CS20B - Concurrent Session 20B: An Alternate Approach for Developing Design Waves for Establishing Freeboard Requirements for Perched Reservoirs Exposed to Extreme Winds

As off-channel reservoirs grow in number, it is increasingly common to construct perched reservoirs surrounded partially or wholly by a ring dam. The cost of these above-ground reservoirs is sensitive to the design freeboard between the reservoir surface and the dam crest. Raising the dam crest widens the dam base and swells the embankment volume. Wind-generated waves from extreme winds, such as from hurricanes, can be the primary driver of freeboard requirements to limit wave overtopping.

Traditional approaches to calculating design wave conditions have limitations when applied to above-ground reservoirs subject to extreme winds. Windspeed adjustments to represent overwater wind conditions and wave calculations using empirical formulas or standard numerical models require engineering judgement and conservative assumptions that can unnecessarily increase capital costs.

Due to uncertainties in traditional methods, land-to-water windspeed adjustments recommended in the U.S. Army Corps of Engineers’ Coastal Engineering Manual, U.S. Bureau of Reclamation Design Standards, and other regulatory guidance documents differ significantly for fetch lengths under 10 miles. Moreover, their recommended adjustments do not capture wind-field changes due to sheltering by the embankment and elevation of the perched water surface above the surrounding natural ground. Additionally, recent advancements measuring wind profiles in hurricanes suggest a limit or even a significant reduction in the drag coefficient between the wind and water at extreme wind speeds. This is not generally represented in standard numerical wave growth models and is not reflected in empirical equations, which are largely based on more moderate wind windspeeds.

This paper describes an alternative approach to developing design waves for perched reservoirs with extreme winds. The approach can be generalized to all reservoirs. We describe how to leverage computational fluid dynamics (CFD) to model changes in the wind field over the surface of the reservoir and provide a wind map for input to a two-dimensional wave growth model. Additionally, we recommend capping the drag coefficient in wave growth models based on a review of available data collected under extreme wind conditions.

Results compared and contrasted with more conventional approaches demonstrate how advanced numerical modeling tools can provide conservative design waves with less uncertainty than standard approaches and potentially lower construction costs.