The inner shelf is an important region in the context of cross-shelf exchange for two reasons. First, it provides the connection between the surface boundary layer and the rest of the water column, closing the cross-shelf circulation pattern (Figure 3). Second, it separates the mid-shelf from the surf zone and beach, consequently anything, such as an oil spill, moving from shelf waters to the beach, or vice versa, must traverse the inner shelf. Lentz [1994] recently examined observations from a mooring in 30 m of water, less than 1 km from shore, in the coastal upwelling region off northern California. The observations showed a relatively simple upwelling circulation with offshore flow near the surface and onshore flow in the lower two-thirds of the water column. This cross-shelf circulation is a very effective mechanism for cross-shelf exchange, as it can completely replace the water inshore of the 30-m isobath in about one day.
There must be a transition from a wind-driven shelf regime to a
wave-driven surf zone [see Holman, this volume], but very little is
known about where or how that transition occurs. Even at midshelf,
Xu and Bowen [1994] have argued that surface gravity
waves may drive significant cross-shelf circulations and hence should be
included in studies of shelf dynamics. Besides directly driving
shelf circulations, surface gravity waves also modify both the bottom
stress (as discussed above in section 3.2) and wind stress, which may have
important consequences over the inner shelf. For example,
Madsen et al. [1993], using bottom tripod measurements taken in 13 m of
water off North Carolina, estimated a wind drag coefficient, based on the
alongshore momentum balance, that was several times larger than typical
open ocean values (
versus 
).