Chapter II: Alternatives
This section presents three alternative approaches for the
future of the Cascades Diversion Dam. The No Action Alternative represents the
status quo; Alternative 2 involves complete removal of the dam, the dam
abutments, the intake structure, and the screenhouse, and restoration of the
dam site; and Alternative 3 involves complete removal of the dam, the river-left
dam abutment, and the screenhouse on top of the intake structures, preservation
of the river-right abutment and the intake structure as a viewing platform, and
restoration of the dam site. Each alternative discussion includes a summary of
the overall approach, a narrative description of the alternative, and graphics
detailing the alternative. This section also provides a brief description of
alternatives initially considered but not selected for further analysis.
Following the alternatives descriptions is a summary comparison of the
alternatives, including key features and the environmental consequences of the
alternatives.
This project tiers off of the Yosemite Valley Plan and its Record of Decision, the latter of which approved
actions to remove Cascades Diversion Dam. The alternatives in this chapter
present a range of options to achieve the goal of dam removal.
Overview of the Alternatives
The No Action Alternative maintains the status quo at
Cascades Diversion Dam, as described in Chapter III, Affected Environment. It
provides a baseline from which to compare the action alternatives, to evaluate
the magnitude of proposed changes, and to measure the environmental effects of
those changes.
Under the No Action
Alternative, Cascades Diversion Dam would remain in its existing condition,
without maintenance or repair (figure II-1). The hydroelectric generating
facility was decommissioned in 1986, and no significant repairs to the
dam have been made since that time. Serious damage to the timber overflow crest
of the dam was sustained during the 1997 flood (see photos). There are voids
under the existing timber crib sheathing, and many boards are on the verge of
collapse. Although no action would be taken under Alternative 1, the National
Park Service would continue to make minor repairs to attendant structures such
as the screenhouse, concrete platform, and safety railing on the intake
structure. The National Park Service would also remove large debris from the
river and banks that has been deposited downstream due to continued
deterioration of the dam over time.
Under the No Action Alternative,
Cascades Diversion Dam would continue to pose a threat to public health and
safety, potentially resulting in human fatality. Although swimming and fishing
are not allowed at the dam and adjacent areas, people occasionally walk on the
dam’s wooden crest and the exposed riverbanks and rocks in close proximity to
the dam and could be exposed to hazards such as falling from the dam structure.
In addition, people recreating in the river near the dam could be exposed to
hazards due to falling dam debris or dam failure.
FIGURE II-1
EXISTING FACILITIES [PDF]
Under the No Action Alternative, no management action
would be taken to repair or remove the dam. This condition of benign neglect
(see Chapter VIII, Glossary) would be expected to eventually result in the
uncontrolled failure of the overflow portion of the structure, due to large
flood flows carrying debris or to continuing deterioration of the timber
components under normal streamflow conditions. Dam failure would likely occur
during or following a period of high flow and could be sudden or could occur
over a course of years or decades.
 An uncontrolled failure or continued
deterioration of the dam over time would result in a release of impounded water
and the deposition of concrete and timber debris, grouted rockfill, and
impounded sediment downstream of the dam. Dam debris – concrete, timber,
grouted rockfill – would litter the downstream channel of the Merced
River, impeding free flow and harming resources such as habitat for
sensitive species and scenic views of the river, and creating possible hazards
to park facilities and visitors. The National Park Service would employ early
warning evacuation procedures for downstream areas in the event of an
uncontrolled dam failure.
 All or portions of the concrete abutment structures on
both banks and the intake structure would likely remain following failure of
the overflow structure. About 5,045 cubic yards of rock, concrete, and timber
debris could be deposited downstream following dam failure.
Following dam failure, sediment
that is impounded behind the dam would be transported downstream with riverflows.
The total estimated sediment behind the dam is 15,000 to 20,000 cubic
yards, including a rock and sediment island that was present before the dam was
constructed (see Chapter III, Affected Environment, Alluvial Processes); rock,
cobble, and sediments that were excavated during dam construction and
left in place upstream of the dam; and sediment that has accumulated since
construction of the dam (Kennedy/Jenks 2002). Some of this material would
likely remain in the existing impoundment area following dam failure, and the
island upstream of the dam is expected to remain relatively stable given its
presence prior to dam construction.
Sediments are expected to wash downstream within the first
year after complete dam failure (Kennedy/Jenks/Chilton and ROMA Design Group
1988). Sediment transport is expected to be highest in March and April during
spring snowmelt. While some sediment would likely remain in the existing
impoundment area, under the worst-case scenario (in which the full 15,000 to 20,000 cubic
yards of sediment would be released), sediment transport would range from approximately
123 tons per day (April) to 173 tons per day (March) during this period.
Sediment concentrations are expected to range from 60 milligrams per liter
(October through February) to 160 milligrams per liter (spring). Most of the
bedload would remain suspended through the steep gorge and sand and gravel
would likely deposit in the vicinity of Cascades Picnic Area, where the slope
is gentle (see figure II-2). Coarse materials would deposit in the upper end of
the reach, with finer material depositing throughout the reach. The depth of
the deposited material would depend upon the specific depositional patterns and
riverflow. Under the worst-case scenario (described above), the depositional
width could range from 100 to 200 feet and the depositional length from 2,000
to 5,000 feet; given these ranges, the average depositional thickness would be
from 0.4 to 2.7 feet at the Cascades Picnic Area.
Less than 1% of the material impounded behind the dam is
finer than 0.07 millimeter in diameter. The fine material would be
transported downstream for some distance and would likely settle out and deposit in areas of low-flow energy, such as
pools or downstream reservoirs. The transported fine material would contribute
minimally relative to background turbidity levels in the lower part of the
river (NPS 2001a). The largest increase of turbidity above background values
(see Chapter III, Affected Environment, Alluvial Processes) would likely
occur the first time the riverflows through the sediments formerly impounded by
the dam. This rise in turbidity would probably last on the order of a few
hours. After the initial rise in turbidity, there may be smaller spikes in turbidity as flood flows pass through the
impoundment region and gradually erode more sediment and expose the fine
material. However, each successive turbidity spike should be smaller than the
previous one until the turbidity levels return to normal (USBR 2001).
The National Park Service would remove dam debris from the
Merced River as soon as feasible following dam failure
or following the release of dam debris due to ongoing deterioration. However,
removal activities would not likely commence until low-flow conditions, which
could be several months after dam failure or debris release. Debris removal
could occur in a single season (following sudden failure) or over the course of
years or decades (continued deterioration). Debris would not be removed where
site access is unavailable. Equipment that could be used during debris removal
activities includes: cranes, excavators, backhoes, skid steer
FIGURE II-2
RIVER PROFILE [PDF]
loaders, trucks, graders, jackhammers, concrete saws, etc.
