What size rock bag do I need?

Size selection for Texas Tuff Rock Bags comes down to flow velocity, water depth, and the application. Everything else — layout, layer count, toe treatment, quantity — follows from the size, which is why it is worth settling first and settling properly.

This page is the starting point. It covers the data needed to size a unit, how each variable moves the answer, what changes when the structure is a bridge pier rather than a culvert outfall or a shoreline, and what to do when the design velocity lands between two sizes. If you already have a design velocity and depth, the sizing calculator will return a size directly, and the flow velocity sizing method explains how it gets there.

One thing to be clear about up front: the published resistance figures are the resistance side of a design check. The load side — the design event and the factor applied to it — comes from the project’s own design basis, not from the product.

On the grouped current resistance figures. These are manufacturer-declared values for units placed in contact as a group; isolated units do not carry the rating. FES does not publish the derivation — flume basis, prototype testing, or factor of safety applied. 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.

The data that drives the answer

Design flow velocity

Velocity is the first filter and usually the controlling one on inland sites. Each size carries a published grouped current resistance, and the calculator matches a design velocity to the smallest size rated to hold it.

“Grouped” means bags placed in contact with each other, which is how armor is built. An isolated unit does not carry the same rating, so a single bag dropped at a spot repair is not covered by the grouped figure.

Water depth

Depth changes the answer in two ways. It affects the velocity profile, so the velocity acting at the bed can differ from a surface or depth-averaged figure — which matters when the number you have came from a model output rather than a bed measurement. Depth also constrains the placement method: deeper work may need crane or ROV handling rather than excavator reach, and that favors fewer, larger units. Both effects are handled in the sizing method.

Wave height

On coastal and port sites, wave conditions often govern over current velocity. Wave attack applies cyclic uplift and drag rather than steady drag, and breaking waves on a slope are a different load case from waves in deep water.

Supply the design wave conditions alongside the current data so both cases get checked, rather than only the one that was easier to measure. Useful inputs are wave height, period, direction, water depth, current velocity, tide range, storm water level, and seabed profile. Wave and current forces should be evaluated together, because their directions, timing, and combined loading can produce a stronger bed response than either condition alone. The controlling load case is then used to evaluate bag weight, grouping, layer arrangement, and toe coverage.

Bed material

Bed material affects the foundation more than the armor. A soft or fine-grained bed may need a filter layer beneath the units to stop material migrating up through the voids, and it may settle more under load — that changes how the layout is built rather than which size is selected. A competent bed usually allows direct placement.

The soil properties that matter most in planning are particle size, cohesion, density, erodibility, bearing strength, and the depth of any soft or loose material. Sand and gravel beds can shift during high flows; soft silt or weak fill may allow greater settlement beneath the bags. Include a description of the bed material and a geotechnical log if one exists. Existing riprap, concrete fragments, buried debris, and exposed rock should be documented too, since they affect toe treatment, underlayer, and transition details.

Seasonal range

Check the layout against the controlling seasonal condition rather than an average water level. High-flow periods may increase scour and movement forces, while low water or rapid drawdown can expose sections of bank that stay submerged for most of the year. Tide range, flood levels, storm surge, ice, debris, and construction access all belong in the same check where they apply.

The four standard sizes

Four standard sizes cover most civil, marine, and offshore duties. Every size uses the same raschel-weave virgin polyester and the same reinforced rope lifting system; what changes is volume, filled diameter, mesh aperture, layer count, and rated resistance.

The full table, including the accompanying test references, is on the specifications page. Individual size pages are under products.

Reading the table

Two rows do more work than the rest. Grouped current resistance sets the hydraulic ceiling. Filled diameter sets coverage — one unit covers roughly the square of its filled diameter, less the packing loss where curved units meet, so a 2-Ton at 1.9 m covers meaningfully more ground per unit than a 1-Ton at 1.5 m.

Mesh aperture matters at procurement rather than sizing: 25 mm on the three smaller sizes and 50 mm on the 8-Ton, which pairs with the larger fill stone used at that scale. Confirm the aggregate gradation against the aperture before ordering stone — the installation guide covers aggregate approval.

