Boundary conditions in HEC RAS

The basic data needed to calculate hydraulic profiles of a channel are the discharge, channel geometry, water elevation at a control section and channel roughness. Discharge is a given data and the channel geometry is obtained by measurements. Usually the water elevation at control section (boundary condition) and channel roughness are unknown and have to be estimated by indirect ways.
Although there are different approaches and recommendations for those data, they are just suggestions and incorporate an error to the vicinity of the boundary station and cross sections. The best approach is to perform a sensitivity analysis of such values, and if needed to perform an uncertainty analysis. Although the task of updating either the boundary condition or manning roughness are simple ones, when repeating many times (sensitivity and uncertainty analysis) it may become a tedious and time consuming task; besides, the possibility of accidental errors when typing values increase.

Boundary conditions
Boundary condition defines the starting water level at the end of the river, which is needed so that the program can begin the calculations. HEC-RAS allows specifying boundary condition in one of the following options:
Known water surface: This is the known water surface for the given profile. This option applies for cases where the water level was measured for a given discharge.
Critical depth: With this option the program calculates the critical depth for the section and uses it as boundary condition. This option applies for cases where there is a control structure such as weir, gate or drop that controls and forces the critical depth.
Rating Curve: In this option the water level is interpolated from the given rating curve. Usually this case applies for control station where water levels and discharges are measured constantly.
Normal depth: In this option the program uses the energy slope to calculate the normal depth with Manning's equation.
The most used boundary condition is the normal depth. Although the normal depth is unknown, usually it is approximated either by using the channel slope or the water surface slope near the boundary station. Nevertheless, these are just approximations that incorporates an error to the vicinity of the boundary station; thus, is suggested to increase the cross sections and channel length (which increases the topographical data required) so that these errors do not affect the study area. Figure 1 shows a river hydraulic profile under normal depth boundary condition located downstream. The hydraulic profile is from a river with a low topographic gradient around 0.0015 m/m. Different values of normal depth slope between 0.001 m/m and 0.015 m/m were tested. The uncertainties due to the boundary condition propagate some 1500 m upstream. The hydraulic profile in those 1500 m is strongly influenced by the selected slope.  
Image 1. Influence of boundary conditions
Source: AHYDRA

The best approach is to perform a sensitivity analysis of the boundary conditions, and if needed to perform an uncertainty analysis. In the present case, if the study area is located more than 1500 m upstream, then the uncertainties of the boundary condition have minor influence. However, if the study area is within the first 1500 m, then it is important to analyze the different profiles. For instance, a visual analysis shows that the highest slope forces a higher velocity and lower depth at the boundary section; thus, the backwater shows a concave curve to fit the imposed condition. On the other side, the other slopes shows a backwater profile with the same pattern over the whole river length. 
Although the task of updating the boundary condition is a simple one, when repeating many times (sensitivity and uncertainty analysis) it may become a tedious and time consuming task. Automating Hydraulic Analysis (AHYDRA) allows updating boundary condition, executing the simulation and viewing the results from a single screen and a single instruction (button click).

AHYDRA
AHYDRA is a freeware application that can be downloaded from its website.
It comes in a compressed .rar file. Just decompress it in the path “C” so that it will create a folder with the path “C:\AHYDRA” and launch it by double clicking the AHYDRA.exe icon. Before executing it, be sure that you have HEC-RAS version 4.1 installed. If you have a previous version the AHYDRA will not work.
NOTE: It is important to have the “C:\AHYDRA” path in order to overcome accessibility limitations with the “C:” and “Program Files” folder.

The following video shows the use of AHYDRA for automating boundary conditions in HEC RAS.

Video 1. Automating boudary conditins in HEC RAS
Source: AHYDRA

References & Further reading
HEC-RAS evolution
HEC RAS hydraulic reference
Open Channel hydraulics
AHYDRA basic tutorial
Automating hydraulic analysis AHUDRA
Youtube tutorials