  We present laboratory experiments that investigate the structure and flow
characteristics of gravity currents travelling through an array of roughness
elements. The roughness elements are of comparable height to the gravity
current such that the current flows through the roughness array rather than
over it. The frontal velocity and density structure are measured as the current
transitions from flowing along a smooth bed to flowing through the roughness
array, and then back to a smooth bed. We find that, upon entering the roughness
array, the gravity current decelerates and the density structure changes from
the head and tail structure typical of smooth bed gravity currents, to a wedge
shape. A model is presented that explains the deceleration and change in shape
based on a dynamic balance between a pressure gradient within the current tail
and a drag force associated with individual roughness elements. This model
accurately predicts the deceleration of the gravity current, supporting the
proposed dynamic balance.
