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Flood Hydrology: Rational Method and Time of Concentration Mechanics

 For small catchments (typically under $50\text{ km}^2),$ the peak surface runoff discharge $(Q_p)$ resulting from a uniform rainfall event is estimated using the Rational Method:

$Q_p = 0.278 \cdot C \cdot I \cdot A$

​Where $Q_p$ is peak flow $(\text{m}^3/\text{s}),$ $C$ is runoff coefficient, $I$ is rainfall intensity $(\text{mm/h}),$ and $A$ is catchment area $(\text{km}^2).$ The critical storm duration occurs when rainfall duration equals the catchment's Time of Concentration $(t_c).$ According to Kirpich’s Equation, $t_c$ (in minutes) is evaluated from physical basin geometry:

$t_c = 0.01947 \cdot L^{0.77} \cdot S^{-0.385}$

​Where $L$ is maximum flow path length (meters) and $S$ is main channel slope $(\text{m/m}).$

​Applying static runoff coefficients $(C)$ in rapidly urbanizing Indian watersheds leads to significant underestimation of peak discharges, causing urban flash flooding.

​Modern urban hydrology workflows dynamically update C values by overlaying GIS high-resolution satellite land-cover maps with SCS hydrologic soil group layers. Furthermore, automated Intensity-Duration-Frequency (IDF) curves derived from real-time automatic weather stations allow engineers to design climate-resilient stormwater drainage systems under national smart city initiatives.

​Note: This technical content was curated and structured with AI assistance to support technical education.

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