When using 2D areas in HEC-RAS, as you decrease cell size, you generally have to decrease computation interval to maintain stability. This is generally related to the amount of time it takes waves to travel through a grid cell. You can usually improve stability by increasing cell size or decreasing computation interval. The two pictures below show the difference in results and computation time between two models of widely varying cell size and computation interval. Which one do you like best?
|2-foot grid, 30-second interval. Runs in a few seconds.|
|2-foot grid, 0.5-second interval. Runs in 37 minutes.|
By carefully adding break lines to your 2D area, you can get good accuracy without going excessively small with your grid sze.