Hec-Ras tutorial.pdf

7/27/2019 Hec-Ras tutorial.pdf

1/40

pag 1 de 40

Hctor Garca Rbade Subject: COMPUTATIONAL FLUID DYNAMICS I Written in October 2012

Universidade da Corua Master in Water Engineering

HEC-RAS tutorial

Index

11.-Introduction.112.-Equations and calculation procedure213.-Program features..314.-Practical example so as to simulate a river..415.-Same example with known downstream boundary conditions1516.-Same example with more spatial discretization2017.-Same example with an unsteady simulation of the river2418.-Same example with a bridge.2719.-Same example with different Mannings values37

1. Introduction

HEC-RAS (Hydrologic Engineering Centers River Analysis System) is public domain softwarewhich was developed by the Hydrologic Engineering Center of the U.S. Army Corps ofEngineers. This program evolved from the known HEC-2 incorporating a few improvements,such as the graphic interface, or the possibility of exchanging data with the GeographicInformation System ArcGIS through Hec-GeoRAS. The program has been so accepted that it isused in civil engineering projects nowadays.

As usual, there are some versions of the software because it is updated by the authorspermanently improving its features. The last one (version 4.1) is the first that incorporate a

water quality module. Therefore, values of temperature (T), dissolved oxygen (DO),carbonaceous BOD, organic nitrogen, ammonium nitrogen (N-NH4), nitrite nitrogen (N-NO2),nitrate nitrogen (N-NO3), nutrient parameters (atmospheric reaeration,), meteorology dataand more information is necessary together an initial condition of all the calculation variables.Below, the Water Quality Data window where this information is included can be seen:


2/40

pag 2 de 40



The version 3.1.3 (of May 2005) will be used because no problems have been detected withtheir use. On the other hand, only the hydrodynamics will be evaluated here. The version can bedownloaded from the internet in the HEC-RAS website, which is:http://www.hec.usace.army.mil/software/hec-ras/

2. Equations and calculation procedure

The Navier-Stokes equations that rule the behaviour of the fluids are formed by the massconservation equation (divergence free condition) and the momentum conservation equations(derived from the Newtons second law particularized for flows). The software has acomputational program that solves (through a finite difference method) the simplification ofthese equations after they are integrated both in depth and width. These one-dimensionalequations are usually known as the Saint-Venant equations. Then, as the Navier-Stokesequations, the equations gives values of velocity and depths and therefore leads tohydrodynamic models that study the movement of the free surface water through the physicfeatures of the bottom and the sources and sinks. According to the hypothesis, the accuracy ofthe solution will be as better as the longitudinal dimension is greater than the other twodimensions. Then, this is a numerical model usually applied for the simulation of rivers andchannels, where this condition is approximately verified. The model uses the Manningscoefficient to evaluate the energy losses and can be used in stationary or transitory simulations.

The software incorporates a lot of improvements that produces more adequate results than theresults provided by the simple one-dimensional model. In fact, the software has the possibilityof a 2D simulation of the surface flow (for one-dimensional flows) which is not based on theresolution of 2D equations but in empirical formulas. The results have been tested with

experimental data for a long time which does the model is very reliable.

In this way the model has an option denoted by flow distribution with which the DividedChannel Method is applied. This tool allows for the subdivision of each of the three existing

parts (main channel, left floodplain and right floodplain) in a determinate number of subsections(defined by the user) in which the mean velocity will be calculated. The calculation in eachsubsection is done with the next expression where Qi, ki, Ai, Riand niare the volumetric flowrate, the hydraulic conductivity, the area, the hydraulic radius and the Manning coefficient ineach subsection, and S0 is the longitudinal slope:

1/2

0i iQ k S with:

2/3i i

i

i

A R

k n

Then, HEC-RAS is a good option for the evaluation of the flood area, this is, the section and thepart of the floodplain which will be occupied for a certain volumetric flow rate. It will dependon the geometry of the bed, the slope and other factors. The problem is that the momentumtransference between the main channel and the floodplains is not considered.

In the case of modeling large areas of free surface flow to obtain the same type of results (withboth velocity components) with better accuracy, the RMA2 module integrated in the SMScomputational software is a good option. This is another software that belong to the U.S. Army

Corps of Engineers too and the RMA2 is an hydrodynamic numerical model that solves(through the Finite Element Method) the simplification of the Navier-Stokes equations after


3/40

pag 3 de 40



they are integrated in depth. These two-dimensional equations are usually known as the 2Ddepth averaged equations or shallow water equations and they need eddy viscosity coefficient tocharacterize the turbulence losses. The accuracy of the solution will be as better as thehorizontal dimensions are greater than the vertical dimension of the fluid (the movement is

mainly horizontal). This condition is usually satisfied in lakes and estuaries besides rivers.Then, this is a model usually applied in the simulation of flow around islands, flow under

bridges, flow in places where rivers are joined, flow in central pumping channels, coastal areas,estuaries, reservoirs, The model uses the Mannings coefficient to evaluate the energy lossesand can be used in stationary or transitory simulations.

This model would be more appropriate so as to know the lateral distribution of velocities inexamples with a main channel and floodplains in which the flood area is evaluated. However, inthe most of applications of hydraulic engineering related to the floods, the longitudinaldimension is still much greater than the transversal dimension. Then, it makes sense to use anone-dimensional model based on the Saint-Venant equation for the longitudinal calculationtreating the transversal calculation in a separate way through lateral distribution 1D models.Moreover, the two-dimensional model, which modifies more parameters because of theconsideration of the hydrodynamics in a more complete way, needs to increment the value ofthe eddy viscosity so as to achieve numeric stabilizations in many cases (being more adequateto simulate subcritical flows). Then, the simplicity with the greater simplification of equationsand the efficiency with the application of the DCM (and other developments) make that HEC-RAS is a better option in this type of cases.

3. Program features

The program allows for:

- The hydraulic calculation of structures (bridges, spillways, culverts)- The graphic visualization of data and results.- The graphic edition of sections.- It is executable in the Microsoft Windows environment.

The program needs the next input data:

So as to use HEC-RAS for simulating a river, geometry data of transversal sections along theriver stretch and volumetric flow rate data (constant or variable in the time) must be available.

Moreover, roughness coefficient and water level at the input or the output section is essential torepresent the real case. In cases more complex (bridges, water quality evaluation) more datamust be provided. Following this, some examples are included so as to see a bit more things ineach one.


4/40

pag 4 de 40



4. Practical example so as to simulate a river

A river stretch through a rural settlement wants to be studied. The initial geometry of theriverbed is modeled with three sections with a separation of 500 m between them. The stretch is

straight and the sections are defined with the next geometry:

- Section 1 (downstream)

X (m) 0 20 100 120 140 150 250 280Y Ground level (m) 101 97 96 92 92 95 97 101

- Section 2

The ground levels of the section 1 are increased in 0.5 m. Section with the same shape as thesection 1, but narrower (90% of the width)

- Section 3 (upstream)

The ground levels of the section 2 are increased in 0.4 m. Section with the same shape as thesection 2, but more wide (30% more wide)

The volumetric flow rate will be 100 m3/s for a return period of 10 years, 300 m3/s for a returnperiod of 100 years and 500 m3/s for that of 500 years. The friction coefficient (Manningscoefficient) will be estimated in 0.1 m-1/3s in the floodplains and 0.03 m-1/3s in the rest (riverbedlimited by the banks).

a) Simulate the river stretch considering slow regime and a normal depth with a slope of0.001.

