The terms below are the ones that appear on drawings, in specifications, and in permit conditions for scour and erosion work, defined plainly. Some are product terms specific to Texas Tuff Rock Bags; most are general civil, hydraulic, and marine engineering terms that a specification will assume you already know.
Where a term is covered in depth elsewhere on this site, the entry links there rather than repeating the argument. For the design-life and durability terms, see service life and failure modes; for placement terms, see the installation guide.
Jump to: A · B · C · D · F · G · H · I · K · L · M · O · P · R · S · T
A
Apparent opening size (AOS)
A measure of the effective opening in a geotextile, used to describe the approximate largest particle that would effectively pass through it. AOS is the property that filter criteria are written against when a geotextile is specified to retain a soil while still allowing water to drain. It applies to the filter layer beneath armor, not to the armor mesh itself — for a rock bag, the equivalent controlling dimension is the mesh aperture.
Apron
A flat or gently sloped area of armor placed downstream of a structure to spread and dissipate the energy of the flow leaving it, most commonly below a culvert or stormwater outfall. Apron dimensions come from design discharge, outlet velocity, flow depth, tailwater, culvert shape, channel slope, bed material, and the predicted extent of the outlet scour hole. An apron that stops short of the full erosion zone simply relocates the erosion to its own downstream edge. See stormwater and drainage. See also: How are apron length and width established below a culvert or stormwater outfall?
Articulating concrete block (ACB)
An armor system built from precast concrete blocks joined by cables or interlocking geometry into a mat that can flex slightly over an uneven surface. ACB mats perform well on prepared, regular slopes where the subgrade can be graded and a filter layer properly installed, and less well on irregular beds, around existing structures, and underwater, where placing and anchoring a mat accurately is difficult. See also: How do rock bags compare to articulated concrete block mats?
As-laid survey
The record of where installed units actually ended up, as opposed to where the drawings said they should go. On offshore and subsea work an as-laid package identifies the coordinate system, vertical datum, survey equipment and accuracy, vessel, installation dates, and pre-lay bathymetric surface, then records each unit’s identifier, size, target coordinate, final position, elevation, placement time, and any anomaly observed at touchdown. It gives future inspectors a reliable comparison instead of relying on memory or construction drawings. See also: What offshore survey records should be included in the final as-laid documentation?
B
Bed
The floor of a channel, lake, or seabed — the surface beneath the water, as distinct from the bank or shore rising above it. The distinction matters in a specification: scour is the removal of bed material at a structure, while erosion more often describes loss from the bank. Protection placed on the bed and protection placed on a bank are different details, even where the two meet.
Bed shear stress
The force per unit area that flowing water exerts on the channel bed, and the parameter many channel stability calculations are expressed in. Shear stress is related to velocity but not interchangeable with it. Because rock bag resistance is published as grouped current velocity, a design worked in shear stress needs the corresponding velocity derived for the design section before it can be checked against the published figures. See also: How does bed shear stress relate to bag sizing?
Bioengineering
Bank and shoreline stabilization that uses live plant material — root systems, live stakes, brush layering — as a structural component rather than as landscaping. Bioengineered designs need the toe held while root systems establish, because plantings that wash out in the first high flow never get the chance to work. Structural armor at the toe and vegetation above and behind it is a common combined arrangement.
Breakwater
An offshore or shore-connected structure built to absorb or reflect wave energy and create sheltered water behind it. Large breakwaters are typically armored with heavy rock or with cast interlocking concrete units such as tetrapods and dolosse, which remain standard for that duty at that scale.
Bulkhead
A vertical retaining wall along a shoreline or property line, commonly built in vinyl, steel, or timber, that holds back soil and maximizes usable land. A bulkhead reflects wave energy downward rather than absorbing it, which often accelerates scour at its own toe, and it eventually needs replacement. Many agencies now favor sloped protection over new vertical walls. See also: Should I use rock bags or a vinyl or steel bulkhead on a lakefront?
