The piano key (PK) weir is a cost-effective structure for flood discharge. Its typical layout comprises a rectangularly cranked crest in planform with up- and downstream overhangs. With the intention to enhance its hydraulic efficiency, the conventional weir is improved. The sloping floor of each key is modified with a downward semi-circle in the cross-section; each overhanging apex is thus assigned an elliptical crest. Thus, the developed crest length of the resulting weir becomes considerably extended. Experiments are performed to compare the hydraulic behaviors of the improved weir with a reference one. The models are 3D-printed to attain high manufacture precision. For the model dimensions chosen in the study, the developed crest is ~36% longer. The study demonstrates that the improvements in geometry lead to appreciably enhanced flow discharge capacity. Within the hydraulic range examined, the augment in flow discharge varies within a range from 30% to 53%. In terms of both discharge capacity and flow patterns, the improved weir clearly outperforms the conventional one. The elliptical overhang apexes noticeably extend the developed crest length. The streamlined upstream overhang without singularity and the lowered inlet key floor reduce the entrance energy loss and improve the inflow to the inlet key and the flow over the crest. The lowered floor also gives rise to extra water volume, which administers to the flow motion towards the crest. For the outlet key, its lowered floor facilitates the outflow and alleviates the liableness of local submergence at high discharges. If the footprint for spillway construction is limited or the increase in the reservoir water stage must be controlled, the use of a more effective PK weir for flood discharge has significant engineering implications.

The updated flood guidelines in Sweden have led to higher design discharges for many existing dams. While the primary function of a spillway chute is to convey floodwater, roughness appurtenances are proposed for installation along the chute. The aim is to dissipate an extra portion of the flow’s energy before release into the tailwater. One straight and three labyrinth roughness configurations are designed and manufactured. Their effectiveness is assessed through model tests. The roughness leads to an increase in water depth and induces an undulating streamwise water-surface profile. Due to their lateral interaction with the flow, the labyrinth shapes exhibit less distinct contours of surface unevenness than the straight one. With an increasing water depth, the free surface becomes gradually smeared out. For all the shapes, the roughness effect on the surface flow almost disappears if the water depth exceeds 6.5–7.0 times the roughness height. Compared to the smooth chute, the straight elements augment the energy loss by a factor of 1.9–3.8; the labyrinth configurations outperform the straight ones by 16–35% more energy dissipation. The differences among the triangular, trapezoidal, and rectangular shapes are, however, minor. Introducing chute roughness is a complementary measure. If the chute is sufficiently long, an adequate number of roughness rows could replace the function of a stilling basin.