When the program is opened the main window appears:

Firstly, a project has to be created by clicking on File>New Project, introducing a file name, aproject name and a path in the new window that appears (click on Create Folder so as to thefiles of the program are not scattered in the path). Remember that this is better to use names andfolders without accents. Now, a name and a path appear for the project in the main window. The

project will be open going to File>Open Project wherever the user wants. Help about optionscan be found in Help>HEC-RAS Help (it is the same as clicking on Help inside any other

window).


5/40

pag 5 de 40



If the System International (depths in m, velocities in m/s, volumetric flow rates in m 3/s) is notused by default, it should be changed in Options>Unit System. Following this, the geometrydata has to be introduced firstly clicking on the next icon (of the main window):

Geometry data

Now a background image can be put clicking on and selecting an image (click on add) suchas a map (there is no problem with bitmap extension .bmp and with .jpeg extension, but .png isnot supported and .tif can have problems). Use the tools View>Full Plot, View>Zoom In orView>Pan to place the image (the image has a determined coordinates and the Geometry Datawindow too). The image can be helpful to plot the river but it is not necessary. Following this,the plotting of the riverbed is drawn with the mouse over the window after clicking . Firstly,the beginning point is put by clicking and the plotting is done clicking on each vertice the userwants to define. Finally by double clicking the name of the river and the reach has to be put

being possible to avoid the edition by selecting cancel (or delete once created in Edit>DeleteReach). The river will appear oriented from the first to the last point. The reach name that

appears near from the plotting can be modified (Edit>Change name, Edit>Move Object).

File>Copy To Clipboard is valid so as to copy a figure from this window. The plotting isindependent of the real distance, which will be indicated when defining the sections. Then, if amap is used it is representative to define the distances according to the representation (in theway that the sum of the lengths that will be defined is the same as the distance of the stretch).

The sections are defined by clicking on . Another window will appear and the sections aredefined from the first section to the last one. The table is prepared to be filled for each sectiondefined (with x, y coordinates that represent the transversal section) and the window contains inthe right side the downstream length, the Mannings values, the position of the banks (in xcoordinates) and the contraction/expansion coefficients. The first section will be added withOptions>Add A New Cross Section and putting a name (River Station).


6/40

pag 6 de 40



The section firstly indicated (downstream or outlet section) will be the correspondent to the lastpoint (observing the river orientation already defined) and there will not be downstream lengthsto a before section. Taking positive length values, if the name is 1 or the number of the lastsection, the sections will be set in the way that the river orientation goes towards the section 1

following a decreasing order (HEC-RAS follow the number order in the way that the outlet isalways the section 1). If the name section defined is 1 the section will be the correspondent tothe outlet section as desirable (and sections are not joined due to the null length).

This is the same to define firstly any section with the previous consideration. Thus, the inputsection will have the downstream length and the last number section so as to it makes sense.

Sometimes the plan view of the river can appear in a strange way, with sections that are notperpendicular to the stretch (see below image). In this case, the data seems to be wrong.Nevertheless, it is well and this is only matter of representation (affects future representations)as it was checked.

The cells of the window are filled by clicking on them (the table is filled with the data of thesection). Once defined the banks (if x point does not exist HEC-RAS interpolate a point),

Manning can be differenced in each part (main channel and floodplains). A value of theMannings coefficient, which depends on many factors, can be found easily in bibliography fordifferent cases (and a table appears clicking on the question mark). In rivers this is usual0.02Plot Cross Section. So as to view the longitudinal

profile Plot>Plot Profile is clicked.

The following sections are added in the same way or through the option Options>Copy CurrentCross Section (all cells are copied). Again, a name is put and cells are filled appropriately. If thedownstream section was firstly defined, as here, the length to the previous defined section is

indicated in all of them.Sections defined can be deleted in Option>Delete Cross Section (andthe lengths are modified so as keep the position of the rest).


7/40

pag 7 de 40



Here, second and third sections are edited by multiplying the factors 0.9 and 1.3 going toOptions>Adjust Stations>Multiply By A Factor and adding the values 0.5 and 0.4 to theelevation with Options>Adjust Elevations (considering that the length is 500 m as this is theseparation section according to the example). The floodplains are a nearly flat land adjacent a

stream or river that stretches from the banks. Here, once the section 1 is observed, the banks ofthe riverbed are considered in X=100 m and X=150 m (90 and 135 in the second and 117 and175.5 in the third following the same points). Red points will show these limits.

The sections are represented in the plan view of the Geometry Data window with a separationequivalent to the lengths defined (since the second cross section is defined). When a section isselected in the cross section data window, will be remarked in the Geometry Data window.Furthermore, as the width appears represented, this is useful so as to compare with a map (if itexists). The geometry of the stretch is shown below.

Following this, the geometry definition must be saved (with a name) going to File>SaveGeometry Data (in File>Open Geometry Data each defined data can be used). Now, a name and

a path appear for the Geometry in the main window (the name could be invisible, formed byspaces). All this saved items can be opened to be modified and saved again.

Following this, the steady flow data icon (of the main window) is clicked to introduce theboundary conditions and the next window will appear.

For steady flow simulations (stationary solutions)


8/40

pag 8 de 40



As known, the slow or subcritical regime is produced when the velocity of the fluid is lowerthan the velocity of the gravity wave ( gh ) that accompanies it. The expression that rules theexistence of this regime is:

1vFrgh

In this case the water level boundary condition must be given downstream. When Fr (theFroude number) is greater than 1 a fast or supercritical regime appears (velocity faster than thevelocity of the gravity wave) and the water level condition must be given upstream. In river, itis usual to find 0.2


9/40

pag 9 de 40



In this case, the regime is slow and Fris increased in the flow direction. A transition from theslow regime to the fast regime is produced (through the critical depth) being possible to modelthis part of the river (boundary condition downstream is adequate). Then, when the solution isobtained, in the Fr tables it must be checked that the behavior is just that. For example ifFr

takes the values in defined sections of 0.2, 0.2, 1 this is wrong, and if 0.8, 0.9, 1 appears this isright. If the transition from fast regime to slow regime (hydraulic jump) is going to be analyzedit has to be taken into account that HEC-RAS cannot model this case.

If the normal depth is selected, the program calculates the water levels for a certain frictionslope (I) which is introduced and which is the same for each flow rate. The value introducedwill be that of the geometric slope (i) at the downstream section as when the geometric slopeand the friction slope is the same, the normal depth is reached (obviously the value will be thesame for all the flow rates). This boundary condition is often applied as this is the depth thatusually appears in this section. In the case of a channel (using the z HEC-RAS axis that followsthe length):

2 2

4/3;

2

n v byI Rh

Rh b y

;

If ;2

nn

n

dy byI i Rh y

dz b y

HEC-RAS calculates yn from the equation(b and n are already defined)

The value given could be that of the mean slope of the river (or the downstream stretch) insteadof that which is calculated through the sections. In rivers the most common slope is 0.001 as inthe example. Here, this value is the same as the slope of the last stretch from section 1 to section2, which is 0.5/500 =0.001. Both values are more near as the discretization is lower. Then, themean slope, which can seem not to be necessary and usually has a different value, can be a

better option when the variation of the bottom levels is large what is usual for a refineddiscretization. Obviously, this will not give exactly the normal depth at the downstream

sections, but the solution will be more representative of the normal depth in nearby sections.

