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OCR for page 259
8
Appurtenant Structures
INTRODUCTION
Appurtenant structures are other structures around a dam that are neces-
sary to the operation of the dam project. These include spillways, outlet
works, power plants, penstocks, gates, valves, trash racks, diversion works,
and switchyards. Generally, these are smaller structures than the dam, but
they can be of considerable importance to the project because they control
the flow of water and power.
Incidents of failure or near-failure of all types of dams that were attrib-
uted to defects in the appurtenant structures have been well documented in
the literature. Often, the defects in appurtenant structures, when identi-
fied in the early stages, can be corrected by taking preventive maintenance
measures without endangering the integrity of the dam. In cases where
more extensive repair work is required, it may be necessary to lower the
reservoir level to provide a sufficient factor of safety during repairs. In ex-
treme instances, defects in appurtenant structures can be of such magni-
tude that they lead to complete failure and subsequent abandonment of the
dam.
This chapter describes some problems common to appurtenant struc-
tures, together with suggested solutions. Table 8-1 summarizes defects,
causes, effects, and remedies.
DEFECTIVE SPILLWAYS
The main appurtenant structure of a dam is usually the spillway. The pri-
marv defect most often indicated is inadequate discharge capacity. Inade-
259
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260
TABLE 8-1 Evaluation Matrix of Appurtenant Structures
SAFETY OF EXISTING DAMS
Type of Defect Causes Effects Remedies
Defective Insufficient Overtoppinga Reevaluate spillway
spillways analysis Erosion or capacity usingpresent-
washout on day hydrologic
downstream techniques
side
Design error Use watershed model
simulation and
prototype studies in
design
New criteria Erosion along and Institute major repairs:
established around spillway Increase spillway
chute capacity
Construction of
auxiliary or
emergency
Major or Breach Alternate methods:&
unpredicted Revise reservoir
events operating procedures
Restrict reservoir
utilization
Require attendance of
dam personnel during
flood events
Establish well-defined
emergency procedures
Obstruction to Excess trashb Overtopping Install log booms or trash
spillways and burden racks based on use of
outlet works reservoir, anticipated
trash burden, etc.
Erosion Perform maintenance as
required to remove
excess trash buildup
- Damage to trash
racks
Defective gates Mechanical Upsets normal Perform regular
and hoists breakdown operation maintenance on
characteristics of mechanical equipment
dam
Inadequate gate Vibrations
seals Fatigue cracking
Cavitation
around gate
guides
Damage to gate
frames and
operating shaft
Check bottom gate seals
for damage
Provide for sharp clean
flow breakoff
Repair cavitated areasC
with steel liners; check
that all gate frames are
securely mounted
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Appurtenant Structures
TABLE 8-1 Evaluation Matrix of Appurtenant Structures (continued)
261
Type of Defect Causes Effects Remedies
Differential
foundation
settlement
Trash and debris
Galvanic
corrosion and/
or mineral
deposits
Poor design and/ Vibration
or inadequate
operational
procedures
Gates becoming
inoperable
Gate frames crack
Vibration
Trash can knock
gates from frames
Corrode moveable
parts; makes
gates inoperable
Repair foundation
Install trash racks
Provide cathodic
protection
Exercise gate to prevent
formation of deposits
Revise operating
procedures
Unbalanced flow Provide adequate air vents
(can cause other
problems to
occur, such as
buckling of steel
liners and
concrete erosion)
Defective Surface Cavitation erosion Grinding surface to
conduits irregularities Piping smoothness that will
(offset joints, prevent cavitation
voids, erosion
transverse Air vents at irregularities
grooves, Require close construction
roughness) tolerances
Provide aeration grooves to
draw air into flowing
Sealing in
conduit
Unsteady flow
conditions
Structural
vibrations
Unsymmetrical Cavitation
flow
Settlement of
foundations
Corrosion
water
Perform prototype studies
and modify
Adequate air vents
Repair concrete
Install guide vanes
Baffle blocks at terminal
structure
Adequate air vents
Stabilize foundations
Replace joint collars
Replace joint seals
Replace or repair conduit
Erosion in stilling
basins
Joint separation
Structural cracking
Piping
Piping of embank-
ment material
through holes
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262
SAFETY OF EXISTING DAMS
TABLE 8-1 Evaluation Matrix of Appurtenant Structures (continued)
Type of Defect Causes Effects Remedies
Defective Inadequate Uncontrolled Investigate and modify
drainage design seepage Install new or improve
system Improper Piping existing drain field
installation Boils Provide relief wells
Reduce reservoir pool level
