Pumps

Some smaller reservoirs such as interior detention ponds or pump stations may use pumps to move water out of the reservoir and into the tailwater when gravity outlets alone are insufficient. Pumps can only be included in reservoirs using the Outflow Structures routing method. Up to 10 independent pumps can be included in the reservoir.

Head-Discharge Pump

The head-discharge pump is designed to represent pumps that are applied in low-head, high-flow situations, such as the centrifugal type. These pumps are designed for high flow rates against a relatively small head. There are options for setting a reservoir pool elevation range for pumping and minimum times for the on or off condition. Figure X depicts the representation of this type of pump in HEC-HMS.

The intake elevation, $//<![CDATA[ \begin{array}{l}elev_{intake}\end{array} //]]>$, defines the elevation at which the pump takes in water within the reservoir pool.
The line elevation, $//<![CDATA[ \begin{array}{l}elev_{line}\end{array} //]]>$, defines the highest elevation in the pressure line from the pump to the discharge point.
The equipment head loss includes all energy losses between the intake and discharge points, including pipe losses related to expansion, contraction, and bends, and losses due to the pump itself. These losses are currently modeled as static losses, not as a function of flow. This total head loss is added to the head difference due to reservoir pool elevation and tailwater elevation to determine the total energy head, $//<![CDATA[ \begin{array}{l}h_{total}\end{array} //]]>$, against which the pump must operate.

A head-discharge curve is used to describe the capacity of the pump as a function of the total head. Total head is the head difference due to reservoir pool elevation and tailwater elevation, plus equipment loss. The head-discharge curve must be defined as an elevation-discharge function, although it actually represents head rather than elevation. If the pump is determined to be active, the pump discharge is determined by looking up and interpolating the flow value based on the total head value.

The pump is set to turn on at a specified on-trigger elevation, $//<![CDATA[ \begin{array}{l}elev_{on}\end{array} //]]>$, and remain on until the pool has dropped below a specified off-trigger elevation, $//<![CDATA[ \begin{array}{l}elev_{off}\end{array} //]]>$, at which point it turns off. In addition, it is possible to constrain the pump such that when it is triggered to turn on, it must stay on for a minimum run time.

The pump operation can also be constrained to stay on or off for a minimum length of time by setting a minimum run time or minimum rest time. If it is used, once the pump turns on it must remain on the specified minimum run time even if the reservoir pool elevation drops below the trigger elevation to turn the pump off. The only exception is if the pool elevation drops below the intake elevation, then the pump will shut off even though the minimum run time is not satisfied.

So HEC-HMS will check to see if the tailwater or headwater is above the intake.