The solution will not give a normal depth in all the sections if the some of the features variesleading to the different known profiles (M1 or M2) for slow regime. This is, the calculation ofthe friction slope in other sections will not be equal to the geometry slope in these sections.

If the rating curve option is selected, pairs h, Q must be defined according to known values for adeterminate river (at the downstream section). Then, for a flow rate the program will interpolatea value of water level. This is approximately the same as the first option (known W.S.). Thedifference is that water levels have not to be defined for each flow rate used once the curve iscreated.

yn

y

z

y

x

b

yn


10/40

pag 10 de 40



Following this, the flow data defined must be saved (with a name) going to File>Save FlowData (in File>Open Flow Data each defined data can be used). Now, a name and a path appearfor the Steady flow in the main window.

The last step is to create a plan which will be created for steady simulation.

For compute the steady example

The plan is constituted with a Geometry Data and a Steady Flow Data (note that the steady flowcannot be defined until the geometry is defined, and the plan cannot be defined until thegeometry and the steady flow is defined). Firstly this is generated going to File>New Plan,using a name (for example Normal_depth_and_FirstGeometry) which can be changed(File>Rename Plan Title) and a short identifier with less than 12 characters (for exampleyn_FirstGeom) that can be modified directly in one of the cells. Then, the last Geometry Dataand Steady Flow Data is put in the adequate cells (the last appear by default), and subcriticalregime (FrEncroachments is very interesting as this is useful in order to analyze in sixsteps the necessary artificial bed to contain a determinate flow rate in certain conditions. Forexample, for the flow rate for the returned period of 100 years, the program will use walls

looking for the walls that let the water level be (for example) a=0.3 m greater than that withoutencroachment. Thus, walls are modified reducing the section in an iterative process untiloptimum emplacement is found for this water level. This option is not used in the simulation ofthe present river.

The option Options>Flow Distribution Locations allows the user to subdivide in subsections(slices) the cross sections. HEC-RAS will do a 2D geometric distribution of energy andvelocities in different points of the cross section will be obtained (remember that it would be

impossible in a 1D model). The number of subsections to be considered, which has a maximumof 45 by cross section, is specified in a separately way for the left overbank, the main channeland the right overbank. Here, three subsections for each part are considered enough. So as tocompute flow distribution for all the cross sections, the numbers (3 for LOB, 3 for Channel and3 for ROB) are specified at the top of the window (Set Global Subsection Distribution) andvalues are stored in the cells. In other case (so as to set slices at specific locations) the other partof the window is used and values are kept in a table. The same is done for the subdivisionindicated (3 for LOB, 3 for Channel and 3 for ROB) if the last section is put as Upstream RSand the section 1 is put as Downstream RS. After defining the slices in this way, the button SetSelected Range has to be clicked. It can be seen that after pressing OK a tip appear in this menuoption.

a


11/40

pag 11 de 40



Following this, the plan must be saved going to File>Save Plan (in File>Open Plan each definedplan can be used). Now, the Compute button is clicked. It can be checked that a name and a pathappears for the Plan in the main window.

Errors can appear before computation due to that the banks or the Mannings coefficient werenot defined. A typical error can appear due to that the ground levels are defined in such a waythat the water is not contained by at least one of the sections.

After the computation is done a window appear (next above figure), and more specifications can

be found by clicking the next icon (in the main window) which leads to another window (rightfigure) with the errors and warnings reached. If less than 5 warnings are obtained this isconsidered a good simulation.

Summary of errors and warnings

Many geometry data files or steady flow data files can have been created (any other defined in

the project can be selected when defining the plan) and then, many plans can be defined for thesame project. When a different geometry data or a flow data is opened or saved from the


12/40

pag 12 de 40



respective window, the name and the path of the plan will disappear. Then, the plan has to bedefined again (with the new files) or an already defined plan has to be selected.

The results can be observed by clicking on the next icon (from the main window).

View cross sections

In Option>Select Plan, the plan can be selected (if more than one plan is defined, they are allcalculated). In Option>Select Variables, the variables that want to be seen are selected (watersurface or W.S. which is the water level, the Energy Grade or E.G. which is the total energy ofthe water as the sum of H+v2/2g = h+P/+v2/2g from the Bernoullis equation, the critical depthwhich appears if that exists). The variable Filled in Water Surface allows for seeing the bluewater color in the images. In Option>Select Profiles>Select All, all the profiles are selected inthe way that three solutions for each variable appear in this case. The option Options>VelocityDistribution>Plot Velocity Distribution allows for seeing the velocities in different colors.Specified information can be obtained clicking over the point of any window of results like thisone.

Thus, after defining the sections the flow rate boundary condition (one for each return period)has been given, establishing the calculation cases. Following this, the depth boundary condition(normal depth, yn) has been given. Finally, a plan has been defined and the solution can becalculated. The next solutions in the three transversal sections are obtained (blue filling forT=10 years, the lowest flow rate by default):

Velocity solutions in the riverbed

Values of the critical depth yc, the water surface (WS) and the energy (EG) can be observed forthe defined return periods of 10, 100 and 500 years. For example, it can be seen that only in thesection 1 there are calculated values for the critical depth (for all the return periods).

So as to see the representation of the blue filling for other return periods, the profile has to bethe only one selected in Options>Profiles (click on the clear all button and select only thedesirable profile by double clicking on the name or using the arrow). For the representation ofthe velocities the same occurs. The water level for the case of T=500 years along with thevelocities for the case of T=10 (all the profiles selected) and T=500 years will be shown in the

next figure.


13/40

pag 13 de 40



As can be seen the water is inside the riverbed only for the lowest flow rate (and maximumvelocities are lower in this case). So as to see the longitudinal representation of the solution, the

next icon (from the main window) has to be selected. The selection of profiles, variables andplans for representations is the same (velocities cannot be represented).

Profile plot of the solution

The next longitudinal profile in a section between the banks can be obtained for all the profiles(flow rates). Here, the variations in the water depth can be seen in a better way.

As a first approach (through the image), the water depth is not near from the critical depth in allthe cases. If it is approaching the critical depth, care has to be taken. The solution could have to

be calculated for a fast regime with other boundary conditions.Now, solution in the cross sections and longitudinal section can also be seen with the sameinterface where sections are edited, in the Geometry Data window (or directly in cross sections).

Now, some variables are represented in a graph with the next icon (from the main window):

Velocities and other variables

The selection of profiles and plans for representations is the same. The variable value is what isrepresented. In the option Standard Plots the different variables can be selected (Velocities,

Flow area,) leading to a representation by variable. The velocities along the river


14/40

pag 14 de 40



(longitudinal representation) in the different areas (left floodplain, main channel and rightfloodplain) for the three return periods (flow rates) are represented in the graph below left.