C
Cable protection
Armor placed over or alongside a subsea power or telecommunications cable to shield it from anchors, fishing gear, dropped objects, and seabed movement. Placement above an existing asset works to horizontal and vertical tolerances set by the asset owner and project engineer from the required cover profile, minimum separation, allowable contact pressure, survey accuracy, and installation equipment. See offshore applications and the 8-Ton rock bag.
Contraction scour
Scour caused by a reduction in the width of the flow area — a bridge opening narrower than the approach channel, for example — which raises velocity through the constriction and removes bed material across the full width of the section. It is one of the standard scour components calculated in a HEC-18 framework, alongside local scour and long-term degradation.
D
D50
The median particle size of a stone gradation: half the material by weight is larger, half smaller. D50 is how riprap is specified, because riprap stability is governed by the individual stone. It does not translate directly into a rock bag size, because a filled bag resists flow as a whole unit rather than as loose particles — which is why bag capacity is published as grouped current resistance rather than as a nominal diameter. To substitute, take the design velocity the D50 was derived from and size against that. See also: How do I translate a specified riprap D50 into a bag size?
Degradation
The long-term, progressive lowering of a channel bed over a reach, as distinct from the localized scour hole that forms at a structure during a single event. Degradation matters to armor design because it moves the elevation the toe has to reach: a toe founded on today’s bed can be left standing proud of a bed that keeps falling.
Depth-averaged velocity
The mean flow velocity through the full depth of the water column at a point, as reported by most hydraulic models. It is not the velocity that loads armor at a structure. A pier, abutment, or outfall accelerates and concentrates the flow, so the local velocity is higher — and using the depth-averaged approach figure is one of the more common ways a design ends up undersized. See also: Should I use depth-averaged velocity or the local velocity at the structure?
Design velocity
The flow velocity the protection is sized against, taken from the design event set by the project’s design basis or the reviewing agency. For rock bag selection it is checked against the published grouped current resistance for each size on the specifications page. Those published figures are a limit rather than a target: where a design velocity falls between two sizes, step up. The sizing calculator makes the comparison.
Detention pond
A stormwater basin that holds runoff temporarily and releases it slowly through an outlet, standing dry or nearly dry between storms. Because the water level rises and falls with every event rather than sitting at one elevation, the bank is worked across a range of levels, and a treatment that is stable at one level can be undermined at another. See also: Retention pond
Drawdown
A fall in water level, whether seasonal on a reservoir or lake, tidal on a coast, or rapid as a flood recedes. Drawdown matters twice. It exposes the toe of the protection at low water, so the toe has to stay supported at the lowest level the bed can draw down to. And rapid drawdown leaves elevated pore pressure in the bank behind an impermeable face, which is a common cause of failure in bulkheads and rigid revetments; a permeable section drains as the level falls.
F
Filter criteria
The set of grading relationships that determine whether a filter layer will retain the soil beneath it while still passing water freely. Filter criteria govern whether a granular or geotextile filter is needed under armor and what its properties must be. On erodible fine-grained soils a filter is often specified to stop material migrating up through the voids between armor units; on competent or coarse beds it is frequently unnecessary. Settle it before the layout is finalized. See also: Do rock bags need a geotextile layer underneath?
Flanking
Erosion that moves around the outside end of an installation instead of through it, undercutting the protection from the edge. It is prevented by extending the layout beyond visible damage and terminating it in stable ground at both ends, with upstream and downstream returns, buried end sections, continuous toe and crest lines, and gradual transitions. An abrupt end placed directly against erodible soil creates a new concentration point even when the central section holds. See also: Which layout details help prevent erosion from flanking the protected area?
Freeboard
The vertical allowance between the design water level and the top of the protection. Set the top of a revetment against the water level that governs rather than the average: on a bank that usually means the design flood level plus an allowance, and on a coastal slope the design still water level plus wave run-up. Armor that stops below the level water actually reaches is attacked from behind, which erodes the bank above and then removes support from the units. See also: How do I handle freeboard and overtopping in the design?
