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