Selecting Standard Plots>Froude the graph above right is obtained and this is possible to see ifthe simulation is right. Froude Chl is the Froude number for the main channel and Froude XS is

the Froude number for the entire cross section. The maximum value is produced for T=500 inthe channel of the second section and for T=100 in the entire second section.

This is obtained a maximum value ofFr=0.5 which is a high value for a typical river. If thisnumber is increased as the downstream section is reached, for example, from 0.4 to 0.9 which isa possibly solution, this is possible that the regime is fast (instead slow reaching the criticaldepth at the downstream). In this case, the another hypothesis should be calculated. This is notthe case.

The rating curve can be obtained by clicking on the next icon (from the main window):

Rating Curve

The rating curve is the curve which relates the water level (W.S.) and the flow rate (accordingto the three cases of the three return periods). These curves give important information in theapplication of boundary conditions for the hydrodynamic models. In fact, one of the water level

boundary condition possibilities is to define a rating curve.

In the graph options, only the selection of the plan can be done (variable is fixed and flow ratesare represented graphically). In the below figure, the curve (obtained postprocess) is shown forthe downstream section. Obviously, the flow rate that goes through section is the same for each

case, but the water surface is not.

Now, it can observed the sensitivity of the solution with other similar boundary conditions suchas the modification of the slope for the calculation of the normal depth, the critical depth,


15/40

pag 15 de 40



5. Same example with known downstream boundary conditions

b) Simulate considering slow regime and a known water level boundary conditions of 92.5,95 and 98 m. Check that with the lowest flow rate critical depth appears.

Now, the Steady Flow Data window is open again, and different water depth boundaryconditions are given after clicking on the button Reach Boundary Conditions. Within the newwindow and observing that the cell downstream is selected, the button known W.S. is clickedand the water levels considered are introduced.

This steady flow data is saved (now, File>Save Flow Data As so as to not overwrite the existingfile) for example with the name known depth (if the water level is known, then the water depthis known because the ground level is data, water level=y+Y). This is the same to save when thewindow is opened (Save As in this case) being necessary to save again before closing thewindow. If now, the user open a steady flow data two files appear (see the right window). Now,the plan has disappeared in the main window. Instead open the defined plan, another plan isdefined. The new steady flow appears by default in the window where it is done. Following this,a new plan is saved (this is the same File>Save or File>Save As, as there are not any plandefined) with a name (for example, known_depth_and_FirstGeometry) and a short identifier(y_FirstGeom). The subcritical regime is selected and now 45 slices are chosen with the option

commented after clicking the button clear all. According to the distribution, 10 slices areconsidered in the LOB, 25 in the main channel and 10 in the ROB (as can be seen with theglobal option the maximum by part is 43 being necessary 1 slice in the other parts are selected).Saving the plan and computing, the results are obtained with 5 warnings.


16/40

pag 16 de 40



The geometry is the same as before, but now a known water level has been given in thedownstream section. These values are of 92.5, 95 and 98 m respectively for the defined return

periods of 10, 100 and 500 years (associated with the flow rates given). The solution in this casein the three cross sections considered is shown below.

Now, the representation of the velocity is better. For T=500 years this is:

Now, according to the results, in the downstream section (section 1) there are values of criticaldepth, in the section 2 there are values for the return period of 10 years and in the upstreamsection (section 3) there is not critical depth value indicated. Curiously, in the downstream

section for 10 years (flow rate of 100 m3

/s) the value of the water level is just the water level forthe critical depth. Then, something strange is occurring.

Again, it is expected that the flow is slow (Fr


17/40

pag 17 de 40



As can be seen, for 93.24 m the critical value is reached for the lowest flow rate. HEC-RAS hasdone a calculation for slow regime and solution is not found so as to reach the imposed

boundary condition of 92.5 m (in fact this is the last warning that appeared). This is the reasonwhy a red dashed line indicates the critical depth for the stretch between section 2 and section 1.

Then, this is checked that for the lowest flow rate and the considered boundary condition inslow regime the critical depth is produced. This is because the critical depth, which is producedfor a water level of 93.24 m, is the minimum water level that allows for a slow regime. As it isknown, this situation is usual in spillways where slow flow (increasing the Froude number inthe direction of the flow) exist upstream appearing the critical depth yc (Fr=1) in the pointwhere the waterbed ends. In this point a transition between slow regime and fast regime couldtake place. As it was explained the other transition that leads to a hydraulic jump from fastregime to slow regime could not be simulated with HEC-RAS.

In the case of having the 92.5 m boundary condition, the flow will be necessarily fast. Thus, thewater level boundary conditions have to be given in the upstream section looking for that onethat allows for this water level downstream.

Now, solution of the two introduced plans can be represented at the same time so as to comparethem. For example for the profile of the greatest flow rate, the next image is obtained.

In the following images, the comparison of the longitudinal profile of velocities for this flowrate (only flow in the main channel) and the rating curve for the last plan are represented.


18/40

pag 18 de 40



Information can be obtained with tables too. The next icon is selected from the main window soas to see some relevant variables:

Tables with results

Here, information of certain variables is indicated. The plan, the profile and the section isselected in the window and a lot of variables related to this case are shown. Some of them are

presented for the total section on the left (energy grade in meters, velocity head, water surface,flow rate which will be the same for each section in a determinate plan and profile due to themass conservation,) and other variables are presented for the LOB (left overbank), the mainchannel and the right ROB on the right (Mannings coefficient considered, flow area as the areaoccupied by the water, the average velocity, the depth,). This table always shows thewarnings of the simulation. In the case of the last plan with the lowest flow rate and thedownstream section the left table appears with the warning message pointed out. The right tableis obtained for the upstream section.

Between these variables, it is the shear stress, that is the stress in the plane of the cross sectionand that can have importance as measure in some way the tangential force of the fluid.

The increment in Froude number can be observed through another type of table instead of thelongitudinal profiles of Froude number. Now, the next icon is clicked from the main window inorder to use this table:

Tables with results by profile


19/40

pag 19 de 40



All the results generated can be shown at the same time clicking on Options>Profiles>Select Allin order to see the profiles and clicking on Options>Plans in order to see all the plans. As can beseen sections are separated which makes easier to view it. In Options>Define Table, the usercan select the desirable variables (by clicking on Delete Column button after select one or

clicking on Insert Column and double click on one of the variables listed below after it). Below,results are shown for all the results on the left, and for the last plan and the lowest flow rate onthe right. As the interest is in the Froude number, in this table fewer variables are put (editedtable). Another variable of interest is the E.G. Slope (slope of the energy grade line) which isthe friction slope defined before (I). Obviously, it is not equal to the geometric slope of0.5/500=0.001 between section 1 and 2, and 0.4/500=0.0008 between section 2 and 3.

This is seen how the velocity of the water is much greater at the downstream section and howthe Froude number is increased as the number of the section is lower (can be considered Fr=1 atthe downstream section, this is, the value for critical depth). Another type of table can be

obtained going to Std Tables. The table which has been used is the Standard Table 1 (appears bydefault). Another Standard table exists (other variables, same configuration), besides specifictables for culverts, bridges, The defined configuration of a table disappears once another tableis selected (for example selecting the same type of table after its definition) if this is not savedin Options>Save Table. The current simulation can be shown in a table opened from a beforesimulation only clicking on the Reload Data button.


