Basics of Reservoir Operations

This post will detail the philosophy of reservoir operations.  The basic philosophy of reservoir operations (when there is a flood control mission) is to not create a greater downstream peak impact than what would have occurred without the dam (or dams) in place.

To understand how this philosophy is implemented, it is important to understand the relationship between the controlled and the uncontrolled areas.

The figure below shows the controlled area for Pine Creek Dam in Oklahoma.  Any rainfall that becomes runoff in the controlled area will flow into the reservoir.

The relationship between the controlled area and the uncontrolled area can be better understood by viewing a profile view of the stream.  The profile view (see figure below) shows reaches both upstream and downstream of Pine Creek Dam.  In dams where there is a flood control mission, the downstream areas are provided some protection due to the dam holding runoff from the controlled area when there is a downstream impact.  Any water that is in the flood pool is released once the downstream conditions allow.

The downstream areas can benefit by keeping transportation routes open (note that there are several highways shown on the profile below).  Additionally, rural and agricultural benefits can be realized along with benefits to urban areas.  Multiple types of benefits can be realized in the areas downstream of a single dam or a single system of dams. 

To more fully understand the philosophy, it is important to understand how one would ensure that the peak downstream impacts are not greater than what would have occurred had the dam not been in place.  First, consider a flood that occurs only in the controlled area.  To ensure that the peak downstream impacts would not be greater, the ordinates of the outflow hydrograph should remain below the ordinates of the inflow hydrograph until the inflow hydrograph has peaked and is on the recession limb (see figure below).  Note that some downstream routing computations may be necessary to ensure that a benefit is derived at downstream locations, but this should generally be true.

If a flood originates in the uncontrolled area while there is water in the flood pool, the release from the dam could be set to zero until the downstream point of interest reaches its peak flow.  Flows could be made from the dam to be timed to coincide with the decreasing limb of the downstream hydrograph.  This would again assure that a benefit is provided to the downstream area, or at least that the conditions are not made worse than what would have occurred had the dam not been in place.  The figure below shows this concept.

Since many flood events occur both upstream and downstream of the dam, it would also be possible to look at both the upstream and downstream hydrograph and pick the greater of the two to determine which one should control.  In reality, a real-time computation could be performed to determine the unregulated (unregulated meaning assumption of no dam in place) flow at the downstream points of interest and then release schedules could be developed to ensure a benefit.  The techniques that are described in the previous paragraphs, however, would require the real-time reservoir operator to make determinations on how much benefit should be provided and can require significant time for the computations.  

For real-time operations, a regulating stage (with a corresponding discharge) is set at a single or multiple downstream locations.  The regulating stage can vary throughout the year and also vary based on conditions within the system.  This makes real-time operations more straightforward, and it  provides the real-time operator with a specific determination of what is expected to occur downstream.  For Pine Creek Dam, the downstream Horatio gage is used to determine allowable releases.  The location of the Horatio gage can be seen in the profile plot that was provided above.

When a downstream point of interest is above its regulating stage, the release from the dam is typically set to zero or to some minimum required release.  If there is water in the flood pool and the downstream point of interest is below its regulating stage, the release from the dam is typically made so that flow at the downstream point of interest is raised to its regulating stage, if needed.  This is a simplistic explanation of the operations, but it is sufficient for understanding the basic concept of reservoir operations.  When operating according to a downstream regulating stage, the concepts stated above will normally happen naturally (as in ensuring a benefit is provided to the downstream areas).  

Some manuals will specify the concept either fully or partially.  For example, the Millwood (located in southwestern Arkansas) Water Control Manual states the following:

"When the lake level is at or below elevation 259.2 and flood estimates indicate that the volume of inflow will exceed the available conservation capacity, releases may be made in amounts which when combined with local inflow below the dam shall not produce flows in excess of those which would have occurred without the Little River System of lakes or 60,000 cfs, whichever is smaller, in Little River ..."