Flood events come from periods of high precipitiation and favourable soil moisture conditions. Based on the basin topography, land cover and precipitation distribution, flood events can be conceptualized as the cummulative sum of a series of peak flows from different affluent rivers.

Flood management involve the prediction of river water elevation and velocitiy from extreme precipitation events. This tutorial shows the procedure to build a unsteady (dynamic) flow simulation of two peak flows with different hydrograms. The channel network configuration for the area of study consists of one main river and a affluent river. Peak flow in the main river is 10 hours ahead of the peak flow in the affluent river.

Three boundary condition have been implemented to represent two inflows and one outlet. Unsteady flow simulations in HEC-RAS requires a mesh; this flow case uses a 4m x 4m mesh. Model is simulated every minute and output is available every 20 minutes. Simulation time is about 3 minutes on a 4 core CPU.

Results from a unsteady flow simulation in HEC-RAS are the water surface, water depth, flow velocity and preferential flow paths. This set output data is important for the determination of river floodpains, the vulnerability assessment of nearby infraestructure and the design of flood barriers.