The influence of coarse woody debris on gravel beach geomorphology

Coarse woody debris (CWD) is an important component of fluvial systems affecting in-channel hydrology and sediment storage which in turn controls channel geometry and evolution. The role of coarse woody debris in coastal geomorphology is an emerging research field and this study investigates its role on mixed sand and gravel beach systems. A case-study approach is used whereby two field areas in Wellington, New Zealand, each with two distinct beaches with differing wood accumulations, were investigated through field surveying. It is found that on gravel beaches CWD is most important during storm events where it causes the berm crests to be 0.5–1.0 m higher than on unwooded beaches, as well as making the beachface twice as steep. During calm periods the berm appears to be inactive as non-storm waves can only rework the lower beachface. A review of the literature indicates that on sandy beaches CWD is most important in the post-storm recovery of beach systems. The same wave event that causes beach erosion also deposits the CWD. During calm periods the woody material is a trap for sand being transported along the beachface and therefore aids in re-establishing foredune systems. The differing sediment composition between sandy and gravelly beaches drives their morphological response to storm events and in turn determines how CWD interacts with the beach profile.

► Coarse woody debris is an important geomorphic agent on the coast.
► Two sets of wooded and unwooded gravel beaches are investigated.
► On gravel beaches wood steepens the beachface and raises berm height.
► Wood on gravel beaches acts as a geomorphic agent primarily during storms.
► On sandy beaches wood is important in post-storm profile recovery.