An updated USACE approach to the evaluation of coastal total water levels for present and future flood risk analysis

Abstract

USACE coastal missions, operations, programs, and projects must be resilient to the full range of forseeable water levels, including extreme water levels, as well as the changing conditions that those water levels can induce at a project location. Water level range, magnitude of extremes, and frequency will all contribute to the stability, operation, and performance of a given project. Understanding which component of total water level or combination of components controls performance (and at what time scale) is critical to the design and evaluation of a project. Being aware of the different cross shore zones of total water level calculation and impacts informs exposure and impact assessments. Estimating future conditions over the project life recognizes that there will be both stationary and nonstationary contributions to the total water level (TWL) over time, necessitating the consideration of scenarios in project alternative development. An adaptive management approach provides a process for dealing with future uncertainties and involves developing plans that envisage a range of futures, incorporates ongoing monitoring, and permits transitions from one approach to another. Identifying thresholds beyond which stability or performance are adversely impacted is an important way to understand current and future vulnerability with respect to water levels, especially within the flood risk mission area. The USACE total water level Engineering Technical Letter (ETL) in development will guide how to evaluate total water levels for USACE coastal missions.