The floodway encroachment capabilities in HEC-RAS were adapted from those found in HEC-2. For the most part, encroachment methods 1-3 in HEC-RAS are the same as methods 1-3 in HEC-2. The following is a list of the differences between the two programs.

  1. HEC-RAS has an additional capability of allowing the user to specify a left and right encroachment offset. While in general the encroachments can go all the way up to the main channel bank stations, the offset establishes an additional buffer zone around the main channel bank stations for limiting the encroachments. The offset is applicable to methods 2-5 in HEC-RAS.
  2. The logic of method 4 in HEC-RAS is the same as method 4 in HEC-2. The only difference is that the HEC-RAS method 4 will locate the final encroachment to an accuracy of 0.01 feet, while the HEC-2 method 4 uses a parabolic interpolation method between the existing cross section points. Since conveyance is non-linear with respect to the horizontal stationing, the interpolation in HEC-2 does not always find the encroachment station as accurately as HEC-RAS.
  3. Method 5 in HEC-RAS is a combination of HEC-2's methods 5 and 6. The HEC-RAS method five can be used to optimize for a change in water surface (HEC-2 method 5); a change in energy (HEC-2 method 6); or both parameters at the same time (new feature).
  4. At bridges and culverts, the default in HEC-RAS is to perform the encroachment, while in HEC-2 the default was not to perform the encroachment. Both programs have the ability to turn encroachments at bridges and culverts on or off.
  5. At bridges where the energy based modeling approach is being used (similar to HEC-2's normal bridge method), HEC-RAS will calculate the encroachment for each of the cross sections through the bridge individually. HEC-2 will take the encroachments calculated at the downstream side of the bridge and fix those encroachment stations the whole way through the bridge.
  6. In HEC-2, if the user specifies a fixed set of encroachments on the X3 record, this would override anything on the ET record. In HEC-RAS, when the data is imported the X3 record encroachment is converted into a blocked obstruction. Therefore any additional encroachment information found on the ET record will be used in addition to the blocked obstruction.