Choosing between adjacent sizes

When is the 1-Ton the right choice?

When access, not hydraulics, is the binding constraint. The 1-Ton fills and places with a compact excavator or skid steer on a single-point lift, which makes it the practical option for residential streambanks, lakefront shorelines, garden and property-line work, and light scour repair where larger machines cannot reach or cannot be justified. Its grouped current resistance is about 13.1 ft/s, so confirm the velocity before selecting it.

How do I choose between the 2-Ton and the 4-Ton?

The velocity check is the first filter: about 15.4 ft/s grouped for the 2-Ton against about 17.4 ft/s for the 4-Ton. Beyond that, it becomes an equipment question. The 2-Ton places with the mid-range excavators most crews already run; the 4-Ton usually needs heavier plant.

If the velocity check is marginal and the equipment already on site is heavy, the 4-Ton is the lower-risk call. The 2-Ton is the most-specified size overall and covers streambanks, shoreline revetment, and light to moderate scour. The 4-Ton steps up for bridges, riverbanks, and port toes.

One limitation applies to the port case. The published grouped current resistance figures are steady-current ratings and have not been demonstrated under jet loading. Berth and quay scour is driven by propeller and thruster wash, and near-bed flow in a propeller jet carries far higher turbulence intensity than a uniform current of the same mean velocity — so a berth should be assessed against a jet-specific criterion rather than by matching a design velocity to the table. PIANC MarCom Report No. 180, Guidelines for Protecting Berthing Structures from Scour Caused by Ships, is the recognized international guidance, and that assessment belongs to whoever is doing the berth hydraulics rather than to the armor supplier.

When should I use the 8-Ton?

The 8-Ton is the offshore and severe-current unit, rated to roughly 19.4 ft/s grouped. It carries a 50 mm mesh aperture and quad-layer construction rather than the 25 mm double-layer used on the mid sizes, which suits larger fill stone and harsher exposure. It is the size specified for subsea cable and pipeline protection, deep water, and oil and gas platform work. See offshore applications.

What if my design velocity falls between two sizes?

Step up. The published resistance figures are the point at which a size reaches its limit, not a target to design to, and real sites deliver debris, turbulence, and events above the design estimate.

If the margin is uncomfortable and the larger size creates an equipment problem, that trade is worth raising through the quote form before the layout is fixed rather than after. There are usually options — a mixed layout, a different placement method, or a revised toe arrangement — but they are much cheaper to explore at design stage.

Sizing for a specific structure

How do I size rock bags for a bridge pier?

Pier scour design starts from the predicted scour depth and the local velocity at the pier, which is higher than the approach velocity because the structure accelerates the flow. Using the approach figure is one of the more common ways a design ends up undersized.

The protection has to extend far enough around and below the pier that its own edge is not undermined. A deeper scour estimate may call for additional toe coverage, more rows, or a grouped layout that can adjust as the bed changes. Bring the hydraulic model output and the scour calculation, and see bridge scour protection for the structural considerations.

How do I size rock bags for a culvert outfall?

Outfall sizing is driven by the discharge velocity leaving the barrel and the energy that has to dissipate before flow re-enters the channel. The apron needs enough plan extent to spread that energy, and enough depth at its downstream edge to stop a headcut walking back toward the structure. Stormwater and drainage covers the arrangement.

How do I size rock bags for a shoreline?

Shoreline work is governed by wave conditions, water-level range, and the slope being protected, rather than by channel velocity. The design has to hold under the design wave while staying stable across the tide or seasonal water-level swing, and the toe has to be set below the level to which the beach or bed can draw down. Larger units or additional layers may be needed on exposed sections where waves act repeatedly across the same face. See coastal protection.

Mixing sizes on one project

Mixing is common and often sensible. Sites rarely present one uniform condition: a channel may need the heavier size at the toe and through the high-velocity section, and a lighter size on the upper bank and at the tie-ins. Mixing keeps cost and equipment demand proportional to the actual loading rather than sizing the whole job for its worst square meter.

The layout drawings should make the transitions explicit so the crew knows where the size changes and does not improvise the boundary in the field.