Inadequate filter Saturated conditions Improve filterlayer
layer Seepage of fines
from foundation
Mineral Clogging Ream drains. Drill
deposition supplemental drains
Erosion Inadequate Fluctuating positive Increase thickness of
design of to negative or concrete slabs
spillways and uplift pressures Impose tailwater elevation
stilling basins can develop on that will force hydraulic
spillways and jump
stilling basins Provide floor drain
(can cause openings in locations to
cracking of avoid subjecting them to
concrete slabs in fluctuating pressures
stilling basins and
subsequent
removal of
embankment
material); this
fluctuation of
pressure can
demolish a
spillway or
stilling basin
Structural cracks Water seepage Pressure grout cracks in
in concrete through slate and slab
slabs of eroding of Replace with thicker slab
spillways and embankment
stilling basins materials
Development of Evaluate effectiveness of
voids under slab energy dissipators and
Loss of slab support replace if necessary
Breakup of slab Fill voids under concrete
slabs
Anchor invert
Unsymmetrical Unsymmetrical Operate gates
operation of loading of symmetrically
outlet gates spillway Repair with erosion
Scour actions in resistant aggregate and
discharge area high-strength concrete
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Appurtenant Structures
TABLE 8-1 Evaluation Matrix of Appurtenant Structures (continued)
263
Type of Defect Causes Effects Remedies
Excessive Abrasion and Repair with special
discharges cavitation erosion concretes end steer
Abrasive objects of concrete in plates
in stilling spillway and Line dissipators with steel
basin (rocks, stifling basins plates
construction
debris, etc.)
Damage to chute Install rip-rap
blocks and energy
dissipators
Breakup of slabs
and destruction
of spillway
a Overtopping is more critical on earth or rockfilled dams. Concrete dams can stand a limited
amount of overtopping.
bLarge trash, such as logs, etc., can damage spillways, stilling basins, and energy-dissipating
blocks as it is carried over the spillway.
CNew techniques for repair: polymer-impregnated concrete has been used to repair cavitation
in concrete tunnels and stilling basins.
New technique for repair: for spillway repair, rollcrete has been used as an alternative repair
method.
quate capacity can lead to overtopping of the dam, which is particularly
critical in earth or rockfill dams because overtopping can cause failure. In
the evaluation of older dams a determination of inadequate spillway ca-
pacity is generally the result of new criteria and updated hydrological pro-
cedures and records rather than design or construction faults.
The subjects of spillway design floods; the ability of the spillway and
reservoir acting together to control safely the design flood, and the general
types of mitigating measures where that ability is lacking are discussed at
length in Chapter 4. Some of the specific defects and poor hydraulic behav-
ior that have been observed and remedies that have been used are discussed
below.
Siphon spillways have been constructed at a few earthfill dams, usually
as towers or imbedded risers in combination with the outlet works conduit.
In some instances subsequent performance has demonstrated that the dis-
charge capacity is much less than what was theoretically predicted. Where
topography permits, a supplemental open channel spillway can be con-
structed beyond one end of the dam with the control elevation above that
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264
SAFETY OF EXISTING DAMS
of the siphon. Vegetated linings may suffice depending on the frequency
and duration of discharge in excess of that which can be handled by the
siphon spillway (Cortright 1970~.
Some spillway stilling basins originally constructed with the basin invert
elevation incorrectly set in relation to tailwater and the conjugate depth of
design discharge have been destroyed or severely damaged. It is sometimes
possible to terminate the spillway discharge channel as a bucket at an ele-
vation above tailwater. The bucket is supported on a foundation level at or
below the expected depth of the eroding plunge basin. The support can be
provided by cast-in-place piling in drilled or cased holes in granular forma-
tions or by a reinforced concrete substructure on a hard rock formation.
Nonsuperelevated horizontal curves have been built in spillway dis-
charge channels where flow velocity is supercritical. As a result, flows have
overtopped the outer wall with consequent erosion and structural damage.
It may be possible to raise the outer wall and accept the transverse slope of
the water surface provided the erratic wave pattern created is safely con-
tained beyond the curve. In one instance the curved portion of the channel
was compartmented with several vertical walls so that the outer rise in the
transversely sloping water surface was diminished sufficiently for contain-
ment within the available freeboard.
