HEC-RAS Bank Stations
HEC-RAS Bank Stations
(OP)
I'm having to model a roadside ditch to satisfy a reviewer for a proposed culvert. I've been told by the reviewer that "The channel bank stations should be at the Ordinary High Water Mark, not at the top of bank" (which is how I've always done it). In essence, moving the Bank Stations from the red dots, down to the red Xs in the attached sketch. Just to satisfy my curiosity, I took my model, copied it and switched bank stations, and re-ran the model. Just by switching the bank stations, the WSEL went down, by about 0.4' (of an 8' deep ditch). Mannings was already set by station, not LOB/Channel/ROB, so that‘s not a factor.
Anyone able to tell my why the WSEL went down, and whether I should or should not move these bank stations?
Anyone able to tell my why the WSEL went down, and whether I should or should not move these bank stations?





RE: HEC-RAS Bank Stations
FWIW, I have never seen the bank stations set for OHW, only for top of bank. It certainly isn't how FEMA does it in my experience (albeit limited). Theoretically, flow in the channel is deep, and the velocities are higher. With the section you have, I would put the bank stations at the dots, not the Xs. There is plenty of depth for high velocities and channel flow between the X and the dot. However, if That's How We Do Things Around Here, it's an uphill battle. They will want consistency across their models.
RE: HEC-RAS Bank Stations
RE: HEC-RAS Bank Stations
It is more of a "that's how we do things" and I know it was an unwinnable battle. Said agency has been overly fixated on OHWM and Bankfull for some time. Fortunately, it was for a small section of the roadside ditch just to show the effects of a proposed culvert, and not a larger scale project where I'm establishing BFEs, so whichever bank stations I used in the proposed was also going to be reflected in the existing. I gave the reviewer what they wanted, along with a little explanation of conveyance and how it was lowering WSELs by moving the stations in.
RE: HEC-RAS Bank Stations
As I recall, the default setting in HECRAS computes the average conveyance for each segment of cross section (LOB, MCH, ROB). Unlike HEC2 where it computed the conveyance between each ground point then averaged for the water surface limits. My thought is that if you have segmented Manning's through the main channel it will average the conveyance based on the distributed Manning's and when you squeez the bank stations your roughness/conveyance changed. Is there still the trigger to compute conveyance like HEC2 in the latest version of HECRAS?
My thought is that bank stations should be geometrical and not dependant on the OHWM or any given water surface. It is important to identify these differences to correctly model the hydraulics of any channel.