To the extent possible, cranes and other equipment would be operated from the
riverbank; however, haphazard debris placement could require equipment to enter
the river channel. Debris removal activities would likely occur downstream of
the Cascades Picnic Area. However, under extreme high flows dam debris,
particularly floatable debris such as timbers, could be transported as far
downstream as El Portal, requiring removal efforts in this access well. Due to
the uncertainty regarding the amount of time required to remove debris and the
amount of dam debris that could be recovered from the river, the cost and
number of truck trips associated with debris removal under the No Action
Alternative cannot be determined.
The channel of the Merced
River
would be expected to naturally narrow and deepen as impounded sediments wash
downstream. There would be a progressive lowering of the base level of the river
channel until the river reach attains equilibrium. Water levels are expected to
lower slightly at the upstream end of the water impoundment, up to approximately
10 feet in some locations immediately upstream from the existing dam
structure. As described above, an island was already present upstream from the
dam prior to dam construction. This island would likely remain following dam
failure.Local scour would occur at
the slope where the diversion dam was located and is expected to develop into a
channel incision that would proceed upstream (headcut) through the impoundment
area. Meander in the reach is anticipated to be minor, since the incision would
tend to follow the channel course that existed prior to dam construction in 1917
(Kennedy/Jenks/Chilton and ROMA Design Group 1988; Kennedy/Jenks 2002). Channel
adjustment would be episodic, with periods of incision followed by sediment
storage and then renewed incision. The channel is expected to stabilize at or
near its natural level over a period of years (Kennedy/Jenks/Chilton and ROMA
Design Group 1988). Due to the nature of the material in the channel –
primarily boulders, cobbles, and gravels – little or no channel incision is
anticipated upstream of the present impoundment. The final bed profile would
likely be a smooth slope connecting the reach downstream of the dam to the reach
upstream of the dam.
Alternative 2:
Complete Dam Removal
(Preferred)
Alternative 2 includes complete removal of the dam, the
dam abutments, the intake structure, and the screenhouse, and restoration of
the related river channel located beneath the dam site (see figure II-3).
Approximately 4,400 to 5,400 cubic yards of sediments (including rocks and
boulders) in the area upstream of the dam would be excavated and repositioned
to stabilize the river-right bank and decrease the potential for sediment
erosion. Figure II-4 indicates the river profile at Cascades Diversion Dam
before and after removal of the dam structure and sediments. Natural river
processes would continue to transport remaining sediments (up to a maximum
range of approximately 9,600 to 15,600 cubic yards of sediment) from the
impoundment area over time, allowing for a gradual re-establishment of the
natural river channel and related riparian habitat. It is expected that the
river would fully recover over time as sediments are transported from the
impoundment area. However, the rate of natural channel recovery and restoration
would be monitored to determine if additional restoration actions were
necessary. Following removal of the river-right abutment, intake structure, and
screenhouse, the river-right bank would be stabilized using a bioengineered bank stabilization system
to prevent erosion. The objective of this alternative would be to restore the
natural river character with a mixture and distribution of boulders, cobbles,
gravels, sand, silt, soil, and vegetation similar to those found in adjacent
riverbank segments.
FIGURE II-3
Alternative 2 [PDF]
FIGURE II-4
Cascades Diversion Dam Profile [PDF]
In-channel work, bank stabilization, and revegetation
would be completed within a two- to three-month period during the fall of 2003
(September through November) when the flow of the Merced River
is typically lowest (less than 200 cubic feet per second). The overall project
duration would be approximately five months. Dam removal would occur in three
phases, as follows:
§
Phase 1 –Dam
Removal Setup
-
Detour traffic to the north side of El
Portal Road to create a staging area adjacent to
the dam site
-
Install interpretive displays within the park
§
Phase 2 – Dam
Removal
-
Draw down impoundment (dewater) using the waste
gates in the existing intake structure
-
Install coffer dam to protect the active removal
site from riverflows
-
Establish temporary diversion channel crossing
bridge near intake structure on the river-right bank
-
Partially excavate impounded sediment near
upstream face of dam
-
Demolish timber overflow section of dam using
large excavator on workpad
-
Remove the river-left abutment concrete
structure
-
Excavate pilot channel upstream of sluiceway
through sediment to river channel
-
Divert riverflow through pilot channel and
remove diversion channel crossing
-
Remove remainder of dam and screenhouse and
mechanical items from the river-right bank
-
Remove the river-right abutment and intake
structure; below-grade intake structure to remain
§
Phase 3 – Site
Restoration and Cleanup
-
Reconfigure excavated sediments along river-right
bank for bioengineered bank
stabilization system
-
Install bioengineered
bank stabilization system using native river rock and vegetation. Match
biological, visual, and structural characteristics of the upstream and
downstream riverbanks
-
Demobilize equipment, remove traffic detour and
contractor use area, and repave parking area
Total estimated excavation volumes for Alternative 2
include:
§
Impounded
Sediments – 4,400 to 5,400 cubic yards
§
Rock,
Concrete, and Timber – 5,160 cubic yards
It is estimated that up to approximately 260 loaded truck
trips would be required to remove excavated rock,
concrete, and timber under Alternative 2. In addition, project activities would
generate additional truck trips between the dam and a secondary project staging
and storage area at Pohono Quarry. The number of daily truck trips between the
dam and the quarry would vary and is expected to be infrequent during most
project activities, but could be as high as 20 trips per day during some
project activities, such as during removal of timbers and rockfill. The total
estimated cost to implement Alternative 2 is between $2 and $2.6 million.
Description of Dam Removal Activities
Phase 1 – Dam Removal Setup
Vehicles and workers required for dam removal would access
the site from either El Portal Road
or Big Oak Flat Road. El
Portal Road would be temporarily realigned approximately 30 feet to the north
through an existing paved parking area and previously disturbed earthen
embankment (see figure II-3). Although realigned to the north, two lanes
of El Portal Road would be
open during dam removal. Staging for equipment access and storage would be
immediately adjacent to the main stem of the Merced River
upstream of the dam along the river-right bank (see figure II-3). The
staging area would occupy approximately 6,250 square feet (25 feet by 250 feet)
of El Portal Road. In
addition, Pohono Quarry, located approximately one mile upstream of the dam, to
the north of Pohono Bridge and El Portal Road, would be established as a
secondary staging area for the storage of equipment that would be used
infrequently during project activities (i.e., not needed on a daily basis), and
for storage and sorting of material removed from the dam that would be reused, recycled,
or disposed (outside the park). Interpretive displays and information regarding
the proposed project would be made available at the Yosemite
Valley Visitor Center
and/or the Cascades Picnic Area.
Phase 2 – Dam Removal
Dam removal activities would be performed during the low-flow
conditions in the Merced River that typically occur from
September through November. The water level in the impoundment would be drawn
down to approximately 7 feet below the dam crest, using the main and
auxiliary waste gates in the existing intake structure (assuming a riverflow of
approximately 200 cubic feet per second). Additional drawdown capacity would be
achieved by removing the stoplogs from the existing sluiceway near the river-left
abutment, and by a partial breach of the timber cribbing near the river-left
abutment. Some transport of impounded sediments would be expected as the
approach channels deepen, as well as headcut due to the lower impoundment level
and higher flow velocities.