Spillways with converging training walls are often susceptible to having
an actual capacity less than theoretical. This is caused by water piling u
along the converging walls and overtopping, often with serious results. Hy-
draulic model testing is often the optimum and only effective way of deter-
mining actual capacity.
In recent years diversion facilities during construction of some rockfill
dams (both impervious core and faced) have included crest and down-
stream face reinforcing. Floods have been successfully passed over the top
and down the slope of the uncompleted embankment with minimal dam-
age. This suggests the possibility of a less costly way of increasing the spill-
ing capability at an existing rockfill dam where the spillway capacity is too
small. In some cases such treatment would be a temporary betterment until
a permanent solution could be financed. In other cases the treatment might
be justifiably considered permanent, for example, where the required spill-
ing capability was expected to operate rarely, if ever, during the project
life.
A decision to adopt this remedy would depend on full consideration of
the quality and size of the rock in the top and face layers of the dam, the
character of the foundation rock along the toe, a limiting dam height, the
length of dam to be so treated, anticipated depth and duration of overtop-
ping, river channel characteristics immediately downstream, and a nearby
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Appurtenant Structures
265
source of additional rock (original abutment or downstream quarries,
perhaps).
The face reinforcing commonly consists of a heavy square steel mesh re-
tained by an orthogonal pattern of spaced horizontal and sloping reinforc-
ing bars. The bar network is retained against the face with bent anchor
bars embedded in the rock mass. The horizontal and sloping bars are an-
chored to bedrock along the toe. During original construction the anchor
bars are embedded as the rock lifts are being placed. At an existing dam the
embedment would have to be made in a sliver fill of rock placed against the
existing face.
Spillway capacities can be increased by constructing vertical concrete
parapet walls on the tops of dam embankments. This can be feasible both
for rockfil1 and earthfill dams. Usually the parapet wall is considered to
provide residual freeboard only and the effective head on the spillway is
thus measured from dam top elevation (Cortright 1970~.
OBSTRUCTIONS IN SPILLWAYS AND OUTLETS
In conjunction with spillway capacity, obstructions in the spillways and
outlet works also can affect the stability and desired operating characteris-
tics of dams. These obstructions can be caused by faulty design, structural
defects, excessive reservoir trash burden, siltation, landsliding, or a combi-
nation of these factors. One documented incident that illustrates the results
of obstructions occurred at the Nacimiento Dam near Bradley, California
(ASCE/USCOLD 1975~. After several intense storms the high-level outlet
slide gate clogged with trash and failed.
Where trash racks are used, their proper design and placement plus reg-
ularly scheduled maintenance and cleaning of debris from the racks can
help prevent such incidents. The design of trash racks generally must con-
sider such factors as the intended use of the reservoir (recreation, water
supply, flood, etc.), types of gates, and maintenance requirements (U.S.
Bureau of Reclamation 1974~. Where log booms are used to prevent ob-
structions to spillways and intakes, accumulated debris should be continu-
ously removed and inspection made for damaged, corroded, or inadequate
log booms.
A frequent deficiency in the outlet works of embankment dams relates to
the elevation at which the intake structure was placed when originally con-
structed. If inadequate dead storage capacity was provided, the intake
structure may be in danger of becoming obstructed by a mixture of water-
logged trash, sediment, and debris. Loss of withdrawal capability is of
great concern when a more serious dam defect appears. Permanently sub-
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SAFETY OF EXISTING DAMS
merged outlet gates in particular can be a problem because they are diffi-
cult to inspect or to maintain. They are assumed to function until sometime
when they no longer do so. By then it may be too late to take corrective
measures short of emergency measures.
Where it is still possible to empty the reservoir, a vertical freestanding
riser or possibly a sloping riser laid on the abutment or possibly the face of
the dam can be constructed to a new and higher intake elevation. The exist-
ing gate control can be set at the new entrance to the structure, or it may be
desirable to modify the type of control at the same time. Vertical risers or
stub towers have been successfully installed on several dams owned by the
Santa Clara Valley Water District in California.
Where the reservoir cannot be drained and where extensive work by div-
ers is impractical, a prefabricated riser can be added underwater. By mak-
ing the riser watertight and by fitting it with connections for hoses through
which ballast water or compressed air can be pumped, it is possible to tip
up and position the riser vertically over the existing intake structure. A bot-
tom end cover temporarily held in position by bolts and clamps is removed
after the riser is in the vertical position and then blown off by compressed
air. With the bottom cover off, the riser can be made to rise or sink by
adding or releasing compressed air. The riser can then be joined to the ex-
isting intake structure and totally flooded. The top cover can then be re-
moved, and a prefabricated trash rack arrangement can be installed by
divers. This remedy was successfully made at Santa Felicia Dam in Califor-
nia by the United Water Conservation District (Bengry and Caltrider
1978~.
