A defining feature of sand closures is the movement and subsequent loss of the construction material. The principle underpinning a sand closure relies on the production of more sand than what is lost during the process. Sand losses occur daily under average flow conditions through the closing gap, contingent upon the flow dynamics. In the context of "strength and load," the "strength" of a sand closure is represented by its production capacity, while the "load" is the resultant loss. A closure is deemed successful when the production exceeds the loss, leading to a gradual narrowing of the closing gap.
The production capacity, which includes a sufficiently large extraction site for the sand, must surpass the maximum anticipated loss during the closure operation. Consequently, the feasibility study for a (complete) sand closure must initially concentrate on identifying the phase associated with maximum losses. Employing hydraulic boundary conditions, the sand loss for each closure phase can be calculated and depicted graphically as illustrated. The horizontal axis in the diagram represents the closing gap's size, indicating that the depicted capacity is insufficient for a sand closure under these conditions.
A sand closure becomes viable if sufficient sand production can be sustained near the closure gap to overcome the phase with the highest losses. The essential criterion is that the average tidal loss remains lower than the production. However, considerable uncertainties exist in both the calculated losses and the anticipated production, necessitating careful attention. The loss curve, as a function of the closing gap area, typically exhibits a single peak. The maximal loss is usually found when the closing gap area is between 0 and 30% of its initial size. Hence, initial loss calculations can be restricted to this range of closing gap sizes.
Interestingly, the peak sand loss does not coincide with the near completion of the closure gap. Despite potentially high flow velocities, the eroded width of the closing hole is minimal, thus keeping overall sand losses low. Hydraulic boundary conditions can be determined using a storage/area approach.
In general, sand closures are theoretically feasible for maximum flow velocities up to approximately 2.0 to 2.5 m/s. Beyond these velocities, achieving a sand closure becomes virtually impossible due to the resulting flow rates, which are influenced by the reference flow rate U and the discharge coefficient μ. The discharge coefficient μ is affected by both friction and deceleration losses within the closing gap, with friction losses being notably significant due to the large dimensions of the sand dams. Consequently, the choice of gradient measurement distance significantly impacts the discharge coefficient, which exhibits considerable variability. However, this variability diminishes during the crucial final phase of the closure, where a value of 0.9 is recommended as a reasonable upper limit for the discharge coefficient. The actual flow velocity within the closing gap is determined by applying the storage area approach, adjusted by the discharge coefficient.