| Abutment |
That part of the valley side against which the dam is constructed. An artificial abutment is sometimes constructed, as a concrete gravity section, to take the thrust of an arch dam where there is no suitable natural abutment. The left and right abutments of dams are defined with the observer viewing the dam looking in the downstream direction, unless otherwise indicated. |
| Adit |
A nearly horizontal underground excavation in an abutment having an opening in only one end. An opening in the face of a dam for access to galleries or operating chambers. |
| Ambursen dam |
A buttress dam in which the upstream part is a relatively thin flat slab usually made of reinforced concrete. |
| Anti-seep collar |
An impermeable diaphragm, usually of sheet metal or concrete, constructed at intervals within the zone of saturation along the conduit that passes through an embankment dam. In theory, anti-seep collars were designed to increase the seepage length along the conduit and thereby prevent backward erosion piping by lowering the hydraulic gradient along the conduit. |
| Arch dam |
A concrete, masonry, or timber dam with the alignment curved upstream so as to transmit the major part of the water load to the abutments. |
| Attenuation |
A decrease in amplitude of the seismic waves with distance due to geometric spreading, energy absorption, and scattering, or decrease in the amplitude of a flood wave due to channel geometry and energy loss. |
| Auxiliary spillway |
Any secondary spillway that is designed to be operated infrequently, possibly in anticipation of some degree of structural damage or erosion to the spillway that would occur during operation. |
| Backward erosion piping |
This occurs when soil erosion begins at a seepage exit point and erodes backwards (upstream), supporting a “pipe” or “roof” along the way. As the erosion continues, the seepage path gets shorter, and flow concentrates in plan view, leading to higher gradients, more flow, and the potential for erosion continues to increase. Backward erosion piping is particularly dangerous because it involves progression of a subsurface pipe towards the reservoir. |
| Baffle block |
A block, usually of concrete, constructed in a channel or stilling basin to dissipate the energy of water flowing at high velocity. |
| Berm |
A nearly horizontal step in the sloping profile of an embankment dam. Also a step in a rock or earth cut. |
| Blanket drain |
A layer of pervious material placed to facilitate drainage of the foundation and/or embankment. |
| Borrow area |
The area from which natural materials, such as rock, gravel or soil, used for construction purposes is excavated. |
| Breach |
An opening through a dam that allows the uncontrolled draining of a reservoir. A controlled breach is a constructed opening. An uncontrolled breach is an unintentional opening caused by discharge from the reservoir. A breach is generally associated with the partial or total failure of the dam. |
| Broad crested weir |
An overflow structure on which the nappe is supported for an appreciable length in the direction of flow. |
| Bulkhead |
A partition or structure separating compartments or to hold back water. |
| Buttress dam |
A dam consisting of a watertight part supported at intervals on the downstream side by a series of buttresses. Buttress dam can take many forms, such as a flat slab or massive head buttress. |
| Caisson |
A watertight chamber or hallow floating box used in construction work under water. |
| Cavitation |
The formation of the vapor phase of a liquid induced by lowering the local hydrodynamic pressure to the liquid’s vapor pressure. At hydraulic structures (such as spillways, gates, and penstocks), cavitation can cause significant noise, vibration and even erosion (or damage). Damage is caused by the implosion of cavitation bubbles near solid boundaries resulting in pitting of the material. Over time, the accumulation of this pitting process can result in significant damage or complete failure of structural components. |
| Chimney drain |
A vertical or inclined layer of pervious material in an embankment to facilitate and control drainage of the embankment fill. |
| Cofferdam |
A temporary structure enclosing all or part of the construction area that construction can proceed in the dry. A diversion cofferdam diverts a stream into a pipe, channel, tunnel, or other watercourse. |
| Compaction |
Mechanical action that increases the density by reducing the voids in a material. |
| Concrete lift |
The vertical distance between successive horizontal construction joints. |
| Conduit |
A closed channel to convey water through, around, or under a dam. |
| Confined Space |
A confined space has limited or restricted means for entry or exit and is not designed for continuous occupancy . Confined spaces include vaults, pits, manholes, tunnels, equipment housings, pipelines, towers, intakes, etc. |
| Construction joint |
The interface between two successive placements or pours of concrete where bond, and not permanent separation, is intended. |
| Contact grouting |
Filling, with cement grout, any voids existing at the contact of two zones of different materials, i.e., between a concrete tunnel lining and the surrounding rock.
