Method 4 of Encroachment

I agree wholeheartedly. I hadn't gone into the smoothing activities in my previous
e-mail. Yes, the large majority of the adjustments are oriented towards the shape of
the floodway, but some adjustments can be made to the surcharge value in the
process. I was trying to indicate that Method 1 "should" be used to finalize the
floodway, not to rely solely upon Method 4. My opinion is that adjusting surcharge
elevations via Method 1 is preferable to adjusting the target in Method 4 for final
tweaking.

I'm currently working on an encroachment analysis with HEC-2 model. Equal conveyance reduction is been used (Method 4). However, in several cross sections a target value greater than 0.30 meters (1.0 foot) need to be used to increase the flood levels up to 0.30 meters, maximum permitted by the regulatory agency. It can happen or it is a modeling error? Thanks in advance.

Miguel Menar, MSCE, PE
Menar Associates

Mr. Moss is correct. The floodway surcharge represents a very complex interaction between two computed water surface profiles. Both of these profiles are affected by downstream conditions. Therefore, the surcharge achieved at one cross-section is almost always affected by the surcharge achieved at the cross-section(s) downstream. In addition, the program is not able to achieve the specified surcharge at certain cross-sections; for example, the program cannot encroach into the area between the channel banks. If the encroachment stations are set to the channel banks and the specified surcharge is not achieved, a smaller surcharge may be the only answer (or you might want to start increasing your target surcharge at cross-sections downstream of this location).

Using Method 4 floodway is not a cookbook procedure; it is a trial-and-error process with many iterations generally required. You should work your way upstream from the mouth of the stream, trying to achieve a surcharge of perhaps 0.8 to 0.9 or more, but without EVER exceeding 1.0 foot of surcharge (and without creating excessive velocities). The target may need to be reduced dramatically at certain locations, and it may also need to be increase dramatically at others.

Remember that your starting condition for the floodway profile should also be 1 foot above the starting condition for the BFE profile, EXCEPT in the following two cases:

1) When the BFE profile is in the channel banks at the starting cross-section, the starting condition of the floodway profile should be the same as the BFE profile;

2) When the BFE profile is out of banks, but encroaching the BFE profile to the channel banks produces a surcharge of less than 1.0 ft. In this case, the surcharge is set to the computed value (less than 1.0 ft above the starting BFE).

I hope that this quick overview of floodway computations is helpful. People always expect the HEC-2 and HEC-RAS floodway routines to be fully automated, but they are not. They are very useful tools for the application of good engineering judgment.

Roy Dodson

I work on FEMA Flood Insurance Studies quite a bit. What you have experienced is not uncommon. You are not doing anything wrong. Method 4 rarely gives perfect or near perfect results. I always have to do some tinkering to get reasonable results. If you are able to achieve a 1.0 foot increase along the entire length of your study stream, you are very lucky.

Christopher Moss
cmoss@taylorengineering.com

I was taught to use Methods 1 and/or 2 (1= enter encroachment stations; 2= fixed top width) to get near satisfactory results, and then tweak to refine those results. I agree with Christopher Moss - if you can get about 1 foot for the reach you are doing very well.

Michael Fanning
mfanning@goodkindinc.com

In my experience with encroachment analysis, Method 4 was utilized to get a starting point for the encroachment stations. Following the initial assessment, I would go back to each individual cross-section and switch to Method 1. If you look at the flows in the left and right overbanks, you should be able to adjust the encroachment stations manually and come fairly close to the required 1 foot of rise.

Derek L. Doughty, P.E.
DoughtyD@Ayres-TPA.com