HD

Highway Drainage System Calculator

This calculator implements key formulas and steps from highway drainage design, including the Rational Method, SCS/NRCS Curve Number Method, and Manning's Equation. It supports runoff estimation, pipe sizing, and basic checks for self-cleaning velocities. All units are as per the document (e.g., m³/s for flow, mm/hr for intensity).

1. Rational Method (for areas < 200 acres)

Formula: Q = C * i * A (where Q in m³/s, i in mm/hr, A in hectares). Convert i to m/s and adjust units internally.

Runoff Coefficient (C, e.g., 0.70-0.95 for asphaltic): Rainfall Intensity (i, mm/hr): Drainage Area (A, hectares):

2. SCS/NRCS Curve Number Method (for larger areas)

Formula: Q = [(P - 0.2S)^2] / (P + 0.8S), where S = (1000 / CN) - 10 (inches). Inputs in mm; converts internally.

Precipitation Depth (P, mm): Curve Number (CN, 30-100):

3. Manning's Equation for Pipe/Channel Sizing

Formula: Q = (1/n) * A * R^(2/3) * S^(1/2) (SI units). Assumes circular pipe; solves for diameter given Q, or Q given diameter.

Mode: Calculate Q (given diameter) Calculate Diameter (given Q) Manning's Roughness (n, e.g., 0.012 for concrete): Slope (S, m/m): Flow Rate (Q, m³/s) - For solving diameter: Pipe Diameter (m) - For solving Q:

4. Self-Cleaning Velocity Check

Checks if velocity is between 0.75 m/s (min) and 3 m/s (max). Use Q from above or enter manually.

Flow Rate (Q, m³/s): Pipe Diameter (m):

5. Climate Change Allowance

Adds 40% to calculated Q.

Original Q (m³/s):