Welcome to the EFDC_Explorer Modeling System community. Thank you for posting.
There are several potential options, and you may wish to try different ones to see which one works for you best. I will describe each one and offer some potential advantages and disadvantages based on my own experience:
- Flow Boundary - this is the basic flow boundary condition, where the user provides a discharge time series (can be positive or negative), for a specific cell. The advantage of this is it is easy to set up, however the size of the withdrawal will always be the same as the dimensions. https://eemodelingsystem.atlassian.net/wiki/spaces/EK/pages/240222648/Flow+-+BC
- Hydraulic Structures - you can more precisely describe the size and orientation of your outlet structure. For your situation, something like a submerged orifice might be appropriate. Keep in mind that hydraulic structures in EFDC+ use the same analytical solutions you will find in many other programs like HEC-RAS for example. https://eemodelingsystem.atlassian.net/wiki/spaces/EK/pages/240320748/Hydraulic+Structures+-+BC
- Jet Plume boundary conditions. Jet plumes allow you to simulate the near/far-field effects of high velocity jets on the ambient flow field. This may be a good option for what you are doing, as you suspected. https://eemodelingsystem.atlassian.net/wiki/spaces/EK/pages/240288146/Jet+Plume+-+BC
Since you are interested in the flow characteristics around the outlet, you should also consider your grid resolution. Depending on the kind of flow rates at your inlet and outlet compared to the size of your grid cells, you might see some differences in results using the different boundary condition types. Thus, I would recommend that you conduct some testing using different grid resolutions and boundary types to determine which combination produces the most realistic and defensible result.
All the best,