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Showing posts with the label Groundwater Mechanics

Hydraulic Structures: Siphon Spillway Mechanics and Priming Dynamics

 A Siphon Spillway is a closed conduit bent over a dam crest that uses atmospheric pressure differentials to discharge high flows under low operating heads. Flow transitions through three distinct operational phases: ​Weir Flow: Initial rising water level overflows the lower lip as a simple weir. ​Priming Phase: Flow seals the downstream leg outlet, entraining and evacuating internal air to form a partial vacuum within the siphon crown. ​Full Siphonic Flow: Continuous liquid column flow established under total differential head (H). ​The ultimate siphonic discharge (Q) is evaluated using pipe flow hydraulics: $Q = C_d \cdot A \cdot \sqrt{2 \cdot g \cdot H}$ ​Where $C_d$ is discharge coefficient $(\approx 0.6\text{ to }0.8)$ and $A$ is throat cross-sectional area. The maximum operating suction head at the crown is limited by water vapor pressure to prevent air pocket formation and cavitation. ​Siphon spillways installed on medium storage dams across India provide rapid automatic dis...
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Coastal Hydraulics: Saltwater Intrusion and the Ghyben-Herzberg Relation

 In coastal unconfined aquifers, dense seawater forms a dynamic wedge underneath fresh groundwater. Under hydrostatic equilibrium conditions, the depth of the fresh-saltwater interface below sea level $(z)$ is governed by the Ghyben-Herzberg Principle: $$z = \frac{\rho_f}{\rho_s - \rho_f} \cdot h_f$$ ​Where $\rho_f$ is fresh water density $(\approx 1.000\text{ g/cm}^3),$ $\rho_s$ is seawater density $(\approx 1.025\text{ g/cm}^3),$ and $h_f$ is freshwater table elevation above sea level. Substituting densities simplifies to: $$z \approx 40 \cdot h_f$$ ​Thus, every meter of freshwater head maintained above sea level supports approximately $40\text{ meters}$ of fresh water column below sea level. Excessive groundwater pumping $(h_f \to 0)$ causes rapid vertical upconing of the saltwater interface toward extraction wells. ​Coastal aquifers in states like Gujarat, Tamil Nadu, and West Bengal suffer severe saline contamination from heavy agricultural and industrial pumping. ​Modern coas...
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