While a reservoir element in HEC-HMS conceptually represents a natural lake or a lake behind a dam, the actual storage simulation calculations are performed by a routing method in conjunction with a storage method, contained within the reservoir. The available options are shown in Table 1.

Table 1.Storage Method and Initial Condition Options for each Routing Method

Selecting a Routing Method

Three different routing methods are available: Outflow Curve, Specified Release, and Outflow Structures. A fourth option is to choose a routing method of None, which assumes no storage and passes inflow, making the model element only nominally representative of a reservoir.

Outflow Curve Routing method

The Outflow Curve Routing method is designed to represent a reservoir with a known storage-outflow relationship. This method does not allow for individual representation of the components of the outlet works, rather, it lumps all outflow structures into one storage-discharge relationship. This routing method is best used for simple reservoirs.

For the Outflow Curve Routing method, regardless of which storage method option is selected, the routing is always performed using only the storage-discharge curve. After the routing is complete using the storage-discharge curve, the program will compute the elevation and surface area for each time step, depending on the selected storage method.

Specified Release Routing method

The Specified Release Routing method is designed to model reservoirs where the total discharge is known for each time interval of a simulation. Usually this method is used when the discharge is either observed or completely specified by an external decision process. The method can then be used to preserve the specified release and track the storage using the inflow, outflow, and conservation of mass. The Specified Release method uses a specified release and computes the storage that would result.
With this method, it is possible to set a maximum capacity limit on the reservoir. The storage at the end of the previous time interval, and the inflow volume and specified outflow volume for the current time interval, are used to calculate the storage at the end of the current time interval (equation below). The calculated storage will not be changed when it exceeds the user setting; however, a warning message will occur when the calculated storage exceeds the setting value.

Outflow Structures Routing method

The Outflow Structures Routing method is designed to model reservoirs with a number of uncontrolled outlet structures. For example, a reservoir may have a spillway and several low-level outlet pipes. While there is an option to include gates on spillways, the ability to control the gates is extremely limited at this time. There are currently no gates on outlet pipes. However, there is an ability to include a time-series of releases in addition to the uncontrolled releases from the various structures. An external analysis may be used to develop the additional releases based on an operations plan for the reservoir.
Additional features in the reservoir for culverts and pumps allow the simulation of interior ponds. This class of reservoir often appears in urban flood protection systems. A small urban creek drains to a collection pond adjacent to a levee where flood waters collect. When the main channel stage is low, water in the collection pond can drain through culverts into the main channel. Water must be pumped over the levee when the main channel stage is high.

Selecting a Storage Method

Once a routing method has been selected, an associated Storage Method must be selected. The Storage Method defines the relationship between detention and discharge. There are five storage methods to choose from: Elevation-Area-Discharge, Elevation-Storage-Discharge, Storage-Discharge, Elevation-Area, and Elevation-Storage. The user must select the functions to be used and which is to be primary if there is more than one curve.

Different sets of storage methods are available, depending on the routing method selected. Each of these combinations are shown in Table XXX. For a simple reservoir, the combination of the Outflow Curve routing method and the Storage-Discharge storage method is preferred. The storage-discharge relationship can be defined with some dead pool storage associated with zero outflow. For the Specified Release Routing Method, a time series of releases is specified. Storage is post-processed using the elevation-area or the elevation-storage relationship. For the Elevation-Area-Discharge and Elevation-Area Storage Methods, the program automatically transforms the elevation-area curve into an elevation-storage curve using the conic formula. The Elevation-Area-Discharge Storage Method option is rarely used.

Because HMS doesn't use triplet curves, if Elevation-Area-Discharge or Elevation-Storage-Discharge is selected, ultimately the storage-discharge relationship is still used to calculate the routing. Elevation is post-processed. The user will select the primary field when entering data (i.e., which curve to key on), and interpolation is used to get the other column.

Setting Initial Conditions

The initial condition sets the amount of storage in the reservoir at the beginning of a simulation. The initial condition can simply be defined using the storage value, or there are other options depending on the method selected for specifying the storage characteristics of the reservoir. Initial conditions can be set using Discharge, Elevation, Inflow-Outflow, or Storage.

When the Elevation-Storage method is selected, you may choose to specify the initial elevation or the initial storage. An initial elevation will be converted to storage using the elevation-storage relationship. When the Elevation-Area method is selected, you must specify an initial elevation. Again, the conic formula is used to develop an Elevation-Storage relationship, and thus the initial elevation is converted to a storage. The initial condition can also be set to Inflow=Outflow, which takes the reservoir inflow at the beginning of the simulation, and uses the storage-discharge curve to determine the storage required to produce that same flowrate as the outflow from the reservoir. If only the elevation-discharge curve is available, it uses the elevation-storage curve to find the matching storage. Typically, the Inflow=Outflow option should not be used. The initial storage or elevation should be defined at the beginning of a simulation.

Some storage methods permit the specification of Elevation as the initial condition. In such a case, the elevation provided by the user is used to interpolate a storage value from the elevation-storage curve. Other storage methods permit the specification of Discharge as the initial condition. In such a case, the storage is interpolated from the storage-discharge curve. The Pool Elevation method can also be selected for the initial condition. In those cases, the elevation provided by the user is used to interpolate a storage value from the elevation-storage curve.