20/40

pag 20 de 40



6. Same example with more spatial discretization

c) So as to obtain a greater discretization it is proposed to insert sections between thatalready defined, in the way that slopes are kept and sections are separated by 100 m.

Analyze the differences for the known water level condition and obtain a 3Drepresentation.

As it can be observed, the previous warnings indicate the need for additional cross sections. Asthe length of the stretch is of 1000 m, the subdivision leads to 11 sections in total:

1000/100+1 (closure) = 11 sections

Another file with another geometry will be defined for the same project (the plan will disappearfrom the main window). So as to do it, click on Geometry Data icon again. The file is generated

previously for example (remember that this is the same as modifying the geometry and Save Aswith other name), going to File>Save Geometry Data As and giving a name (for exampleSecondGeometry). Here, an interpolation between defined sections can be done, going toTools>XS Interpolation>Within a Reach in this Geometry Data window. The user will indicatethe maximum distance between sections in a stretch (between first and last sections or betweenany two sections that define a part of the entire stretch) and HEC-RAS will put the sectionsautomatically using a lower than or equal distance between each pair of sections in the way thatthe originally defined sections are maintained. The option Between 2 XS allows forinterpolating in the same way but only between two defined sections and allowing the user tohave much greater control over the interpolation.

Here, the first option is used, and this is taken into account that with the separation of 100 m,original second section is just in the subdivision (HEC-RAS uses this value) and there will be asection each 100 m along the river stretch. In this example, the distance would always beconstant in the entire stretch though the user wanted a maximum distance of 22 m (in whichcase HEC-RAS would use 21.739). The river, the reach, the sections between which sectionsare added, and the distance are written. Then, the Interpolate XS button is clicked and thewindow is closed. Following this, the section is saved in File>Save Geometry. As it can be seen,sections are added interpolating all the defined variables (manning, banks, geometry of the crosssection,). In fact, sections are put with a downstream length (to a before section) of 100 m,modifying lengths in originally defined sections (from 500 to 100 m in the section 2 and thesection 3) so as to maintain their position.

The names of the original sections are maintained, and new names appear with asterisk in aformat in which the number of an original section is followed by a reference number. Previous


21/40

pag 21 de 40



calculations are still in the original sections (the section 3 has water, the 2.8* still has not).Now, a new plan (File>Save As) with this interpolated geometry and the known depth boundaryconditions is generated going to the appropriate icon. Subcritical simulation is done after the

plan is saved. Therefore, a known downstream value of 92.5, 95 and 98 m is given respectively

for the return periods of 10, 100 and 500 years.

If the discretization is not good, the user can overwrite the second file opening the first file andsaving with the name of the second (instead of modifying and saving the second). If the planwas already created, it only has to be selected after the previous step.

Now, the geometry is defined with more discretization (8 new sections) leading to a moreaccurate solutions. However, according to the data, the geometry is not defined with more

precision as the cross sections have interpolated values (for example, the waterbed of thesection has interpolated values of the points that define the waterbed of the known sections).Following this, the results in the cross sections 1, 1.2* and 2.2* are shown (four warnings wereobtained). Here, it is better observedhow the water depth is adjusted to the critical depth for thelowest flow rate.

A little bit differences due to the discretization are observed. Below, the upstream section isincluded, comparing this plan with the previous plan (same boundary conditions) for the lowestflow rate (last plan appears by default). In Options>Zoom In a zoom is done (Option>Full Plotreturn the complete view) and a little outline appears in the left upper part of the window.


22/40

pag 22 de 40



Now, the water level longitudinal profile has a better appearance as can be seen in the leftimage. A big refinement (sections each 10 m) has been done so as to obtain the right image(three warnings in this case). Take into account that if a plan is selected for the representationdeselecting the plan used when computing, the discretization is kept. Thus, if the plan used

when computing has a greater discretization, all the cross sections will still appear though someof them without solution. The geometry shown on the right is that of the last simulation.

Again, the Froude number will be increased as the distance to the downstream section is lowerfor the lowest flow rate case. This can be better observed now. The graphic representation of theFroude number along the stretch (only Froude in the main channel) and the table (Standard table1) with this variable are shown below on the left for this case. On the right, the graph for thegreater refinement is included.

A 3D representation of the water level scalar field is obtained by clicking in the next icon (mainwindow):

For 3D representation of the solution

The selection of profiles and plans for representations is the same. The variable to berepresented is only the water surface. Here, a customized perspective view of the solution isobtained with the interface that HEC-RAS offers. The image can be rotated around two axisthanks to buttons prepared for it. In this way, the solution can be observed from different angles.Moreover, only a part of the stretch can be seen by selecting the upstream and downstreamsections. After running with the geometry for the 100 m discretization, the representation for the

lowest flow rate and all the flow rates is included below.


23/40

pag 23 de 40



After running the program for the discretization of 10 m, the figure below is obtained for thelowest flow rate through the animation option. As it was indicated, the program is executedtwice so as to the discretization in each case appear. The animation can be done in each resultswindow and allows for a video with the solutions obtained. Profile plots can be animatedthrough different flow rates for steady flow (this is the case). With Options>Animate thesolution for each profile appears one by one (though only a profile is selected). Profile plots can

be animated through a time series for unsteady flow too, though this option is not carried outhere. Then, when a transitory solution (unsteady) is obtained, HEC-RAS solutions for each timestep during a certain time interval are used to make the video, which is more representative.When the animation is selected, a window with the scheme of the animation and a control barappear besides the original window. In the animation control bar, the right button let the userselect the animation speed.


24/40

pag 24 de 40



7. Same example with an unsteady simulation of the river

d) Obtain the solution supposing an initial condition of flow of 100 m3/s, a downstreamwater level boundary condition that depends on the flow rate and a variable upstream

flow condition from 100 m3/s to 500 m3/s with a lineal variation during four hours and aconstant value of 500 m3/s in the next four hours. See the evolution and calculate thedownstream water level in five and seven hours.

Select the unsteady flow data icon from the main window. If the boundary conditions weredefined for unsteady flow this icon has to be used:

For unsteady flow simulations (transitory solutions)

Go to the Initial Conditions menu and set an initial flow in the upstream (by default) by doubleclicking on the appropriate cell and clicking on the Apply Data button. Then, go to theBoundary Conditions menu. In the cell for downstream section a water level is defined througha rating curve clicking on the appropriate button (the depth will depends on the flow during thesimulation). The curve is defined with the values of each return period modifying the last one soas to not reach the critical depth. Following this, a flow curve is defined upstream in the waythat a different flow appears at each different time as boundary condition. Then, the FlowHydrograph button is clicked and the curve is introduced after selecting the cell for theupstream section.

Now, the unsteady flow name and the path appear in the main window. Following this, theunsteady flow data is saved in File>Save Unsteady Flow Data. Now, the plan is defined clickingon the icon (main window):

For compute the unsteady example

The geometry with the discretization of 10 m will be used along with the unsteady flow dataand are chosen (set by default) in the window that appears. Then, all the programs to run are

selected. Following this, the time and date that define the start and the end of the simulationhave to be entered.


