Freeboard Allowances for Chute Spillways

Allowances must be made for the physical phenomena which cannot be modelled, such as wave action, air bulking, splash and spray; the simplifications of behaviour implicit in the calculations; and potential inaccuracies in assumed project parameters. These allowances are covered by the freeboard which is added to the water surface profile and should be sufficient to prevent overtopping of the walls at the design flow. In establishing the profile of the top of the walls the designer will include a safety factor which depends on the probability of the design flow, the consequences of overtopping and the confidence with which the maximum water surface profile can be predicted. A literature review was undertaken to establish the current state of the art for calculating freeboard allowances. Water surface profiles for the chutes, determined both analytically and by hydraulic model testing, have been compared. Also observed prototype behaviour has been compared to analytically derived profiles to profiles obtained from hydraulic models. Study results are discussed.