A temporary coffer dam would be installed to protect the
active removal site from riverflows. Riverflows would be shifted toward the
river-right bank. The river-left portion of the channel (active removal area)
would be dewatered. A temporary bridge would be installed near the entrance to
the intake structure to permit equipment access across the diversion channel
from the river-right bank to the exposed island immediately upstream of the
central portion of the dam. The temporary bridge would be constructed of a
railroad flatcar (or similar structure) supported by precast concrete blocks or
steel sheetpiling (installed by vibratory methods).
The sediments at the upstream face of the dam would be
removed to expose the concrete headwall, with an assumed 3:1 backslope
excavated between the base of the headwall and a level workpad. For a 10-foot
depth, this would place the top of the backslope approximately 30 feet from the
headwall. The workpad would be used to support equipment and would be built
upstream of the dam using excavated sediments.
The concrete headwall would either be
broken into pieces using a hoe ram attachment on a large tracked excavator
(Caterpillar 375 or equivalent), or in sections along vertical joints. Timber
cribbing would be demolished in lifts from the workpad. As the work progresses,
additional sediment would be removed to expose the lower portions of the
structure. All other materials would be stockpiled for reuse on site and/or
removed from the site for use within the park or disposal at an approved and
licensed facility, as described below under Phase 3 – Site Restoration and
Cleanup.
It is assumed that some of the interior
rockfill has been grouted and would require greater effort to excavate. Two rows
of timber sheetpiles approximately 80 feet in length and 10 feet deep
beneath the river-right half of the structure would also be removed, either
using a mobile crane or an excavator bucket with a thumb attachment. Unnecessary
disturbance of boulders or bedrock within the river channel would be avoided.
All debris would be loaded into trucks for removal from the site, as describedbelow under Phase 3 – Site Restoration
and Cleanup.
The river-left abutment structure consists of an L-shaped
gravity wall with a total estimated concrete volume of 600 cubic yards. The
structure rises about 30 feet above its foundation to an elevation of
3,816 feet and is surrounded by trees and other riparian vegetation.
Following removal of the river-left abutment and overflow structures, a pilot
channel would be excavated within the impounded sediments upstream of the
existing sluiceway and roughly parallel to the river-left shoreline. This
channel would divert riverflow away from the river-right shoreline and
facilitate removal of the river-right abutment, the intake structure, and the remainder
of the dam. The pilot channel would have a bottom width of 5 to 10 feet
and sideslopes of 2:1, extending approximately 400 feet upstream; approximately
1,000 to 2,000 cubic yards of material would be excavated. The invert of the
pilot channel would be approximately 10 feet below the dam crest and would intersect
the river channel approximately 500 feet upstream of the dam. Excavated
materials would be placed in temporary stockpiles for reuse during site
restoration. The pilot channel would convey approximately 500 cubic feet of
water per second, with a variable depth of approximately 3 feet. Minor
transport of sediment is expected during the changeover in flow channels from
the river-right to the river-left side of the river, and during initial
operation of the pilot channel. It is expected that periods of higher water
flow would gradually widen the pilot channel to convey larger flows.
Following redirection of the riverflow away from the river-right
side of the river and into the newly excavated pilot channel, removal of the
remainder of the dam (approximately 40%), the river-right abutment, intake
structure, and screenhouse would begin (see photo). In-river activities and mechanical
equipment would include individual operators for the two waste gates and for
the penstock slide gate, a traveling fish screen, and a trashrack. The
reinforced-concrete deck, sidewalls, and air-vent pipe encasement (above the
concrete penstock pipe) would be removed. The below-grade portion of the intake
structure would be retained. The total volume of concrete and rock removed from
the intake structure would be approximately 110 cubic yards. Approximately 60
linear feet of stone wall would be installed along the roadway to connect
adjacent wall segments.
Phase 3 – Site Restoration and Cleanup
The last phase of the project is site restoration. Once
the dam and attendant structures are removed, the riverflow characteristics in
the area are expected to change. The approach channel immediately upstream of
the dam would undergo gradual degradation from increased flow velocities. The
river is expected to cut a channel through the remaining impounded sediments,
narrowing and deepening the river channel. The river would divide into a
river-right and river-left channel around the existing island above the dam
site. The decrease in riverbed elevation would necessitate the placement of
bank slope protection on the river-right side of the channel (USBR 2001).
Exposed soil would be covered with a combination of
locally acquired native duff and forest litter from adjacent riparian sites to
provide immediate groundcover and facilitate natural revegetation of the site. On
the river-right bank, a bioengineered slope protection system would be
installed to protect the riverbank from erosion upstream and downstream of the
removed dam, and at the below-grade intake structure that would remain
following dam removal (see figure II-3). The bioengineered slope protection
would be constructed of native vegetation and boulders and designed to match
the natural conditions of upstream and downstream riverbanks to the extent
possible (see figure II-5).
The bioengineered bank stabilization
system would be constructed using standard techniques, including brush layering
incorporated into a boulder structure. Other materials may be incorporated,
including logs and root wads. Boulders would not be grouted into place.
Approximately 4,400 to 5,400 cubic yards of excavated sediment (including
native river rock/boulders) would be reconfigured on the river-right bank and
incorporated into the bioengineered bank stabilization system. Sediments
excavated during dam removal are expected to generate enough rock and sediment
to meet the needs of the river-right bank stabilization system. No imported rock
fill or riprap would be used (USBR 2001). Equipment used to perform restoration
activities may include excavators, bulldozers, loaders, cranes, dump trucks,
pumps, and water trucks. Restoration would be consistent with the project’s
U.S. Army Corps of Engineers Special Site Permit (refer to Chapter VI,
Consultation and Coordination).
FIGURE II-5
BIOENGINEERED SLOPE PROTECTION [PDF]
The project area would be contoured and finished with
sediments to facilitate both natural and assisted revegetation by native
species that grow in the site-specific conditions present in the Cascades
Diversion Dam area. Appropriate planting prescriptions for revegetation have
been developed, including appropriate plant species and their placement in
relation to Merced River water levels. Species suitable
for planting adjacent to the water would include a variety of rush species
(such as small-fruited bulrush, sedge, and beaked sedge), intermixed with
willows (including sandbar willow, red willow, and arroyo willow). Other
species planted in this area may include horsetail, dogbane, and goldenrod.
Native grasses would be planted throughout the floodplain. Herbaceous species
would be re-established through hand-application of locally acquired mulch from
adjacent sites that support these species and hand-application of seeds,
minimizing the potential for introduction of non-native species. Seedlings of
appropriate species, such as white alder, would be planted within the
bioengineered slope at the bankfull mark. Black cottonwood and bigleaf maple cuttings
would be planted near the upper limits of the riparian zone to match adjacent
riparian vegetation patterns.
