Experimental evaluation of discharge coefficient of the combined culvert-piano key weir

Increasing demand for water has challenged governments to manage water resources by optimizing and designing highly efficient hydraulic systems. One common water-management device is the weir which is used to modify the water-flow within rivers. Existing weirs are not typically responsive to water demand. Consequently, it would be advantageous if the water supply from these structures could be modified to meet fluctuating water demand. Therefore, new structures could replace existing structures or modifications could be made to current weirs to achieve this desired control. Candidate changes could include an extension of the weir crown or an opening up of the weir body to modify the flow. When the flow can pass over and under the structure, one of the proposed options is a combined culvert-weir, which also increases the sediment and flow rate. Choosing the appropriate overflow to achieve this goal is very important. In this research, the piano key weir has been used as the overflow of this combined structure due to its high performance. The innovative shape of a non-rectilinear weir, known as the Piano-Key weir PKW, increases the total effective crest length, escalating the discharge capacity of the weir. A PKW is a modified form of a labyrinth weir with the specific geometrical characteristics including inlet and outlet overhangs or inclined inlet and outlet keys floors, forming a new set of variables.

The experiments have been performed in hydraulic laboratory of Shahid Ashrafi Esfahani University. The tests were conducted in a rectangular channel with 10 m long, 0.6 m wide, and 0.75 m high. Models were built with galvanized sheet with 0.9 mm thickness. Laboratory models of the combined culvert-weir were examined by considering the varied height and length of the overflow, as well as the different openings for culverts. In addition, the performance of the culvert and piano key weir was investigated and compared with the combined structure. Experiments had been done in free flow conditions and in discharge range of 5 to 50 lit/s, for piano key weirs, culverts and combined culvert-weir structures. The geometry parameters of models are shown in Table (1).

The results indicated that in the PKW the discharge 25% and 12.5% is increased with %50 and 63% increasing in height and width of the weir respectively. Thus, the effect of the height on increasing the discharge of the weir is more considerable. In combined structures the effects of culvert opening are more than weir height. The results show that for a specific head, the discharge of combined structure is greater than the discharge of weir and culvert and the efficiency of combined structure is approximately 10% more than culvert and weir.The results showed that in piano key weirs, the discharge coefficient decreases with the dimensionless ratio of head to the weir height and in the combined structure, this trend is increasing. Therefore, in situations where the flow rate is high and it is not possible to use a high piano key weir, a combined culvert-weir is a good option.