Piano-key weirs can be used instead of classic rectangular side-weirs (CRSWs) to increase the discharge capacity of side weirs. So far, no research has been done on trapezoidal piano-key side weirs (TPKSWs) in a curved channel. This study examines the effect of using one or two cycles in TPKSWs on discharge capacity, having the same total width and upstream–downstream length. Dimensional analysis has been performed to determine the dimensionless parameters affecting the discharge coefficient related to the developed length (CdL) of TPKSWs in a curved channel. An empirical equation for CdL has been proposed based on the experimental results. There is a good agreement between estimated and measured data. Results showed that the discharge coefficient related to the total width of a TPKSW is 1.7 to 5.6 times higher than that of a CRSW. Also, the CdL of a one-cycle TPKSW is 1.4 to 2 times higher than that of a two-cycle TPKSW.