25/40

pag 25 de 40



The time step for calculation which is the computation interval has to be small enough (this isgood that at least the time of the hydrograph divided by 24 is considered). The hydrographoutput interval should be equal or greater than the selected computational interval. The detailedoutput interval makes that the program has not to print the solution for each computational step

being able to avoid the storage of large amount of information (then, it will be equal or greaterthan the computational interval). Here, a computational interval of 1 minute (instabilities for agreater time were seen), a hydrograph output interval of 20 minutes and a detailed outputinterval of 20 minutes are taken. The mixed flow regime is an option that is used if there is atransition from subcritical regime to supercritical (draw downs) or from supercritical tosubcritical (hydraulic jumps). This is not the case. The plan is saved and the computation isdone.

Now, all fields of the main window are filled. Only solutions from the animation of the 3D viewfor this plan are going to be shown. Now, as indicated, there is a profile per printed solution(detailed output interval). The first profile is that in which the maximum water level is

produced.


26/40

pag 26 de 40



The water depth is always growing though the flow rate is constant at the end. However, in thelast hour almost is the same (stationary solution will be reached if boundary conditions areconstant). The water level is for example observed through the longitudinal profile consideringthat for five and seven hours the profiles for 24:00 and 2:00 hours have to be selected. A waterlevel of 97.77 m is obtained at 24:00 hours and 97.98 m is obtained at 2:00 hours (see the figure

below left). At 19:00 hours the water level will be 93.5 m which is in correspondence with100m3/s according to the rating curve and this is right because the initial condition is a flow rateof 100 m3/s (see the right figure).

For the constant boundary condition of 500 m3/s, the stationary solution will be reached for awater level of 98 m because this water level is in correspondence with 500 m3/s according to the

rating curve.


27/40

pag 27 de 40



8. Same example with a bridge

e) Near from the central part between the section1 and the section 2 a bridge with threepiles of 1 m of diameter wants to be defined. One of the piles will be placed in the

riverbed and two in the floodplains. Propose a design with a flat deck which has a levelof 98 m in the lower part and 98.6 m in the upper part. Moreover, define ineffective flowareas in the stations X=80 and X=200 m with a level of 98.6 m. Apply the Energys,Momentums and Yarnells Methods. Simulate with the know water level boundaryconditions and the geometry with sections each 100 m.

The geometry with the discretization indicated is opened from the Geometry Data window.Then, in the same window, the icon Bridge and Culvert is selected. Now, in the BridgeCulver Data window the option Options>Add Bridge is chosen and the river station has to beintroduced.

After observing the sections it is decided that the bridge will be collocated in the section 1.5which is the number to be introduced. This section does not exist and, as the number is betweenthe 1.4* and 1.6* defined sections, HEC-RAS will place this section between them afterinterpolating (which is just the middle of the first stretch). Thus, the number has to be definedwith a value between the number of the two existent sections which will be the boundingsections (1.55 is between 1.5 and 1.6). This is important to know that this number cannot be oneof the existents.

Automatically, the new interpolated section appears in the window (only this section can be

seen here because as many sections as bridges are defined appear) with two buttons to see thebounding sections. The upstream and downstream parts of the new section are drawn in thewindow. There are some possibilities in this Bridge Culver Data window. For example, theCulvert icon could be clicked to define a culvert. Here, the Deck/roadway icon will beclicked so as to define the deck.

The deck of the bridge has a thickness of 0.6 m (by resting values) and will be designed with awidth of 4 m and the bridge symmetry axis (the section 1.5) exactly placed in the middle

between the sections 1.4* and 1.6*. In this window, the distance is referred to the distance fromthe next section (1.6*) to the bridge, and the width is the width of the deck. In this way, as thedistance from the bridge axis to the section 1.6* will be 50 m, if the distance is 48 m and thewidth is 4 m (combination as 40 m and 20 m will lead to the same axis position with otherwidths). Now, the position of the section 1.5 is defined.


28/40

pag 28 de 40



It is important to take into account that no section can be crossed after defining the distance andthe width (width + distance


29/40

pag 29 de 40



Bridge Culver Data window is selected. The piles will be defined upstream and downstream inthe same way and there will not be differences between them. Then, at the top of the windowthat appear, the X station of the first pile is put for the upstream position. In the table thatappears below, the pile will be defined through two sections for the upstream section. The first

row will have the width (diameter) and a given level for which the position is in the ground asbefore. The level of Y=0 will assure that the defined point is in the ground. The second row willhave the width and a level in such a way that the position is in the deck. The level Y=98.1 isright. Thus, the shape of the section of the pile is constant and more positions would not makesense. Finally, the pile will be painted from the ground to the lower part of the bridge. Now, thedownstream information is added (same as upstream) by clicking on Copy Up To Down button.

The second pile can be generated by clicking on the button Add and defining it (in the sameway) or clicking on the button Copy. As the pile will have the same geometry, the button Copyis used and automatically a second pile will appears with the same values as the previous. Onlythe value of the X station has to be modified (changing it in the two places where appear orchanging it in one place and copying with Copy Us to Down). The third pile is defined in thesame way. Now, for the geometry with the indicated discretization, a bridge has been defined ascan be observed in Geometry Data window.

Ineffective flow areas are defined so as to quantify the energy losses. According to the HEC-RAS implementation, in the ineffective flow areas water will be contained but the velocity isassumed to be zero (the flow rate will be null through them) while the water level is lower thana value. In other words, water is accounted for volumetrically but it is not considered in theconveyance until an elevation is reached. If the depth is less than the ineffective flow elevation,they will be considered as ineffective.

The ineffective flow areas could be used for representing zones in which the flow is stagnant

due to narrowings of the riverbed (this is a 1D model). In this case, over a level that will bedefined for the ineffective area consideration, the narrowing will disappear. Then, a section (or


30/40

pag 30 de 40



sections) will have already a narrowing and this areas will be defined where the stagnation isproduced, in the upstream and downstream sections.

In the case of a culvert, the ineffective area is used to restrict the flow area to the area of the

culvert until flow overtops the roadway. It makes sense as another stagnant water zone (inwhich in this case the water is accumulated) appears until the water reach the roadway and allthe section works as if the culverts does not exist (according to the flow rate). Now, thenarrowing will be represented through ineffective areas in the culvert and they will be definedwhere the stagnation is produced in the upstream and downstream sections too.

Near bridges as the designed zones where the water is stagnated appear. The first case can beconsidered in the river shores as with little depth and the side piles the water is approximatelystagnant. When the water level is enough, the second case can be considered, as the water will

be accumulated (contained by the deck) forming vortexes until the water overtops the deck. Inthis way, as the ineffective flow areas will become effective when a defined value is exceeded,in bridges it makes sense to define this value as that of the elevation of the deck. Over thisvalue, all the water in the section is working. The user could take the ineffective areas since anX station near the side piles as an approach.

This is more representative if the bridge has anchorage blocks on the sides (more similar to aculvert). In this case, it is good to keep a 1:1 proportion in the way that if the upstream section is48 m from the upstream bridge face (as in this case), the ineffective areas should be placed 10 maway from each side of the bridge opening. In the downstream section the position will dependson the bridge being possible that the areas begin at the openings, before or farther.

