The unsteady module of HEC-RAS (UNET-based) will simulate floodplain storage as will SWMM.

With HEC-RAS you have several options to simulate the braided channels

1) as distinct river reaches - This approach requires a set of cross-sections for each channel. Advantages of this approach include: (1) the flow split will be calculated automatically and (2) the length of channel vs length of overbank will be accurate. Disadvantages include: (1) the HGL will only match at the upstream and downstream ends of the wetland - where you have a single-thread stream and (2) it may be difficult to accurately simulate storage in the wetland - you will have 3 sets of cross-sections through the wetland and may have gaps or overlaps in the floodplain.

2) as multiple topographic lows within a single river reach - This approach requires a single set of cross-sections through the wetland. Advantages include: (1) the HGL will match at all cross-sections through the wetlands. Disadvantages include: (1) If the channels are very sinuous, this choice will not accurately simulate channel length vs overbank length, (2) HEC-RAS calculates Q-channel, Q-left overbank, Q-right overbank. You can "trick" the model into giving you the flow splits by using ineffective flow areas (on floodplain surfaces) and choosing bank stations to place the left and right channels onto the left and right floodplain.

There may be other approaches for simulating braided channels with HEC-RAS or other model. I've tried the above and neither is are perfect. Depends on what you really want to get from the model for design.

When designing the channels it will be important to consider what happens as the hydrograph rises and falls - Specifically, how will flow exit the channels, move across the floodplain, and reenter the channels? Braided channels are susceptible to headcuts and avulsions. Is this acceptable for your design? If not, consider how will you minimize potential for occurrence in your design?

Just some food for thought. Good Luck!
Janet Corsale, P.E.
Inter-Fluve, Inc.