Development of Induced Surcharge Envelope Curve

This post provides further information on the development of an induced surcharge envelope curve.  The material from this post is taken from the Corps of Engineers publication, EM-1110-2-3600, Management of Water Control Systems, dated 10October2017.

The operation of a spillway gate in a surcharge operation is shown below.  As the gate is raised, additional storage is created in the pool due to the higher top of gate elevation while, at the same time, the release from the reservoir is increasing. 

According to EM-1110-2-3600, the induced surcharge envelope curve defines the maximum permissible reservoir elevation for a given reservoir release.  Basically, this means that as the reservoir operator is developing a maximum pool forecast using both anticipated inflow from observed rainfall and the projected release schedule, he or she must determine if the maximum permissible reservoir elevation will be exceeded based on the proposed maximum release.  If it is, the release schedule will need to be altered iteratively until a solution is found that satisfies the maximum permissible reservoir elevation condition.

The development of an envelope curve is performed by following the steps below (from EM 1110-2-3600):

Once the spillway rating curve is developed, the top of gate elevation is plotted on the rating.  In developing the envelope curve, once must ensure that the top of gates will not be exceeded by the water surface elevations on the envelope curve.  Typically, some freeboard is also provided.  The comparison of the envelope curve (blue line) and the top of gate elevation (red line) is provided in the figure below.

The next step is to identify the greatest non-damaging flood risk management release.  EM 1110-2-3600 states that, usually, the non-damaging release equals the downstream channel capacity or the flow associated with flood stage at a downstream regulating station.  In the figure below, it appears that the release at the top of the flood pool is zero (shown by green arrow).

The next step is to select the fully-open-gate pool elevation. This involves consideration of both the upstream and downstream conditions.  EM-1110-2-3600 states the following: 

“Selection of the fully open elevation must consider economics, critical infrastructure needs, and life loss risks. The economic portion of the decision must balance upstream and downstream flood damage risks. Often, the upstream property damage risks are identified by setting the project real estate taking line (see EM 1110-2-1420). The downstream property damage risks are identified by overlaying economic data (e.g., Hazus mapping) on downstream inundation maps. Critical infrastructure and life loss concerns are often connected, with releases or pool elevations impacting evacuation routes, capacity of downstream dams, dam safety concerns, areas of high population density, and national security sites. The water manager should exercise proper engineering judgment to ascertain whether the spillway gates should be completely opened. Consideration should include current and forecasted inflow and corresponding reservoir elevations as well as recommendations from the district and division dam safety officers.”

The location of the fully open elevation is shown in the figure below (shown by green arrow).  This is where the envelope curve intersects with the rating for gates fully open.

The two points (non-damaging flow at top of flood pool and fully open elevation) could be connected with a straight line as shown in the figure below (shown by the red dashed line).

However, in the actual drawing of the line, one can notice that at the lower elevations, the rate of increase in discharge from the dam is not as great as that at the higher elevations.  This is shown in the figure below (look at portion of the envelope curve traced in green).  This is to attempt to provide some downstream benefit at the more frequent events.  It also acknowledges that at the higher, less frequent events, the benefits of reservoir are diminished due to the significant required increase in release with increases in elevation. 

EM 1110-2-3600 also provides guidance on making spillway releases ahead of the releases required by the spillway gate regulating schedule.  The following is given in Section 4.5.6.1.1.

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4.5.6.1  Rising Reservoir Levels

4.56.1.1.  If forecasts based on observed precipitation indicate that runoff from a storm will appreciably exceed the remaining reservoir storage capacity, the opening of spillway gates may begin before the time required using the spillway gate regulating schedule.  This could occur on the rising or falling limb of the inflow hydrograph.  See Section 7.3.2 for further discussion on operating based on water-on-the-ground.

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It should be noted that the guidance provides for making these releases based on a forecast using observed precipitation.