This version of HEC-RAS allows the user to enter two Manning's n values inside of the culvert, one for the top and sides, and a second for the culvert bottom. The user defines the depth inside the culvert to which the bottom n value is applied. This feature can be used to simulate culverts that have a natural stream bottom, or a culvert that has the bottom portion rougher than the top, or if something has been placed in the bottom of the culvert for fish passage. An example of this is shown in the figure below.
Culvert With Multiple Manning's n Values

When multiple Manning's n values are applied to a culvert, the computational program will use the bottom n value until the water surface goes above the specified bottom n value. When the water surface goes above the bottom n value depth the program calculates a composite n value for the culvert as a whole. This composite n value is based on an equation from Chow's book on Open Channel Hydraulics (Chow, 1959) and is the same equation we use for computing a composite n value in open channel flow (see (Composite Manning's n for the Main Channel:1) of this manual).