures, concrete channel sections and other structures can greatly influence local hydraulics and sediment transport which can lead to enhanced sedimentation conditions. Most common are bridge and culvert crossings.

In general, bridges form constrictions and often include flow obstruction structural support piers that extend from the bridge deck to the channel bed. Under sediment moving flow conditions, the bridge constriction and obstructing piers can cause flow to "back up" and create a backwater condition upstream. Backwater conditions generally reduce sediment transport thereby causing sediment deposition. In some conditions, recessional flows occurring after a flow peak may be capable of flushing deposited material when the backwater is removed, but not always. There may be riffles developed downstream. Debris or drift accumulation on the support piers or the bridge deck can worsen backwater and sedimentation problems. In cases where culvert pipes form the stream crossing, backwater may be present throughout sediment-moving flows of a storm.

Table IV-A-3 shows the geomorphic setting and the causal factors for most of the District streams with routine sediment removal. Nearly all of these maintenance reaches have been considerably altered by urbanization.

Table IV-A-3

Geomorphic Setting and Causal Factor of Sediment Removal Sites

   
Causal Factor Present
Stream Name Geomorphic Setting 1. Alluvial Fan Slope Transition 2. Delta Slope Transition 3. Tidal Sediment Source 4. Channel-ized 5. In-channel Structure
Adobe Cr. Delta   X   X X
Matadero Cr. Delta/Tidal   X X X X
Permanente Cr. Delta/Tidal   X X X X
Permanente Cr. Alluvial Fan X   X X  
San Francisquito Cr. Delta X X   X X
Stevens Cr. Delta   X   X X
Calabazas Cr. Delta   X   X X
Calabazas Cr. Alluvial Fan X     X X
Calabazas Cr. Foothill Canyon(Debris Basin) X       X
Regnart Cr. Alluvial Fan X     X X
San Tomas Aquino Cr. Delta/Alluvial Fan X X X X X
San Tomas Aquino Cr. Alluvial Fan X     X X
Saratoga Cr. Alluvial Fan (Debris Basin) X     X X
Saratoga Cr. Alluvial Fan X     X X
Sunnyvale East Ch. Artificial Channel   X X X  
El Camino Storm Drn. Artificial Channel/Alluvial Fan       X X
Guadalupe R. Delta/Tidal X X X  
Guadalupe R. Delta   X   X X
Guadalupe R. Alluvial Fan X       X
Los Gatos Cr. Alluvial Fan X     X X
Ross Cr. Alluvial Fan X     X X
Berryessa Cr. Delta/Alluvial Fan   X X X X
Berryessa Cr. Alluvial Fan X     X X
Berryessa Cr.

@ Sierra Cr. Conf

 Alluvial Fan X     X X
Calera Cr. Alluvial Fan   X   X X
Lower Silver Cr. Alluvial Floodpln>Alluvial Fan X     X X
Lower Silver Cr. Alluvial Fan/Artificial Channel X     X X
Lower Penitencia Cr. Delta/Tidal X X X X
Lower Penitencia Cr. Delta/Alluvial Fan X     X X
Upper Silver Cr. Alluvial Fan X       X
Norwood Cr. Alluvial Fan X     X X
Coyote Cr. Bypass Delta/Tidal   X X X X
Golf Cr. Foothill Valley/Alluvial Fan X     X X
Permanente Diversion Alluvial Fan/ Artificial Channel       X X
Junipero Serra Channel Alluvial Fan/ Artificial Channel       X X
Rodeo Creek Alluvial Fan X     X X
Canoas Creek Alluvial Fan X     X X
Greystone Creek Alluvial Fan X   X X
Guadalupe Creek Alluvial Fan X     X X
Randol Creek Foothill Valley/Alluvial Fan X     X  
Fisher Creek Alluvial Fan X   X
Los Coches Creek Alluvial Fan X     X X
Miguelita Creek Alluvial Fan X     X
Upper Penitencia Crk Alluvial Fan X     X X
Sierra Creek Alluvial Fan X     X X
Tularcitos Creek Alluvial Fan X     X
Thompson Creek Alluvial Fan X     X
Llagas Creek Alluvial Floodpln X     X
Jones Creek Alluvial Floodpln X     X
West Little Llagas Crk Alluvial Fan/Alluvial Floodpln X     X
West Little Llagas Crk Alluvial Floodpln X     X
East Little Llagas Crk Alluvial Floodpln X     X
Madrone Channel Artificial Channel X     X
San Martin Creek Alluvial Floodpln X        

