The National Academies Press

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From page 99... ... 99 CHAPTER 5 DESIGN CRITERIA AND OTHER PROJECT CONSIDERATIONS Other factors which enter into the decision process of an engineer when selecting a numerical model include potential design applications that the engineer may examine in conjunction with the hydraulic modeling, as well as resource related and other considerations. Design considerations may include a type of study (e.g., a FEMA "norise" study, a FHWA scour evaluation, etc.) Read the entire page →
From page 100... ... 100 For example, HEC-23 (Lagasse et al., 2001) contains the standard for design of rock riprap at abutments. Read the entire page →
From page 101... ... 101 -100% -50% 0% 50% 100% 150% 200% 250% 300% -100% -75% -50% -25% 0% 25% 50% 75% 100% Percentage Change in Velocity P er ce nt ag e C ha ng e in D 50 , W 50 D50 W50 Figure 51 Ishbash Equation Sensitivity to Velocity Read the entire page →
From page 102... ... 102 Armor Units HEC-23 (Lagasse et al., 2001) provides design guidelines for protection comprised of armor units. Read the entire page →
From page 103... ... 103 -100% 0% 100% 200% 300% 400% 500% 600% 700% -150% -100% -50% 0% 50% 100% 150% Percentage Change in Velocity P er ce nt ag e C ha ng e in D u an d # of U ni ts /A re a Du #/Area Figure 52 Toskane Design Dependence on Velocity Read the entire page →
From page 104... ... 104 Concrete Blocks Articulating concrete block (ACB) provides an alternative to traditional riprap in many protection applications. Read the entire page →
From page 105... ... 105 -150% -100% -50% 0% 50% 100% 150% 200% 250% 300% 350% -150% -100% -50% 0% 50% 100% 150% Percentage Change in Velocity and Depth P er ce nt ag e C ha ng e in S he ar S tre ss Shear Stress Error as a Function of Velocity Shear Stress Error as a Function of Depth Figure 53 Shear Stress Sensitivity to Hydraulic Inputs Read the entire page →
From page 106... ... 106 -150% -100% -50% 0% 50% 100% 150% 200% 250% 300% 350% -150% -100% -50% 0% 50% 100% 150% Percentage Change in Velocity P er ce nt ag e C ha ng e in L ift & D ra g Fo rc es Figure 54 Lift and Drag Force Sensitivity to Velocity Read the entire page →
From page 107... ... 107 Abutment Scour Calculation According to the FHWA, calculation of abutment scour at a bridge site follows either the Froehlich equation or the HIRE equation. To demonstrate the sensitivity of these calculations to hydraulic inputs, the HIRE equation is examined. Read the entire page →
From page 108... ... 108 -120% -100% -80% -60% -40% -20% 0% 20% 40% 60% 80% 100% -150% -100% -50% 0% 50% 100% 150% Percentage Change in Velocity and Depth P er ce nt ag e C ha ng e in A bu tm en t S co ur D ep th % Error as a Function of Depth % Error as a Function of Velocity Figure 55 Abutment Scour Equations Sensitivity to Hydraulic Inputs Read the entire page →
From page 109... ... 109 From the above figure, it is important to note that equations that employ both depth and velocity present a unique problem in error analysis. In general, if the depth is over estimated, the velocity is under estimated. Read the entire page →
From page 110... ... 110 -100% -80% -60% -40% -20% 0% 20% 40% 60% -150% -100% -50% 0% 50% 100% 150% Percentage Change in Velocity and Depth P er ce nt ag e C ha ng e in S co ur D ep th % Error as a Function of Velocity % Error as a Function of Depth Figure 56 Pier Scour Sensitivity to Changes in Approach Flow Velocity and Depth Read the entire page →
From page 111... ... 111 One of the correction factors, K2, addresses angle of attack. This factor is a function of angle of attack via the following equation: 0 65 2 LK a . Read the entire page →
From page 112... ... 112 -80% -60% -40% -20% 0% 20% 40% 60% -150% -100% -50% 0% 50% 100% 150% Percentage Change in Angle of Attack P er ce nt ag e C ha ng e in S co ur D ep th 10 degrees 20 degrees 30 degrees 40 degrees Initial Angle of Attack Figure 57 Pier Scour Sensitivity to Angle of Attack for L/a = 8 Read the entire page →
From page 113... ... 113 FEMA "No-Rise" Studies Prior to construction of bridges near or over regulatory floodways, a hydraulic engineer is often asked to perform hydraulic modeling to ensure that the new construction will not impact pre-project base flood elevations. Based on this requirement accurate determination of water surface elevations in and around the bridge is paramount. Read the entire page →
From page 114... ... 114 with the model. Also relevant is the engineer's demonstrated performance with the model as rated by past project performance as well as internal and external QA/QC reviews. Read the entire page →

From page 99...

... 99 CHAPTER 5 DESIGN CRITERIA AND OTHER PROJECT CONSIDERATIONS Other factors which enter into the decision process of an engineer when selecting a numerical model include potential design applications that the engineer may examine in conjunction with the hydraulic modeling, as well as resource related and other considerations. Design considerations may include a type of study (e.g., a FEMA "norise" study, a FHWA scour evaluation, etc.)