HEC-RAS Basic HECRAS Principles - PowerPoint Presentation

HEC-RAS Basic HECRAS Principles~Steady 1D Flow Jon Fripp NDCSMC 2016

Hydraulic Analysis Components What the program does when it runs steady, 1D simulationsModule 2: HEC-RAS – Basic PrinciplesWe want to be sure everyone is on the same page before we jump into HECRAS applications

A necessary discussion of theory necessitates some coffee

What about water... Incompressible fluidHigh tensile strengthNo shear strength- must increase or decrease its velocity and depth to adjust to the channel shape- allows it to be drawn smoothly along while accelerating- does not decelerate smoothly, results in standing waves, good canoeing, air entrainment, etcWater is treated as an ideal fluid that behaves according to mathematical/physical principles

A free surface Liquid surface is open to the atmosphereBoundary is not fixed by the physical boundaries of a closed conduit What about open channel flow... Being open channel does not make it simpler!

Q is flow V is velocityA is cross section areaVA = constantDischarge is expressed as Q = VASince the flow is incompressible, the product of the velocity and cross sectional area is a constant. (conservation of mass) Continuity Equation 10’ 5’ 10’ 1’ Velocity = 1 fps Velocity = 5 fps Q=VA=1 fps x 5’x 10’= 50 cfs Q=VA=5 fps x 1’x 10’= 50 cfs

Open Channel Flow - Controls Definition: A control is any feature of a channel for which a unique depth - discharge relationship occurs. Weirs/Spillways Structures Abrupt changes in slope or width Friction - over a distance

When in Balance Uniform or Normal Flow The gravitational forces that are pushing the flow along are in balance with the frictional forces exerted by the wetted perimeter that are retarding the flow. Gravity Friction If the flow is a function of the slope and boundary friction, how can we account for it?

Manning's Equation One of the most widely used to account for friction losses Area Wetted Perimeter (P) R = A/P

Manning’s n A bulk term, a function of grain size, roughness, irregularities, etc… Values have been suggested since turn of century (King 1918)

n=0.014 n=0.016n=0.018n=0.018 n=0.020 n=0.060 n=0.080 n=0.110 n=0.125 n=0.150 n=0.050 USGS -Water Supply Paper 1849 Guidance Available (calibrated photographs) SCS - Fasken, 1963

Can compute many of the parameters that we are interested in: VelocityTrial and error for depth, width, area, etc What can we do with Manning’s Equation?

So…why make things any more complicated?

W=100’ d=5’S=0.004n=0.035Q=3700 cfsn=0.03 to 0.04 13% to 17% d=4.5 to 5.5 ft 16% to 17% w =90 to 110 ft 11% S=0.003 to 0.005 12% to 13% All 40% to 70% How sensitive is the equation? It gets worse!

Limitations of a Normal Depth Computation: Constant Section - natural channel?Constant Roughness - overbank flow?Constant SlopeNo Obstructions - bridges, weirs, etc

F 16Hydraulic Jump Critical Subcritical Subcritical Supercritical Lynn Betts , IA NRCS Subcritical Hydraulic Jump Supercritical Subcritical Critical Tim McCabe, IA NRCS Both man made and natural channels Open Channel Flow is typically varied

So….in natural gradually varied flow channels…. Velocity and depth change from section to section. However, the energy and mass are conserved.Simply put – sections and slopes are not constant in natural channels, the flows are not uniform….

HEC-RAS uses the one dimensional energy equation with energy losses due to friction evaluated with Manning’s equation to compute water surface profiles. This is accomplished with an iterative computational procedure called the Standard Step Method.

(V 2/2g)2+Y2Z2+ =(V2/2g)1 + + + Y 1 Z 1 h e Water Surface Energy Grade Line Y 2 Y 1 h e ( V 2 /2g) 1 (V 2 /2g) 2 Z 2 Z 1 Datum Channel Bottom Energy Equation in Standard Step Method Start at a known point. Know Ws1, Q and V1 How many unknowns? Trial and error

HEC-RAS - Computation Procedure Assume water surface elevation at upstream/ downstream cross-section.Based on the assumed water surface elevation, determine the corresponding total conveyance and velocity headWith values from step 2, compute and solve equation for he.With values from steps 2 and 3, solve energy equation for WS2.Compare the computed value of WS2 with value assumed in step 1; repeat steps 1 through 5 until the values agree to within 0.01 feet, or the user-defined tolerance. Simply put – a trial and error approach

Energy Loss - important stuffLoss coefficients Used:Manning’s n values for friction lossvery significant to accuracy of computed profilecalibrate whenever data is availableContraction and expansion coefficients for X-Sectionsdue to losses associated with changes in X-Section areas and velocitiescontraction when velocity increases downstreamexpansion when velocity decreases downstreamBridge and culvert contraction & expansion loss coefficientssame as for X-Sections but usually larger values

