Lateral migration and bank erosion in a saltmarsh tidal channel in San Francisco Bay, California

Saltmarsh tidal channels have often been recognized as being stable landsca pe features, despite highly sinuous planforms, severely undercut banks, and high rates of bank erosion. In an effort to solve this paradox, a saltmars h tidal channel in the San Francisco Bay was monitored from March 1995 to M arch 1996. The short-term rate of bank erosion was measured using erosion p ins and found to be 57 +/- 10 mm yr(-1) on the outside banks of meander ben ds. In addition, a long-term maximum lateral migration rate of 23 +/- 23 mm yr(-1) was estimated from aerial photos, producing a dimensionless channel migration rate (defined as the rate of migration divided by channel width) of 0.5% yr(-1). The difference in the rates of lateral migration and bank erosion is attributed to the persistence of failed bank material (slump blo cks) in the channel. The slump blocks induce sedimentation, protect the ban ks, and prevent further bank erosion. A published stability analysis method for undercut banks is applied to determine a maximum overhanging width. Us ing the measured compressive and tensile strengths of rooted bank material, 16.55 +/- 1.16 Wa and 2.93 +/- 0.71 kPa, respectively, the maximum width o f an undercut bank is calculated to be 0.69 m. The average width of slump b locks measured in the field is 0.67 +/- 0.25 m. A simple numerical model pr edicting the rate of lateral migration is derived using the results from th e stability analysis and data from sedimentation and erosion pins inserted throughout the channel. This model accurately predicts a rate of 23 +/- 3 m m yr(-1).