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6 o Assumes that flow continues to be somewhat uniform. o Erosion is estimated based on effective and critical stresses, and bond strength of underlying soils. o Critical stress is a function of clay and density properties for soils and particle size for rock.  Phase 3: Headcut Advance and Deepening o Flow is turbulent. o Headcutting is considered in two parts: downward movement and headward movement o Rate of headcut migration is a function of the material strength (erodibility index Kh) and hydraulic power being dissipated. o Numerous layers of material require determination of a representative value using a depth-weighted log averaging scheme. Figure 3 – SITES Output of Predicted Erosion Source: SITES 2005 Water Resources Site Analysis Computer Program User Guide The SITES software is available for public use. NRCS, ARS and Kansas State University have developed WinDAM B which can incorporate data from SITES into analysis of full breach development. Hydraulic Models There are a number of hydraulic models that could be used to help assess the spillway erosion potential, from the popular one-dimensional model HEC-RAS (USACE HEC), to the two-dimensional models such as MIKE-21 (Delft Hydraulics Institute) and RiverFlow2D (Hydronia), to the more complex three dimensional Computational Fluid Dynamics (CFD) models. CFD is becoming a popular and powerful tool for evaluating spillway erosion potential in recent years because of the increasing accuracy of CFD against prototype measurements, ease of use, and dramatic increase in computer processing speed. CFD programs, such as FLOW-3D, FLUENT, CFX, OpenFOAM, STAR-CD, and others, simulate hydrodynamic characteristics of flow, such as velocity, pressure, shear stress, etc., over the spillway and further downstream in three-dimensions and thus provide more detailed information that can be used to assist the evaluation of spillway erosion potential. Some CFD programs, such as FLOW-3D, also have sediment scour modules that could be used to evaluate sediment erosion. Typical outputs from CFD model include flow velocity, dynamic pressure, bed shear stress, shear velocity, turbulence energy, etc. The stream power used in the Annandale Method could then be easily calculated based on the results from the CFD, using equations such as the following (Annandale, 2010): P =7.853 ρ (√τo ρ ) 3 where P = stream power in w/m2  = fluid macro density in kg/m3, 1000 kg/m3 for clear water o = bed shear stress in N/m2. CFD is widely used in spillway design to help identify alternatives that could minimize adverse hydraulic conditions leading to potential erosion or other undesirable hydraulic conditions. However, the current cost of the analysis may be prohibitive for small projects. Figure 4 – CFD Model Estimated Velocities