Sizing armor: the data the calculation needs, and what it cannot tell you.
Size selection starts with the flow condition at the structure, not the average through the reach. Quantity then comes from geometry: bag diameter sets linear coverage, the protection depth sets the number of stacks, and the slope decides whether they go vertical or stairstep.
This hub covers the sizing method, the hydraulic inputs the engineering team needs from you, and the design references a reviewer will expect you to have worked from. Where our own data does not support a claim, the limitations page says so rather than filling the gap.
Articles
Bed shear stress and armor sizing
Velocity is what gets specified; shear stress is closer to what actually moves material. The two approaches answer the same question differently.
Depth-averaged vs. local velocity
A model reports the average through a section. Armor sits where the flow is fastest and most disturbed. Using one for the other under-sizes the protection.
Choosing a design storm
The return period is a policy decision about acceptable risk, not an engineering constant. And the event that sizes the armor is not always the biggest one.
Taking a design velocity from a model
A model output is a number with a pedigree. Whether it is usable for armor sizing depends on the geometry, the calibration, and which velocity it reports.
Freeboard and overtopping
A crest set to the water you saw on the site visit is the most common defect in a revetment. Flow gets over the top and takes the bank from behind.
HEC-18 and bagged armor design
HEC-18 evaluates scour; HEC-23 selects countermeasures. Neither has a chapter on bagged armor, and knowing that is more useful than pretending otherwise.
Predicting scour depth and layout
The predicted depth decides the toe, the stack count, and the plan extent. It is also the input with the widest uncertainty in the whole design.
Translating a specified riprap D50
A D50 is not convertible to a bag size, because the element resisting the flow is different. Go back to the hydraulic condition the D50 came from instead.
Safety factors in armor sizing
A factor applied to a confident input means something different from one applied to a guess. Stacking conservative assumptions is not the same as a stated factor.
Sizing without measured data
Most projects have no measured velocity and no scour evaluation. That is workable. What is not workable is an estimate that stops being labelled as one.
Slope steepness and stacking
Slope decides whether units stack vertically or stairstep, and a slope steeper than the arrangement will hold is a stability problem before it is an erosion one.
Supercritical flow and spillways
Fast, shallow, high-energy flow on a steep slope behaves differently, and a hydraulic jump concentrates energy at one location. This is a design case for hard structures.
Combined wave and current loading
Waves and current together are worse than either alone, and this is the condition where our own evidence is thinnest. Stated rather than glossed.
What hydraulic data is needed
Design velocity at the structure, depth, bed material, scour depth, and water levels. Five inputs, and where each one usually comes from.
Sizing flow velocity to bag size
How we recommend bag sizes from flow data, depth, and substrate type — with worked examples from a streambank, a port toe, and an offshore monopile.
Where this topic already lives on the site.
The product pages, applications, and documentation that bear on design & sizing.
36 questions on design & sizing, answered.
Part of the full rock bag FAQ. Each question below opens at its answer.
Design Calculations and Hydraulics
- What hydraulic data does the engineering team need to size a job?
- What is HEC-18 and does it apply to rock bag design?
- What is HEC-23 and how does it relate?
- How do I translate a specified riprap D50 into a bag size?
- How does bed shear stress relate to bag sizing?
- Should I use depth-averaged velocity or the local velocity at the structure?
- Can I take the design velocity straight from a HEC-RAS model?
- What design storm or return period should the protection be sized for?
- How is scour depth predicted, and why does it drive the layout?
- Do I need to represent the armor itself in the hydraulic model?
- Does adding armor change the hydraulics elsewhere in the channel?
- How do I handle freeboard and overtopping in the design?
- What about supercritical flow at steep sections and spillways?
- How is turbulence around a structure accounted for?
- How do combined wave and current conditions get handled?
- How does slope steepness affect the design?
Sizing and Size Selection
- What size rock bag do I need?
- How do I choose between the 2-Ton and the 4-Ton?
- When should I use the 8-Ton bag?
- When is the 1-Ton the right choice?
- Do I need a custom size?
- How many rock bags will my project need?
- How much area does one rock bag cover?
- What flow velocity can each size handle?
- Does water depth change the size I need?
- Does wave height affect the size selection?
- How does bed material affect size selection?
- Should I mix bag sizes on one project?
- What if my design velocity falls between two sizes?
- How do I size rock bags for a bridge pier?
- How do I size rock bags for a culvert outfall?
- How do I size rock bags for a shoreline?
- What safety factor should I apply to the sizing?
- Can I use the sizing calculator without measured flow data?
- How does bag size affect the equipment I need?
- Does the size change the mesh construction?
FES Solutions is a supplier, not the engineer of record. This output is a starting point for discussion, not a site-specific design. Scour depth, foundation conditions, hydraulic loading, and the governing specification are the engineer of record's call, not ours. Where rock bags are not the right answer.
Working on a project like this?
Send the site conditions and the engineering team will come back with sizing, lead time, and pricing within one business day. For storm and flood emergencies, call directly.