A critical concern along the Pajaro River and the Salsipuedes and Corralitos Creeks is the amount and maintenance of vegetation in the channel. Vegetation slows water flow in rivers and creeks, which has an impact on the levee or floodwall heights and distance of setback areas needed to contain a 100-year flood flow.
Vegetation and maintenance issues were key factors in the floods of 1995 and 1998. Although the 1998 floods released more water into the system than the 1995 floods, the levee system handled the water more effectively because of the significant channel clearing, vegetation removal, and levee work that occurred after the 1995 floods. This maintenance must remain a high priority to maintain the effectiveness of current or new levees and floodwalls.
Roughness refers to the amount and character of vegetation that is allowed to develop in a creek or river’s channel bed and along its banks and setback areas. Roughness is measured using a number called Manning’s co-efficient n. A high level of roughness indicates a relatively developed amount of vegetation (n = .08 to .10). A low level of roughness indicates that a channel is heavily maintained and kept clear of vegetation (n = .03 to .045).