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10/7/2015 10:29:45 AM
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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Research, Thesis, Technical Publications
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<br /> <br /> <br /> <br />6 <br />o Assumes that flow continues to be somewhat <br />uniform. <br />o Erosion is estimated based on effective and <br />critical stresses, and bond strength of <br />underlying soils. <br />o Critical stress is a function of clay and density <br />properties for soils and particle size for rock. <br /> Phase 3: Headcut Advance and Deepening <br />o Flow is turbulent. <br />o Headcutting is considered in two parts: <br />downward movement and headward <br />movement <br />o Rate of headcut migration is a function of the <br />material strength (erodibility index Kh) and <br />hydraulic power being dissipated. <br />o Numerous layers of material require <br />determination of a representative value using a <br />depth-weighted log averaging scheme. <br /> <br />Figure 3 – SITES Output of Predicted Erosion <br />Source: SITES 2005 Water Resources Site Analysis Computer <br />Program User Guide <br />The SITES software is available for public use. NRCS, <br />ARS and Kansas State University have developed <br />WinDAM B which can incorporate data from SITES into <br />analysis of full breach development. <br /> <br />Hydraulic Models <br />There are a number of hydraulic models that could be <br />used to help assess the spillway erosion potential, from <br />the popular one-dimensional model HEC-RAS (USACE <br />HEC), to the two-dimensional models such as MIKE-21 <br />(Delft Hydraulics Institute) and RiverFlow2D <br />(Hydronia), to the more complex three dimensional <br />Computational Fluid Dynamics (CFD) models. CFD is <br />becoming a popular and powerful tool for evaluating <br />spillway erosion potential in recent years because of <br />the increasing accuracy of CFD against prototype <br />measurements, ease of use, and dramatic increase in <br />computer processing speed. CFD programs, such as <br />FLOW-3D, FLUENT, CFX, OpenFOAM, STAR-CD, and <br />others, simulate hydrodynamic characteristics of flow, <br />such as velocity, pressure, shear stress, etc., over the <br />spillway and further downstream in three-dimensions <br />and thus provide more detailed information that can <br />be used to assist the evaluation of spillway erosion <br />potential. Some CFD programs, such as FLOW-3D, also <br />have sediment scour modules that could be used to <br />evaluate sediment erosion. <br />Typical outputs from CFD model include flow velocity, <br />dynamic pressure, bed shear stress, shear velocity, <br />turbulence energy, etc. The stream power used in the <br />Annandale Method could then be easily calculated <br />based on the results from the CFD, using equations <br />such as the following (Annandale, 2010): <br />P =7.853 ρ (√τo <br />ρ ) <br />3 <br /> <br />where P = stream power in w/m2 <br /> = fluid macro density in kg/m3, 1000 kg/m3 for <br />clear water <br />o = bed shear stress in N/m2. <br />CFD is widely used in spillway design to help identify <br />alternatives that could minimize adverse hydraulic <br />conditions leading to potential erosion or other <br />undesirable hydraulic conditions. However, the current <br />cost of the analysis may be prohibitive for small projects. <br /> <br />Figure 4 – CFD Model Estimated Velocities
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