2018 – Review of the Impacts of Dam Operations on Flood Hydrology

D Stephens, M Scorah and P Hill

There are many dams in Australia with appurtenant features such as spillway gates, large capacity outlet works, power stations and transfer tunnels. These features can play a significant role in how these dams are operated during flood events and allow for additional flexibility to implement flood mitigation activities such as pre-releases and surcharge depending on authorised operating procedures for the dam.

Typical practice in many dam flood hydrology studies has been to significantly simplify or even ignore the impacts of these features on the dam water level frequency curve. For example, it may have been assumed that spillway gates were either fully open or changed from fully closed to fully open in a uniform manner regardless of inflow rate. Whilst this approach significantly simplifies routing of floods through these storages, it may produce results which are inconsistent with the expected flood probability of the dam given its current operating procedures, especially for relatively frequent flood events. This is particularly critical for risk assessment where definition of the flood loading probabilities requires robust estimates of water level AEPs for all events.

In a number of recent studies, greater emphasis has been placed on detailed modelling of the effects of spillway gates and other outlet works on dam flood hydrology. This has required site-specific algorithms to be developed which incorporate the characteristics of the spillway gates or other features at each structure, as well as the flood operations procedures for the dam. This paper presents a number of case studies where explicit simulation of dam flood operations has had a significant impact on the resulting flood frequency curve and downstream flow rates and discusses the implications of that on dambreak modelling and risk assessment for those dams.