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| Core wall |
A wall built of relatively impervious material, usually of concrete or asphaltic concrete in the body of an embankment dam to prevent seepage. |
| Crib dam |
A gravity dam built up of boxes, crossed timbers or gabions, filled with earth or rock. |
| Cutoff trench |
A foundation excavation later to be filled with impervious material so as to limit seepage beneath a dam. |
| Cutoff wall |
A wall of impervious material usually of concrete, asphaltic concrete, or steel sheet piling constructed in the foundation and abutments to reduce seepage beneath and adjacent to the dam. |
| Dead storage |
The storage that lies below the invert of the lowest outlet and that, therefore, cannot readily be withdrawn from the reservoir. |
| Design water level |
The maximum water elevation, including the flood surcharge, that a dam is designed to withstand. |
| Design wind |
The most severe wind that is reasonably possible at a particular reservoir for generating wind setup and run-up. The determination will generally include the results of meteorologic studies that combine wind velocity, duration, direction and seasonal distribution characteristics in realistic manner. |
| Dewpoint temperature |
The temperature at which dew begins to form or vapor begins to condense into a liquid. |
| Differential settlement |
Settlement that varies in rate or amount, or both, from place to place across a structure. |
| Dike |
A subsidiary dam of any type constructed across a saddle or low point on the perimeter of a reservoir. |
| Dispersive clay |
Dispersive clays differ from 'normal' clays because of their electrochemical properties. Dispersive clays usually have a preponderance of sodium cations on the clay particles compared to a preponderance of calcium and magnesium on 'normal' clays. The imbalance of electrical charges that result from this makeup causes dispersive clays to deflocculate in the presence of water. This deflocculation occurs because the interparticle forces of repulsion exceed the attractive forces. The clay particles go into suspension even in slowly moving or standing water. This means that dispersive clays are extremely erosive, and flow through cracks in dispersive clays can quickly erode the cracks and lead to rapid enlargement of the cracks. Failures caused by internal erosion in dispersive clay dams are common. Dispersive clays are also subject to severe rilling and jugging on exposed natural and constructed slopes because they are so erosive. Dispersive clays are not detectable with normal soil tests, such as mechanical analyses and Atterberg limit tests, and special tests, such as the crumb test, double hydrometer, and pinhole test, are required to detect the presence of dispersive clays. |
| Diversion dam |
A dam built to divert water from a waterway or stream into a different watercourse. |
| Double curvature arch dam |
An arch dam that is curved both vertically and horizontally. |
| Drainage curtain |
A line of vertical wells or boreholes to facilitate drainage of the foundation and abutments and to reduce water pressure. |
| Drainage wells or relief wells |
Vertical wells downstream of or in the downstream shell of an embankment dam to collect and control seepage through and under the dam. A line of such wells forms a drainage curtain. |
| Drawdown |
The difference between a water level and a lower water level in a reservoir within a particular time. Used as a verb, it is the lowering of the water surface. |
| Drum gate |
A type of spillway gate consisting of a long hollow drum. The drum may be held in its raised position by the water pressure in a flotation chamber beneath the drum. |
| Earth dam |
An embankment dam in which more than 50% of the total volume is formed of compacted earth layers are generally smaller than 3-inch size. |
| Embankment dam |
Any dam constructed of excavated natural materials, such as both earthfill and rockfill dams, or of industrial waste materials, such as a tailings dam. |
| Emergency spillway |
Emergency spillways are not intended to be used for the routing of the design flood. They are provided where there is a desire to protect against a malfunction of another feature required to safely pass the design flood. In the past, auxiliary spillways which are used to route the design flood but typically designed for infrequent use have been referred to as 'emergency spillways'. This should be should be discontinued. Media references to flow through 'emergency spillways' often leads to a misconception by the public that an emergency condition exists at a dam when the dam is safely functioning as designed. |
| Energy dissipater |
A device constructed in a waterway to reduce the kinetic energy of fast flowing water. |
| Failure mode |
