# Surface Water Routing:Surface Water Routing

GSSHA uses similar two-step explicit finite volume schemes to route water for both 1-D channels and 2-D overland flow, where flows are computed based on heads, and volumes are updated based on the computed flows. Compared with more sophisticated implicit finite difference and finite element schemes, the algorithms used in GSSHA are simple. The friction slope between one grid cell and its neighbors is calculated as the difference in water surface elevations divided by the grid size. Compared with the kinematic wave approach, this diffusive wave approach allows GSSHA to route water through pits or depressions, and regions of adverse slope. The Manning formula is used to relate flow depth to discharge. Use of the Manning formula implies that the flow is both turbulent and that the roughness is not dependent on flow depth. Neither of these assumptions may be valid on the overland flow plane. While being simple, the method is powerful because it allows calculations to proceed when only portions of the stream network or watershed are flowing. This is an important attribute as rainfall may occur on only a portion of the watershed. Though simple, the methods used in GSSHA are very robust allowing model time steps in the range of 1 to 3 minutes (Downer, 2002a; Downer et al., 2000) and minimal processing of the DEM and channel thalweg elevations.

## GSSHA User's Manual

- 5
**Surface Water Routing**