An Experimental Study of Tidal Bores and Positive Surges: Hydrodynamics and Turbulence of the Bore Front

A positive surge results from a sudden change in flow that increases the depth. Positive surges are commonly observed in man-made channels while some estuaries may be subjected to a positive surge process called tidal bore during flood tides. The aim of this study is to detail the hydrodynamic and turbulence characteristics of tidal bores that were rarely studied under controlled flow conditions. New experiments were conducted with positive surges in a large channel (L = 12 m, W = 0.5 m). Detailed turbulence measurements were performed with high-temporal resolution (50 Hz) using side-looking acoustic Doppler velocimetry and non-intrusive free-surface measurement devices. The experiments were designed to study a range of positive surges with a minimum number of dependant variables. Most tests were conducted with a horizontal bed slope, a constant flow rate (Q = 0.040 m3/s) and uncontrolled flow conditions. The only dependant variable was the downstream gate opening after closure. Two main types of positive surge were observed. For surge Froude numbers Fr less than 1.7, the bore was an undular surge. For Fr 1.7), a weak (breaking) surge was observed and the surge front had a marked roller. Detailed turbulent measurements were conducted in the initial flow at one location. Instantaneous velocity measurements beneath surges showed a marked effect of the surge passage. Streamwise velocities were characterised by a rapid flow deceleration at all vertical elevations, and some flow reversal were measured next to the bed in the weak surge flow. Large fluctuations of transverse velocities were recorded beneath the surges. Turbulent stresses were deduced from high-pass filtered data. The results showed large normal and tangential Reynolds stresses beneath the surge front. A comparison between undular and weak surge data suggested two main mechanisms of bed scour and scalar transport. In weak surge flows, the data showed some rapid flow separation beneath the surge front. In undular surges, maximum Reynolds stresses were observed beneath and just before each wave crest. Experimental results were compared with data sets obtained in stationary hydraulic jumps and in tidal bores. The latter field measurements and observations emphasised very-strong turbulence levels behind bore fronts that might be underestimated in small-size laboratory channels and in bore experiments propagating in stationary fluid. Overall the present study demonstrated the uniqueness of positive surges and some key differences with hydraulic jumps flows.