Source: Swanson, 1998; TRA 3/01

f. Riparian Vegetation Management

The type and amount of vegetation in District streams varies considerably. Some channels are chronically inhabited by invasive, non-native plants. Others have mature stands of native riparian species. Many District streams require routine vegetation management in order to maintain the flood capacity of the channels. Section 2 of the SMP provides an overview as well as specific vegetation management procedures that the District conducts. Generally the District uses hand removal, mechanical, and chemical control as its primary methods for management on about 130 acres of in-channel vegetation.

g. Streambank Erosion and Stability Management

Erosion can occur because of hydraulic forces and geotechnical instabilities, and can be accelerated by human intervention and land uses. Accelerated erosion is typically a result of particular land uses that affect the stream corridor, including grazing, agriculture, and road and utility construction. Erosion of banks can result in increased sediment deposition, which can lead to decreased flood flow capacities and potential flood hazards. Erosion on banks may also cause vegetation and soil loss, damage to private or public property, transportation impacts, introduction of pollution, safety hazards, and turbidity injurious to fish and aquatic life. Levee erosion may lead to failure of the structure and flooding.

Bank protection work may either occur as repair of an existing bank protection project which is failing, or as new work along a bank which is eroding. The new work is considered routine maintenance because it is either restoring the flood protection function of a modified channel or it is repairing a natural bank to its approximate condition prior to becoming an erosion problem.

Repair of existing bank protection structures occurs when these structures fail and are replaced with in-kind, in-place materials. New bank protection projects are those that repair or protect the watercourse from further degradation or erosion using the softest method appropriate. This type of protection is considered maintenance if the maintenance does not significantly alter the flood conveyance capacity of the streams.

The District estimates that an average of 5,000 linear feet of banks may be repaired annually based on historical records, District experience, and current levels of funding. This is an average annual quantity and will vary from year-to-year. Facilities are inspected after the winter storms for damage and maintenance needs. Bank protection work is identified and work conducted in any given year is balanced with annual budget constraints.

In the past 14 years, the total length of bank protection activities in an individual year ranged from approximately 1,500 to 13,000 feet. The District has completed an average of 38 bank protection jobs per year, based on historical records, but there is considerable deviation. For example, there were nine jobs in 1994 and 73 in 1987. A more detailed summary of historical bank protection activities is provided in Chapter 5 of the SMP.

3. Significance Criteria

The threshold of significance for a geomorphic impact is defined as a project condition or activity that alters stream channel dimensions, profile, and stability (including the removal of vegetation) to the point where significant new erosion and sediment production would result. The manifestation of the geomorphic impacts may take years to develop and/or could be overshadowed by regional changes such as subsidence. It also can happen in a very short period as during a flood. However, it is important to examine the District's SMP and its associated sediment removal, vegetation management, and bank protection activities to determine if there is a potential for geomorphic impacts. A maintenance project would have a significant geomorphic impacts if it would:

Criterion Geo-1 Accelerate erosion downstream of the maintenance site;

Criterion Geo-2 Instigate new bank instability problems that could lead to loss of property or damage structures;

Criterion Geo-3 Cause a blockage to hydraulic structures or fish passages.

