Induced Surcharge Rule

In the context of watershed hydrology, the word surcharge refers to the extra storage that naturally occurs behind an uncontrolled spillway when the pool elevation exceeds the spillway's crest (and inflow exceeds outflow from the reservoir). Thus, surcharge storage is the volume of water in the reservoir above the spillway crest elevation. This concept is illustrated in "Figure: Surcharge Storage".

Induced surcharge is the extra storage that occurs behind a controlled spillway when the spillway gates are at least partially open. The adjective induced is used to indicate that the extra storage was caused by man's actions and not a natural occurrence. So, since induced surcharge is caused by opening the gates, then induced surcharge storage is the volume of water in the reservoir above the elevation of the top of the closed gates. As the gates are opened (together), the elevation of the top of the gates rises providing more room for water to be held behind the dam (as long as inflow exceeds outflow). "Figure: Induced Surcharge Storage" below shows both the minimum and the maximum positions of the gates on a spillway, closed and fully open, and the maximum size of the induced surcharge pool.

Induced surcharge operation is the process of adjusting the position of the spillway gates for the specific purpose of inducing surcharge. When the gate opening is set to limit the spillway flow to less than free overflow, water is intentionally surcharged—or stored—behind the gates.

To water managers (reservoir regulators), Induced Surcharge Operation is a flood risk reduction operation that calls for larger releases (than would be made under normal flood operations) when the current pool elevation together with a rapidly rising inflow (or pool elevation) threaten dam stability. As such, it could be considered a "save the dam" operation. Induced surcharge operation allows operators and regulators to manage an extreme flood event by utilizing the additional volume above the top of the (closed) gates.

Although induced surcharge operations are predicated on the assumption that the reservoir has a gated spillway, this is not necessarily a requirement. The actual requirements are (a reserved quantity of storage at the top of the usable reservoir pool and enough release capacity to pass the peak inflow of the PMF or reservoir design storm. For example, a reservoir may be constructed without a gated spillway but with one or more lower level outlets have far more release capacity than would ever be utilized under normal conditions. If a portion of storage at the top of the pool of this reservoir is identified for use only under extreme inflow conditions, an operating plan could be developed that, when needed, follows the induced surcharge operation described above to utilize the reserved reservoir storage and the full release capacity of the reservoir.

The induced surcharge operation in ResSim is a special operation whose primary objective is to "protect the dam without making the downstream flooding worse than if the dam were never there". To do this, ResSim will utilize the last remaining "safe" storage in the reservoir to store the peak of the incoming flood event while passing the rest of the inflow through the reservoir. The assumption is that by holding back the peak of the inflow, the downstream peak will be reduced. In addition, during the rising limb of the inflow hydrograph, the Induced Surcharge rule will not call for releases that are greater than inflow; thus, this operation won't make the flood worse than it would have been if the reservoir weren't there.

The induced surcharge, or emergency gate regulation, operation described in the water control manual for most USACE reservoirs usually specifies a release that must be made. In general, a release that must be made can usually be interpreted as a minimum release requirement. However, under the conditions that call for induced surcharge operation, the release requirement is usually interpreted as a specified flow, i.e., no more and no less. Since specified flow rules can be more heavy-handed than necessary, the Induced Surcharge rule in ResSim was designed to produce a desired minimum release limit. However, it was not designed to stand alone, it needs a partner.

To model induced surcharge operation in HEC-ResSim, you MUST specify TWO rules:

1. An Induced Surcharge rule placed at the top of the rule stack in each operating zone from which the induced surcharge operation could be triggered. Remember, the trigger is a combination of pool elevation and rapidly rising inflow or elevation so the rule could potentially be activated even if the pool is relatively low but inflow is high and rising fast.
2. An accompanying maximum limit rule (Release or Downstream Control Function) placed below the Induced Surcharge rule in the rule stack of each zone that includes the Induced Surcharge rule. This maximum limit rule should correspond to the maximum discharge that would occur if surcharge operations were not in effect; as such, the maximum limit rule(s) you select to partner with the induced surcharge rule may vary per zone.

It is important to understand why the two rules listed above are necessary to model induced surcharge operation. If you don't understand why, you will find it difficult to build an operation set that can smoothly transition from normal flood control operation to induced surcharge and back again.

First, the Induced Surcharge rule is a minimum limit release rule. And, it should be placed as the highest priority rule so that, when it determines that the dam is at risk, it can force higher releases than normal flood control operations would allow.

The maximum limit rule is the rule (or set of rules) that describe normal flood operation. The maximum limit rule must be placed at a lower priority so that it can prevent the reservoir from releasing more than it should while still allowing the higher priority Induced Surcharge rule to override the more restrictive normal flood control operation when necessary.

When the Induced Surcharge rule becomes active, the two rules function together as a specified limit rule. To explain this, let's see how the two rules will affect the allowable release range:

• For simplicity, assume the rule stack has only two rules, an Induced Surcharge rule followed by a Maximum Release rule representing channel capacity.
• When the Induced Surcharge rule becomes active, it will produce a desired release that is usually equal to or greater than the limit of the Maximum Release rule.
• The desired Induced Surcharge (minimum) release is then applied to the allowable range. If the desired release is greater than the current minimum limit of the allowable range, the minimum limit will be assigned the desired release value.
• Next, the lower-priority Maximum Release rule will be evaluated, and its value will be applied to the allowable range.
• As the allowable range attempts to lower its maximum limit to the new value called for by the Maximum Release rule, it will bump into the bottom (minimum limit) of the range which has been set by the higher-priority Induced Surcharge rule. Since the lower-priority maximum rule cannot reduce the minimum limit of the allowable range, the maximum limit of the range will be assigned the same value as the minimum limit, resulting in a single-valued allowable range equal to the value of the desired Induced Surcharge release. Thus, combined, the two rules effectively produce a specified limit.