Safety factor

The safety factor is set by the reviewing engineer or agency, not by the product, and it varies with the consequence of failure. A bridge carrying an interstate is treated differently from a private lakefront bank.

Published grouped current resistance figures are the resistance side of the check. The load side — the design event and the factor applied to it — comes from the project’s own design basis. FES supplies the resistance data and the sizing recommendation; the factor of safety belongs to the design.

Sizing without measured flow data

You can size from an estimate, and that is often how a project starts. Hydraulic models, stream gauge records, design discharges, flood studies, drainage-area data, channel dimensions, slope, and documented high-water marks all support a first estimate. The calculator will return a starting size from any of them.

Photographs of erosion, debris lines, exposed foundations, and previous storm damage help identify where flow concentrates, but they do not replace hydraulic calculations.

Treat the result as a scoping number rather than a design output. Compare the estimated velocity against the grouped current-resistance values in the specifications, then confirm it against real hydraulic calculations before the size goes into a specification.

The same applies to the survey. The topographic or bathymetric survey behind the layout should represent the site’s current bed, bank, and structural elevations, and a new one may be needed after a flood, storm, dredging operation, bank failure, or construction activity. A bathymetric survey is needed where protection extends below the waterline and the submerged bed cannot be mapped accurately from land; where both survey types are used, they should share the same horizontal and vertical datum so the layout connects across the waterline.

How bag size drives equipment selection

Directly, and it is often the deciding factor. The 1-Ton is placeable with compact plant. The 2-Ton suits the mid-range excavators most civil and marine crews already have. The 4-Ton generally needs heavy-civil equipment. The 8-Ton is normally crane or vessel work.

Confirm machine capacity against the filled weight at the working radius, not just the nominal lift rating — the setdown point frequently sits at a larger radius than the filling area. Contractor support covers the planning side, and the installation guide covers lift planning in detail.

Because equipment availability is often fixed before the hydraulics are finalized, it is worth checking early whether the size the velocity implies is one the site can actually place. Where it is not, the choice is between bringing in heavier plant, changing the placement method, or revisiting the layout — and all three are design-stage decisions.

From size to quantity

Bag count is a function of the area and depth of protection, not just the linear extent. Once the protected footprint is set, the count follows from the filled diameter of the chosen size — 1.5 m for the 1-Ton through 3.0 m for the 8-Ton — plus any second layer the design calls for.

A preliminary quantity starts with the length, width, slope, and total surface area requiring protection, then adjusts for the nominal footprint of the proposed size, the number of layers, toe coverage, transitions, irregular site geometry, and placement around structures. A reasonable allowance may be added for uneven bed conditions or field adjustments. The quantity stays preliminary until the survey, hydraulic conditions, bag size, and final placement pattern are confirmed.

Toe and crest elevations should be precise enough to define a continuous protected area, calculate quantities, and give the placement crew clear limits in the field. Preliminary elevations can be approximate during early budgeting, but construction drawings should reference a confirmed survey datum and identify acceptable placement tolerances. Rock bags conform to irregular surfaces, but that flexibility does not remove the need for reliable toe, crest, and transition elevations — poor elevation control leaves an exposed edge for flow or wave action to move behind.

Do you need a custom size?

Usually not. The four standard sizes cover most civil, marine, and offshore duties, and standard units carry the published test data that reviewers expect.

Custom bags make sense when a dimension is genuinely fixed by the site — a confined slot around an existing structure, an unusual fill material, or a rigging arrangement dictated by the placement vessel. Send the constraint through the quote form and the engineering team will say whether a standard size solves it.

What to send us

For a sizing recommendation, the useful set is: project location, application, estimated area requiring protection, available plans or surveys, photographs, water depth, flow or wave data, and the proposed construction schedule. Site access, placement equipment, nearby structures, and locally available fill stone also affect the recommendation.

If some of it is still being collected, send what exists — the engineering team will identify the remaining items needed for sizing and pricing. What comes back is a recommended size and quantity.

Related: Sizing calculator · Flow velocity sizing method · Installation guide · When not to use rock bags · Cost guide · Full FAQ