Lidar-Measured Bernoulli Response to Tidal Flow Over a Sill

An airborne scanning topographic lidar is shown to detect a local sea-level change induced by tidally forced flow over a 60-m-deep estuarine sill. As the flow speeds up over the sill, the sea level can be expected to decrease to conserve total flow energy according to the Bernoulli equation. The measurements resolve this decrease, showing a drop in sea level over the sill crest of about 0.1 m near the start of flood tide, increasing to about 0.25 m an hour later. These values are shown to be consistent with the Bernoulli effect, but only if a fraction (about 60%) of the upstream water column flows over the sill. By inference, the remaining fraction must deflect horizontally around the sill to flow through an adjacent channel, an effect consistent with laboratory studies of stratified flow past a three-dimensional (3-D) barrier with a gap. The lidar measurements, therefore, provide some insight into the 3-D flow response.