The site would be monitored in July (when maximum biomass is
present, and to ensure the correct identification of herbaceous species) for
five years to determine the success of the revegetation. Based on monitoring
results, additional planting or stabilization could be required, including
mulching, seeding, and planting of seedlings and cuttings. Any non-native plant
species discovered would be removed. Successful revegetation would establish a
self-sustaining cover of native species that stabilize soil, trap sediment,
provide wildlife habitat, and fulfill other basic functions of riparian
ecosystems. The natural regeneration of vegetation would be deemed successful
if, after five years, the herbaceous species composition and cover is within
90% of the composition and cover of adjacent native riparian areas, and if the
stem density of shrubs and trees is within 90% of natural stem densities of
adjacent areas. In addition, monitoring of river-channel morphology, bank
conditions, and water quality (turbidity) would continue.
Following revegetation and bank stabilization, all dam-removal-related
materials and equipment would be removed from the site. El
Portal Road would be realigned to its former
location and the paved parking area would be returned to its pre-removal state.
In addition, all equipment stored at Pohono Quarry would be removed.
It is assumed that excavated sediment
would be used for the site restoration activities described above. However, if
excess sediment remains, it would be dried on site at the island upstream of
Cascades Diversion Dam and subsequently removed from the site for disposal at an
approved and licensed facility or for reuse within the park. Rockfill removed
from the dam structure that is free of concrete would be used to support the
site restoration activities described above or would remain in the riverbed.
Other material removed may be temporarily stored at Pohono Quarry, where
materials would be sorted for reuse within the park, recycling, or disposal.
Consistent with the National Park Service’s Guiding
Principles of Sustainable Design (1993b), all infrastructure materials
removed from the site (e.g., concrete, rock rubble, wood) would be recycled to
the extent possible, at an approved and licensed facility, or reused within the
park. No metal, concrete, or timber materials would be disposed within the
boundaries of Yosemite National
Park. All project materials that would not be reused
within the park would be removed from Pohono Quarry upon completion of the
project.
Once the dam is removed, some portion of the remaining
sediment not removed by mechanical means would be transported downstream with
riverflows. The channel of the Merced River would be
expected to naturally narrow and deepen as impounded sediments (up to a maximum
range of approximately 9,600 to 15,600 cubic yards) wash downstream. As
described in Chapter III, Affected Environment, Alluvial Processes, and
under the No Action Alternative, sediment and materials were present prior to
dam construction, and some of these would likely remain following dam removal.
The island located upstream from the dam would likely remain following dam
removal and would naturally become vegetated with native riparian species,
further stabilizing sediments on the site. Downstream sediment transport and
evolution of natural channel dynamics under Alternative 2 would be the same as
described under the No Action Alternative, with the following exceptions:
§
Up to a maximum range of approximately 9,600 to
15,600 cubic yards of sediment would wash downstream (compared to a maximum range
of 15,000 to 20,000 cubic yards of sediment under the No Action Alternative)
§
Dam-related debris would not be washed
downstream (compared to approximately 5,045 cubic yards of rock, concrete,
and timber debris that could be deposited in the river and along the banks to
the Cascades Picnic Area under the No Action Alternative)
§
Average sediment depositional thickness at
Cascades Picnic Area would range from 0.3 to 2.1 feet (compared to 0.4 to
2.7 feet under the No Action Alternative)
Wild and Scenic River
Segment Reclassification
Under the Wild and Scenic Rivers Act,
classifications (Wild, Scenic, or Recreational) are applied to
each segment of the river corridor based on the existing conditions in that
portion of the corridor. The river segment at the dam from the top of the pool
to 200 feet below the dam is classified as recreational due to the manmade
impoundment at the site. The river segments upstream and downstream of the dam
are classified as scenic, a designation reserved for sections of rivers that are
“free of impoundments, with shorelines or watersheds still largely primitive
and shorelines largely undeveloped, but accessible in places by roads.” Upon
implementation of Alternative 2, the river segment through the dam site would be
restored to a free-flowing state, near natural conditions. It would then be
reclassified as scenic, as is called for in the Merced River Plan.
Alternative 3 includes complete removal of the dam, the river-left
dam abutment, and the screenhouse on the river-right intake structure, and restoration
of the related river channel located beneath the dam site (see figure II-6). Under
this alternative, the river-right dam abutment and intake structure would be
retained for use as a river viewing platform. Approximately 4,400 to 5,400
cubic yards of sediments (including rocks and boulders) in the area upstream of
the dam would be excavated and repositioned to stabilize the river-right bank
and decrease the potential for sediment erosion. Figure II-4 indicates the
river profile at Cascades Diversion Dam before and after removal of the dam
structure and sediments. Natural river processes would continue to transport
remaining sediments (up to a maximum range of approximately 9,600 to 15,600
cubic yards of sediment) from the impoundment area over time, allowing for a
gradual re-establishment of the natural river channel and related riparian
habitat. It is expected that the river would fully recover incrementally over
time, as sediments are transported from the impoundment area. However, the rate
of natural channel recovery and restoration would be monitored to determine if
additional restoration actions were necessary. Following removal of the dam and
screenhouse, the river-right bank would be stabilized upstream and downstream
of the intake structure using a
FIGURE II-6
ALTERNATIVE 3 [PDF]
bioengineered bank stabilization system to prevent erosion
of the river-right bank. The objective of this alternative would be to restore
the natural river character with a mixture and distribution of boulders,
cobbles, gravels, sand, silt, soil, and vegetation similar to those found in
adjacent riverbank segments.
In-channel work, bank stabilization, and revegetation
would be completed within a two- to three-month period during the fall of 2003
(September through November) when flow of the Merced River
is typically lowest (less than 200 cubic feet per second). The overall project
duration would be approximately five months. Dam removal would occur in three
phases, as follows:
§
Phase 1 – Dam
Removal Setup
-
Detour traffic to the north side of El
Portal Road to create a staging area adjacent to
the dam site
-
Install interpretive displays within the park
§
Phase 2 – Dam
Removal
-
Draw down impoundment (dewater) using the waste
gates in the existing intake structure
-
Install coffer dam to protect the active removal
site from riverflows
-
Establish temporary diversion channel crossing
bridge near intake structure on the river-right bank
-
Partially
excavate impounded sediment near upstream face of dam
-
Demolish timber overflow section of dam using
large excavator on workpad
-
Remove the river-left abutment concrete
structure
-
Excavate pilot channel upstream of sluiceway
through sediment to river channel
-
Divert riverflow through pilot channel and
remove diversion channel crossing
-
Remove remainder of dam and screenhouse
-
Install concrete apron over the top of the intake
structure and install a safety railing to create river-viewing platform
-
Install interpretive displays within the project
area
-
Install sidewalk and curbing between vehicle
turnout and the river-viewing platform
§
Phase 3 – Site
Restoration and Cleanup
-
Reconfigure excavated sediments along
river-right bank for bioengineered bank stabilization system
-
Install bioengineered bank stabilization system
using native river rock and vegetation; match biological, visual, and
structural characteristics of the upstream and downstream riverbanks
-
Demobilize equipment and remove traffic detour
and contractor use area
-
Remove and revegetate the former parking area
Total estimated excavation volumes for Alternative 3
include:
§
Impounded
Sediments – 4,400 to 5,400 cubic yards
§
Rock,
Concrete, and Timber – 5,050 cubic yards
It is estimated that approximately 255 loaded truck trips would be required to remove excavated
materials under Alternative 3. In addition, project activities would generate
additional truck trips between the dam and a secondary project staging and
storage area at Pohono Quarry. The number of daily truck trips between the dam
and the quarry would vary and is expected to be infrequent during most project
activities, but could be as high as 20 trips per day during some project
activities, such as during removal of timbers and rockfill. The total estimated
cost to implement Alternative 3 is between $2 and $2.6 million.