Free span
A length of subsea pipeline or cable that is unsupported because the seabed beneath it has been scoured away or was never in contact. Free spans introduce fatigue and stress concentrations and generally get either supported or eliminated. Rock bags can form a controlled support or protection berm beneath and around a short span where the pipeline engineer approves the layout, arranged to avoid creating a hard point or an abrupt change in support along the pipe. See also: Can rock bags be used to support or protect a short pipeline free span?
G
Gabion
A wire-mesh basket filled with stone and stacked to form a wall, revetment, or channel lining. Gabions can serve for a long time above water in dry or intermittently wet settings with no significant bedload abrasion and a stable foundation, and they give a defined near-vertical face that a rounded bag cannot. Their vulnerability is the wire rather than the stone. See rock bags vs gabions and where wire baskets fail.
Gabion mattress
A wide, shallow gabion — typically a large plan area at low thickness — laid as a continuous blanket over a bed or slope rather than stacked as a wall. It shares the gabion’s failure mechanism: corrosion, bedload abrasion, differential settlement that racks the basket, and a persistently wet foundation all attack the cage, and once the wire opens the stone runs out even though every rock in it is intact. See also: What actually fails first in a gabion basket?
Geotextile filter
A fabric filter layer placed between armor and the soil beneath it, sized by filter criteria to retain the underlying soil while allowing drainage, and with enough strength to survive subgrade preparation and placement. FES specifies no underlayer requirement for the bags themselves; whether a project needs one is a design decision that follows from the soil, the flow, and the drawdown regime.
Geotextile sand container (GSC)
A fabric container filled with sand or a sand slurry, used for shoreline and dune work. A GSC relies on the containment fabric for all of its integrity, so a puncture can cost it its fill and its shape. That is the structural difference from a rock bag, where the graded stone fill is larger than the mesh aperture and carries load itself, so damage stays local. See also: Are rock bags the same as geotextile sand containers?
Geotextile tube
A large fabric tube filled hydraulically with sand or dredged slurry, efficient where large volumes and a pumped fill source are available — typically beach and dune work. Its weakness is the fill: sand-filled units are vulnerable to puncture and vandalism, and a breach empties the tube. See also: How do rock bags compare to geotextile tubes?
Granular filter
A graded layer of sand or gravel placed between armor and the subgrade to perform the same retention-and-drainage function as a geotextile filter, sized by filter criteria against the soil below and the armor above. It is the traditional alternative to a fabric filter and is still specified where a thicker, more robust transition layer is wanted.
Grouped current resistance
The flow velocity a rock bag size is rated to hold when units are placed in contact with each other as a group, which is how armor is actually built. Bags in contact support one another, so the grouped value is the one a design velocity is checked against; isolated units do not carry the same rating. The published figures run from approximately 13.1 ft/s for the 1-Ton to 19.4 ft/s for the 8-Ton, with the full table on the specifications page. These are manufacturer-declared values: FES does not publish the derivation — flume basis, prototype testing, or factor of safety applied — so treat them as one input requiring project-specific verification, not as a certified design value, and ask for the basis where the figure governs a submittal. See also: What does grouped current resistance mean on the spec sheet?
Grout bag
A fabric bag filled with cementitious grout that cures into a rigid mass, used subsea for permanent pipeline supports and void fill. The rigidity is the point where a hard, fixed support is required, and it is the liability where armor is required, because a cured unit cannot follow a seabed that continues to move. See also: How do rock bags compare to grout bags for subsea work?
Groyne
A structure built out from the shoreline, roughly perpendicular to it, to interrupt longshore sediment transport and hold sand on the updrift side. Also spelled groin in US practice. Groynes redistribute sediment along a frontage rather than adding to it, so the downdrift effect is part of the design question.