DEFECTIVE CONDUITS
Surface irregularities such as offset joints, voids, and roughness create tur-
bulence within a conduit, which can cause cavitation, leakage, and piping.
Grinding surfaces to a smooth finish, applying a smooth coat of epoxy, and
providing air vents and/or aeration grooves to draw air into the flowing
water are some solutions for turbulence problems.
Sealing or the transition from a free surface flow to full pipe flow in hori-
zontal or inclined conduits can result in structural vibrations because of
unsteady flow conditions and in an undesirable variation of the water flow
(ASCE 1978~. Sealing can be mitigated by providing adequate air vents in
the conduit. In any case it is a condition to avoid through adequate design
of the conduit and the use of prototype studies.
Unsymmetric flow conditions through conduits caused by bends or irreg-
ular gate operation can result in cavitation in the conduit and erosion in the
stilling basin. Guide vanes installed in the conduit and adequate air vents
OCR for page 267
Appurtenant Structures
267
can help streamline the flow. Baffle blocks and energy dissipators at the
terminal structure can help control erosion.
Settlement of conduit foundations can cause joint separation and struc-
tural cracking that can lead to leaking and piping. In such cases the foun-
dations need to be stabilized and joint collars and seals repaired or re-
placed. In corroded metal conduits, embankment material can be piped
through the corroded holes and may require total lining/grouting or re-
placement of the conduit.
Bare metal conduits are frequently found in and beneath embankment
dams especially in smaller, older, privately owned dams storing water for
farm use and recreation. The conduits consist of welded or riveted steel or
corrugated metal pipe. The transverse joints are welded, riveted, banded,
or even slip-jointed. Bell and spigot cast iron pipe have been used. The con-
duits were installed by bedding them either on embankment or granular
foundation surfaces and surrounding them with the materials of the overly-
ing embankment zones. Little or no compaction was achieved beneath the
overhanging portions of the pipe sections. The overlying embankments
themselves may not have been placed with controlled moisture and con,-
paction procedures. The outlet discharge is often controlled only by down-
stream gates or valves, and the conduits are subjected to full reservoir head
when the outlet is closed.
The steel pipes are corroded by electrolysis and/or chemical action exter-
nally and are pitted internally. The rivets are no longer in intimate contact
with the surrounding plate material. The banded joints are loose and
rusted. Slip joints and bell and spigot joints have been opened by the base
spreading forces of the embankment.
These defects are reason for great concern and have caused a number of
dam failures. An outlet conduit is subjected to full hydrostatic reservoir
pressure when closed downstream and transmits that pressure directly to
all portions of the embankment and foundation along its entire length. The
conduit is subjected to the lesser pressure of the hydraulic grade line when
flowing. If the point of free discharge is far beyond the downstream end of
the conduit the pressure can approach reservoir head. Any leakage under
pressure from the conduit into the surrounding embankment or foundation
can cause failure by internal erosion.
The existence of the defect can be determined by physical examination
and reference to any reliable construction records. The deterioration of the
conduit interior can be examined by closed-circuit television if the conduit
is too small for entry. Inspectors can be pulled on wheeled dollies through
dewatered conduits as small as 30 inches in diameter. Precautions should
be taken to provide adequate air supply to the inspectors. Leakage appear-
ing about the periphery of the conduit at the downstream face may have its
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SAFETY OF EXISTING DAMS
source in the conduit. Temperature and chemical comparisons of the water
may verify that source. By varying downstream gate settings, correspond-
ing changes in the leakage rate may help identify the source.
If the conduit is of sufficient diameter and its full discharge capacity is
not needed, a smaller-diameter pipe can be inserted and centered in the
conduit, the annular space bulkheaded at both ends, and the space pressure
filled with a sand-cement mixture. If the existing gate or valve was installed
on the downstream end, it should be relocated to the upstream end of the
conduit (Cortright 1970~.
Conduits can be taken out of service by filling them with a sand-cement
mixture under pressure. A drain and filter system can be installed around
the exterior of the conduit near the downstream end to protect the embank-
ment against piping from any leakage that may tend to flow along the exte-
rior surfaces of the conduit. If drawdown limitations allow, a new, shorter
outlet works can be constructed and founded on an abutment by removal
and replacement of a portion of the embankment.