Description of Dam Removal Activities
Phase 1 – Dam Removal Setup
Dam removal setup under Alternative 3 would be the same as
described under Alternative 2.
Phase 2 – Dam Removal
 Dam
removal activities under Alternative 3 would be the same as described under
Alternative 2, with the exception of activities at the river-right dam abutment
and intake structure. Following redirection of the riverflow away from the
river-right bank and into a newly excavated pilot channel (see description of
dam removal activities under Alternative 2), removal of the remainder of the
dam (approximately 40%) and screenhouse would begin (see photo). In-river
activities and mechanical equipment would include individual operators for the
two waste gates and for the penstock slide gate, a traveling fish screen, and a
trashrack.
The river-right dam abutment and the intake structure
would be retained as a viewing platform by covering the structure with a 6-inch
reinforced-concrete pad and installing approximately 75 linear feet of
handrail around the perimeter of the intake deck. Exhibits documenting the
history of the dam and its relationship to park history would be installed in
the river-viewing platform area. In addition, a 5-foot-wide sidewalk would be
installed between the vehicle turnout to the west and the viewing platform,
with concrete curbing installed between El Portal
Road and the sidewalk.
Phase 3 – Site Restoration and Cleanup
Site restoration, cleanup, downstream sediment transport, and
evolution of natural channel dynamics under Alternative 3 would be the same as
described under Alternative 2. Bank stabilization along the river-right,
upstream and downstream of the intake structure, would be required under
Alternative 3, as the retained dam abutment and intake structure would not
provide bank protection from erosion as the Merced River
returns to more natural conditions upstream and downstream of the structure. See
the description of bank stabilization under Alternative 2.
Following revegetation, bank stabilization, and realignment
of El Portal Road to its
former location, the paved parking area, public telephone, and trash cans north
of El Portal Road would be removed.
The existing parking lot would be contoured to create a variety of microhabitats
and would be revegetated with a combination of upland tree and shrub species,
primarily canyon live oak, California black oak, ponderosa pine, and Mariposa
manzanita. Drier-site riparian species, including California bay-laurel and
black cottonwood, would also be used. The tree and shrub species would be
planted as seedlings in densities representative of nearby areas with similar
slope and aspect characteristics. Herbaceous species, including native lupines,
lotus, California fushia, and
grasses, would be seeded onto the site following contouring. Native forest
litter and duff from adjacent areas would be applied to the site to protect
seeds from predation and to minimize soil moisture loss as seedlings become
established. Further seeding, mulching, and planting could be required as the
result of site monitoring, which would occur for five years to determine the
success of the revegetation. Success of this revegetation would be evaluated in
conjunction with the river-right bank stabilization and revegetation. Visitor
parking in the El Portal Road/Big Oak Flat Road
intersection area would be restricted to the turnout to the west of the
intersection, along the river-right bank.
Wild and Scenic River
Segment Reclassification
Under the Wild and Scenic Rivers Act,
classifications (Wild, Scenic, or Recreational) are applied to
each segment of the river corridor based on the existing conditions in that
portion of the corridor. The river segment at the dam from the top of the pool
to 200 feet below the dam is classified as recreational due to the manmade
impoundment at the site. The river segments upstream and downstream of the dam
are classified as scenic, a designation reserved for sections of rivers that are
“free of impoundments, with shorelines or watersheds still largely primitive
and shorelines largely undeveloped, but accessible in places by roads.” Upon
implementation of Alternative 3, the river segment through the dam site would be
restored to a free-flowing state, near natural conditions. It would then be
reclassified as scenic as is called for in the Merced River Plan.
Alternatives Considered but Dismissed
The following alternatives are not addressed in the Cascades
Diversion Dam Removal Project Environmental Assessment for one or more of the
following reasons:
§
The alternative does not meet the project’s Purpose
and Need
§
Less environmentally damaging or less expensive
options are available
§
The alternative would cause unacceptable
environmental, cultural, or social impacts
§
The alternative conflicts with the guidance and
direction provided in the Merced River Plan (NPS 2001a)
§
The alternative conflicts with the guidance and
direction provided in the Yosemite Valley
Plan (NPS 2000a)
Remove Cascades Diversion Dam and Complete Sediment Removal
Removal of the Cascades Diversion Dam in conjunction with
complete removal of the 15,000 to 20,000 cubic yards of sediment in the
upstream impoundment would remove sediments that were present prior to
construction of the dam in 1917. As described in Chapter IV, Environmental
Consequences, Alluvial Processes, a rock and sediment island already existed
immediately upstream of the dam prior to construction. The quantity of sediment
present prior to dam construction is not known. Removal of sediment present
prior to dam construction would not result in the re-establishment of a near-natural
river channel and related riparian habitat. In addition, this alternative would
require 700 to 750 truck trips to remove dam materials and sediment and would entail
a project duration of seven months. This would result in greater impacts to air
quality, traffic and access, and recreation-related experience than the action
alternatives considered in this environmental assessment.
Restore Cascades Diversion Dam and Hydroelectric Generating Facility
Repair of Cascades Diversion Dam and the entire
hydroelectric generating facility to fully functioning capacity would require
complete removal and reconstruction of the dam overflow structure;
reconstruction of approximately one mile of penstock, which would need to be
installed under El Portal Road (Highway 140); reconstruction of the
tailrace (or outlet channel); and purchase and installation of new generating
equipment in the powerhouse.
Removal of the dam overflow structure
would occur as described under Alternative 2 above. The new overflow structure
could utilize a system of lightweight galvanized steel members bolted together
in a series of adjoining closed-face bins backfilled with native gravel and
cobble materials. Although this alternative would alleviate the high-hazard
condition of the dam, consistent with the project’s Need, it is inconsistent
with the Purpose of the project to remove an unnatural obstruction on the Merced
River and to restore the river’s free-flowing condition, consistent with the
Merced River Plan (NPS 2001a) and Yosemite Valley Plan
(NPS 2000a).