A potential failure mode is a physically plausible process for dam failure resulting from an existing inadequacy or defect related to a natural foundation condition, the dam or appurtenant structures design, the construction, the materials incorporated, the operations and maintenance, or aging process, which can lead to an uncontrolled release of the reservoir. |
| Fat Clay |
Cohesive and compressible fine-grained soil with a high liquid limit that tends to be difficult to compact when saturated and very stiff when dry. |
| Fetch |
The-straight-line distance across a body of water subject to wind forces. The fetch is one of the factors used in calculating wave heights in a reservoir. |
| Filter |
One or more layers of granular material graded (either naturally or by selection) so as to allow seepage through or within the layers while preventing the migration of material from adjacent zones. |
| Flap gate |
A gate hinged along one edge, usually either the top or bottom edge. Examples of bottom-hinged flap gates are tilting gates and fish belly gates so called from their shape in cross section. |
| Flashboards |
Structural members of timber, concrete, or steel placed in channels or on the crest of a spillway to raise the reservoir water level but intended to be quickly removed, tripped, or fail in the event of a flood. |
| Flip bucket |
An energy dissipater located at the downstream end of a spillway and shaped so that water flowing at a high velocity is deflected upwards in a trajectory away from the foundation of the spillway. |
| Flood hydrograph |
A graph showing, for a given point on a stream, the discharge, height, or other characteristic of a flood with respect to time. |
| Flood storage |
The retention of water or delay of runoff either by planned operation, as in a reservoir, or by temporary filling of overflow areas, as in the progression of a flood wave through a natural stream channel. |
| Flood surcharge |
The storage volume between the top of the active storage and the design water level. |
| Flume |
An open channel constructed with masonry, concrete or steel of rectangular or U shaped cross section and designed for medium or high velocity flow. Also, a channel in which water is accelerated for purposes of measurement. |
| Flyash |
The finely divided residue resulting from the combustion of ground or powdered coal and which is transported from the firebox through the boiler by flue gases; known in the United Kingdom as pulverized fuel ash (pfa). |
| Foundation |
The portion of the valley floor that underlies and supports the dam structure. |
| Gabion |
Gabions are rectangular-shaped baskets or mattresses fabricated from wire mesh, filled with rock, and assembled to form overflow weirs, hydraulic drops, and overtopping protection for small embankment dams. Gabion baskets are generally stacked in a stair-stepped fashion, while mattresses are generally placed parallel to a slope. Gabions have advantages over loose riprap because of their modularity and rock confinement properties, thus providing erosion protection with less rock and with smaller rock sizes than loose riprap. |
| Gallery |
A passageway in the body of a dam used for inspection, foundation grouting, and/or drainage. |
| Gate |
A movable water barrier for the control of water. |
| Geomembrane |
An essentially impermeable geosynthetic composed of one or more synthetic sheets (ASTM D-4439, 2004). |
| Geosynthetic |
A planar product manufactured from polymeric material used with soil, rock, earth, or other geotechnical engineering related material as an integral part of a man-made project, structure, or system (ASTM D-4439, 2004). |
| Geotextile |
Any fabric or textile (natural or synthetic) when used as an engineering material in conjunction with soil, foundations, or rock. Geotextiles have the following uses: drainage, filtration, separation of materials, reinforcement, moisture barriers, and erosion protection. |
| Gravity dam |
A dam constructed of concrete and/or masonry, which relies on its weight and internal strength for stability. |
| Groin |
The area along the contact (or intersection) of the face of a dam with the abutments. |
| Grout |
A fluidized material that is injected into soil, rock, concrete, or other construction material to seal openings and to lower the permeability and/or provide additional structural strength. There are four major types of grouting materials: chemical; cement; clay; and bitumen. |
| Grout blanket |
An area of the foundation systematically grouted to a uniform shallow depth. |
| Grout cap |
A concrete filled trench or pad encompassing all grout lines constructed to impede surface leakage and to provide anchorage for grout connections. |
| Grout curtain |
One or more zones, usually thin, in the foundation into which grout is injected to reduce seepage under or around a dam. |