Although not favored as a permanent solution, an interim siphon outlet
can be installed over the top of the dam until a permanent gravity-flow
outlet works can be financed. If downstream releases are not required and
the project has no other defects, the defective outlet can be taken out of
service as described earlier and its replacement deferred temporarily until
it can be financed.
DEFECTIVE GATES AND HOISTS
Defective gates and hoists, especially those under high head, can cause un-
expected problems and threaten dam stability when malfunctions occur.
Being mechanical devices, these gates and hoists are subject to breakdown.
Generally, two types of gates can be found on dams, depending on the de-
sign: (1) spillway gates used to control flow releases over the spillway if
reservoir storage above the spillway crest is desired and (2) gates that func-
tion as regulating and guard gates in conduits.
One major problem that can occur with gates is induced vibrations from
hydraulic forces during opening and closing. The problem has been most
acute with radial gates on spillways. In Japan in 1967 (journal of Fluids
Engineering 1977), oscillations due to fluid-induced structural loadings
caused the collapse of a radial gate. This resulted in a sudden rise in the
water level downstream, with a subsequent loss of human lives. In the
United States it was reported that vibrations on spillway gates on the
Arkansas River were severe enough to cause fatigue cracks (ASCE 1972~.
An investigation determined that to eliminate the vibrations a sharp, clean
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Appurtenant Structures
269
flow break-off point was required. Soft rubber seals should not be used on
the bottom of the gates. They should be rubber bar seals rigidly attached.
Differential foundation settlement of the gate structure can crack gate
frames or skew the frames, so that the gates become inoperable. Cavitation
erosion of concrete around gate frames can weaken the supports and cause
subsequent failures. Cavitated areas can be repaired by using steel plates.
Gate operation can be stopped by formation of ice in the guideways or
by reservoir ice. Galvanic action and/or mineral deposits can corrode mov-
able parts on gates, rendering them inoperable. Cathodic protection and
regular exercising of gates can help prevent formation of deposits and elim-
inate the problem. If trash racks are damaged or not provided, large trash
and debris also can knock gates from frames.
Poor gate design and/or inadequate operational procedures can cause vi-
bration and unbalanced flow. An unbalanced flow condition can lead to
other problems such as cavitation and abrasive erosion and damage to gate
frames. In such cases a revision of operating procedures may be all that is
required to solve the problem. Providing adequate air ventilation behind
the gate also will help mitigate such problems. Basic hydraulic design
guides and criteria have been established by the U.S. Army Corps of Engi-
neers and U.S. Bureau of Reclamation (ASCE 1973~.
Other problems related to gate structures have been caused by down-
slope movement of riprap due to frost action/creep, causing the gate struc-
ture to tilt or crack. Where service bridges are used to gain access to the
gate structure, any movement of the gate structure or any displacement of
the bridge support foundation may induce stress or buckling of the struc-
tural elements of the bridge.
DEFECTIVE DRAINAGE SYSTEMS
Defective drainage systems are often a source of problems. Clogged or
plugged drains can lead to saturated conditions and create uplift pressures
on spillways. Inadequate filter systems also can cause saturated conditions
and allow piping of fines from the foundation. When such conditions occur
the problems can be temporarily lessened by a reduction of the reservoir
pool level. Long-term solutions can include providing a new design with an
improved drain field or installation of new field and/or relief wells.
EROSION
Erosion in and around dams is sometimes associated with defective appur-
tenant structures. Erosion can play a dual role in that it can be both the
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270
SAFETY OF EXISTING DAMS
cause and the effect of defects and, if left untreated, can lead to dam fail-
ure. Erosion is far more evident with spillways and stilling basins because
the tremendous force of moving water makes the effects of erosion highly
visible.
An inadequate spillway or stilling basin design can lead to erosion with
subsequent undermining of the dam itself. If uplift pressures on the spill-
way are not adequately controlled, the fluctuating positive to negative
pressure can lead to cracking of the concrete slabs and removal of founda-
tion materials (ASCE 1972~. Structural cracks in spillways and stilling ba-
sins or poorly constructed joints allow seepage through the slab; this can
cause piping of embankment material with a subsequent loss of slab sup-
port. If corrective measures are not taken, complete breakup of the slabs
may result.