Thus, ineffective flow areas will be defined in the upstream section (1.6*) and the downstreamsection (1.4*). The areas can be defined directly from Bridge Culver Data window by clickingon the Bounding XSs buttons (1.6* and 1.4* in this case) or from the Cross Section Datawindow where the original sections were defined (selecting the section and clicking then in

Options>Ineffective Flow Areas).

According to the example this areas will be defined from the stations X=80 and X=200 (sameground level, the areas are reached at the same time) in both sections. For the first section, the Xvalues are written in the first row, the values for the ineffective consideration (98.6) are writtenin the second row and the ticks are put on the boxes (permanent). Then, the procedure isrepeated for the other section. The Ineffective Flow Areas HEC-RAS option automaticallyconsiders the areas for X200 in each section (see the representation of them whenthe Apply Data button is clicked).

The representation of the ineffective flow areas can be seen in the Geometry Data and the Cross

Section Data windows. Moreover, they appear in the Bridge Culvert Data as they are consideredover the section (in the upstream and downstream parts) of the bridge too.

Q1

Q2>Q1

X=80 X=200

Ineffectiveuntil here


31/40

pag 31 de 40



The contraction and expansion coefficients that HEC-RAS has by default will be used (they arethe same as before and could be changed in this case using 0.3 and 0.5). Finally the geometry issaved (File>Save Geometry Data As if it was not still saved). Following this, a new plan isgenerated (Known_depth_BridgeGeometry in this case) using this geometry (and the steadyflow for known water level boundary conditions) and the example is executed. The results in thecross sections 1.0, 1.4*, 1.6* besides in the upstream and downstream parts of the section 1.5(sections 1.5 BR D and 1.5 BR U in the locations 1.5+2m and 1.5-2m) are shown below.Remember that if the bridge was defined with a distance of 96 m, the section BR D would bethe section 1.4*. The ineffective flow areas appear here too.


32/40

pag 32 de 40



Following this, the longitudinal representation of the water level and the 3D representations forthe lowest and the greatest flow rate are shown.

The same water level value is obtained downstream (93.24 m) for the lowest flow rate. The

water level varies near the bridge appearing a bit greater depth value upstream of the bridge. Ascan be observed the water reaches the bridge (without overtopping it) for the greatest flow rateand is under the bridge in other case. On the other hand, the ineffective areas are working forthe two greatest flow rates. Below, the graph with the velocities along the stretch and thevelocity distribution in the upstream and downstream bridge cross sections for the greatest flowrate are included. The option of the flow distribution in the cross section with 45 slices (thedefined before) was used when defining the plan. Note that solutions are not different whencomputing with this option, only this representation is available (a postprocess treatment of thesolution is done).

As was explained, the cross sections have the different velocities which are produced (this issomething that is impossible through the 1D equations derived from the simplification).Moreover, the calculation near the bridge is done in an approximate way because though thevelocity variations are not simulated, a lower velocity is represented near the pile (greater zonein the downstream section). Enough slices are necessary to observe it. On the other hand, thenull velocities in the ineffective areas are also observed.

With the steady flow for normal depth boundary conditions warnings are not obtained in thesimulation and this is observed that the water does not reach in any case the bridge.


33/40

pag 33 de 40



As can be seen (in the figure with the first profile) the ineffective areas are not remarked withgreen lines if they are not used (both in the velocity representation and water levelrepresentation) appearing only the green arrows.

Again, the Standard Table 1 will show the problems for finding the solution for the lowest flowrate in slow regime, reaching the critical depth. Following this, this table is included for thegreatest return period. Note that Froude numbers could be too high compared to the numbersthat usually appear in river (FrSix XS Bridge in the same window that the previous table wasobtained. Here, the six sections where the bridge has influence (from 1.2* to 1.8*, twodownstream sections, the sections of it and two upstream sections) are shown with the reached

values for other variables. Below, the table obtained going to Std. Tables> Bridge Only ispresented. This table shows values for the stretch from the section 1.6* to 1.5 BR U.


34/40

pag 34 de 40



Low flow in bridges occurs when the flow goes under the deck. HEC-RAS has four methods forcomputing losses through the bridge (from 1.5 BR D to 1.5 BR U) when resolving the equationsin this case. They are the Energy (standard step method), Momentum (momentum balance),Yarnell (Yarnell equation) and FHWA WSPRO methods. HEC-RAS first will use the

momentum equation to identify one of the next three cases and the possibility of applying eachmethod will depend on the case.

The water surface through the bridge is completely subcritical (class A for HEC-RAS). Energylosses through the expansion (sections 1.5 BR D to 1.4*) are calculated as friction losses andexpansion losses. The average friction slope (I) is based on one of the available alternatives,with the average-conveyance method being the default (as always, see in the Plan window theoption Options>Friction Slope Methods). Energy losses through the contraction (sections 1.5BR U to 1.6*) are calculated as friction losses and contraction losses. The four methods can beused in this situation.

The water surface passes through critical depth in the bridge constriction for either subcritical orsupercritical profiles (class B). For a subcritical profile, the momentum equation is used tocompute an upstream water surface above critical depth and a downstream water surface belowcritical depth, using a momentum balance through the bridge. For a supercritical profile, the

bridge is acting as a control and is causing the upstream water surface elevation to be abovecritical depth. Momentum is used again to calculate an upstream water surface above criticaldepth and a downstream water surface below critical depth.

The water surface through the bridge is completely supercritical (class C). HEC-RAS can useeither the energy or the momentum equation to compute the water surface through the bridge.

For the case that the flow goes over the bridge (high flow) there are two options, to use theEnergy Only option, or to use the Pressure and/or Weir option, with which:

Pressure flow computations are done when the flow comes into contact with the low chord ofthe bridge. Once the flow comes into contact with its upstream side, a backwater occurs andorifice flow is established. HEC-RAS will handle the case in which only the upstream side ofthe bridge is in contact with the water (a sluice gate type of equation is used) and the case inwhich the bridge constriction is flowing completely full (the standard full flowing orificeequation is used). Weir flow computations are done when the flow goes over the bridge. Flowapproaching the bridge will be calculated using the standard weir equation. For high tailwater

elevations the program will automatically reduce the amount of weir flow to account forsubmergence on the weir. When the weir becomes highly submerged, the program willautomatically switch to calculating losses based on the energy equation.

For the discussion of the methods it is recommended to read the chapter 5 of the HEC-RASHydraulic Reference Manual, in the pages from 9 to 17 and from 26 to 28. Here, the flow will

be slow and under the bridge section in the case of the two lowest flow rates (see beforesolutions) and then the four methods can be used. This is done going to the Geometry Datawindow, clicking on the icon Bridge/Culvert, going to Bridge Modeling Approach icon andgoing to the low computation part which is in the top. The options Use and Computation has to

be ticked (the class reference can be seen in the window that appear, which is called the

modeling approach editor) to obtain the solution for a method. The user can select a group ofthese methods in the computations (by ticking some or all the compute options) choosing either


35/40

pag 35 de 40



a single method as the final solution or tell the program to use the method that computes thegreatest energy loss through the bridge at section 1.5 BR U (by ticking use). This allows theuser to compare the answers from several techniques all in a single execution of the program.Minimal results are available for all the methods computed, but detailed results are available for

the method that is selected as the final answer. Then, the methods are selected one by one.

