Study of Labyrinth Spillways with Curved Planform

A labyrinth spillway is an overflow spillway to regulate and control flow in canals, rivers and reservoirs. The main hypothesis for the development of such a spillway is to increase the discharge per unit width of structure for a given headwater. This type of structure is often an efficient alternative to a gated-spillway type where either the increase of the flood-passage capacity or the control of the water surface upstream is concerned. This study was aimed to investigate the hydraulic performance of labyrinth spillways of general trapezoidal planform with simple curved apexes. In the experimental work, twelve spillway models with double cycles were considered using three different curved apexes (R/w= 0.15, 0.2, 0.25), each with four different crest heights (w/P= 1.5, 2, 3, 4). Based on the cited recommendations, the length magnification was set to a constant ratio of (l/w= 3) the crest shape was to be of a semi-circular form with simple radius (r= 15 mm) and the spillway walls were vertical with the thickness of T= 2r. An intensive experiment was carried out over a wide range of flows, providing 720 flow data ranging from free flow to submerged flow conditions. 1D flow equation was presented using combined mathematical and dimensional analysis. A coefficient of discharge, Cd, was introduced to represent the influence of the effective geometric and hydraulic parameters on the flow capacity over the spillway. Modular limit was also controlled to see whether the flow over the spillway would be submerged. The results of the study indicate that the modified curved planform of the spillway apexes with consistent divergence in the downstream channel introduces a significant improvement in the flow efficiency over the labyrinth spillways. Spillways with narrower curved apexes (i.e. R/w≤ 0.2), and with the vertical-aspect ratio of (2≤w/P<3) provide more stable and higher hydraulic performance than any other labyrinth planforms over a wide range of flows (i.e. 0.1<h0</h/P<0.6). In terms of the flow capacity, the proposed spillway model is shown to be more efficient than other zig-zag planforms (i.e. triangular and trapezoidal shapes) with an identical crest length.