Climate change is causing the global sea level to rise. Research and discussion of the effects of sea level rise are often focused along coastlines. However, the effects of higher water level and changing morphodynamics can reach far inland via rivers. This study uses a one-dimensional numerical model to analyze bed level response to sea level rise. The model simulates 100 years of steady sea level rise on a 1000 km long, fixed width channel. Sea level rise creates a backwater curve which grows in the upstream and vertical directions. The transient response of the channel bed is an aggradation wave the grows in the upstream and vertical directions. We studied different cases which vary in sediment flux, flow discharge, grain size, and rate of sea level rise and found changes in the rate of growth of the aggradation in both directions. All runs start in an equilibrium state and run for 100 years.

This study finds a close relationship between the equilibrium slope and depth of the channel and the shape of the backwater curve. This relationship drives the aggradation patterns. For example, for the same amount of sea level rise, a flat, deep channel has a longer backwater curve with a smaller relative increase in depth than that of a steeper, shallower channel. The backwater curve drives the aggradation patters, such that the flat, deep channel then has aggradation over a longer reach, and a smaller increase in bed level than the steeper shallower channel.

With the cases modeled in this study, three general trends in aggradation rates emerge: (1) an aggradation mound that grows quickly upstream, with slower increase in bed level as found in flatter, deeper channels; (2) a faster increase in bed level with slower upstream growth, found in cases with steeper slope and shallower depths; and (3) faster growth in both bed level and upstream direction caused by an increased rate of sea level rise.

Since natural channels are often complex with multiple sources of discharge inputs, a tributary case is also
included. Starting from equilibrium state and applying sea level rise to the downstream boundary, the model
shows aggradation waves in the regions downstream and upstream of the confluence, starting from the downstream boundary and the confluence. There is also degradation just downstream of the confluence. The scour hole grows at first, in depth and the downstream direction then reduces. The aggradation moving upstream intersects with the degradation moving downstream and fills in the scour hole. In some cases, we see the scour fills in within the 100 year time frame, resulting in a net increase in bed level. In a tributary system, the risk of scour is greatest for tributaries with high flow discharge or low sediment flux.

The transient response of the bed level to sea level rise is aggradation. As the water level continues to rise, the bed level is expected to do the same, but at a lower rate. In this study, the nominal rate of sea level rise is 10 mm/yr. The fastest rate of bed level rise in the model results is less than 6mm/yr, creating an ever increasing water depth. This is beneficial for shipping and navigation in the channel, which would not require dredging of the aggradated material. However, the reduction in bankfull volume with rising water levels is dangerous for flood control.