D--048966 - CALFED Bay-Delta Program - State of California

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VOLUME II, CHAPTER 30
Failure
1940s turn out to be conservative, society witl have a long
time to think about what to do with the dams of the Sierra
Catastrophic failure of impoundments is always a concern of Nevada.
those living below dams. Sudden releases of water also have
great potential for dramatic environmental change. During
the gold-mining era, dam failures were fairly common, both
because of design flaws and because of intentional releases
to rearrange gold-bearing sediments in the practice known
R O A D S
as booming. Early debris dams were also intentionally destroyed
to allow fresh access to impounded gravels and to Roads provide the most intensive modification of land surcreate
new storage space. Unintentional collapse of the En- face properties relevant to the hydrology of common landglish
Dam on the Middle Fork of the Yuba (Ellis1939; McPhee management practices. All vegetation is removed and
1993) in June 1883 released almost 18 million m 3 (15,000 AF) prevented from reestablishment. Dirt-surfaced roads are cornof
water suddenly and cleaned out much of the stored min- pacted to a near-impervious state, and sealed and paved roads,
ing debris in that channel (James 1994). Excessive water re- are completely impervious. Runoff from the surface is colleases
from an upstream dam washed out a small dam on lected and discharged as potentially erosive flows at points
Bishop Creek in June 1909. Following failure of the Saint below the road. Roads that are cut into slopes intercept sub-
Francis Dam in the Ventura River Basin in 1929, the Division surface water flow and bring it to the surface. Fill materials
of Dam Safety of the Department of Water Resources has regu- cover additional portions of the slope and often contribute to
fated larger dams and inspected them at least annually. Dams sediment yields slowly over time or catastrophically if they
that are either more than 7.6 m (25 ft) tall and store more than become saturated from subsurface water entry and then fail.
62,000 m 3 (50 AF) or, alternatively, more than 1.8 m (6 ft) tall Erosion from the actual roadbed of unpaved roads may be
regardless of capacity or impound more than 19,000 m 3 (15 significant as well (Garland 1993; Adams 1993). Unauthorized
AF) regardless of height are regulated by the Department of use during wet surface conditions adds to the erosion of the
Water Resources (1988). Modern dams have little risk of fail- road. A principal side effect of an extensive road network is
ure; however, failures are not unknown. Tl~e best-known dam the access that is provided to allow additional alterations. Few
collapse in the Sierra Nevada in recent decades was that of adverse impacts occur in the absence of roads. Avoidance of
the Hell Hole Dam on the Rubicon in December 1964 (Scott new road construction can minimize other potential impacts
and Gravlee 1968). Failure of the North Lake Dam during a in currently unroaded areas.
storm in September 1982 produced the largest flood of record
on Bishop Creek and severely damaged one of the power- Stream Crossings
houses. During the massive floods of February 1986, the coffer
dam at the Auburn Dam site failed when diversion tunnels The most serious impacts of roads occur where roads are in
became clogged and the dam was overtopped. Structural fail- close proximity to streams or wetlands. Stream crossings by
ure of a penstock during high-pressure testing at the Helms ford, culvert, or bridge have direct effects on the channel and
Creek pumped storage facility in 1982 resulted in massive local sediment regime. Although virtually any stream crossscouring
of Lost Canyon (Chan and Wong 1989). Even partial ing will have some impact on the channel, careful engineerfailures,
such as the gate damage on Folsom Dam in July 1995, ing, construction, and maintenance can limit the severity. The
can result in large releases of water and prolonged difficul- basic problem just comes down to disturbing the bed, banks,
ties in project operation, floodplain, and terraces. Because the crossing is coincident
Eventually, some larger dams will become filled with sedi- with the channel, there is little opportunity to buffer the inment
and no longer worth operating. The Federal Energy adequacies of design or construction. Also, roadside ditches
Regulatory Commission now has the authority to take dams near the crossing drain directly into the stream, often conout
of service when they come up for relicensing. We have no tributing sediment to the stream. In past decades, very little
real experience with what to do about a dam filled with sedi- attention was paid to stream crossings, and the cheapest alment.
Early debris dams on the Yuba and Bear Rivers just ternative was usually chosen. Often, that choice was merely
failed or were intentionally destroyed, and the sediments pushing a stack of cull logs into the channel and covering
eventually moved downstream or became semistable terraces, them with dirt. Installation of culverts sized only for summer
However, that option probably won’t be acceptable in the flow, with anticipated reconstruction, was often a more costfuture.
Plans are being made to decommission a dam on the effective choice than a properly engineered crossing. Fortu-
Elwha River in Olympic National Park in Washington. Initial nately, engineering and construction practices have improved
estimates suggest that removal of the dam could cost $60-80 dramatically since crossings have become widely accepted
million and sediment removal could cost $150-300 million. If as a potential problem (Furniss et al. 1991).
estimates of reservoir sedimentation rates made during the
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