Unsymmetrical operation of outlet gates and unsymmetrical loading of
the spillway can result in scouring action in the discharge area. Excessive
discharges due to major storm events likewise can cause abrasion, erosion,
and cavitation of concrete on spillways and in stilling basins. Chute blocks
and energy dissipators can be damaged. Remedial measures for erosion vary
extensively depending on the size of the dam and severity of the erosion.
Corrective action as simple as the placement of riprap in the discharge
area may be all that is required to solve minor problems. For more complex
problems a redesign and reconstruction of the spillway and stilling basin
may be necessary. In cases where uplift pressures are determined to be the
cause of erosion, slab thickness can be increased or rock anchors used to tie
down the slabs to underlying rock. In addition, spillway and floor drains
can be installed to relieve the excess pressure. Voids under spillway and
stilling basin slabs need to be filled and the cracks grouted. In such grout-
ing, care must be exercised to avoid having grout fill underlying filters and
uplift the slab. If slabs are to be replaced, the use of erosion resistant aggre-
gate and high-strength concrete is advantageous. Eroded concrete slabs
and energy dissipators can be repaired and strengthened by use of steel
plates. Nonstructural solutions to erosion can include better reservoir man-
agement through the controlled releases of water.
EARTHQUAKES
After an earthquake it is essential that appurtenant structures continue to
function in order to keep water and power flow under control. The uncon-
trolled water flow could result in damage to all or part of the dam and the
surrounding terrain. The location of these structures is significant in engi-
neering studies because much more shearing energy may be transmitted to
the base of one of these structures if it is located on a ridge than if it were
OCR for page 271
Appurtenant Structures
271
located on firm, level ground. Likewise, its foundation may indicate the
susceptibility to damage of the structure, e. g., a structure on alluvium may
suffer considerably more damage than one located on bedrock.
Freestanding structures, such as intake towers, tend to magnify ground
motions. If they are submerged in the reservoir, the motion of such a tower
causes a certain mass of the lake to move with it. This increases the appar-
ent mass of the structure and, consequently, its response to the earthquake.
For all these reasons the appurtenant structures must be studied for their
response to earthquake. They must be structurally stable, i.e., the earth-
quake-induced stresses in the concrete and steel must be within acceptable
limits. Also a check must be made to see if the machinery will continue to
function at the conclusion of the seismic incident.
REFERENCES
ASCE (1978) Size Determination of Partly Full Conduits. Proceedings 104 (HY 7 No. 13862~.
ASCE (1973) High Head Gates and Valves in the U.S. Proceedings 99:1727-75, October.
ASCE (1972) Spillway Gate Vibrations on Arkansas River Dams. Proceedings (HY) 99:219-
238, January.
ASCE/USCOLD (1975) Lessons from Dam Incidents, USA, American Society of Civil Engi-
neers, New York, pp. 259-61.
Bengry, E. O., and Caltrider, W. T. (1978) "Reservoir Outlet Extended Above Silt to Prevent
Clogging," Civil Engineering-ASCE, September.
Cortright, C. J. (1970) "Reevaluation and Reconstruction of California Dams," Journal of the
Power Division, American Society of Civil Engineers, January, pp. 63, 65.
Journal of Fluids Engineering (1977) "Instability of Elastically Suspended Tainter-gate System
Caused by Surface Waves on the Reservoir of a Dam," Vol. 99, December, pp. 699-708.
U.S. Bureau of Reclamation (1974) Design of Small Dams, Government Printing Office,
Washington, D.C.
RECOMMENDED READING
ASCE (1973) Reevaluation Spillway Adequacy of Existing Dams, Proceedings 99 (HY), pp.
337-382, February.
ASCE Proceedings 98 (1972) Damage to Kannajuli Dam Spillway, (HY 12 No. 9452), Decem-
ber, pp. 2155-2170.
Chopra, A. K., and Liaw, C-Y. (1975) "Earthquake Resistant Design of Intake-Outlet Tow-
ers," Journal of the Structural Division, ASCE, Vol. 101, No. 577 (July).
Engineering News Record 200:11 (1978) Brazil Blames Earth Dam Collapses on Failure to
Open Spillway Gates, February 2.
U.S. Army Corps of Engineers (1964) Structural Design of Spillways and Outlet Works, EM
1110-2-2400.
U.S. Bureau of Reclamation (1974) Safety of Dams, ASCE Proceedings 100 (HY), February,
pp. 267-277.
Representative terms from entire chapter:
stilling basins
