Abstract
A wave friction coefficient applicable to short and long waves is theoretically derived for laminar and rough turbulent flow. This friction coefficient approaches the conventional wave friction factor for short waves with the increase of water depth and/or with the decrease of wave period. At another limit, it asymptotically approaches the friction law for steady current with the decrease of water depth and/or with the increase of wave period. A criterion is proposed to determine the condition that the bottom friction coefficient for steady flow can be used in place of that for waves.
