Friday, April 21, 2017

Back to the Basics: Bank Station Placement

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants 
Copyright © The RAS Solution 2017.  All rights reserved. 

Lately, I’ve seen a lot of basic bank station issues for models I have reviewed.  Some real basic stuff.  So I thought it would be good to go back to the basics a bit here and review proper placement of bank stations for cross sections in HEC-RAS. 

What do bank stations do for us?  First of all, they separate your channel into three distinct conveyance zones.  One for the left overbank, one for the main channel and one for the right overbank.  Not every application has multiple conveyance zones (i.e. canals), but most natural systems do.  By segregating out the different conveyance zones, we are using Manning’s equation to more appropriately determine energy loss through the system.  Here’s an example of a simple cross section with properly placed bank stations:


Notice the bank stations (the red dots on the plot) also reside at the grade break between the physical channel and the flatter overbanks.  While this is typically what is done, remember the correct placement should always be made based on the location of the change in conveyance.  For example, if you have a lot of thick vegetation down the banks of the channel, you might conclude that the excessive roughness there pushes the boundary between conveyance zones down closer to the toe of the banks like so:



Sometimes locating the bank stations are not as obvious as these examples.  For example, where should the bank stations be placed for a cross section like this?


One might initially conclude that the deeper channel should get the bank stations in which case you may place them like this:


However, it is important to know what is happening upstream and downstream of this location before you can make this decision.  Perhaps the smaller channel is actually the main conveyance and there just happens to be a large low-lying area in the left overbank. 


You would only know this by studying the reach above and below this spot.  Having nice aerial imagery behind the geometry schematic can help to make this decision for you. 


Notice in the figure above, the main channel is very obvious.  Even though there may be some low spots in the right overbank, we can clearly see where the main channel is and the bank stations have been placed accordingly.  It’s also important to point out that as you move through your reach, the placement of bank stations should be fairly consistent from cross section to cross section.  Changes in main channel width should generally be gradual from one cross section to the next. 

One of the most basic steps in constructing your HEC-RAS model is to go through every cross section and properly place bank stations.  If you are importing your cross sections from GIS (e.g. via GeoRAS), make sure that your bank line delineation placed the bank stations properly.  While your bank lines may look like they follow the conveyance boundaries well, you may see a very different picture once you’ve imported your cross sections and look at them in cross section view.  It’s always important to fine-tune your bank station placement in HEC-RAS after importing cross sections. 

As with most things in HEC-RAS, there are always exceptions to the rule.  The key thing to remember is that you want to place bank stations so that they capture the change in conveyance between the main channel and the overbanks and that the resulting main channel width doesn’t change too drastically from one cross section to the next.  

Friday, March 10, 2017

Rating Curves for Dams

Written by Jesse Rufener, P.E., CFM | GPD Group
Copyright © The RAS Solution 2017. All rights reserved.


Version 5 of HEC-RAS allows the use of rating curves for inline structures (and lateral structures).  Rating curves can be added through the Outlet RC feature in the inline structure editor.  Inline structure rating curves can be useful for evaluating the impacts of structures, such as labyrinth dams, where it may be difficult to correctly capture the geometry and/or flow properties within the inline structure editor.



Click the Outlet RC button and in the following menu you can add your rating curve based on upstream flow or water surface elevation. 


Please note that the rating curve must account for any influence of downstream tailwater as HEC-RAS does not with the rating curve option. 

When using the inline structure rating curve, the top of the dam must be above the highest elevation on the rating curve if you only want the rating curve to account for flow over the outlet feature.  You can check the Stage and Flow hydrograph to see how the rating curve is contributing to flow past the inline structure.  In the image below, the total flow is a combination of the rating curve and flow over the structure as I didn’t have the top of dam elevation set high enough in the first iteration.



Note that the peak flow is 34,471 cfs with a peak HW stage of 927.86.  When I raised the top of the dam to be above the top elevation of the rating curve, the peak flow is 33,967 cfs with a peak HW stage of 930.47 as shown in the Stage and Flow hydrograph below.  The peak flow rates are within 1.5%, but the peak HW elevations differ by almost 3’.





The Outlet RC feature is also available for lateral structures.

