I am modelling cross sections at a culvert and I know that 4 cross sections are required. How far should the cross sections be apart? Is there a general rule?
Yes there are general rules and are outlined in detail in the HEC-RAS technical reference manual. Download the manual and search for Cross Section Locations and it should pop up. Its a short read but understanding it is imperative for a beginner in culvert modeling. If im not mistaken i believe it is 6 cross sections though.
You need at least 4 cross-sections that the user creates and HEC-RAS creates 2 internal ones which gives a total of 6. You can have more than 4 user cross-sections, you just need to make sure that ineffective flow areas are set correctly.