H
Hard armor
Armor that resists hydraulic attack primarily through mass and interlock — riprap, concrete revetment, precast units, grouted mats. The category is usually set against soft armor, which protects a soil surface while vegetation establishes. Rock bags sit between the two: like hard armor they deliver the mass to resist high-velocity flow and wave attack, and like soft armor they settle and conform as the bed moves beneath them. Reviewers sometimes classify them as flexible armor for that reason. See also: Are rock bags hard armor or soft armor?
Headcut
A near-vertical step in a channel bed that migrates upstream as flow plunges over it and erodes the bed at its base — the mechanism by which erosion below a culvert outlet works its way back toward the structure. Protection has to extend beyond the predicted outlet scour hole and connect to a stable downstream grade or bed control; a short apron placed only at the pipe moves the erosion to the apron’s downstream edge. See also: How far should protection extend below a culvert to control an advancing headcut?
I
In-water work window
The range of dates a permit allows in-water construction, set to avoid spawning, migration, incubation, or juvenile rearing periods for local species. A short window drives aggregate delivery, bag filling, equipment mobilization, daily production targets, and the sequence used to protect unfinished sections. Materials and staging should be ready before the window opens, with contingency for high water, storms, and required biological monitoring. See also: What is an in-water work window and how does it affect scheduling?
K
Keying in
Setting the lowest row of armor into a trench excavated below the bed or slope surface, so that flow cannot work beneath the leading edge. Keying is specified where the lower edge is exposed to active scour, wave drawdown, a mobile bed, or concentrated flow. It is not required on every site: competent rock, a stable graded bed, or a broad apron designed to settle into a developing scour hole can all justify surface placement instead. See also: Does the bottom row need to be keyed into a trench?
L
Launchable stone
Armor stone placed in a mound or windrow at or above the toe, intended to be undermined deliberately so that it falls — launches — into a developing scour hole and armors it. It is a design strategy for beds expected to lower, and it depends on having enough stone volume in the mound to cover the predicted scour depth. Bagged armor addresses the same condition differently, by conforming to the scour hole as it develops rather than spanning it.
Living shoreline
A shoreline stabilization approach that uses natural or hybrid materials — plantings, sills, oyster or reef structures, graded slopes — to hold a shoreline while preserving habitat and the intertidal gradient. Rock bags are frequently specified within these designs to provide a structural toe without the impermeable, ecologically dead face a concrete wall creates, with vegetation established behind and above them. See also: Can rock bags be part of a living shoreline or bioengineered design?
Local scour
Scour concentrated immediately around an obstruction — a pier, an abutment, a spur — caused by the vortices and flow acceleration the structure itself generates. It is the component that drives armor layout around a structure, because the protection has to extend deep and wide enough that its own edge is not undermined by the hole it exists to prevent. Distinct from contraction scour, which acts across the whole section, and from degradation, which acts over the whole reach.
M
Mesh aperture
The clear opening size of the rock bag mesh, and the dimension that governs which fill gradations are acceptable. Texas Tuff 1-Ton, 2-Ton, and 4-Ton bags use a 25 mm aperture; the 8-Ton uses 50 mm, which pairs with the larger fill stone used at that scale. Approved aggregate must be large enough to stay securely inside the mesh, including its narrowest particle dimension. Confirm a gradation against a sieve analysis, not against nominal stone size — see the specifications page.
Mobilization
The cost and time of getting crew, plant, and material to a site and away again, as distinct from the cost of the work itself. Mobilization is largely fixed regardless of how much work is done once there, which is why a short reach rarely costs proportionally less than a long one, and why staging a repair across several visits usually costs more in total than completing it in one.
Monopile
A single large-diameter steel tube driven into the seabed as the foundation for an offshore structure, most commonly an offshore wind turbine. Flow accelerating around a monopile generates local scour at its base, which is why targeted scour protection is placed there. Discrete, accurately located units suit that duty. See offshore wind.