Replace Cascades Diversion Dam with Boulders
This option would require complete removal of the dam
overflow structure and installation of large boulders at the current dam site
to impound water and limit downstream sediment transport. This option would
perpetuate unnatural conditions at the site, limiting free flow of the Merced
River and other natural processes. Similar to the above scenario,
this option is inconsistent with the purpose of the project.
Remove Portions of the Dam Over Time
This option would remove the dam
overflow structure, abutments, and the intake structure in phases over a series
of years, ultimately resulting in complete removal of all structures and
impounded sediments. Although this option meets the purpose of the project, it
has potential to create substantial environmental effects. Phased removal of the
dam over a period of years would expand removal-related impacts. For example,
transportation- and recreation-related effects of removal would last years
instead of one season. Removing only a portion of the dam’s overflow structure
in the first season could destabilize remaining structures and could result in
uncontrolled failure. Uncontrolled failure of the remaining structures would
have downstream impacts on resources and free flow of the Merced
River
and has potential to adversely affect park facilities, recreational uses, and
visitors, similar to the No Action Alternative. Therefore, this alternative
would not be consistent with the Need for this project.
Combine the Removal of Cascades Diversion Dam with the Final Phase of the
El Portal Road Improvement Project
The National Park Service intends to
remove Cascades Diversion Dam, let natural processes prevail through this reach
of the Merced River, and allow the river to stabilize. If the National Park Service decides to
proceed with the El Portal Road Improvement Project – Cascades Dam to Pohono
Bridge, potential future designs
will need to comply with the Wild and Scenic Rivers Act as well as other
legislation and park planning documents. Although the Cascades Diversion Dam
Removal Project is completely distinct from the El Portal Road Improvement
Project, the road improvement project is included in the cumulative impact
analysis in this document because it is a reasonably foreseeable project that
the National Park Service may decide to pursue (see Appendix E, Projects
Included in the Cumulative Impact Analysis).
Mitigation Measures Common to All Action Alternatives
To ensure that implementation of the proposed action
protects natural, cultural, and social resources and the free-flowing condition
of the Merced River corridor, a consistent set of
mitigation measures would be applied during project implementation to avoid,
minimize, and mitigate adverse impacts.
The project shall avoid or minimize impacts to natural,
cultural, and social resources. The project shall be designed to work in
harmony with the surroundings, particularly the Merced River
transition between Yosemite Valley and the downstream
gorge. The project shall reduce, minimize, or eliminate air and water
nonpoint-source pollution. The project shall be sustainable whenever
practicable, by recycling or reusing materials, by minimizing materials, and by
minimizing energy consumption during the project.
Best Management Practices During Dam Removal
The National Park Service (and its contractors) shall
implement the following best management practices, as appropriate, prior to,
during, and/or after dam removal. Specific tasks would include, but are not
limited to, the following:
§
Inspect the project to ensure that impacts stay
within the parameters of the project and do not escalate beyond the scope of
the environmental assessment, as well as to ensure that the project conforms
with the U.S. Army Corps of Engineers Special Site Permit, Cascades Dam Removal
(as amended), Merced River Cascades Restoration Report, the Central Valley
Regional Water Quality Control Board Waiver of Waste Discharge Requirements and
Water Quality Certification, Cascades Dam Removal, and other applicable permits
or project conditions.
§
Implement compliance monitoring to ensure the
project remains within the parameters of National Environmental Policy Act and
National Historic Preservation Act compliance documents, U.S. Army Corps of
Engineers Section 404 permits, etc. Compliance monitoring would ensure
adherence to mitigation measures and would include reporting protocols.
§
Implement natural resource protection measures.
Standard measures include demolition scheduling, biological monitoring, erosion
and sediment control, use of fencing or other means to protect sensitive
resources adjacent to the work area, and revegetation. The measures include
specific monitoring by resource specialists as well as treatment and reporting
procedures.
§
Implement the requirements of the 1999 Programmatic
Agreement between the National Park Service, the State Historic Preservation
Officer, and the Advisory Council on Historic Preservation for the “Resolution
of Adverse Effects” associated with planning construction, operations, and
maintenance activities within Yosemite National Park (i.e., review of project
design, avoidance of sensitive cultural resource areas, monitoring of project
activities as appropriate, ongoing tribal consultation).
§
Implement the requirements of the 1999 Agreement
between the National Park Service and the American Indian Council or Mariposa
County, Inc. for conducting traditional activities.
§
Confine work areas within the river channel,
such as workpads to support demolition equipment, to the smallest area
necessary.
§
Limit the amount of rock and sediment required
for the river-right bank bioengineereed bank stabilization to the minimum
required to stabilize and protect the slope from erosion. Amount shall be
determined in consultation with National Park Service resources management staff
during final project design.
§
Steam-clean heavy equipment prior to its entry
into the park to prevent importation of non-native plant species, and repair
all petroleum leaks prior to work near the Merced River.
Tighten hydraulic hoses and ensure they are in good condition.
§
To minimize the possibility of hazardous
materials seeping into soil or water, check equipment frequently to identify
and repair any leaks, as directed in the spill prevention and countermeasure
plan. Standard measures include hazardous materials storage and handling
procedures; spill containment, cleanup, and reporting procedures; and
limitation of refueling and other hazardous activities to upland/nonsensitive
sites. Provide an adequate hydrocarbon spill containment system (e.g.,
floatable absorption boom, absorption materials, etc.) on site, in case of
unexpected spills in the project area. Ensure equipment allowed within the
river channel is equipped with a hazardous spill containment kit. Ensure that
personnel trained in the use of hazardous spill containment kits are on site at
all times during dam removal activities.
§
Store all construction equipment within the
delineated work limits.
§
Implement measures to reduce effects of dam
removal on visitor safety and experience. Safeguard visitors, contractors, and
park personnel from removal activities. Implement a barrier plan indicating
locations and types of barricades to protect public health and safety.
§
Provide information about recreational closures
and the location, timing, and duration of work activity to visitors as they
enter the park. Flag and/or fence off work areas to maintain visitor safety
during both work and nonwork hours.
§
Implement an interpretation and education
program. Continue directional signs and education programs to promote
understanding among park visitors.
§
Implement a traffic control plan, as warranted.
Include strategies to maintain safe and efficient traffic flow during the
project work period.
§
Ensure an emergency notification program is in
place. Standard measures include notification of utilities and emergency
response units prior to demolition activities. Identify locations of existing
utilities prior to removal activity to prevent damage to utilities,
particularly the wastewater lines that pass under El
Portal Road within the project area. The
Underground Services Alert and National Park Service maintenance staff shall be
informed 72 hours prior to any ground disturbance. Demolition shall not
proceed until the process of locating existing utilities is completed
(wastewater, electric, and telephone lines). An emergency response plan shall
be required of the contractor for measures that will be taken during all high-water
events during dam removal, such as evacuation of personnel, equipment, and
materials from the river, etc.
