The gap between a layout that fits the site and one that gets revised by a crew standing in the water is almost always information that was available and not gathered.
The five inputs that decide the layout
1. Survey
Topographic above the waterline, bathymetric below it. The layout is built on elevations, so this is the foundation.
Two questions the answer bank addresses directly: how recent the survey needs to be before a layout is developed, and when a bathymetric survey is needed rather than a standard land survey.
The short version: if the bed has moved since the survey, the survey describes a site that no longer exists. On an actively eroding site that can be a matter of months. And anywhere the protection extends below water, you need bed elevations below water, which a land survey does not give you.
2. Water levels through the year
Not the level on the day of your visit.
- Normal, low, and high water
- Design flood level
- Seasonal range, and on a managed water body the operating range
- Tidal range where applicable
This sets where the toe goes, where the crest goes, and how tall the treated band has to be. A treatment sized to summer water is unarmored in the condition that matters.
3. Hydraulic conditions
Design velocity at the structure, and where that figure came from. Depth. Wave conditions on coastal and open water sites.
The common gap: measured velocities frequently do not exist. The answer bank covers what information can be used when measured current velocities are unavailable, and the practical answer is that a hydraulic model, a gauged record, a regional estimate, or a documented professional judgement are all workable starting points. What is not workable is a guess presented as a measurement.
4. Anticipated scour depth
The number that drives toe treatment and stack count. See toe protection and how to read a scour evaluation.
Where a scour evaluation exists, use it. Where one does not, the depth of protection is being estimated, and that assumption belongs in writing.
5. Bed and bank material
Which soil properties matter most in planning is covered in the bank. The practical points:
- Grain size and cohesion, which affect both scour behaviour and the filter requirement
- Whether there are fines that can be winnowed out from beneath armor, which decides whether a geotextile underlay is needed. See layouts, single, stacked and keyed
- Bearing capacity where plant will stand
Access, which is not a design input but decides everything else
Gather it at the same time:
- What plant can reach the work, from where, and at what reach
- Whether the standing ground is sound
- Staging space for empty units, aggregate, filling, and lifting
- Overhead constraints, particularly under a bridge
- Whether a barge is more practical than working from the bank
- Delivery access and offloading capability
The answer bank has a section on equipment and access covering each of these. See site access and staging.
Fill stone availability
Confirm before the layout is finalised, not after:
- Source, distance, and whether they can supply the gradation
- Whether quarry-run material is usable or needs screening
- Whether locally sourced aggregate can be submitted for approval before construction
That last one matters on agency work. See fill stone selection and when local stone changes the economics.
What to send for an engineering review
The answer bank covers exactly this. In practice:
Essential
- Location and what is being protected
- The problem: what is eroding or scouring, and its extent
- Survey, or rough dimensions if no survey exists
- Water levels
- Access description and available plant
- Timescale
Valuable
- Photographs, which are worth more than descriptions
- Design velocity and its source
- Bed material
- Any scour evaluation, inspection report, or drawings
- The governing specification
- Local stone source
Useful
- Permitting position and which agencies are engaged
- Whether this is emergency, planned, or feasibility
- Who the engineer of record is
You do not need all of it to start. Photographs, rough dimensions, and access are enough for a substantive first answer. See what a quote includes.
The preliminary estimate
Before a final layout exists, a preliminary quantity can be estimated, and the bank covers how. The method is published: bag diameter sets linear coverage, the protection depth sets the number of stacks, and the slope decides vertical or stairstep arrangement. The calculator applies it.
Treat the output as a shortlist rather than a design. Final selection belongs to the engineer of record. We are a supplier, not the engineer of record.
The precision question
How precise the proposed toe and crest elevations need to be is one of the bank’s questions, and it is worth asking on every job, because the answer changes what survey you need. Elevations that matter to the tenth of a foot need a different survey from elevations that matter to the foot.
Where to go next
- Fill stone selection
- Site access and staging
- Layouts, single, stacked and keyed
- Design and sizing
- The rock bag FAQ for the full site-assessment section
- The installation hub for the rest