Appendix A—Case Study 21
Case Study 21. Capps Low-Water Bridge
Location
North central California. El Dorado National Forest, Placerville Ranger
District. North Fork Cosumnes River. Meiss Cabin Road, Primary Forest
Route 52.
Crossing Description
This low-water bridge was constructed in 1998 as an Emergency Relief
for Federally Owned Roads (ERFO) project (figure A132). It replaced a
vented concrete slab ford that plugged and caused substantial channel and
flood plain damage during the 1997 flood. Because the old ford blocked
sediment transport during high flows, it had caused dramatic channel
aggradation and flood plain erosion. To reestablish channel stability at the
site, the designers decided to bridge the entire flood plain, and to provide
for overflow in case of debris jamming. The bridge has 13 concrete piers
set on bedrock and the deck is square steel tubing with two smooth steel
plate runways.
Figure A132. Looking downstream at the Capps low-water bridge. July 2002.
Setting
Sierra Nevada Section (M261-E). Rocks are mixed granitic, volcanic,
and metasedimentary. Ponderosa pine and mixed conifer forest. Riparian
vegetation includes cottonwood, alder, willow, dogwood, and cedar.
Appendix A—191
Appendix A—Case Study
21
Why Was This
Structure Selected?
The forest considered relocating the road, but nearby archeological
resources and private property eliminated relocation as a feasible
alternative.
This crossing structure was expected to accomplish the following
objectives.
• Remove the sediment transport blockage associated with the old
crossing structure and allow stream processes to restore a more nearly
natural channel size, shape, and substrate.
• Provide room for the channel to migrate within the flood-prone area.
• Provide passage for aquatic organisms.
• Reduce the need for maintenance.
Crossing Site History
Historically, this road was a stagecoach route across the Sierra divide.
It forded the North Fork Cosumnes River at this location, and probably
widened the channel by breaking the banks down. Sediment deposited in
the widened channel and flood flows were diverted across the rutted flood
plain. With time, the flood plain progressively lowered in elevation as
floods washed more material away.
Subsequent ford improvements did not solve the problem. The structure
that existed prior to the 1997 flood of record was a concrete slab with
an 8-foot-wide concrete box culvert, which filled with bed material and
needed annual cleaning even during normal years. During floods, the
ford plugged and flow spread over the flood plain, eroding and changing
channel location, and washing out the road approaches. To reopen the road
after floods, onsite stream-deposited material was routinely heaped up into
a turnpike across the eroded flood plain, topped with an aggregate base,
and either paved or oiled. This practice, combined with erosion during
floods, explains why the flood-plain surface at the crossing site is several
feet lower than in adjacent sections upstream and downstream of the
site. Because of the widened section, sediment deposition was a serious
problem, and the channel became so embedded it seemed to be paved. The
vented ford washed out in 1997, during the flood of record (figure A133).
Appendix A—192
Appendix A—Case Study 21
Figure A133. Capps ford after the 1997 flood. Flood flows spread out across the
entire valley and road is washed out on far side.
Road Management
Objectives
Forest Route 52 is maintained for passenger cars (maintenance level 3),
and is used for recreation, timber haul, administrative access, and access
to private land. Occasional closures due to severe weather are acceptable,
but dependable summer seasonal access is required. There is little or no
winter use.
Stream Environment
Hydrology: Large floods occur on the North Fork Cosumnes River
during rain-on-snow events. The approximately 15 square mile watershed
was mined in California’s gold rush days, and early roads followed the
intermittent streams up each tributary draw. (Crossing structures on these
tributaries are described in case study 13.) Current roads are located on
both sides of many of these draws, and are in only moderate condition.
Loose bed material is readily available to all these streams because of
these disturbances, and when flows rise, bedload transport can be very
high. Blocking sediment transport with a ford is a particularly bad idea in
this system.
