Friday, March 18, 2016

Induced Surcharge

This post details the modeling of induced surcharge operations in HEC-ResSim.  It is probably helpful to start with the definition of induced surcharge.  Basically, induce means to force and surcharge mean extra.  So, with an induced surcharge operation, you are forcing extra storage in the reservoir by opening the spillway gates.  With the top of the spillway gates at a higher elevation, additional storage has been created.  Obviously, with the spillway gates open, the release from the reservoir has also increased.

This example has one reservoir on a stream with no tributaries.  The watershed setup is shown in the figure below:



Two outlets are modeled at this reservoir.  The outlets are named "Controlled Outlet" and "Gated Spillway".  They are shown in the figure below:



There are four zones modeled at this reservoir.  Pertinent elevations of these zones are as follows:

Top of Dam = 1140.4 ft
Flood Pool = 1130.4 ft
Conservation = 1120.4 ft

The inactive zone was assigned a very low elevation since it doesn't affect these results.  The zones are shown in the figure below:



A maximum release rule of 500 cfs was assigned to all of the zones.  This maximum release rule is of lower priority than the induced surcharge rule.  Typically, you will want the induced surcharge rule to be the highest priority rule.  The induced surcharge rule specifies a minimum release for a given pool elevation.  The maximum release rule is shown in the figure below:




The induced surcharge curve for this reservoir is shown in the figure below.  Notice how the discharge increases less rapidly with changes in pool elevation at the lower elevations and more rapidly with changes in pool elevation at the higher elevations.  This is in response to the operation changing from a flood mitigation operation early in the event to more of a dam safety response as the event becomes more severe.



The curve is modeled numerically using the induced surcharge rule.  The relationship between the elevation and the minimum required release is shown in the figure below.

There is also a constant titled, "Time of Recession (hrs)".

The HEC-ResSim User's Manual defines this constant as follows:

"This constant describes the length of time an incoming flood is expected to recede.  The program uses this time to compute the volume of water that must be evacuated to prevent overtopping of the dam."

The Corps of Engineers' EM 1110-2-3600 provides the following method for computing this value:

"The recession constant can be obtained by plotting the recession curve as a straight line on semilog paper, with the flow on a logarithmic scale and time on an arithmetic scale.  The recession constant, T, is defined as the time required for the discharge to decrease from any value, say Qa, to a value, Qb, where Qb equals Qa / 2.7."




The falling pool options describe when HEC-ResSim will transition from the induced surcharge operation over to the falling pool options and what operations will take place under the falling pool options.

The "Time for Pool Decrease (hrs)" tells HEC-ResSim when to transition from the induced surcharge operation to the falling pool operation.  For this example, the pool must be falling for six consecutive hours for this transition to occur.

The "Falling Pool Transition Elev (ft)" tells HEC-ResSim when to discontinue the falling pool operation and to transition to the other rules of operation in the rule set.

HEC-ResSim provides a choice of four different falling pool release options.  HEC-ResSim uses only the option selected even if there are numbers in the other options.  For this simulation HEC-ResSim is using the average of inflow and previous release with inflow averaged over the past 3 hours.    



In this simulation, the maximum release of 500 cfs is maintained until the induced surcharge operation becomes necessary.  It can be seen that the difference between inflow and outflow is not very large on the falling pool options causing a very slow fall in pool elevation.  The operation is shown in the figure below.



In this next simulation, the falling pool option is only dependent on inflow.  The release is set to be 3 times the inflow averaged over the past 3 hours.  This is shown in the figure below.


  

In this simulation, the difference between the inflow and outflow is greater on the falling pool options than in the previous simulation.  This causes a greater drop in pool elevation; however, once the falling pool transition elevation is reached, the maximum release of 500 cfs governs the operations. Since the inflow is greater than 500 cfs, the pool begins to rise.  This operation is shown in the figure below.




In the final simulation, only the transition pool elevation is changed.  A lower value is used to continue the falling pool options for a longer period of time.  This is shown in the figure below.  The falling pool transition elevation was changed from 1129.5 ft to 1128.0 ft.



The effect of this change is that the pool no longer rises in this time period since the falling pool operation is in effect for a longer period of time.