how culverts fail - californiabofdata.fire.ca.gov/board_committees/monitoring_study_group/msg... ·...
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How Culverts Fail
Sam FlanaganNOAA Fisheries
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…and what to do about it
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Chronology of Studies
• 1992-1996: Woody Debris Transport through Low-order Channels: Implications for Culvert Failure
• 1997: Field Indicators of Culvert Capacity• 1996-1998: Response of Road-Stream Crossings
to Large Flood Events in the PNW
• Funded by: CDF, HSU, USFS, NCASI
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The Environmental Risk of Road-Stream Crossings
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How do culverts fail?What can we do about it?
A consideration of inputs and capacity
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Failure Mechanisms for the 1996/97 PNW storms (n=258)
(a very large, infrequent storm)
Debris torrent26%
Sediment slug36%
Wood debris17%
Wood / sediment12%
Hydraulic exceedence
9%
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Failure Mechanisms for NW California (<12 year event)
Debris torrent2%
Sediment slug7%
Wood debris61%
Wood / sediment18%
Hydraulic exceedence
12%
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Failure Mechanisms I. The Phenomenon of Wood
Plugging Culverts
A. B.
E.D.
C.
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Examine Wood Size Transported Through Low-order Channels
• 26 “Culvert-sized” streams• Channel width ranging from 0.3 to 3.5m• Construct debris screens across each stream
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Debris Screens 15cm square mesh wire
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The Routine
• Collect wood accumulated in screens after each peak flow event
• Measure length and diameter of each piece• Re-measure channel dimensions• The joys of Storm Patrol
– Sipping hot Tang– Access problems in and OUT due to landslides
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Findings
• 99.5 % of the wood transported through low-order channels is < channel width– n = 3,114– 95th percentile wood length increases with
channel width– Suggests woody debris supply to culvert is
transport limited– Findings apply for storms < ~12 year event
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Fluvially transported pieces are rarely greater than the channel widthIn length (i.e. L/w < 1).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Piece L/w
Pilot CreekBull CreekCoyote Creekmean distribution
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Wood lengths initiating plugging relative to channel width
0
1
2
3
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
Piece length/channel width
num
ber o
f fai
lure
s
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Sizing Culverts to Accommodate Wood: Implications of Findings
• Culverts should be sized in relation to the “bed width”: the zone of active, annual scour and deposition
• Suggests that a culvert diameter that approximates the channel width reduces “fluvial wood” plugging potential - but does not eliminate the hazard
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Anecdotes and Hypotheses
• Multiple barrel installations are not a suitable means of providing wood passage
• Inlet geometry influences wood orientation as it approaches
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Failure Mechanisms II. Sediment
• Larger storms trigger a greater proportion of sediment-related failures
• Discuss (a) Fluvial sediment delivery versus (b) catastrophic sediment delivery (e.g. slides and debris flows)
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A. fluvial sediment failure hazard
• Avoid placing relatively flat pipes on steep streams
• Two things working against this:– Flat pipe = a shorter pipe = less installation $$– Roads often built on natural hillslope benches
that coincide with a short depositional reach in the channel
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B. Debris Flow Failure Hazard
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Addressing Debris flow failure hazard
• Minimize the interference that the crossing presents in the path of the debris flow
• Avoid rolling dips directly over the crossing or in the potential path of the debris flow
• The “cascading failures” observed by Grant et al.:
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New slide initiated
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Water diverts to next crossing with further erosional consequences
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Failure mechanisms III. Insufficient hydraulic capacity
• Typically least frequent cause of failure– For forested, mountainous settings
• Is only a minimally useful measure for addressing failure hazard
• Why we shouldn’t hang our hat on hydraulic sizing alone:
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10
100
1000
10000
100000
0 10 20 30 40 50 60 70 80 90 100
Initial T at HW/D ≥ 1.0
45 cm culvert (increase to 61 cm)
91 cm culvert (increase to 107 cm)
Effects of increasing pipe diameter one increment (6”)
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Sizing for 100 year “water” does not ensure adequate channel sizing for
woody debris passage
012345678
0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5
culvert diameter/channel widthfor pipes sized for 100-year flow
#
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Sizing for channel width typically ensures adequate hydraulic capacity
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Failure mechanisms IV. Culverts on small alluvial fans
• Roads, particularly low slope position, often traverse small alluvial fans
• The channel is prone to avulsion at the apex of the fan
• Channel is abandoned and no culvert is present on the road to accommodate the new channel location
• Relief culverts on each edge of the fan
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Summary I – Styles of culvert failure
• More frequent storms cause failures by fluvial mechanisms – wood transport and fluvial sediment – we can reduce failure probability for these events
• Large, infrequent storms cause hillslopefailures that initiate slides and debris flows – we can reduce failure consequences for these
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Reducing fluvial failure probability
• Channel dimensions should drive culvert sizing– Channel width– Channel slope– Inlet basin configuration / approach angle
• Avoid sizing that creates ponded conditions
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SIZING FOR A DESIGN DISCHARGE SUCH AS THE 100-
YEAR PEAK FLOW
DOES NOT ENSURE ADEQUATE CAPACITY FOR DEBRIS AND
SEDIMENT
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Reducing failure consequences for large, infrequent storms
• AVOID DIVERSION POTENTIAL! – And be aware of the potential consequences of
debris flows and adjacent slides overwhelming remedial dips
• Keep the watershed products moving downhill, not across the slope
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(sidenote)
Existing culverts and channels often indicate past failures and types
• Dented inlets from repeat excavations• Depositional terraces in the inlet basin• Debris flow levees (may be well vegetated)• A small pipe on a wide stream
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Where roads cross streams, failure hazard can only be reduced, but not avoided, through careful design that accommodates water, wood and sediment, and potential erosional consequences when they do fail.