| Hazard creep |
Also referred to as risk creep, this phenomenon is caused by new development downstream of existing dams resulting in increased potential consequences that would occur if a dam were to fail. This evolution can result in the reclassification of many dams to a higher hazard category which often requires greater spillway capacity and/or reservoir storage volume, often at substantial cost to the dam owner. |
| Hazard potential |
The possible adverse incremental consequences that result from the release of water or stored contents due to failure of the dam or misoperation of the dam or appurtenances. Impacts may be for a defined area downstream of a dam from flood waters released through spillways and outlet works of the dam or waters released by partial or complete failure of the dam. There may also be impacts for an area upstream of the dam from effects of backwater flooding or landslides around the reservoir perimeter. |
| Hazard potential classification |
A system that categorizes dams according to the degree of adverse incremental consequences of a failure or misoperation of a dam. The hazard potential classification does not reflect in any way on the current condition of the dam (i.e., safety, structural integrity, flood routing capacity). |
| High severity flooding |
High severity flooding occurs when the flood sweeps the area clean and nothing remains. This type of flooding is rare, and is appropriate only for locations flooded by the near instantaneous failure of a dam. |
| Hollow gravity dam |
A dam constructed of concrete and/or masonry on the outside but having a hollow interior and relying on its weight for stability. |
| Hydraulic Fill Method |
Method used to construct earthen dams in which materials are conveyed by suspension in flowing water and placed without any compactive effort applied. |
| Hydraulic fill method |
Method used to construct earthen dams in which materials are conveyed and placed by suspension in flowing water. |
| Hydraulic fracturing |
Separation in a soil or rock mass that occurs if the applied water pressure exceeds the lateral effective stress on the soil element. Hydraulic fracture may occur if differential foundation movement is allowed. Soils compacted dry of optimum water content are more susceptible to hydraulic fracture. |
| Hydraulic gradient |
The change in total hydraulic head per unit distance of flow (ASTM D 653, 2005). |
| hydraulic roller |
One of the most significant risks to public safety at low head dams is the hydraulic roller phenomenon that occurs at the downstream face of the dam under certain flow conditions based on flow rate and tailwater depth. This rotational flow phenomenon is created by a submerged hydraulic jump. The length of the standing vortex or “hydraulic roller” can be about three to four dam heights in length and feature an upstream surface velocity comparable in magnitude to the maximum human swimming velocity of about 7 feet per second. The rolling water takes floating objects, including a person, to the bottom of the stream, releases it to the surface, draws it back to the face of the dam, and then pushes it back to the bottom. This cycle is a function of streamflow and can continue for an extended period. Air bubbles mixing in the water can decrease buoyancy by as much as one third. The victim may therefore have a difficult time staying afloat, even with a life jacket. These factors combined with the recirculating current create a nearly inescapable drowning machine. Rescuers have also been trapped and killed in hydraulic rollers. |
| Hydrology |
One of the earth sciences that encompasses the natural occurrence, distribution, movement, and properties of the waters of the earth and their environmental relationships. |
| Hydrometeorology |
The study of the atmospheric and land-surface phases of the hydrologic cycle with emphasis on the interrelationships involved. |
| Hydrostatic pressure |
The pressure exerted by water at rest. |
| Inclinometer |
An instrument, usually consisting of a metal or plastic casing inserted in a drill hole and a sensitive monitor either lowered into the casing or fixed within the casing. This measures at different points the casing’s inclination to the vertical. The system may be used to measure settlement. |
| Industrial waste dam |
An embankment dam, usually built in stages, to create storage for the disposal of waste products from an industrial process. The waste products are conveyed as fine material suspended in water to the reservoir impounded by the embankment. The embankment may be built of conventional materials but sometimes incorporates suitable waste products. |
| Inflow Design Flood |
The flood hydrograph used in the design of a dam and its appurtenant works particularly for sizing the spillway and outlet works and for determining maximum storage, height of dam, and freeboard requirements. |