Channel Description: At the crossing site, the channel is best described
as a Rosgen C3 because of channel widening and flood plain modifications
due to the crossing. It is a B3 or B4 upstream and downstream (figure
A134), with some bedrock-controlled sections. Bankfull width is between
20 and 30 feet. The low terrace adjacent to the natural sections of the river
has been lost at the crossing site, and the bridge crosses a 300-foot flood
Appendix A—193
Appendix A—Case Study
21
plain that is about 2 to 3 feet lower than the ground adjacent to the stream
elsewhere. Even in undisturbed upstream reaches, abandoned channels
and side channels are evident across the valley bottom. Clearly the stream
is dynamic and prone to shifting across the valley floor.
Figure A134. Downstream view of Capps bridge showing cobble bars and debris
in channel.
Because the previous structure obstructed sediment transport, bed material
at the site was much finer than in up- and downstream reaches. Cobbleembeddedness was very high. The new structure has allowed fines to
migrate through, opening up the cobble bed again, and improving fish
habitat.
Aquatic Organisms: This structure provides passage for all species, at
all life stages. By providing for free downstream transport of bed material
and wood, the structure is also working to maintain downstream and
onsite stream habitats. Foothill yellow-legged frogs, tree frogs, western
pond turtles, and trout are among the aquatic species that are likely to use
these habitats.
Water Quality: This structure was designed to restore channel functions
and it helps maintain good water quality by reducing channel and bank
erosion.
Structure Details
Appendix A—194
Structure: The Capps low water bridge is 224 feet long, with 13 piers
spaced on 16-foot centers. The piers are up to 12 feet deep to reach
9+70.89
BEGIN TURNOUT RIGHT
49
00
49
05
4860
4880
10+00
10+20
10+40
2
00
10+60
49
CONSTRUCT
ABUTEMENT
BM
CH
IN
1T
1
00
49
PINRIPRAP
PLACE
E
@ 2:1 SLOPE
OR FLATTER
32
STA
05
49
11+00
11+20
CONSTRUCT CONCRETE
ABUTEMENT
16'
10+80
4900
8'
8'
PLAN VIEW
49
Scale
11+40
11+80
12+00
5'
1" = 40' HORZ.
1" = 40' VERT.
11+60
REMOVE EXISTING
2 EA. - 24" CMP
12+20
13 PIERS @ 16' CENTER TO CENTER
224'
STA
3
4900
16'
12+40
12+60
12+80
PROVIDE FOR STREAM CHANNEL
AT THIS ELEVATION
Scale
1" = 40'
INSTALL - 28 cattle guard grates, 2 rows of 8' road widths
14 rows of grates x 16' span = 224 L.F.
4900
REMOVE EXISTING
2 EA. - 24" CMP
16'
4900
REMOVE EXISTING
CONCRETE LOW-WATER
CROSSING WITH STEEL GRATE
4900
224'
EXCAVATION UNSUITABLE FOR USE AS BACKFILL
IN ABUTMENT & ABUTMENT APPROACHES SHALL BE
DISPOSED OF AT "VAN HORN" PIT & NECESSARY BORROW
CAN BE OBTAINED FROM STOCKPILE AT PIT.
���
E
4900
PROFILE VIEW
TURNOUT
24'
30'
CONSTRUCT
STA
INSTALL OBJECT MARKER
& STEEL POST
4900
PR
4900
EXISTING GROUND
4900
4920
EXISTING
ASPHALT
95
48
4900
AT
E
PR
4900
CONSTRUCT
B2
ABUTEMENT
SLA
4900
13+00
13+20
4860
4880
4900
4920
REMOVE
EXISTING
CONCRETE
13+40
13+60
13+80
14+00
LOW WATER CROSSING
WITH STEEL GRATE
CONSTRUCT CONCRETE
ABUTEMENT
EXISTING GROUND
4905
INSTALL OBJECT MARKER
& STEEL POST
28'
CONSTRUCT TURNOUT
40'
��
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��
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CONTROL POINTS
APPROXIMATE PROPERTY LINE
EDGE DIRT ROAD
EDGE AGGREGATE ROAD
EDGE PAVED ROAD
CENTERLINE DRAINAGE
MINOR CONTOURS
MAJOR CONTOURS
���
3 of 8
SHEET NO.
HYDROLOGY: DRAINAGE AREA = 9 SQ.MI.
