What is hydraulic grade line?
Hydraulic grade line (HGL) is a measure of flow energy. According to the Oregon Department of Transportations’ (ODOTs) Hydraulics Manual, HGL is a line coinciding with the level of flowing water at any point along an open channel.
In closed systems flowing under pressure, the HGL is the level to which water would rise in a vertical tube at any point along the pipe.
HGL is determined by subtracting the velocity head (V2/2g) from the energy gradient (or energy grade line).
What is a hydraulic grade line used for?
HGL can be used by engineers to determine the elevation water will rise to in a system. HGL can also be used to assess the performance of an existing system.
HGL can be used for:
- Helping designers determine whether a proposed water system is acceptable.
- Monitoring where a system might be under stress – such as potential overflow locations or bottlenecks in a system.
- Indicate the static pressure of a system.
- Show how energy is lost in a water system.
What is the difference between hydraulic grade line and total energy line?
According to the University of Dayton, the hydraulic grade line is defined as the sum of elevation head and the pressure head.
The energy grade line (EGL) is the total head in the system, or the sum of the elevation head, the pressure head and the velocity head.
HGL represents the static pressure level in a pipe or channel. EGL represents the total energy available in the fluid flow at any given point.
HGL can be viewed as the level to which water would rise in a static pressure cap, or piezometer. EGL shows the total energy available for flow, including both potential and kinetic energy.
Is the hydraulic grade line the water level?
The HGL is closely related to water level, although not exactly the same.
HGL represents the level to which water would rise in a small vertical tube connected to a pipe at a given point – indicating the pressure head at the location measured.
In pressurized water systems, the HGL is an imaginary line that represents the pressure level within the pipe.
According to the city of West University Place, Texas, when extreme events such as heavy rainfall occur, values can exceed the HGL – which result in ponding in streets and flooding of property and structures.
When pressure is added to a system, HGL and water level become less related.
Can hydraulic grade line be negative?
According to the University of Texas, an HGL can be drawn to show the variation of the piezometric head – a measurement of liquid above a vertical datum. The distance from the centerline of the pipe to the HGL is the pressure head.
An HGL above a pipe corresponds to positive pressure, while an HGL below the centerline means that the pressure is negative.
What is tailwater?
Tailwater is the body of water that receives flow water from a system.
According to the city of West University Place, tailwater refers to the level of water in a receiving stream of body of water. Increasing tailwater levels have the effect of reducing the effective capacity of a system and increasing HGL elevations.
Increased HGL elevations can result in overflows and flooding.
