Rip Rap Calculator

Designing an effective rip rap revetment requires balancing hydraulic forces with stone stability. The calculation begins with the Isbash formula to determine the median stone diameter (D₅₀) capable of resisting displacement from flowing water: Average Rock Diameter = (Water Velocity²) / (2 * Gravitational Acceleration * (Isbash Constant²) * (Specific Gravity - 1)). Next, the tool multiplies the project surface area by the specified blanket depth to find the raw volume. It then factors in a wastage percentage for voids and spillage to yield the total volume, converts this volume into mass using material density, and multiplies by the unit price to determine the total cost.

Imagine you are protecting a sloped embankment with a surface area of 500 square feet and a design rip rap depth of 2 feet. The local water velocity reaches 5 feet per second, with standard rock specific gravity of 2.65, an Isbash constant of 1.2, gravitational acceleration of 32.2 ft/s², a stone density of 100 lbs/ft³, a 10% wastage allowance, and a unit price of $0.05 per pound. First, volume = 500 ft² * 2 ft = 1,000 cubic feet. Accounting for 10% wastage gives a total volume of 1,100 cubic feet. Multiplying by a density of 100 lbs/ft³ results in a total weight of 110,000 pounds (55 tons). At $0.05 per pound, the estimated material cost comes to $5,500.

Always incorporate a high-strength geotextile filter fabric underneath your rip rap layer to prevent underlying soil from washing out through the voids between stones. When ordering materials, never skip adding a 10% to 15% wastage factor to account for irregular packing, sub-grade settling, and fines. Finally, ensure your average rock diameter matches peak hydraulic velocity ratings to prevent structural failure during major storm events.