Challenges of Modeling a Tidal Influence Conveyance System, a Seattle, Washington Case Study

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Hydraulic modeling of a drainage basin usually involves system representation of a network that flows either by gravity downhill or a force main via an outfall to a receiving water body or a treatment plant. And calibration of such a system is usually straight forward. Occasionally, engineers encounter a “City of New Orleans” type of conveyance system, where most of the storm drain network is heavily influenced by tides or in most cases below sea level.

The challenge in this instance is to avoid flooding from interior runoff during a high tide cycle when gravity drainage is impossible even when there are tide gates within the storm drain system to prevent reverse flow of river water into the basin. Therefore, drainage improvements to reduce local flooding must deal with three distinct types of events namely; direct tidal flooding, conveyance of peak flows and interior flooding which produce local flooding and drainage complaints in the basin. Model calibration of such a basin become a challenge as engineers consider matching peak flow, volume and high tide simultaneously in order determine design return period and acceptable level of service. In addition, Seattle Public Utilities (SPU) considers the triple bottom line strategy of social, economic and environmental impacts to the execution of its projects.

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