In some locations, regional subsidence drops the land elevation relative to sea level. Regional subsidence can complicate mobile bed calculations for systems where the river datum is changing relative to the tail water. Therefore, HEC-RAS includes a Subsidence feature. Users can define constant subsidence rates for each cross section. HEC-RAS will lower all the cross-section nodes (inside and outside of the movable bed limits) at this rate over the duration of the simulation.

Modeling Note: Using Subsidence with Fixed Bed Models

HEC-RAS only includes subsidence in the sediment model. But users sometime want to apply subsidence for fixed-bed hydraulic models. Modelers can apply subsidence to a hydraulic model by creating a "dummy" sediment file with simple, synthetic, sediment data, and then set all the cross sections to Pass Through Nodes (see next section). This will signal that HEC-RAS should adjust cross sections due to subsidence but not erosion or deposition.