Turning the tide: creation of tidal inlet systems in experimental scale models

Abstract

Ever since 120 years people have been trying to create estuaries and tidal inlet systems in experiments, starting with Osborne Reynolds in the 1880’s. So far, no group of scientist succeeded creating a physical scale model of a dynamic tidal system. Low‐sediment mobility, ebb‐dominance and the emergence of unrealistic deep scour holes and large ripples as a result of smooth flow were major problems of earlier experiments. Our objective was to create a tidal inlet system in a laboratory physical scale model by applying a novel method to generate flow, to determine what the possible scale effects are, and whether equilibrium morphology exists, in case of a dynamic state. In our experiments morphology emerged that resembled nature with the existence of an ebb‐tidal delta, channel network and inlet channel(s). In contrast to the regular pumping of water, the entire basin (3.8m by 1.2m) was tilted vertically, over the short axis. This novel method of tilting the experimental set‐up caused an increased sediment mobility in the ebb‐ and flood phase because the bed surface slopes in downstream direction, both during flood and ebb phases. This reduced experimental time from weeks to days. Channels formed through backward erosion and started to bifurcate after retreating from the basin margin to form a network of up to four orders. Channels in the back‐barrier area were found to migrate cyclically. Resemblance with natural cyclic behaviour such as the number of inlet channels existed and typical ebb‐ and flood channels separated by sills were present. The use of light‐weight grains, an initial sediment basin level considerably higher than mean sea level, a start perturbation and non‐erodible barriers were required to create a dynamic tidal inlet system. The parameters most sensitive to the equilibrium morphology were sediment bed level, average water level, inlet width and tidal amplitude. Low average water level with respect to the sediment bed caused long sharply curved channels. These channels had few branches and persisted on the irregularly shaped ebb‐tidal delta. Higher average water levels increased channel migration rates, number of branches and a regularly shaped ebb‐tidal delta formed. Inlet width and tidal amplitude most importantly affected the shape and extent of the back‐barrier basin and ebb‐tidal delta, since they determined the strength of the flow. The effect of a rise in water level was that the back‐barrier area widened and lengthened, channels deepened and the number of channels increased. The ebb‐tidal delta built up in the vertical. In this report possible scale effects, equilibrium state of the system and how the experimental results can be translated to natural systems has been discussed.