| Instrumentation |
An arrangement of devices installed into or near dams that provide for measurements that can be used to evaluate the structural behavior and performance parameters of the structure. |
| Internal erosion |
A general term used to describe all of the various erosional processes where water moves internally through or adjacent to the soil zones of embankment dams and foundation, except for the specific process referred to as 'backward erosion piping'. The term internal erosion is used in place of a variety of terms that have been used to describe various erosional processes, such as scour, suffosion, concentrated leak piping, and others. |
| Inundation map |
A map showing areas that would be affected by flooding from releases from a dam’s reservoir. The flooding may be from either controlled or uncontrolled releases or as a result of a dam failure. A series of maps for a dam could show the incremental areas flooded by larger flood releases. |
| Inundation zone |
Area downstream of the dam that would be inundated by the released water. This zone is typically demarcated by a boundary reflecting the vertical elevation of the peak flow of water for both a flood failure and “sunny day” failure situation. |
| Liquefaction |
A condition whereby soil undergoes continued deformation at a constant low residual stress or with low residual resistance, due to the buildup and maintenance of high pore water pressures, which reduces the effective confining pressure to a very low value. Pore pressure buildup leading to liquefaction may be due either to static or cyclic stress applications and the possibility of its occurrence will depend on the void ratio or relative density of a cohesionless soil and the confining pressure. |
| Low level outlet (bottom outlet) |
An opening at a low level from a reservoir generally used for emptying or for scouring sediment and sometimes for irrigation releases. |
| low-head |
Low head or run-of-river dams usually span the entire river or stream and spill over all or nearly all the dam crest length. They present a safety hazard to the public because of their capability of producing dangerous recirculating currents, large hydraulic forces, and other hazardous conditions sufficient to trap and drown victims immediately downstream from the overflowing water. Low head dams act like uncontrolled spillways and provide little storage function. |
| Masonry dam |
Any dam constructed mainly of stone, brick, or concrete blocks pointed with mortar. A dam having only a masonry facing should not be referred to as a masonry dam. |
| Maximum wind |
The most severe wind for generating waves that is reasonably possible at a particular reservoir. The determination will generally include results of meteorologic studies that combine wind velocity, duration, direction, fetch, and seasonal distribution characteristics in a realistic manner. |
| Meteorology |
The science that deals with the atmosphere and atmospheric phenomena, the study of weather, particularly storms and the rainfall they produce. |
| Mine tailings dam |
An industrial waste dam in which the waste materials come from mining operations or mineral processing. |
| Minimum operating level |
The lowest level to which the reservoir is drawn down under normal operating conditions. The lower limit of active storage. |
| Multiple arch dam |
A buttress dam comprised of a series of arches for the upstream face. |
| Normal reservoir level |
For a reservoir with a fixed overflow sill the lowest crest level of that sill. For a reservoir whose outflow is controlled wholly or partly by moveable gates, siphons or other means, it is the maximum level to which water may rise under normal operating conditions, exclusive of any provision for flood surcharge. |
| Observation well |
A hole used to observe the groundwater surface at atmospheric pressure within soil or rock. |
| Ogee |
A reverse curve, shaped like an elongated letter “S.” The downstream faces of overflow spillways or weirs are often made to this shape. |
| Orographic |
Physical geography that pertains to mountains and to features directly connected with mountains and their general effect on storm path and generation of rainfall. |
| Outlet gate |
A gate controlling the flow of water through a reservoir outlet. |
| Outlet works |
A dam appurtenance that provides release of water (generally controlled) from a reservoir. |
| Parapet wall |
A solid wall built along the top of a dam (upstream or downstream edge) used for ornamentation, for safety of vehicles and pedestrians, or to prevent overtopping caused by wave runup. |
| Peak flow |
The maximum instantaneous discharge that occurs during a flood. It is coincident with the peak of a flood hydrograph. |
