Using Principles of Momentum Conservation to Develop a Coastal Storm Impulse Scale (COSI)

Coastal engineers have long recognized the need for a storm classification system that combines wave conditions, storm surge, and the length of time for the storm event. A new classification system has been developed that is based on the conservation of total horizontal momentum. This research concentrated on developing the theory and then applying that theory for a 10-year period of data from the Army Corps of Engineers Field Research Facility (FRF) in Duck, North Carolina. When fully developed, the Coastal Storm Impulse scale (COSI) may be applied to all previous Hurricane and Northeaster storm events along a coast and can be calculated during future events as the storm moves in space and time. Newton's second law of motion relates the forces, F, needed to cause an acceleration, a (dV/dt), of a given mass, m. A lesser-known form relates the impulse, I (F*dt), required to cause a change in the momentum (m*dV) of the mass. Radiation stress theory (Longuet-Higgins and Stewart, 1964) can be used to calculate the depthintegrated and time-averaged horizontal momentum flux of the waves. Hughes (2004) refined the theory and introduced the maximum wave momentum flux that needs only the wave height, period, and water depth in the computation. Uniform, open-channel flow theory can be used to calculate the depth-integrated horizontal pressure and flow-induced momentum of the current. The still-water depth (hydrostatic) component can be removed to leave only the storm surge momentum. Wave-induced momentum and surge-induced horizontal momentum are simply added together to produce the total storm momentum in units of force per unit crest width. When integrated over the storm duration, the storm impulse, Is, is the result for a particular location at the coast. Both a synthetic Hurricane and synthetic Northeaster were developed. These are similar to the scales used for the calibration of the Army Corps of Engineers Storm-Induced Beach Change (SBEACH) software (Larson, Kraus, and Byrnes, 1990). These synthetic storms were used ...