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5 P = γUhS γ = Unit weight of water U = Flow velocity h = Flow depth S = Slope Annandale developed an erosion threshold curve based on approximately 150 field observations from spillways and plunge pools. The curve is shown on Figure 2, along with confidence curves developed by Wibowo et al. (2005). As with any simplified analysis method, engineering judgment is required in using the curves and multiple conditions and assumptions should be considered. The data are particularly sensitive to the Kb value. However, the curves are helpful in providing a range of likelihood for erosion and progression of headcutting. Unit Conversion 1 kW/m2 = 0.093 kW/ft2 Figure 2 – Annandale Likelihood of Erosion Source: Best Practices Dam and Levee Safety Risk Analysis, Chapter 15 SITES The SITES spillway erosion analysis software was developed by the National Resource Conservation Service (NRCS), the Agricultural Research Service (ARS) and Kansas State University. It is a one-dimensional hydraulic simulation of flow through the spillway channel. It was developed based on lab testing and field data of headcutting in soil- and grass-lined spillways, but has also been applied to rock channels. The analysis estimates whether headcutting will occur and whether flow duration is long enough to deepen the headcut and advance upstream. The model assumes failure when the erosion reaches the spillway sill. The model represents a three-phase analysis as follows:  Phase 1: Surface Erosion o Flow persists long enough to initiate erosion and the flow concentrates at a location and removes vegetation. o The model can account for surface discontinuities. o Erosion is estimated based on effective stresses and bond strength of underlying soil. o If no vegetation exists, Phase 1 is negligible.  Phase 2: Concentrated Flow Erosion o Flow enlarges and deepens erosion Annandale Wibowo 1% Wibowo 99% Wibowo 50%