When evaluating potential geomorphic impacts it is important to keep in mind that many of the channels that the District maintains have been significantly altered and the equilibrium status of the channel is deliberately kept permanently out of balance to maximize flood water conveyance capacity. If maintenance were ceased in many of these channels they would strive to regain the equilibrium state. The channel would naturally tend to develop a more meandering pattern and the evolution of a low flow channel would begin. Tidal channels would fill with sediment and the channel would change alignment.

These more natural streams forms function only in the context of a defined flood plain. In most of the urban area of Santa Clara County, intensive land uses have encroached on the flood plain. Thus, many of these responses though natural are obviously unacceptable because of increased flood threat. The District is required to intervene and maintain conditions that would be an unstable channel in a natural, unaltered environment.

The salient question then becomes, does the District's ongoing maintenance program simply maintain the channels or does it increase the overall geomorphic instability of the system leading to increase maintenance and environmental degradation? There is a sharp distinction between realigned, constructed channels and natural or quasi-natural channels. Realigned channels typically do not have significant meander patterns and do not exhibit naturally occurring geomorphic conditions. Their initial construction created a geomorphically unstable condition; maintenance activities in these channels have little incremental geomorphic effect. Maintenance activities in natural and quasi-natural channels, however, have more potential to effect the geomorphic properties in the adjacent channel reaches. The broad impact categories are discussed below as they pertain to the SMP procedures.

4. Impacts and Mitigation

The geomorphic impacts associated with the SMP are presented according to the maintenance categories. The potential geomorphic impacts identified in this section could occur at some locations and may not be apparent for many years. In general, the remedy for geomorphic impacts is ongoing maintenance; ideally the potential for impact is avoided through the maintenance project design process. The mitigation for these impacts is presented as Best Management Practices (BMPs). These BMPs are specified by number cross-referenced on the BMP list accompanying this the SMP and EIR.

a. Sediment Removal

Impact Geo-1. Sediment removal in certain locations may increase erosion downstream of the removal site. Removing sediment from a channel fundamentally alters the quasi equilibrium state described in the setting section of this chapter. River and streams have a potential to move and transport sediment based on their slope and flow regime. When sediment supply is out of balance with transport ability, the channel will adjust its geometry to the sediment supply. If the sediment supply is reduced, and the stream maintains its transport potential, the channel will adjust its dimensions and path to restore the balance with its potential. This adjustment may cause erosion to occur in the vertical and/or lateral directions of the stream course. This impact pertains to Criterion Geo-1 Accelerate erosion downstream of the maintenance site.

This type of impact would occur in the steeper channel slope found in the transport zone of the stream courses where sediment is actively moving. This effect may also occur without sediment removal where sediment is trapped behind culverts and bridges. The basic premise is that the sediment transport system has been altered. This potential impact may take several rounds of sediment removal before significant noticeable erosion features begin to develop downstream. This potential impact could occur at locations that require semi-regular removal projects (e.g. every 3 to 5 years). Sediment removal could reduce long-term sediment delivery rates to downstream channel reaches. The sediment removal at these sites reduces the sediment supply and size in the channel bottom. Each sediment removal site needs to be evaluated individually over the long term to identify trends.

Most of the District's sediment removal activities occur in the depositional zone of the channels and are not prone to this type of impact. If accumulated sediment is not removed in the natural accumulation zones (i.e. low slope and tidal) then the channel will adjust and that adjustment may instigate lateral movement of the channel, increasing erosion.

Mitigation

Mitigation for this potential impact is primarily accomplished by observing sediment removal work in the transport zone of District streams and modification of future maintenance to prevent adverse changes downstream.

1.13 Prevent Scour Downstream of Sediment Removal

Consistent application of the BMP will reduce potential impact to a less than significant level.

b. Vegetation Management

The District conducts extensive vegetative management activities, most outside of the active stream channel. Vegetation management within the active channel is done to reduce channel roughness to restore hydraulic capacity to a channel reach. In-channel vegetation and sediment are usually related: accumulated sediment provides the substrate for emergent vegetation and the vegetation holds the sediment. In many reaches, the District alternates vegetation removal and sediment removal as part of the ongoing maintenance of flood capacity.

