Flowpath Engineering Portal
Riprap Apron Sizing — Pipe Outfall Scour Protection
Size riprap scour protection aprons for stormwater pipe outfalls. Calculate rock size, apron geometry, and volume estimates per Auckland Council TP10 and FHWA HEC-14.
What this tool does
NZ Stormwater Standards
Design riprap aprons in accordance with Auckland Council TP10 and FHWA HEC-14 energy dissipation guidance for pipe outfall scour protection.
Dual Stone Sizing Methods
Calculate d50 rock size using both the Isbash equation and the HEC-14 method, with automatic selection of the governing (conservative) value.
Interactive Plan & Profile Diagrams
Visualise your riprap apron design with reactive SVG plan and profile drawings that update as you change inputs, showing key dimensions and features.
Volume & Cost Estimates
Automatically calculate rock volume, rock mass, and filter area to support material ordering and cost estimation for construction.
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Open Riprap Apron Sizing CalculatorPart of the Flowpath Engineering suite of free civil engineering tools for New Zealand.