STRUCTURAL STEEL: ALL STRUCTURAL STEEL EXCEPT THE FORMED RAIL
SECTIONS SHALL CONFORM TO ASTM DESIGNATION A-36. FORMED RAIL SECTIONS
SHALL CONFORM TO ASTM DESIGNATION A-441. ANCHOR BOLTS AND NUTS SHALL
CONFORM TO AASHTO M 314-90. ANCHOR BOLTS & NUTS SHALL BE ZINC COATED
BY HOT DIP OF MECHANICAL DEPOSITION. ALL WELDING SHALL BE DONE IN
CONFORMANCE WITH THE LATEST EDITION OF THE AMERICAN WELDING SOCIETY
SPECIFICATIONS FOR WELDED HIGHWAY & RAILROAD BRIDGES. ALL STRUCTURAL
STEEL EXCEPT ANCHOR BOLTS SHALL BE PAINTED WITH ONE COAT OF RUST
INHIBITIVE METAL PRIMER AND ONE COAT OF FOLIAGE GREEN BRIDGE PAINT,
AASHTO M-67.
REINFORCING STEEL: REINFORCING STEEL SHALL CONFORM TO AASHTO M31
(ASTM A-615), GRADE 60. MINIMUM COVERING TO FACE OF DIMENSIONS SHOWN
RELATING TO SPACINGS OF REINFORCING STEEL ARE TO CENTER OF BARS
UNLESS OTHERWISE SHOWN. MINIMUM COVERING TO THE FACE OF ANY
REINFORCING BAR IS 2" UNLESS OTHERWISE SHOWN. WHERE CONCRETE IS
PLACED AGAINST EXISTING SOIL THE MINIMUM COVERING SHALL BE 3".
CONCRETE: CLASS "A" CONCRETE WITH A 28 DAY MINIMUM COMPRESSIVE
STRENGTH OF 3500 P.S.I. AND SHALL BE VIBRATED. EXPOSED CORNERS SHALL BE
CHAMFERED 3/4". ALL VERTICAL SURFACES SHALL BE FORMED. ADDITIVES
CONTAINING CALCIUM CHLORIDE SHALL NOT BE USED. AIR ENTRAINMENT SHALL
BE 4 % TO 7 %.
SPECIFICATIONS: CONSTRUCTION - U.S.FOREST SERVICE GENERAL PROVISIONS &
STANDARD SPECIFICATIONS FOR CONSTRUCTION OF ROADS & BRIDGES, 1985
EDITION WITH ACCOMPANYING SPECIAL PROJECT SPECIFICATIONS.
DESIGN LOAD : HS 20-44.
�
LEGEND:
PLAN & PROFILE
GENERAL NOTES:
05
49
PROJECT
Capps Crossing ERFO
INSTALL SIGN & POST
Figure A135—Site plan and cross section for Capps Crossing replacement project. A full size drawing may be found on the CD included in the
back of this publication.
INSTALL SIGN & POST
13+25.00
BEGIN TURNOUT LEFT
10+01.00
SAW CUT A/C
BLEND GRADE
SAW CUT A/C
10+51.00
ELEV. 4904.00
10+51.00
BEGIN ABUTEMENT CONSTRUCTION
13+00.00
13+00.00 ELEV. 4902.00
BEGIN ABUTEMENT CONSTRUCTION
10+76.00
00
ELEV. 4902.00
IV
A
S. OPTE
F.S ERT
. Y LIN
U.
OX
IM
00
10+01.00
49
10+76.00
00
END ABUTEMENT CONSTRUCTION
49
ELEV. 4904.00
P
E
CM CKPIL
STO A
ARE
END ABUTEMENT CONSTRUCTION
14+00.00
END TURNOUT LEFT
4905
BLEND TO EXISING GROUND
14+00.00
05
49
13+25.00
AP
PR
Appendix A—Case Study 21
Appendix A—195
Appendix A—Case Study
21
bedrock. The deck consists of 16-foot-long sections of box iron grating 16
feet wide. Steel beams support the rectangular steel tube decking. The 25foot-long concrete abutments slope down to the deck at 8 percent (figure
A135).