Multibeam sonar
An acoustic survey method that maps the seabed or channel bed across a swath rather than along a single line, producing a bathymetric surface. It is one of the standard methods for documenting submerged armor where the water is not clear, alongside side-scan sonar, acoustic imaging, ROV video, diver inspection, and physical probing. Repeat surveys should use the same datum, transects, and reference points so changes in unit position and bed elevation can actually be measured. See also: What inspection methods work below the waterline when visibility is limited?
O
Ordinary high water mark (OHWM)
The line on a bank or shore established by the fluctuations of water and indicated by physical characteristics such as a clear natural line impressed on the bank, changes in soil, and changes in vegetation. It is one of the boundaries used to define federal jurisdiction over work in waters of the United States, and a USACE submittal typically has to show it, or the tidal boundary, along with the proposed quantity and area of fill.
P
Propeller wash
The high-velocity jet a vessel’s propeller or thruster directs at the bed, and a primary cause of scour at berths, quay walls, and ferry terminals. Designing berth protection needs the vessel classes using the berth, propeller diameter, installed power, operating RPM, propeller and thruster locations, clearance above the bed, and jet direction, plus bow and stern thruster use, tug activity, berthing frequency, water depth across the tide range, quay geometry, seabed material, and recent bathymetric surveys showing existing scour. The published grouped current resistance figures are steady-current ratings and have not been demonstrated under jet loading, so a propeller-wash case cannot be checked against them by velocity equivalence alone. PIANC MarCom Report No. 180, Guidelines for Protecting Berthing Structures from Scour Caused by Ships, is the recognized international guidance for berth-scour assessment. See marine and ports.
R
Raschel weave
A warp-knit construction in which the strands are interlooped rather than woven over and under. The practical consequence for armor is that a cut or abraded strand does not unravel across the fabric the way it can in a woven mesh. Texas Tuff mesh is raschel-weave virgin polyester in every size, and that resistance to tear propagation is one of the properties covered by tear-strength testing. See also: What does raschel weave mean?
Retention pond
A stormwater basin that holds a permanent pool, with storm runoff raising the level above it before it draws back down. The permanent waterline concentrates wave and wake attack at a single elevation, which is one reason a pond commonly fails along one shore — the one the prevailing wind blows toward — while the rest of the perimeter stays intact. See also: Detention pond
Revetment
A facing placed on a slope to absorb and deflect the energy of flowing water or waves, protecting the bank behind it. Rock bags placed in a graded pattern down a bank face function as a revetment and are specified that way on river, streambank, and shoreline projects. See rivers and streambanks and the New Zealand river and bridge revetment.
Revetment transition
The detail where one protection system ends and another begins — new armor tying into existing riprap, concrete, or sheet pile, or a change of unit size along a layout. Transitions are where installations fail, so coverage should extend into sound existing protection far enough that no exposed seam is left for water to concentrate in, with elevations, filter layers, and toe support kept continuous. Loose riprap, broken concrete, and projecting debris get corrected before new units are placed.
Riprap
Loose quarried stone of a specified gradation placed as armor on a bed or bank. Riprap is proven, inexpensive, and well understood, and it remains the right answer where a local quarry produces the specified gradation cheaply, haul is short, access is easy, and the hydraulic loading is comfortably within what loose stone holds. Its characteristic failure mode is migration: under high flow individual stones move and the protection is no longer where it was specified. See rock bags vs riprap for bridge scour.
Rock filter bag
The general industry term for a permeable fabric bag filled with rock. Most products sold under the name are small sediment-control units placed at inlets and in ditches. Texas Tuff Rock Bags work on the same principle but at a different scale and for a different purpose — 1, 2, 4, and 8 tons, built to armor against scour and wave energy. If a specification calls for rock filter bags, request the spec sheet so the fit can be confirmed against the intended duty.