Friction loss is evaluated as the product of the friction slope and the discharge weighted reach length Channel Conveyance

Friction Slopes in HEC-RAS Average Conveyance (HEC-RAS default) - best results for all profile types (M1, M2, etc.) Average Friction Slope - best results for M1 profiles Geometric Mean Friction Slope - used in USGS/FHWA WSPRO model Harmonic Mean Friction Slope - best results for M2 profiles In HECRAS, stand between sections and look half way up and down to the adjoining sections then average. In WSP2, look downstream while standing at an upstream section. WSP2 uses friction from upstream section for entire reach

Mild slope: normal depth above critical Steep slope: normal depth below criticalFlow Classification

Friction Slopes in HEC-RAS HEC-RAS has option to allow the program to select best friction slope equation to use based on profile type.

Other losses include: Contraction lossesExpansion losses C = contraction or expansion coefficient

Note 1: WSP2 uses the upstream section for the whole reach below it while HEC-RAS averages between the two X-sections. Note 2: WSP2 only uses LSf in older versions and has added C to its latest version using the “LOSS” card. Contraction and Expansion Energy Loss Coefficients

Expansion and Contraction Coefficients Contraction 0.0 0.1 0.3 0.6 Expansion 0.0 0.3 0.5 0.8 No transition loss Gradual transitionsTypical bridge sectionsAbrupt transitions Notes: maximum values are 1. Losses due to expansion are usually much greater than contraction. Losses from short abrupt transitions are larger than those from gradual changes.

How about hydraulic jumps? Water surface “jumps” upTypical below dams or obstructions Very high-energy loss/dissipation in the turbulence of the jump

A rapidly varying flow situation Going from subcritical to supercritical flow, or vice-versa is considered a rapidly varying flow situation.Energy equation is for gradually varied flow (would need to quantify internal energy losses)Can use empirical equationsCan use momentum equation

Momentum Equation Derived from Newton’s second law, F=maApply F = ma to the body of water enclosed by the upstream and downstream x-sections. Difference in pressure + weight of water - external friction = mass x acceleration

Momentum Equation 2 2( V / 2g ) + Y + Z = ( V / 2g) + Y + Z + hm 2 2 2 1 1 1The momentum and energy equations may be written similarly. Note that the loss term in the energy equation represents internal energy losses while the loss in the momentum equation (hm) represents losses due to external forces.In uniform flow, the internal and external losses are identical. In gradually varied flow, they are close.

HEC-RAS can use the Momentum Equation for Hydraulic jumpsHydraulic dropsLow flow hydraulics at bridgesStream junctions.Since the transition is short, the external energy losses (due to friction) are assumed to be zero

When using HEC-RAS to model a 1-Dimensional steady flow situation, the modeler is assuming… Flow is constant with respect to time (unsteady is an option but a different computational engine)Flow is gradually varied with distance (except at structures where momentum or empirical equations are used)Dominant flows in x direction ( 1-D, because total energy head is assumed to be the same for all points in a cross section)Channel slopes less than 1:10H ( because pressure head is assumed to be represented by the water depth measured vertically .) And the modeler is not concerned with super elevation in bends secondary currents

Unsteady Examples Dam breach routingEstuaries and BaysFlood waveothers...Natural streams are always unsteady - when can the unsteady component not be ignored?Later in this class we will talk about this in more detail

Two Dimensional Examples Estuaries and bays Breach into a wide floodplainJunctionsOthers…Later in this class we will talk about this in more detail

Other assumptions in HEC-RAS HEC-RAS is a fixed bed modelThe cross section is staticStructures are fixedHEC-6 (or HECRAS in sediment mode) allows for changes in the bed.HEC-RAS can not by itself reflect upstream watershed changes. HEC-RAS is a simplification of the natural, chaotic system.

Q: What is an appropriate model approach to use? A: A normal depth assumption would be sufficient. At least in the reach that can be seen in this photo…

Q: What is an appropriate model approach to use? A: In these photos, we can see a confluence, variable section and a bridge. To model different design flows a steady flow, 1-D approach would be good. Standard HECRAS would be suitable

Q: What is an appropriate model approach to use? Q1Q3A: If the modeler in interested in flood heights below the dam at this dam discharge, a HECRAS steady, 1D model could be used (if one knows the breach shape through the road). If the breach itself is to be modeled – then this is unsteady. If the modeler is interested in specifics of flooding through the water plant below the dam, it may be a 2D situation. Above and through the dam is a reservoir routing condition. Q1<Q2=Q3Q2

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