Dholera Stormwater Culvert Sizing: DICDL Flood Drainage Codes

1. Hydrological Runoff Calculations (The Rational Method)

DICDL drainage engineers determine the peak stormwater discharge ($Q$) for catchments within Dholera SIR using the Rational Method formula:

$$\Large Q = C \cdot I \cdot A$$

Where: * $Q$ = Peak Runoff Rate ($\text{m}^3/\text{s}$): The maximum volume of water entering the drainage channel per second. * $C$ = Runoff Coefficient: Dimensionless factor representing surface imperviousness. $C = 0.85\text{ to }0.90$ for paved roads and industrial roofs, $C = 0.60$ for residential plots, and $C = 0.30$ for green parks. * $I$ = Rainfall Intensity ($\text{mm/hr}$): Sourced from Indian Meteorological Department (IMD) intensity-duration-frequency (IDF) curves for a 100-year storm return period (typically $120\text{ mm/hr}$ peak design intensity). * $A$ = Catchment Drainage Area ($\text{m}^2$): Total surface area contributing runoff to the specific culvert inlet.

2. Hydraulic Capacity & Manning’s Equation

Once the design flow ($Q$) is established, the required internal dimensions of the reinforced concrete (RCC) box culvert are calculated using Manning’s Equation:

$$\Large V = \frac{1}{n} \cdot R^{2/3} \cdot S^{1/2}$$

Where: * $V$ = Flow Velocity ($\text{m/s}$): Maintained between $0.9\text{ m/s}$ (self-cleansing velocity to prevent siltation) and $3.0\text{ m/s}$ (maximum non-erosive velocity). * $n$ = Manning’s Roughness Coefficient: Fixed at $n = 0.013$ for smooth, cast-in-situ reinforced concrete box culvert surfaces. * $R$ = Hydraulic Radius ($\text{m}$): $R = A / P$, where $A$ is wet cross-sectional flow area and $P$ is wetted perimeter. * $S$ = Hydraulic Gradient / Slope ($\text{m/m}$): The invert slope of the culvert bed, laid at a minimum gradient of $1\text{ in 1,000}$ ($0.001$) towards coastal discharge outfalls.

The table below outlines DICDL stormwater culvert hydraulic dimensions across road classes:

3. Freeboard Safety Margins & Tidal Gate Protection

• 0.50m Minimum Freeboard: To prevent hydraulic surcharging and structural uplift during peak storm surges, box culverts must maintain a vertical freeboard clearance of at least 0.50 meters between the peak design water level (DWL) and the underside of the culvert top slab (soffit level).
• Automatic Flap / Tidal Gates: At outfalls discharging into the Sabarmati estuary or Gulf of Khambhat, culvert exits are fitted with automatic counter-weighted flap gates. These gates close automatically during sea high tides to prevent seawater from backing up into the city’s drainage network.

Frequently Asked Questions (FAQ)

#### What formula is used to calculate stormwater runoff for Dholera culverts?

DICDL drainage engineers use the Rational Method formula ($Q = CIA$) to calculate peak runoff, incorporating catchment area, rainfall intensity, and land surface runoff coefficients.

#### What is the minimum self-cleansing velocity for Dholera drainage culverts?

Culvert bed slopes are engineered to ensure a minimum flow velocity of 0.9 m/s to prevent silt and sediment from settling inside the concrete box cells.

#### Why are freeboard clearances mandatory in Dholera box culverts?

A minimum vertical freeboard margin of 0.50 meters above peak water levels prevents water surcharging and protects road embankments from hydraulic pressure during 100-year storm events.