ROV (remotely operated vehicle)
An unmanned, tethered submersible used to observe and assist subsea work. On rock bag placement an ROV confirms asset position and separation, tracks the unit during descent and touchdown, and inspects lifting components before a planned recovery. ROV visibility is one of the factors that sets realistic placement tolerances, and loss of position or visibility is normally a stop-work condition. See offshore applications.
S
Scour
The localized removal of bed material by flowing water, usually concentrated where a structure accelerates or redirects the flow — a bridge pier, an abutment, a culvert outfall. Scour is stopped by placing enough mass over the vulnerable bed that the flow can no longer lift and carry the material away. It is distinct from erosion, which is the broader loss of bank, bed, or shoreline material over time: scour protection is designed around a specific structure and a design flood event, while erosion control is designed around a length of bank and a range of conditions. See also: What is scour, and how do rock bags stop it?
Scour hole
The depression that scour excavates around or downstream of a structure. Its predicted depth and extent drive the armor layout, because protection that stops short of the predicted scour depth fails from the edge inward regardless of how well the units are sized. Flexible units settle into a developing hole and keep covering the bed; rigid armor holds its shape while the bed drops away beneath, creating a void that eventually undermines the edge. See also: What does it mean that rock bags conform to scour?
Seawall
A substantial vertical or near-vertical structure built along a shoreline to resist wave attack and hold the land behind it. Like a bulkhead, a vertical face reflects wave energy rather than absorbing it, offers little colonization surface, and eliminates the intertidal gradient that supports shoreline life — which is why many coastal regulators have shifted policy toward sloped, permeable protection.
Sheet pile
Interlocking steel, vinyl, or composite sections driven to form a continuous vertical wall that retains soil. Sheet pile is a structural retaining element and solves a different problem from armor: it holds back lateral earth pressure and creates a vertical face, while armor protects a slope or bed from hydraulic attack. Where the requirement is retention or a vertical wall line, sheet pile is the correct system — and rock bags are frequently used at its toe. See also: Are rock bags an alternative to sheet pile?
Soft armor
Surface protection intended to hold soil while vegetation establishes — erosion control blankets, turf reinforcement mats, and similar products for low-velocity overland flow and freshly graded slopes. Soft armor is not a competitor to hard armor but addresses a different energy range, and it will not survive scour at a pier, an outfall, or a high-flow bank toe. Projects with both conditions commonly specify mats on the upper slope and armor units at the toe.
T
Toe embedment
How far the lowest row of armor is set below the surrounding bed or slope surface. There is no standard depth. The toe is commonly set at or below the design scour elevation, or paired with enough apron coverage to settle into a developing scour hole without exposing the protected slope. Local degradation, bed material, wave or flow loading, drawdown, and expected settlement all move the number. See also: How much toe embedment is typically needed to reduce undermining?
Toe protection
Armor placed at the base of a slope, where the bank meets the bed. It matters because most bank failures start there: once flow undermines the toe, everything above it loses support and slides regardless of how well the upper face is protected. Armoring the toe first is standard practice, and the Texas Tuff Rock Log exists specifically to run continuous protection along that line — see the residential Rock Log case study. See also: What is toe protection and why does it matter?
Turbidity
Cloudiness in water caused by suspended particles, and a water-quality parameter permits commonly regulate during in-water construction. Placement disturbs the bed locally and causes short-term turbidity, as any in-water work does. Controls may include clean aggregate, stabilized equipment access, controlled lowering, turbidity curtains, temporary isolation, staged work, spill prevention, and monitoring upstream and downstream of the placement area — matched to the current, depth, substrate, and permit limits, since a curtain that works in still water may fail or redirect flow in a channel. See also: Which turbidity and sediment controls may be required during placement?
Missing a term?
If a term in your specification or permit condition is not defined here and you want to know how it applies to bagged armor, send it with the project details through the quote form and the engineering team will answer it directly.
Related: Service life & failure modes · Installation guide · Sizing method · Test data & documentation · When not to use rock bags · Full FAQ
Term count: 54