Impact Geo-2. Elimination of in-channel vegetation may increase sediment accumulation downstream. This impact pertains to Criterion Geo-3, Cause a blockage to hydraulic structures or fish passages.

In areas of generally active sediment transport, low velocity caused by vegetation may prevent sediment release downstream. Mechanical or hand vegetation management to reduce roughness leaves the roots in place. Systemic herbicide application would also kill plant roots and would be more prone to the effect. Release of trapped sediment could increase sediment accumulation downstream.

A sudden increase in sediment could affect downstream geomorphic stability. This could 1) cause more rapid siltation and possible blockage at a culvert or fish ladder, or 2) cause deposition as point bars that cause flows to be deflected to channel banks, which could result in scour/erosion and bank instability. Both of these types of impacts would be addressed by the maintenance practices described in the SMP. However, it is the potential of impact increased sediment release that could possibly increase the frequency and severity of these routine maintenance actions.

The movement of the trapped sediment can not be precisely predicted: weather and runoff patterns will affect the process. Moderate winter flows or drought conditions may move the sediment slowly not causing any significant downstream effects. The greatest effect would likely occur if a significant flow event immediately followed a significant vegetation removal.

Many areas in the lower reaches of the District's channels are in natural deposition zones. Vegetation management in these areas is unlikely to have significant effects on sediment transport.

In many locations vegetation management both from herbicide applications and manual methods is consistent and routine; these areas are less likely to experience this effect. Creek reaches which have not undergone routine vegetation maintenance, or reaches where the vegetation control system changes may have greater impact potential, because they may have more trapped sediment within the vegetation. This effect could occur in the southern portions of the County that will be switching from mechanical to herbicide vegetation management.

Alternatively, vegetation removal within the transport zone of a stream reach may have beneficial effects. Typically, large stands of emergent vegetation can trap significant amounts of bedload sediment. In these locations the release of sediment into the system can prevent the sediment starvation erosion impacts described above.

Mitigation Prior to herbicide application within channels, the potential for significant sediment release will be assessed.

1.14 Minimize Sediment Transport Downstream from In-channel Herbicide Sites

Consistent application of the BMP will reduce potential impact to a less than significant level.

Impact Geo-3. Removal of vegetation may increase local erosion. The objective of vegetation removal is the decrease in channel roughness and, consequently, an increase in flow velocity. The greater flow velocity has a tendency to increase the erosive force on the banks. This impact pertains to Criterion Geo-2 Instigate new bank instability problems that could lead to loss of property or damage structures.

Mitigation The effect is controlled by minimizing vegetation control to that required by the maintenance guidelines.

1.16 Minimize Local Erosion increase from In-channel Vegetation Removal

Consistent application of the BMP will reduce potential impact to a less than significant level.

c. Bank Protection

A basic maintenance activity is the repair and maintenance of the stream banks within District's streams. The variety of banks and forces within the districts channels mandates significant variability in the techniques which are applicable to each individual site.

The District strives to utilize biotechnical or soft bank stabilization approaches to the greatest extent possible. The design of bank protection under the SMP is intended to follow the site evaluation and treatment selection process described in SMP Appendix E. Programmatic Impact Assessment and Mitigation for Routine Bank Protection Activities. In some instances the erosional forces and land use constraints mandate the use of harder structures that utilize rock and concrete for channel stability. The impacts of bank stabilization structures on the immediate channel area and downstream of the site need to be considered.