| Penstock |
A pressurized pipeline or shaft between the reservoir and hydraulic machinery. |
| Phreatic surface |
The free surface of water seeping at atmospheric pressure through soil or rock. |
| Piezometer |
An instrument used to measure water levels or pore water pressures in embankments, foundations, abutments, soil, rock, or concrete. |
| Piping |
The progressive development of internal erosion by seepage. |
| Plunge pool |
A natural or artificially created pool that dissipates the energy of free falling water. |
| Pressure relief pipes |
Pipes used to relieve uplift or pore water pressure in a dam foundation or in the dam structure. |
| Probable Maximum Flood |
The flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the drainage basin under study. |
| Probable Maximum Precipitation |
Theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location during a certain time of the year. |
| Radial gate |
A gate with a curved upstream plate and radial arms hinged to piers or other supporting structure. Also known as tainter gate. |
| Riprap |
A layer of large uncoursed stone, precast blocks, bags of cement, or other suitable material, generally placed on the slope of an embankment or along a watercourse as protection against wave action, erosion, or scour. Riprap is usually placed by dumping or other mechanical methods, and in some cases is hand placed. It consists of pieces of relatively large size, as distinguished from a gravel blanket. |
| Risk analysis |
A procedure to identify and quantify risks by establishing potential failure modes, providing numerical estimates of the likelihood of an event in a specified time period, and estimating the magnitude of the consequences. The risk analysis should include all potential events that would cause unintentional release of stored water from the reservoir. |
| Risk assessment |
The process of deciding whether existing risks are tolerable and present risk control measures are adequate and, if not, whether alternative risk control measures are justified. Risk assessment incorporates the risk analysis and risk evaluation phases. |
| Rock anchor |
A steel rod or cable placed in a hole drilled in rock, held in position by grout, mechanical means, or both. In principle, the same as a rock bolt, but usually the rock anchor is more than 4 meters long. |
| Rock bolt |
A tensioned reinforcement element consisting of a steel rod, a mechanical or grouted anchorage, and a plate and nut for tensioning or for retaining tension applied by direct pull or by torquing. |
| Rock reinforcement |
The placement of rock bolts, untensioned rock dowels, prestressed rock anchors, or wire tendons in a rock mass to reinforce and mobilize the rock’s natural competency to support itself. |
| Rock-fill dam |
An embankment dam in which more than 50% of the total volume is comprised of compacted or dumped cobbles, boulders, rock fragments, or quarried rock generally larger than 3-inch size. |
| Roller compacted concrete dam |
A concrete gravity dam constructed by the use of a dry mix concrete transported by conventional construction equipment and compacted by rolling, usually with vibratory rollers. |
| Rubble dam |
A stone masonry dam in which the stones are unshaped or uncoursed. |
| run-of-the-river |
Low head or run-of-river dams usually span the entire river or stream and spill over all or nearly all the dam crest length. They present a safety hazard to the public because of their capability of producing dangerous recirculating currents, large hydraulic forces, and other hazardous conditions sufficient to trap and drown victims immediately downstream from the overflowing water. Low head dams act like uncontrolled spillways and provide little storage function. |
| Saddle dam (or dike) |
A subsidiary dam of any type constructed across a saddle or low point on the perimeter of a reservoir. |
| Safety Evaluation Flood |
The largest flood for which the safety of a dam and appurtenant structure is to be evaluated. |
| Scour |
The loss of material occurring at an erosional surface, where a concentrated flow is located, such as a crack through a dam or the dam/foundation contact. Continued flow causes the erosion to progress, creating a larger and larger eroded area. |
| Seepage |
The internal movement of water that may take place through the dam, the foundation or the abutments. |
| Seiche |
An oscillating wave in a reservoir caused by a landslide into the reservoir or earthquake-induced ground accelerations or fault offset or meteorological event. |
| Settlement |
The vertical downward movement of a structure or its foundation. |
| Sinkhole |
A depression, indicating subsurface settlement or particle movement, typically having clearly defined boundaries with a sharp offset. |
| Siphon |
