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WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 3, ISSUE 3, JULY 2015
SPILLWAYS, DESIGN CONSIDERATIONS FOR OUTLET CONTROL, INSTRUMENTATION FOR SMALL EARTH DAMS
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<br /> <br /> <br /> <br />4 <br />Table 2: Erosion Potential Class <br />Spillway Characteristic Erosion Potential Class <br />AAAA AAA AA A <br />Vertical Consistency (ft) >6 6-2 2-0.25 <0.25 <br />Lateral Consistency (#) 1 2 >2 >2 <br />Tectonics: <br />Unit Orientation Related to <br />Flow Direction <br />Flat Dip <br />Toward Dip Parallel Dip Away <br />Rock Mass <br />Fracture Spacing (ft) >3 3-1 1-0.5 <0.5 <br />Particle Diameter (ft) 3-5 1-3 1-0.5 <0.5 <br />Fracture Size/Opening (in) <1/8 1/8-1/2 >1/2 Open/clean <br />Fracture Sets (No.) 2 2-3 >3 shattered <br /> <br />For each factor, an A rating is assigned and the number <br />of A’s (between 1 and 4) is averaged for the erosion <br />risk and the erosion potential. If the erosion risk <br />average is higher than the erosion potential average, it <br />is estimated that the spillway is likely to erode. If there <br />are multiple distinctly identifiable geologic units within <br />the spillway this process should be repeated for each <br />unit to identify the most critical. Refer to USACE <br />Technical Report REMR-GT-3 Supplement (1998) for a <br />more detailed description of the method. It is <br />important to note this analysis method is empirical and <br />engineering judgment is required to make any <br />decisions regarding the safety of a spillway. <br />Cohesionless Soil <br />If the spillway includes cohesionless materials with a <br />D50 larger than 4 inches, the curves developed by <br />Frizell et al. (1998) can be used to estimate the flow at <br />which erosion could occur. The data is based on the <br />slope (S) of the spillway, the D50 grain size, the <br />coefficient of uniformity (Cu = D60/D10) and the unit <br />discharge. <br /> <br />Unit Conversion 1 m3 = 35.3 ft3 <br />Figure 1 – Erosion Potential of Cohesionless Soil <br />Source: Best Practices Dam and Levee Safety Risk Analysis, Chapter 15 <br /> <br /> <br />Plotting on the line represents a 20 percent chance of <br />erosion beginning and below the line means an <br />increase in probability. These curves do not represent <br />the probability of breach, only of erosion, and the data <br />are based on testing with uniformly sized angular <br />riprap in ideal conditions. <br />Annandale <br />The analysis method developed by Annandale (1995 <br />and 2006) quantifies two properties: the erodibility <br />index (Kh) and stream power (P). The erodibility index <br />Kh represents the susceptibility of a material to erode <br />and is computed as follows: <br />Kh = Ms Kb Kd Js <br />Ms = Material strength number, relates to <br />unconfined compressive strength <br />Kb = Block or particle size, based on RQD/Jn <br />where Jn is the joint set number <br />Kd = Inter-particle bond shear strength, taken <br />as Jr/Ja (joint roughness/joint alteration) <br />Js = Relative shape and orientation of blocks, <br />ease with which water can penetrate <br />discontinuities and dislodge blocks (is equal to <br />1 for soils) <br /> <br />Values associated with the J variables can be obtained <br />from tables developed by Annandale and are available <br />in the USBR and USACE Best Practices Dam and Levee <br />Safety Risk Analysis (Chapter 15, tables 15-1 through <br />15-4). The stream power P represents the rate of <br />energy dissipation per unit of surface area and is <br />computed as follows: <br /> <br />
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