Skew bridges and multiple bridges

[Gengan]

a) Skew bridges
I am currently modelling a river with a very wide flat floodplain. The proposed structure and road embankments cross the direction of flow at a significant skew angle due to various factors affecting the site. How can this structure be modelled in RiverCAD as the general requirement is that structures cross perpendicular to teh flow direction.

b) When modelling multiple bridges, is it required to establish the in-effective flow areas as for single bridges? What is the difference between stagnation points and inneffective flow areas?

[Chris Maeder]

Regarding skewed bridges, you want to make certain you cut your cross section perpendicular to the flow so that you accurately capture the flow area. Note that there might be additional complexities, such as ineffective flow areas as the flow begins to intrain itself through the bridge opening or as it expands after exiting the bridge opening. Therefore, you may need to add additional transitional cross sections just upstream and downstream of the bridge structure in order to define the ineffective flow areas.

When defining multiple bridge openings, you need to define the area between the openings as being ineffective. In that way, HEC-RAS will know that there is no conveyance within this area. However, you need to assist HEC-RAS so that it can determine how much flow to appropriate for each opening. Therefore, you need to place your "best guess" at where the stagnation point. Note that HEC-RAS used to be able to iterate and determine where the stagnation point was if you allowed the bridge opening stagnation stations to overlap. However, it seems that the newest HEC-RAS engine 3.1.3 does not work correctly with this.

Sincerely,

Chris E. Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com

[Gengan]

Chris

Thank you for your speedy replies. It is much appreciated.

With regard to defining the stagnation points for multiple structures, can this be done by interpolating ineffective flow boundaries on cross sections away from the structure (both upstream and downstream) until these boundaries for the individual structures intersect?