An inverted U-shaped pipe or conduit, filled until atmospheric pressure is sufficient to force water from a reservoir over an embankment dam and out of the other end. |
| Slickensided |
A smoothly polished surface created by frictional movement between rock along the two sides of a fault or bedding plane. |
| Slide gate (sluice gate) |
A gate that can be opened or closed by sliding in supporting guides. |
| Spillway |
A structure over or through which flow is discharged from a reservoir. If the rate of flow is controlled by mechanical means, such as gates, it is considered a controlled spillway. If the geometry of the spillway is the only control, it is considered an uncontrolled spillway. |
| Stability |
The condition of a structure or a mass of material when it is able to support the applied stress for a long time without suffering any significant deformation or movement that is not reversed by the release of the stress. |
| Stilling basin |
A basin constructed to dissipate the energy of rapidly flowing water, e.g., from a spillway or outlet, and to protect the riverbed from erosion. |
| Stoplogs |
Large logs, timbers, or steel beams placed on top of each other with their ends held in guides on each side of a channel or conduit so as to provide a cheaper or more easily handled means of temporary closure than a bulkhead gate. |
| Strainer |
Excessive seepage or leakage paths through a dam can create a danger to swimmers or divers during low flow conditions. A swimmer or diver can become trapped against the upstream face of the dam, trashracks or other features, by the suction created as the result of a substantial seepage or leakage. Swimmers and divers can also become trapped in strainers created by fallen trees and other debris that tend to accumulate at a dam. Water passes through the debris, but solid objects like boats or people can become trapped. |
| Submerged hydraulic jump |
One of the most significant risks to public safety at low head dams is the hydraulic roller phenomenon that occurs at the downstream face of the dam under certain flow conditions based on flow rate and tailwater depth. This rotational flow phenomenon is created by a submerged hydraulic jump. The length of the standing vortex or “hydraulic roller” can be about three to four dam heights in length and feature an upstream surface velocity comparable in magnitude to the maximum human swimming velocity of about 7 feet per second. The rolling water takes floating objects, including a person, to the bottom of the stream, releases it to the surface, draws it back to the face of the dam, and then pushes it back to the bottom. This cycle is a function of streamflow and can continue for an extended period. Air bubbles mixing in the water can decrease buoyancy by as much as one third. The victim may therefore have a difficult time staying afloat, even with a life jacket. These factors combined with the recirculating current create a nearly inescapable drowning machine. Rescuers have also been trapped and killed in hydraulic rollers. |
| Tailwater |
The water immediately downstream from a dam. The water surface elevation varies due to fluctuations in the outflow from the structures of a dam and due to downstream influences of other dams or structures. Tailwater monitoring is an important consideration because a failure of a dam will cause a rapid rise in the level of the tailwater. |
| Thrust block |
A massive block of concrete built to withstand a thrust or pull. |
| Toe drain |
A system of pipe and/or pervious material along the downstream toe of a dam used to collect seepage from the foundation and embankment and convey it to a free outlet. |
| Toe of the dam |
The junction of the downstream slope or face of a dam with the ground surface; also referred to as the downstream toe. The junction of the upstream slope with ground surface is called the heel or the upstream toe. |
| Trashrack |
A device located at an intake to prevent floating or submerged debris from entering the intake. |
| Tributary |
A stream that flows into a larger stream or body of water. |
| Unit hydrograph |
A hydrograph with a volume of one inch of runoff resulting from a storm of a specified duration and areal distribution. Hydrographs from other storms of the same duration and distribution are assumed to have the same time base but with ordinates of flow in proportion to the runoff volumes. |
| Uplift |
The hydrostatic force of water exerted on or underneath a structure, tending to cause a displacement of the structure (ASTM D 653, 2005). |
| Vajont |
In some literature, Vajont is also referred to as ‘Vaiont’. For consistency, ‘Vajont’ is used according to current Italian toponymy. |
| Wave runup |
Vertical height above the stillwater level to which water from a specific wave will run up the face of a structure or embankment. |
