HD1

Enhanced Highway Drainage System Calculator

This enhanced calculator includes dropdowns for standard values based on research from sources like FHWA, NRCS, and engineering manuals (e.g., runoff coefficients from NYSDOT and CED Engineering guidelines, Manning's n from USGS and Chow tables, CN from TR-55). It allows calculating once and transferring values (e.g., Q) to other sections. Added Time of Concentration (Tc) using Kirpich formula (metric adapted from standard). All units as per original (metric).

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 * A / 360 (m³/s, i mm/hr, A ha).

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: Q = [(P - 0.2S)^2] / (P + 0.8S), S = (1000/CN)-10 (inches, converted).

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):

5. Climate Change Allowance (40%)

Original Q (m³/s):