While a reach element conceptually represents a segment of stream or river, optional modeling of interactions with the subsurface is performed by a loss/gain method contained within the reach. A loss/gain method represents losses from the channel, additions to the channel from groundwater, or bi-directional water movements, depending on the specific implementation of a method. The following loss/gain methods are provided in GeoHECHMS:
- Constant Loss/Gain
- Percolation Loss
Each method included in the software provides a different level of detail and not all methods are equally adept at representing a particular stream. Further, some loss/gain methods are only compatible with certain routing methods because of differing data requirements.
The user can select the loss/gain method for a reach using the Reach Data dialog box.
Follow the steps given below to select a loss/gain method:
- From the Input ribbon menu, select the Reach Data command.
Alternatively, the user can either double click on the reach polyline from the Map View or choose the Reach Data command from the Routing Reaches dropdown menu of the Input ribbon menu.
- The Reach Data dialog box will be displayed.
- Click on the Loss/gain method dropdown combo box to select a loss/gain method from the list of available choices.
- To enter the data for the selected loss/gain method, select the Loss/Gain Data option from the Reach Specifications dropdown combo box.
- The Loss/Gain Data panel will be displayed. Note that the Loss/Gain Data panel content changes based upon the loss/gain method selected in the General Specifications section.
The following sections describe different loss/gain methods and how to enter the data for each method in the Loss/Gain Data panel.
Loss/Gain Method: None
When None is selected for the Loss/gain method, the Loss/Gain Data dropdown combo box entry is disabled (i.e., grayed out).
If the user chooses the None method, the reach will perform routing calculations without including any losses or gains to the channel.
Loss/Gain Method: Constant Loss/Gain
The constant loss/gain method uses an empirical relationship to calculate channel loss using a fixed flow rate reduction and a ratio of the flow. It does not include any capability to represent gaining streams. A fixed flow rate is subtracted from the routed flow, and then the remainder is multiplied by a ratio. The reduced flow becomes the outflow for the reach.
Note that this method is compatible with all routing methods.
When Constant Loss/Gain is selected for the Loss/gain method, the following data panel will be displayed.
The following input parameters are provided in the data panel:
- Infiltration rate
This entry field specifies the amount of flow to be subtracted from the inflow. It may be zero or greater than zero. If the specified value is zero, then no flow rate reduction will occur. If the specified value is greater than the inflow for a time step, then no outflow will result for that interval.
- Infiltration reduction fraction
This spin control is used to reduce the inflow in linear proportion to the flow rate. After the constant value is subtracted from the routed inflow, the remainder is multiplied by the value one minus the fraction. The fraction must be between zero and one.
Loss/Gain Method: Percolation Loss
The percolation method uses a constant infiltration rate in combination with the inundated area in the reach to compute channel loss. It does not include any capability to represent gaining streams.
Note that this method is only compatible with the Modified Puls and Muskingum Cunge routing methods.
When used with the Modified Puls method, the optional elevation-discharge function and invert elevation must be specified in the routing parameters. The inundated area is calculated differently depending on the routing method. In combination with the Modified Puls method, the current storage and outflow are combined with the elevation-discharge curve to compute the inundated area. With the Muskingum-Cunge method, the wetted perimeter is multiplied by the reach length to compute area.
To learn more about different routing methods, refer to this article in our knowledge base.
When Percolation Loss is selected for the Loss/gain method, the following data panel will be displayed.
The following input parameter is provided in the data panel:
- Infiltration rate
This entry field specifies percolation in terms of a flow rate per area. The inundated area is multiplied by the rate to determine the channel loss for each time interval.