Installation of bank protection measures can change the hydraulic geometry of a channel. For example, bank full channel geometry is important to the overall function of the stream course from a flow and sediment transport perspective. Eliminating a bankfull channel can destabilize the channel and result in unpredictable erosion patterns

Impact Geo-4. Bank protection measures can direct flows downstream, resulting in new erosion and bank instability problems at locations downstream of the repaired site. The redirected flows may result in prolonged bank instability problems to downstream locations. These downstream impacts could include toe scour and/or bank sloughing, and may include loss of riparian vegetation associated with erosion. This causes a need for additional bank protection downstream in response to effects of the initial bank protection placement. Earthen banks devoid of vegetation downstream are particularly sensitive. This impact pertains to Criterion Geo-2 Instigate new bank instability problems that could lead to loss of property or damage structures.

The effect can occur with hard bank protection that creates a smooth channel surface such as sacked concrete or small rock. Even biotechnical measures can have an impact, as vegetation may not be fully established for several years. Therefore, a biotechnical bank repair may act as a "hard" structure for the first few years, and potential downstream impacts need to be considered.

Mitigation Bank stabilization site design will assess hydraulic effects immediately upstream and downstream of the work area. If the hardscape revetment would cause significant increase in erosion potential, downstream energy dissipation features such as pools or grade control structures would be considered in the design. In some instances, proactive protection of downstream areas would include, but are not limited to, coir logs, riparian enhancement planting, strategic placement of rock, and flow deflectors.

In many cases, biotechnical techniques that stress vegetation in design are the most applicable. These are called for in BMPs intended to implement policies protecting riparian and aquatic habitat.

1.15 Prevent Erosion Downstream of Bank Protection Sites

Consistent application of the BMP will reduce potential impact to a less than significant level.

d. Minor Maintenance

With the exception of debris removal, most minor maintenance does not affect the in-channel area.

Impact Geo-5 Removal of woody debris may reduce channel bed diversity. In some cases maintenance involves the removal of downed or woody debris in the channel. Woody debris in the channel creates channel bed diversity which in turn creates habitat, and is part of the natural stream morphologic process. Removal of debris may have environmental benefits by improving fish passage in the channel. Where debris is clogging hydraulic structures impacting capacity or poses a threat to bank erosion and property then the debris will be removed.

Mitigation Implementation of BMPs directed at aquatic habitat conservation will also serve to limit geomorphic effects.

3.9 Retain Woody Materials and Vegetation

Consistent application of the BMP will reduce potential impact to a less than significant level.

e. Cumulative Effects

Within the SMP

The various aspect of stream maintenance all interactively define the channel shape, cross-section, and surface roughness, and hence act cumulatively to produce the baseline condition of managed channels in the urban areas of Santa Clara County. The potential impacts described above are examples of cumulative interaction of maintenance activities.

The District seeks to minimize the adverse interactions of maintenance through the maintenance design process. As described in the SMP, maintenance is initiated when channel conditions exceed prescriptions set in the Maintenance Guidelines. The maintenance design process evaluates the fluvial geomorphic conditions of the site, taking into account current site conditions and likely future trends in the channel. The evaluation includes determining:

With Other Activities

Other District activities affect channels, as defined in Chapter II, Program Description. Capital Improvement Projects (CIP), such as construction to provide increased flood capacity on a certain reach of stream, undergo a separate engineering design and environmental review. As set forth in the SMP, the CIP must include an assessment of the maintenance implications of the new channel design based on an analysis of the hydraulics and fluvial geomorphology of the reaches up- and down-stream. This process is intended to minimize adverse cumulative effects on channel stability and the maintenance obligation of the District.

Activities by others may adversely affect channel stability by improper storm water discharge, erosion adjacent to the channel, or improper activities in the channel itself which can produce increased sediment. The District uses its jurisdiction, owned right-of-way and the encroachment permit review process to minimize the cumulative effects of the activities by others.

In the long term, District participation in the Watershed Management Initiative, and sediment source reduction has the potential to cumulatively reduce the excessive sediment input into District managed channels and reduce the component of geomorphic instability that requires sediment removal and consequential environmental impact.

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