Hydraulic Gradient (HGL)

Definition - What does Hydraulic Gradient (HGL) mean?

The hydraulic gradient (HGL) can be defined as the locus of elevations to which water from a pipe will rise when subjected to atmospheric pressure. In trenchless technology, it is used to determine the design flows for a new drainage system such that maximum flow does not cause flooding, or in the case of old pipes, to determine if wastewater is exiting improperly, which would need to be corrected.

The hydraulic gradient is also referred to as the hydraulic grade line.

Trenchlesspedia explains Hydraulic Gradient (HGL)

A sewage system is designed to effectively remove the sewage from one area to another low-lying area where it enters the treatment plant. The hydraulic gradient is equal to the inverted slope for uniform flows.

A sewer system needs to be installed in a continuously sloping gradient to ensure that waste is effectively discharged by gravity at the discharge point. Sewers are designed as open channels. The hydraulic gradient is measured when designing sewers in order to assess the needed elevation of the conduit according to the slope of the land towards which its load will be discharged.

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