Here, three of the methods have to be used as it is indicated in the exercise. Moreover, thecomputation of the Energy method was selected (by default) in the previous calculations usingthe option Highest Energy answer (computes the greatest energy loss upstream) which leads tothe same as if the option Energy is computed and used.

Below, the table obtained going to Std. Tables> Bridge Comparison is shown for the Energymethod. This table shows values of the water level for the different calculation methods. Here,the water level (as W.S.) is seen.

Then, the solution is presented for the other two computation methods. For high computationthe Energy Only option (by default in the below part of the window) is kept. This method onlywill be used for the greatest flow rate because (slow) flow over the bridge exists. The modelingapproach editor will be shown in each case.

When using the two methods indicated which are Momentum and Yarnell methods, a losscoefficient value in the pile has to be introduced. The value can be choosing observing the tablethat appears when clicking on the question mark taking into account that circular shaped piles

are considered in this example. The momentum method (only one option for critical depth in thebridge) is firstly used with a coefficient of 1.2 for circular piles leading to the next results for allthe flow rates.


36/40

pag 36 de 40



Finally the Yarnell method is used with a coefficient of 0.9 for semicircular nose and tail pilesleading to the next results.

Now, the Six XS Bridge Table is included above for the Energy method, below on the left forthe Momentum method and below on the right for the Yarnell method.

As can be seen the water level result in the bridge for the greatest flow is the same in all thecases, as it is computed as high flow. This value, which is 98.06 m, is the same upstream anddownstream. For the lowest flow rate this is obtained a value from 94.27 to 94.29 (upstream anddownstream) for the energy method, from 94.41 to 94.29 for the momentum method and thesame value of 94.19 for the Yarnell method. For the medium flow rate this is obtained a valuefrom 95.69 to 95.72 for energy method, from 95.86 to 95.72 for the momentum method and thesame value of 95.61 for the Yarnell method. Thus, the momentum method is the method thatobtains the greatest water level upstream (94.41 and 95.86) and the energy and the momentummethods give the greatest water level downstream (94.27 and 95.72). On the other hand the

Yarnell method is the only one that gives the same result upstream and downstream. The bestmethod should be chosen by comparing the solutions with measurements.


37/40

pag 37 de 40



9. Same example with different Mannings values

f) After the zone is studied, actuations are going to be done in order to reduce the frictionboth in the riverbed and in the floodplains, generating a concrete channel and

promenades in the floodplains. Study the effect of considering 0.04 m-1/3s in thefloodplains and 0.015 m-1/3s in the rest. Compare the solutions in the cases a), c) and e)for the energy method.

The values of n=0.015 m-1/3s in the riverbed and n=0.04 m-1/3s in the floodplains for theMannings coefficient are typical values for concrete channels and no wooded floodplainsrespectively. Following this, some points are repeated with these decreased coefficients.

Some coefficients can be changed easily, replacing the information in all the sections at thesame time, through the tables that can be seen in the Geometry Data window such asTables>Contraction and Expansion Coefficients and Tables> Mannings n or k Values. Thevalues can be replaced by other ones in the part of the table the user wants. So as to reduce theMannings values (required in this example), this is necessary go to Tables>Mannings n or kValues and press the Replace button. Then, the value to be changed and the new value arewritten and the option Entire Table is selected.

The effects of the Mannings coefficient reduction are studied case by case comparing theresults with that already obtained (with the water level longitudinal profile and the StandardTable 1). The previous results are shown on the left, and the new results with the reduction areshown on the right. Below, the results for the case a) are included.


38/40

pag 38 de 40



This is observed that when the Mannings coefficient is decreased the normal depth will belower (for the same slope of 0.001). Now, the water depth obtained downstream for the lowestflow rate is 93.55 m instead of 94.27 m. For an intermediate value as n=0.018 in the riverbed(the Manning in the floodplains has not influence for this flow rate), the water level would be

93.72 m. Thus, the floodplains are flooded with lower amount of water and are not alwaysflooded for the medium flow rate.

Moreover, greater Froude numbers are observed. This is because, as there is lower friction, thevelocities are greater as can be seen too. Values as high as Fr=0.7 are obtained in the section 2for the lowest flow rate and Fr=0.9 for greatest flow rate. However, the flow is still slow as thenumber is decreased as the flow goes ahead. Following this, the solutions for case c) areincluded, representing the critical depth too in the longitudinal profile and showing the tableonly for the lowest flow rate.

Again, the water depth is lower in all the sections apart from the section 1 in which the waterlevel is imposed (floodplains less flooded) or the water level is that of the critical depth againcalculated in the case of the lowest flow rate. The differences are smaller for the greatest flowrate and in the case of the medium flow rate the water depth profile has lower progressionreaching a water depth almost constant in the last stretch. In the case of the lowest flow rate it isobserved that the water depth yc is not dependent of the Mannings coefficient as it is the same

in both cases. Now, the progression is better near the downstream section (the water depthprofile is more smooth and the Froude numbers are increased more progressively). Finally, thesame results are shown for the discretized case which has the bridge using the energy method

both in low flow computations and high flow computations (previously it was always used) as itis by default.


39/40

pag 39 de 40



Comparing the results, now it is observed that, due to the lower water depth, the water level ispractically the same as the level of the low chord of the bridge (a little bit lower) for the greatestflow rate. Moreover, it is observed a disconnection of the water level under the bridge with asignificant variation for the lowest flow rate, possibly because the critical depth was reachedhere (momentum equation would have been applied) and a solution is not reached for theequations. Again, the critical depth is reached in the downstream section for the lowest flowrate.

In conclusion, with the Mannings coefficient decrease, the velocity of the flow is greater andthe flow is under the bridge for the maximum flow rate considered in the project, that for thereturn period of 500 years, and for the known water levels downstream (values can be knownthanks the existence of a reservoir, for example).


40/40

pag 40 de 40

References

The exercise is based on the exercise proposed by Enrique Pea. This exercise, which is muchmore simple was used in 2010 for the subject Programas Comerciales en Ingeniera

Hidrulica that belonged to the Master En Ingeniera Del Agua.

Alberto Andreotti, Gerardo D. Hillman, Cecilia E. Pozzi P., Andrs Rodrguez, e Indigo SA,2009. Optimizacin de un modelo hidrodinmico Bidimensional.This is an article written in Spanish in where RMA2 is presented and used for simulating a

particular case. The aim is to present some improvements for the model.

F. Javier Snchez San Romn, 2007. Manual introductorio a HEC-RAS. Dpto. Geologa, Univ.Salamanca (Espaa).This is a HEC-RAS manual very basic, written in Spanish, in where the first steps of thistutorial are commented with more detail.

Juan F. Weber and ngel N. Menndez, 2003. Desempeo de modelos de distribucin lateralde velocidades en canales de seccin compuesta. Primer simposio regional sobre hidrulica deros.In this document, written in Spanish, a comparison between HEC-RAS and RMA2 is carriedout.

HEC-RAS Hydraulic Reference Manual. US Army Corps of Engineers.Nowadays the Manual that can be found in the internet is the Manual for the HEC-RASversions 4.0 and 4.1. This is not a problem as the information will be more complete.

Documents

Hec-Ras tutorial.pdf