§
Avoid damage to natural surroundings in and
around the work limits. Provide temporary barriers to protect existing trees,
plants, and root zones, if necessary, as determined by vegetation management
staff. Trees and other vegetation shall not be removed, injured, or destroyed
without prior written approval. Ropes, cables, or fencing shall not be fastened
to trees. All existing resource protection fencing (post and rope) shall be
left in place and protected from heavy equipment.
§
Remove all tools, equipment, barricades, signs,
surplus materials, and rubbish from the project work limits upon project
completion. Repair any asphalt surfaces that are damaged due to work on the
project to original condition. Remove all debris from the project site,
including all visible concrete, timber, and metal pieces. Grade disturbed areas
and rake them smooth to eliminate tire tracks and tripping hazards.
§
Locate, contain, and stabilize excavated and
stored materials within the upland staging areas and prevent re-entry into the
river.
§
Implement standard noise abatement measures
during work. Standard noise abatement measures include the following elements:
a schedule that minimizes impacts to adjacent noise-sensitive uses, use of the
best available noise control techniques wherever feasible, use of hydraulically
or electrically powered impact tools when feasible, and location of stationary
noise sources as far from sensitive uses as possible (see Chapter III, Affected
Environment, Noise). Ensure all construction equipment is equipped with
mufflers kept in proper operating conditions, and, when possible, shut off equipment
rather than allowing it to idle.
§
If deemed necessary, demolition work on weekends
or federal government holidays may be authorized, with prior written approval
of the Superintendent. To the extent possible, perform all on-site noisy work
above 76 dBA (such as the operation of heavy equipment) between the hours of
8:00 a.m. and 5:00 p.m. to minimize
disruption to nearby park users.
§
Use silt fences, sedimentation basins, etc. in
work areas to reduce erosion, surface scouring, and discharge to water bodies,
as defined in the erosion control plan prepared for this project.
§
Delineate wetlands and apply protection measures
during construction. Wetlands shall be delineated by qualified National Park
Service staff or certified wetland specialists and clearly marked prior to
work. Perform activities in a cautious manner to prevent damage caused by
equipment, erosion, siltation, etc.
Resource-Specific Measures
Hydrology, Floodplains, and Water
Quality
§
Prepare an erosion control plan specifying
measures to prevent erosion/sedimentation problems during project construction.
Include a map of the project site delineating where erosion control measures
will be applied. Include the following minimum criteria, as listed in the Guidelines for Protection of Water Quality
During Construction and Operation of Small Hydro Projects (CVRWQCB 1983):
-
Construction equipment shall not be operated in
flowing water, except as may be necessary to construct crossings or barriers.
-
Where working areas are adjacent to or encroach
on live streams, barriers shall be constructed that are adequate to prevent the
discharge of turbid water in excess of specified limits.
-
Material from construction work shall not be
deposited where it could be eroded and carried to the stream by surface runoff
or high stream flows.
-
All permanent roads shall be surfaced with
materials sufficient to maintain a stable road surface.
-
All disturbed soil and fill slopes shall be
stabilized in an appropriate manner.
-
Surface drainage facilities shall be designed to
transport runoff in a nonerosive manner.
-
Riparian vegetation shall be removed only when
absolutely necessary.
-
There shall be no discharge of petroleum
products, cement washings, or other construction materials.
-
Erosion control measures shall be in place prior
to dam removal and maintained in good repair.
-
Stream diversion structures shall be designed to
preclude accumulation of sediment. If this is not feasible, an operation plan shall
be developed to prevent adverse downstream effects from sediment discharges.
§
Erosion control measures shall be inspected
daily during dam removal and monthly following removal, and repaired as
required.
§
Waters shall be free of changes in turbidity
that cause a nuisance or adversely affect beneficial uses. Increases in
turbidity attributable to controllable water quality factors shall not exceed
the following limits, as described in The
Water Quality Control Plan for the Central Valley Regional Water Quality
Control Board (CVRWQCB 1998). In determining compliance with the limits below,
appropriate averaging periods may be applied, provided that beneficial uses
will be fully protected:
-
Where natural turbidity is between 0 and 5
Nephelometric Turbidity Units (NTUs), increases shall not exceed 1 NTU.
-
Where natural turbidity is between 5 and 50
NTUs, increases shall not exceed 20%.
-
Where natural turbidity is between 50 and 100
NTUs, increases shall not exceed 10 NTUs.
-
Where natural turbidity is greater than 100
NTUs, increases shall not exceed 10%.
§
Implement stormwater management measures to
reduce nonpoint-source pollution discharge. This could include measures such as
oil/sediment containment or street sweeping.
§
Remove hazardous waste materials generated
during implementation of the project from the project site immediately.
§
Dispose of volatile wastes and oils in approved
containers for removal from the project site to avoid contamination of soils,
drainages, and watercourses. Keep absorbent pads, booms, and other materials
onsite during projects that use heavy equipment to contain oil, hydraulic
fluid, solvents, and hazardous materials spills.
Vegetation
§
Implement a noxious weed abatement program.
Standard measures include, as appropriate, the following elements: ensure that
vehicles and equipment arrive onsite free of mud or seed-bearing material,
certify all seeds and straw material as weed-free, identify areas of noxious
weeds before dam removal, treat noxious weeds or noxious weed topsoil prior to
work (e.g., topsoil segregation and removal), and revegetate with appropriate
native species.
§
Cover exposed soil with a combination of locally
acquired native duff and forest litter from adjacent riparian sites to provide
immediate groundcover and facilitate natural revegetation.
§
Implement the planting prescriptions prepared
for this project.
§
Develop and implement a monitoring plan to
ensure successful revegetation, maintain plantings, and replace unsuccessful
plantings.
§
Use native or seed-free mulch to minimize
surface erosion and introduction of non-native plants.
§
Confine all construction operations to specified
project work limits. Install temporary barriers to protect natural surroundings
(including trees, plants, and root zones) from damage. Avoid fastening ropes,
cables, or fences to trees.
§
As much as possible, removed plants and
materials (cuttings) shall be salvaged and stored on site for revegetation
following dam removal.
Wildlife
§
Implement measures to reduce bear/human
encounters. Measures include worker education on bear behavior; enforcement of
park regulations; and removal of regular trash, all food-related items, and rubbish
to bear-proof containers.
§
Minimize night lighting during work. Where night
lighting is necessary, design lighting to be minimal, directed downward, and
shielded.
§
Educate workers on the dangers of intentional or
unintentional feeding of park wildlife, and on inadvertent harassment through
observation or pursuit.
Bird Species
§
To avoid conflicts with nesting birds, conduct
activities outside the breeding season (typically from March to August).
