Calculator
Formula used
The calculator treats the revetment as a sloped rectangular face plus optional toe trench volume. The face slope length is computed from the vertical height and the slope ratio.
- Vertical total:
Htot = H + Htoe + Hfree - Slope multiplier:
M = √(1 + (H:V)2) = √(1 + (H/V)2) - Slope length:
Ls = Htot × M - Slope face area:
A = Lbank × Ls - Rock volume (face):
Vface = A × t - Toe trench volume (optional):
Vtoe = Ltoe × w × d - Wastage:
Vrock = (Vface + Vtoe) × (1 + W/100) - Mass:
T = V × γbulk(bulk unit weight) - Bulk from solid + voids:
γbulk = ρsolid × (1 − n)
How to use this calculator
- Choose your unit system and enter the bank length.
- Enter the vertical height, plus any toe embedment and freeboard.
- Set the slope ratio (H:V) from your design drawing.
- Enter placed rock thickness and a realistic wastage percent.
- Use bulk unit weight from supplier tickets if available.
- Optionally add a toe trench and filter layer for totals.
- Press Calculate to see results and download reports.
Example data table
| Scenario | Bank length (m) | Vertical height (m) | Slope (H:V) | Thickness (m) | Wastage (%) | Bulk unit weight (t/m³) | Rock volume (m³) | Rock mass (t) |
|---|---|---|---|---|---|---|---|---|
| Typical riverbank | 50 | 3.0 | 2:1 | 0.45 | 7 | 1.65 | ≈ 121 | ≈ 200 |
| Steeper slope | 50 | 3.0 | 1.5:1 | 0.45 | 7 | 1.65 | ≈ 105 | ≈ 173 |
| Heavier thickness | 50 | 3.0 | 2:1 | 0.60 | 7 | 1.65 | ≈ 161 | ≈ 265 |
Slope geometry drives area
Rock demand starts with slope face area. The tool converts vertical height, toe embedment, and freeboard into a vertical total, then multiplies by the slope multiplier √(1+(H/V)²) to get slope length. A flatter slope such as 3:1 increases face area and volume compared with 2:1 at the same height. Base the slope ratio on as-built grades. Confirm heights using survey shots and benchmarks.
Thickness and gradation selection
Placed thickness should reflect the specified gradation and placement method. A common check is 1.5–2.0 times the D50 stone size, with added thickness where hand placement is limited. If the project includes a second layer or an armor/bedding split, run the calculator twice using each thickness and then add totals for procurement.
Bulk unit weight and voids
Suppliers may quote by volume or by mass, so the bulk unit weight links both. Typical riprap bulk unit weight ranges from about 1.55 to 1.75 t/m³, depending on rock type and voids. If you only know solid density (often 2.60–2.80 t/m³), estimate bulk using γbulk=ρsolid×(1−n) with void fraction n around 0.35–0.45 for dumped rock.
Toe trench and filter allowances
Toe features stabilize the slope and can add meaningful quantity. The calculator treats the trench/apron as a rectangular volume using length, width, and depth; set trench length to zero to assume it matches bank length. Filter bedding is computed from face area and filter thickness; typical granular filters are 0.10–0.20 m and should be matched to the filter design criteria.
Procurement and hauling planning
After wastage is applied, the placed rock volume becomes the order quantity. Use wastage to cover sorting, breakage, handling losses, and trimming; 5–12% is common depending on access and quality control. Enter truck capacity to estimate loads from total mass and compare haul plans. Export CSV or PDF results to support takeoffs, bid tabs, and delivery tracking.
FAQs
What height should I enter for the bank?
Enter the vertical height from toe to crest. Add toe embedment for below-bed depth and freeboard for extra crest height above the design level. The calculator sums these to compute the sloped face length.
Why does the tool ask for bulk unit weight?
Bulk unit weight converts placed volume to delivered mass. Use supplier ticket averages when possible. If you only know solid density, select the solid mode and enter a reasonable void fraction to estimate bulk behavior.
What wastage percentage is reasonable?
Use 5–8% for good access and controlled placement, and 8–12% for difficult access, heavy sorting, or tight trimming. If you are unsure, run two scenarios and use the higher value for budgeting.
How is the toe trench quantity handled?
When enabled, the toe is treated as a rectangular prism: length × width × depth. Leave trench length at zero to automatically use the bank length. This toe volume is added to face rock before wastage.
Why are results shown in metric even in imperial mode?
Calculations are performed in metric internally for consistency, then converted for display. You can still enter feet in imperial mode, and the table will show converted values beside the metric results.
Can I estimate multiple reaches or varying slopes?
Yes. Run the calculator for each reach, slope, or thickness change and export CSV for each run. Sum the rock volumes or masses in a spreadsheet to build a project total and hauling plan.