Highway Drainage System Calculator

This calculator integrates climate change allowance at the initial runoff calculation phase, as per research from sources like UK GOV (20-40% for England), FHWA, and engineering standards (e.g., 15-20% in some cases). The allowance increases rainfall intensity/depth before computing discharge (Q). Default is 40% reference s, with dropdown for common values.

Climate Change Allowance

Select or enter percentage to add to rainfall (e.g., i or P).

Allowance (%): 40% (Document Default, UK Upper End) 30% (Common UK Central Estimate) 20% (Lower End, e.g., Some US Adaptations) 15% (Some Highway Guidelines) 0% (No Allowance)

Note: Applied to rainfall inputs in Rational (i) and SCS (P) methods.

0. Time of Concentration (Tc) - Kirpich Formula

Adapted metric: Tc (min) = 0.948 * (L / 1000)^{0.77} / S^{0.385} (L in m, S in m/m). For rural/highway areas.

Drainage Length (L, m): Slope (S, m/m):

Use Tc to determine rainfall intensity (i) from IDF curves (enter manually in Rational section).

1. Rational Method (for areas < 200 acres)

Formula: Q = C * (i * (1 + allowance/100)) * A / 360 (m³/s).

Runoff Coefficient (C): Asphaltic (0.70-0.95, avg 0.85) Concrete (0.80-0.95, avg 0.875) Brick (0.70-0.85, avg 0.775) Drives/Walks (0.75-0.85, avg 0.80) Roofs (0.75-0.95, avg 0.85) Grass/Lawn (0.05-0.35, avg 0.30 per FHWA) Cultivated Soil (0.10-0.30, avg 0.20) Rainfall Intensity (i, mm/hr) - Based on Tc: Drainage Area (A, hectares):

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

Formula: Uses adjusted P = P * (1 + allowance/100).

Precipitation Depth (P, mm): Curve Number (CN): Paved Roads/Streets (98 per TR-55) Residential (77-92, avg 85) Commercial/Industrial (89-95, avg 92) Open Space Good (39-79, avg 76 for urban per NRCS) Dirt Roads (72-87, avg 82) Cultivated Agricultural (60-80, avg 70) Urban Mixed (70-90, avg 80)

Note: Convert depth to volume by multiplying by area (manual step).

3. Manning's Equation for Pipe/Channel Sizing

Formula: Q = (1/n) * A * R^(2/3) * S^(1/2) (SI, full flow assumed).

Mode: Calculate Q (given diameter) Calculate Diameter (given Q) Manning's Roughness (n): Concrete Pipe (0.010-0.013, avg 0.013 per USGS/Chow) PVC/Smooth Plastic (0.009-0.011, avg 0.010) Corrugated Metal Pipe (0.022-0.026, avg 0.024) Earth Channel Smooth (0.018-0.025, avg 0.022) Grassed Channel (0.025-0.035, avg 0.030) Natural Stream Minor (0.030-0.040, avg 0.035) Slope (S, m/m): Flow Rate (Q, m³/s) - For solving diameter: Pipe Diameter (m) - For solving Q:

4. Self-Cleaning Velocity Check

Checks 0.75-3 m/s.

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