Bank stabilization and approaches: Riprap placed at the two abutments
provides protection against scouring. Near the abutment in the active
channel, large long rocks were placed at an angle to help turn flow toward
the middle of the stream, away from the abutment and banks. Other than
the riprap, the banks will be allowed to stabilize naturally.
Cost: $300,000 in 1997.
Safety: Early cold weather in this area makes icing a hazard while the
road is still open. Wooden guard rails were added to keep vehicles from
sliding off the roadway (figure A136). Safety signing is limited to type III
object markers on the ends of the bridge.
Flood and Maintenance
History
On January 1 2006, the bridge underwent a flood estimated to have an
85-year return interval. Large amounts of debris were caught under and
on the bridge, and sediment accumulated around the structure as a result.
Perhaps due to sediment accumulation upstream of the bridge during the
flood, flow spread over the entire flood plain and has concentrated into
two principal channels, one on either side of the flood plain. Maintenance
work will include debris removal and in-channel work to confine low-flow
to a single channel and remove some sediment deposits.
Figure A136. Note concrete abutment with riprap, grating with tire runways, and
wood curbs.
Appendix A—196
Appendix A—Case Study 21
Summary and
Recommendations
This site presents challenges for crossing structures because historic
disturbances in the drainage—combined with recent floods—mean that
very large amounts of boulder and cobble-sized material are available for
transport during high flows. The bridge is an appropriate structure type
for this disturbed site because it minimizes interruption of sediment and
debris transport and should permit the channel to stabilize over time.
This low-water bridge is a solid heavy duty structure that is expected to
sustain flooding well. In retrospect, forest personnel believe it would have
been advisable to space the piers at a distance equivalent to the current
active channel width to minimize the potential to block floating wood
debris.
Ken Pence, engineering technician (retired); Cheryl Mulder, zone
hydrologist; David Jones, engineer; and Richard Adams, facilities
engineer from the Eldorado National Forest provided information for this
case study.
Similar Structures In
Other Locations
The Eldorado National Forest had experience with low-water bridges
before constructing one at the Capps crossing in 1997. A very similar
structure has been in existence since 1971 where the Jones Wreckum road
crosses Jones Fork of Silver Creek. Originally, the crossing was probably
an unimproved ford. It accesses private property.
The site is at about the same elevation as Capps, and has a similar runoff
regime. According to Steve Brink, the designer, flow at this site fluctuates
from 15 feet wide and 1-foot deep to 180 feet wide and 5-foot deep (Brink
1974, 2000). Crossing objectives were to support log haul and recreation,
provide for free fish passage during low flows, protect good trout
spawning habitat up- and downstream, and avoid flow obstructions that
might cause channel shift. This bridge was designed to pass 80 percent
of the estimated 100-year flow under the deck. It was overtopped the first
winter without damage or any need for maintenance. It has since been
overtopped at least three times, and the only maintenance on record is
removal of woody debris.
The bridge rests on 10 concrete spread footings placed 5 feet below the
streambed surface. The approach slabs have cutoff walls 5 feet below the
streambed and were riprapped. The deck consists of twenty 8-foot by 16foot cattleguards.
Appendix A—197
Appendix A—Case Study
21
Figure A137. Looking downstream at Capps bridge with debris trapped under
deck, 1988.
This reach is flatter than Capps, bed material is finer (coarse gravels about
1 to 12 inch), and the channel appears to be more stable. The bridge
is located just upstream of a right-angle bend in the river and crosses
the point bar leaving enough space for natural adjustments in sediment
storage. Woody debris trapped under the bridge on the point bar side has
not been removed, and has contributed to sediment accumulation (figure
A137). Both upstream and downstream bars have enlarged since the
1970s (figures A138a, A138b). Nonetheless, comparing current channel
conditions to Brink’s 1974 description, the channel does not seem to have
changed much, indicating that the stream is functioning naturally and is
stable. None of the pier footings are exposed.
Appendix A—198
Appendix A—Case Study 21
A
circa 1975
B July 2002
Figures A138a and A138b. Looking downstream at the Jones Wreckum Bridge.
A138a. Bridge inspection photo.
Appendix A—199