Objectives

Reservoir design demands special care because of the potential risk to human life. The economic efficiency objective described in the Flood-Loss Reduction Studies requires that the location and capacity of a reservoir be selected so that the net benefit is maximized. However, the capacity thus found may well be exceeded by rare meteorologic events with inflow volumes or inflow rates greater than the reservoir's design capacity. In fact, simple application of the binomial equation demonstrates that if the USACE constructed and operates 200 independent reservoirs, each designed to provide protection from the 0.005 AEP (200-year) and smaller events, the probability of capacity exceedance at one or more reservoirs in any given year is 0.63.

This capacity exceedance may present a significant risk to the public downstream of the reservoir. Unless the reservoir has been designed to release the excessive water in a controlled manner, the reservoir may fill and overtop. This may lead to catastrophic dam failure. Accordingly, USACE policy is to design a dam, and particularly the dam's spillway, to pass safely a flood event caused by an occurrence of a rare event - one much larger than the design capacity of the reservoir (ER 1110-8-2 - USACE, 1991). A spillway capacity study provides the information necessary for this design.

Spillway capacity studies are required for both proposed and existing spillways. For proposed spillways, the studies provide flow rates required for sizing and configuring the spillway. For existing spillways, the studies ensure that the existing configuration meets current safety requirements. These requirements may change as additional information about local meteorology becomes available, thus changing the properties of the likely extreme events. Further, as the watershed changes due to development or natural shifts, the volume of runoff into the reservoir due to an extreme event may change, thus rendering a historically safe reservoir unsafe. In that case, the spillway will be modified or an auxiliary spillway may be constructed.

Extreme Events

Performance of a water-control measure can be evaluated with 3 broad categories of hydrometeorologic events: (1) historical events; (2) frequency-based events; and (3) an estimated limiting value event. Evaluation with historical events is useful for providing information that is easily understood by and relevant to the public. For example, a useful index of performance of a USACE reservoir is a report of the damage reduction attributable to that reservoir during the flood of record. The utility of frequency-based events was demonstrated in earlier chapters of this document; they permit computation of EAD and regulation or operation to meet risk-based objectives. The final category of event, the estimated limiting value, is described by Chow (1988) as follows:

The practical upper limit on the hydrologic design scale is not infinite…Some hydrologists recognize no upper limit, but such a view is physically unrealistic. The lower limit of the design scale is zero in most cases…Although the true upper limit is usually unknown, for practical purposes an estimated upper limit may be determined. This estimated limiting value (ELV) is defined as the largest magnitude possible for a hydrologic event at a given location, based upon the best available hydrologic information.

Thus the utility of the ELV event is to demonstrate how a damage reduction measure would perform in the worst reasonable case - a case that is very unlikely, but still possible. This is the approach used for spillway studies.

The ELV used for USACE studies is the Probable Maximum Precipitation (PMP) event, and the corresponding Probable Maximum Flood (PMF). The PMP is the "…quantity of precipitation that is close to the physical upper limit for a given duration over a particular basin" (WMO, 1983). In the United States, the properties of a PMP commonly are defined by NWS.

Procedural Guidance

The following USACE guidance is particularly relevant to reservoir design studies:

  • EM 1110-2-1411 (USACE, 1965) provides background for the development of the standard project precipitation method used by USACE.
  • ER 1110-8-2 (USACE, 1991) describes the regulations for selecting the appropriate inflow design flood for dam safety. Required assumptions, such as initial water surface elevation and operation of control structures for reservoir analysis, are also described.
  • EM 1110-2-1603 (USACE, 1992) provides guidance for the hydraulic design of spillways for flood control or multipurpose dams.

Study Procedures

To meet the objective of a reservoir spillway capacity study, the following steps are typically taken:

  1. Develop a model of the contributing watershed and channels.
  2. Define the extreme-event rainfall: the PMP.
  3. Compute the inflow hydrograph to the reservoir: the PMF.
  4. Develop a model of the performance of the reservoir and spillway.
  5. Use the model to simulate reservoir performance with the hydrograph from Step 3, routing the PMF through the reservoir, over the spillway, and through downstream channels.
  6. Compare the performance of the spillway to the established criteria to determine if the spillway adequately meets the criteria.

HEC-HMS is a convenient tool to use for this analysis. Its application within this procedure is illustrated with the case study below.