§
Remove trees or structures with unoccupied nests
(stick nests or cavities) prior to March 1, or following the nesting season.
Alternatively, if activities take place during the breeding season, a qualified
biologist shall conduct a pre-work survey for individuals no more than two
weeks prior to construction in March through August. If any special-status
species is observed nesting, a determination shall be made as to whether or not
the proposed action will impact the active nest or disrupt reproductive
behavior. If it is determined that the action will not impact an active nest or
disrupt breeding behavior, work shall proceed without any restriction or
mitigation measure. If it is determined that dam removal activities will impact
an active nest or disrupt reproductive behavior, then avoidance strategies shall
be implemented. Dam removal activities could be delayed within 500 feet of such
a nest until a qualified biologist determines that the subject birds are not
nesting or until any juvenile birds are no longer using the nest as their
primary day and night roost.
Mammal Species
§
Ensure excavation sites (trenches or pits) have
suitable ramps to allow small mammals to exit these areas.
§
A qualified biologist shall be available to
inspect all excavations before refilling occurs, ensuring that special-status
species are passively relocated to avoid incidental take.
§
Erect exclosure fencing prior to activities to
ensure that no special-status species are within the work area.
Special-Status Species
Special-Status Aquatic Species
Implementation of the following conservation and
protection measures would reduce or eliminate potential taking of special-status
aquatic species.
§
Work activities within potential special-status
aquatic species habitat shall be completed during low-flow conditions.
§
All work adjacent to or within aquatic habitats shall
be regularly monitored.
§
All fueling and maintenance of vehicles and
equipment shall occur at least 65 feet from any aquatic habitat.
§
The total area of activity shall be limited to
the minimum necessary to achieve the project goal, as determined
collaboratively with contractors and National Park Service staff (including resources
management staff).
§
During dewatering, intakes shall be completely
screened with wire mesh not larger than 5 millimeters to prevent aquatic
species from entering the system. Release or pump water downstream at an
appropriate rate to maintain downstream flows during work. Upon completion of
activities, remove barriers to flow in a manner that allows flow to resume with
the least disturbance to the substrate.
§
Fence the downstream work boundary to limit the
movement of aquatic species into the work area to the actively flowing water
area of the channel and to control creek siltation and disturbance to
downstream riparian habitat. An exclosure fence shall be installed in the creek
channel both upstream and downstream of activities, as appropriate. Install
fences at least four weeks prior to the commencement of any activities. Immediately
after installation of the exclosure fence, a qualified biologist shall inspect
all areas within the fence for aquatic species.
Special-Status Species of Bats
§
A qualified biologist shall conduct surveys in
the spring and immediately prior to dam removal to determine whether trees or
other habitat (e.g., crevices) that would be affected by the proposed action
provide hibernacula or nursery colony roosting habitat.
§
If special-status species of bats are found
breeding within the vicinity of the proposed action, no blasting shall occur
between May 1 and September 1.
§
If spring surveys reveal that the site is being
used as a nursery colony, the action shall not occur until after August 15,
when the pups are weaned and are volant.
§
If surveys conducted immediately prior to dam
removal do not reveal any bat species present within the project area, then the
action shall begin within three days to prevent the destruction of any bats
that could move into the area after the survey.
§
Snags shall not be removed without prior
approval from a National Park Service wildlife biologist and/or plant
ecologist. Riparian vegetation shall be retained to the extent possible to
preserve important foraging habitat.
Air Quality
§
Implement a dust abatement program. Contractors
shall implement the following measures:
-
Water all active work areas, access roads and
paths, parking areas, and staging areas at least twice daily (use of dust
abatement products would not be allowed). Ensure that applied water does not
enter the Merced River.
-
Cover all trucks hauling dam debris and other
loose materials that could spill onto paved surfaces, or require all trucks to
maintain adequate freeboard.
-
All paved areas that are subject to vehicle and
pedestrian traffic shall be kept clean of debris and soils. Sweeping of these
areas shall be implemented as necessary.
-
Cover all stockpiles.
-
Limit traffic speeds on unpaved roads and paths and
around the project site.
§
Implement vehicle emissions controls. Contractors
shall implement the following measures:
-
Use California
on-road diesel fuel for all diesel-powered equipment.
-
Use equipment that is properly tuned and maintained in
accordance with manufacturers’ specifications.
-
Avoid unnecessary emissions. Engines of trucks and
vehicles in loading and unloading areas shall be turned off when not in use.
This section compares the key features of each of the
alternatives and summarizes the potential impacts. Table II-1 shows the
comparison of key features, and table II-2 displays potential impacts
summarized from Chapter IV, Environmental Consequences. The three alternatives
presented in this document represent a reasonable range of options for Cascades
Diversion Dam. Under the No Action Alternative, existing conditions and
management practices would continue for Cascades Diversion Dam.
Summary of Environmental Consequences
Table II-2 summarizes the key impacts
that could result from each of the alternatives, including the No Action
Alternative. Detailed descriptions of these impacts are provided in Chapter IV,
Environmental Consequences.
Summary Comparison of Key
Features of the Alternatives
|
Key Features
|
Alternatives
|
|
Alternative
1:
No Action
|
Alternative
2: Complete Dam Removal
|
Alternative
3: Partial Dam Removal
|
|
Estimated
cost
|
Continue existing repairs
|
$2 to $2.6 million
|
$2 to $2.6 million
|
|
Estimated
total duration of removal
|
0
|
5 months
|
5 months
|
|
Estimated
duration of in-channel removal
|
0
|
2 to 3 months
|
2 to 3 months
|
|
Estimated
total volume of dam materials (rock, concrete, and timber) removed from the
site
|
0
|
up to 5,160 cubic yards
|
up to 5,050 cubic yards
|
|
Estimate
total volume of sediment excavated (including rock/boulders)
|
0
|
4,400 to
5,400 cubic yards
|
4,400 to
5,400 cubic yards
|
|
Estimated
total volume of material reconfigured onsite
|
0
|
3,000 cubic yards of sediment and 1,500 cubic
yards river rock
|
3,000 cubic yards of sediment and 1,500 cubic yards river rock
|
|
Estimated
total volume of materials deposited downstream
|
Up to 20,000 to 25,000 cubic yards of
dam-related debris, rock and sediment (includes rocks/boulders)
|
9,600 to 15,600 cubic yards of sediment
(includes rocks/boulders)
|
9,600 to 15,600 cubic yards of sediment
(includes rocks/boulders)
|
|
Estimated
total thickness of sediment deposited in the vicinity of Cascades Picnic Area
|
0.4 to 2.7 feet
|
0.3 to 2.1 feet
|
0.3 to 2.1 feet
|
|
Estimated
number of loaded truck trips
|
--
|
260
|
255
|
|
Site restoration
and stabilization
|
No
|
Yes
|
Yes
|
|
Materials
recycled
|
No
|
Yes
|
Yes
|
|
