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<br /> <br /> <br /> <br />3 <br />Figure 1. Total rainfall-runoff process with losses <br />Flow Equations Energy (Modified Bernoulli) Equation [4]: Applied for <br />steady (i.e., constant and no attenuation) flow <br />applications in HEC-RAS. It is generally more <br />appropriate for gradually varied flow conditions as <br />compared to rapidly varied flow conditions (i.e., rapid <br />transitions between subcritical and supercritical flow). <br /> <br />Momentum Equation [4]: <br /> <br />Kinematic wave: The simplest form of the momentum <br />equation. It assumes that flow is steady, uniform (i.e., <br />constant flow depth and velocity for a given cross <br />section and channel slope) and gravity forces and <br />friction forces balance each other (i.e., friction slope is <br />equal to the channel bed slope). This applies to <br />uniform channels with steep slopes and no backwater <br />effects. This is the only version of the momentum <br />equation available in HEC-HMS. If backwater effects <br />are important, HEC-RAS should be used. <br />Diffusion wave: This form of the momentum equation <br />incorporates pressure forces in addition to gravity and <br />frictional forces. This is the default equation used in 2D <br />HEC-RAS analyses and is generally suitable for most <br />applications. However, in complex flow situations, the <br />full dynamic wave equation could be more <br />appropriate. <br />Dynamic wave: This is the most accurate equation <br />available in HEC-RAS (both one- and two-dimensional) <br />and should be used when backwater effects are