May, 1999 Low Flow Bridge Modeling Yarnell’s Pier Coefficient, K Semi-circular nose and tail Twin-cylinder piers with connecting diaphragm Twin-cylinder piers without diaphragm 1.05 90 degree triangular nose and tail 1.05 Square nose and tail Ten pile trestle bent Taken from Table 5.3 of Hydraulic Reference Manual (page 5-14) ġ2 Low Flow Bridge Modeling Class A Low Flow - WSPRO Should be used only when piers are the predominant obstruction ( such as a RR trestle) ġ1 Low Flow Bridge Modeling Yarnell’s Pier Coefficient, K
May, 1999 Low Flow Bridge Modeling Class A Low Flow - Yarnell Equation Based on 2,600 lab experiments on different pier shapes Requires entering pier shape coefficient, K Should only be used where majority of losses are due to piers. May, 1999 Low Flow Bridge Modeling CD Coefficients for Piers Circular Pier Elongated piers with semi circular ends Elliptical piers with 2:1 length to width 0.60 Elliptical piers with 4:1 length to width 0.32 Elliptical piers with 8:1 length to width 0.29 Square nose piers Triangular nose with 30 degree angle 1.00 Triangular nose with 60 degree angle 1.39 Triangular nose with 90 degree angle 1.60 Triangular nose with 120 degree angleđ.72 Taken from Table 5.2 of Hydraulic Reference Manual - page 5-13 ġ0 Low Flow Bridge Modeling Class A Low Flow - Yarnell Equation ĩ Low Flow Bridge Modeling CD Coefficients for Piers Usually accept program defaults Momentum is best for when piers and friction losses are predominant. Requires entering coefficient of drag for piers, CD Check “Options” menu under “Bridge & Culvert Data” window for momentum equation options.
May, 1999 Low Flow Bridge Modeling Class A Low Flow - Momentum Method Friction losses are external skin friction = wetted perimeter times length times shear stress.
Ĩ Low Flow Bridge Modeling Class A Low Flow - Momentum Method If piers are large part of obstruction to flow, this method is probably not appropriate. Energy losses are empirical coefficient times change in velocity head (expansion and contraction losses). May, 1999 Low Flow Bridge Modeling Class A Low Flow - Energy Method Friction losses are computed as length times average friction slope. ħ Low Flow Bridge Modeling Class A Low Flow - Energy Method You will see in the output from time to time. May, 1999 Bridge piers are small obstruction to flow, friction losses predominate - Energy, Momentum, or WSPRO Pier and friction losses predominate - Momentum Flow passes through critical depth in vicinity of bridge - Energy or Momentum Pier losses are dominant - Yarnell Supercritical flow without piers - Energy or Momentum Supercritical flow with piers - Momentum Again, not that important. Ħ Low Flow Bridge Hydraulics Appropriate Methods Yarnell - empirical formula developed to model effects of bridge piers. Developed for bridges that constrict wide floodplains with heavily vegetated overbank areas. FHWA WSPRO - energy based as well as some empirical attributes. Momentum - physically based, accounts for friction losses and geometry changes through bridge. May, 1999 Energy - physically based, accounts for friction losses and geometry changes through bridge, as well as losses due to flow transition & turbulence.
ĥ Low Flow Bridge Hydraulics 4 methods of modeling
May, 1999 Low Flow Bridge Modeling 3 Types of Flow Class A Low Flow - Subcritical Class B Low Flow - Passes through critical depth Class C Low Flow - Supercritical Again, not that important. Ĥ Low Flow Bridge Modeling 3 Types of Flow Ģ 2 Types of Bridges May, 1999 Low Flow - Flow where the water surface does not reach the low beam High Flow - Flow where the water surface reaches the deck or higher There are sub-types for both low and high flow Often both types of flow occur in single simulation with different profiles These are not that important to know - except you must specify analysis methods for each. Presentation on theme: "May, 1999 Bridges This module will cover bridges and how they are input into HEC-RAS."- Presentation transcript:ġ May, 1999 Bridges This module will cover bridges and how they are input into HEC-RAS.