Tuesday, December 6, 2016

Time Series Outflows for Dams

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © The RAS Solution 2016. All rights reserved.

Want to simulate a time series of outflows for a dam?  It's easy.  In Version 5.0.3 there's a new feature called Outlet T.S. (Outlet Time Series) in the inline structure editor.  



Click the Outlet TS button and give your new time series a name.  You might name it "Powerhouse" or "Fish Bypass" or something like that.  Once you name it and click OK, it seems like nothing happens.  But what you've done is added the ability for the inline structure to have a flow hydrograph assigned to it in the unsteady flow editor.  

Open up the unsteady flow editor and select the "Add RS..." button.  Then select the inline structure to add it to the list of Boundary Conditions.  Notice in this example, I already have a time series of gate openings assigned to this inline structure.  No matter...just add another boundary condition.  If you click on the empty cell next to the inline structure, you'll see that "Flow Hydrograph" is now an available boundary condition type.


Select the Flow Hydrograph button and now you can enter in a time series of flows for whatever feature you are modeling.  



By the way, this new feature is also available for lateral structures.  

Wednesday, November 23, 2016

Resources for 2D Modeling

Now that we've had 2D modeling in HEC-RAS for almost 2 years, I'm curious to know what your favorite 2D Hydraulic Modeling resources are. Of course, for HEC-RAS modeling, the HEC-RAS 2D Manual is excellent, but what about for a more general discussion of 2D Hydraulic Modeling?  Books?  Journal articles?

I like the Australian Rainfall & Runoff Revision Projects, Project 15:  Two Dimensional Modelling in Urban and Rural Floodplains http://arr.ga.gov.au/revision-projects/project-list/projects/project-15

I also like reading the works of Vincenzo Casulli.  Google him.

If you have a favorite resource for 2D modeling, please share it in the comments section below.


Wednesday, October 19, 2016

Creating Static Results Maps (Shapefiles or Rasters) using RAS Mapper

Written by Mikell Warms  |  WEST Consultants
Copyright © The RAS Solution.  2016.  All rights reserved.
Many readers have been asking how to create shapefiles and other static maps in RAS Mapper. One of the main differences between RAS Mapper in RAS 4.1 and RAS 5.0 is the existence of Dynamic Maps. What makes these maps dynamic is that when you have that Results Layer selected in RAS Mapper, you can use the slider bar (top right) to animate through the simulation and see the results of that layer at each mapping output interval (or each profile for steady flow models). These maps are not saved to the disk, they are temporarily stored in memory.



With this change, you can no longer “Export Layer to Shapefile” as you could previously, due to the fact that these dynamic layers are not one map, they are many maps.

To create static maps, such as shapefiles or rasters, we must first tell RAS Mapper what type of static map we want (Inundation Boundary, Velocity, Shear Stress, etc.) and whether we want the data from a specific timestep, or max/min. To do this, right click on the bold “Results” heading in the Layer Manager in RAS Mapper, then choose “Manage Results Maps”.  All of your plans and default layers will load (like below):


Choose the plan you are interested in, and click on “Add New Map.”

If you’re running a Steady model, the window that pops up will look like this: 

If you’re running an Unsteady model, the window that pops up will look like this: 

Choose the Map Type you are interested in creating; in this case we are creating a maximum Inundation Boundary. Notice we have chosen “Maximum” under the Unsteady Profile options, but we could have chosen “Minimum” or a specific point in time. On the right hand side of the window, make sure to change the Map Output Mode to one of the “Stored (saved to disk)” options, depending on your needs. In this case, we are creating a shapefile polygon. Then click Add Map.

One last step. The Manage Results Maps layer should now look like below:


Notice the static map we have added has appeared. However, the Inundation Boundary map status says “Map Not Created”. You will need to select that map, and click on “Compute/Update Stored Maps” at the top right. If you have multiple static maps, you can select them all by holding the CTRL key and selecting multiple maps prior to computing them.

Once the computations have completed, the status will change to “Map files up to date” and RAS will save your shapefile in your HEC-RAS project folder, in a subfolder that has the same name as the plan from which the shapefiles were created.


Thursday, October 13, 2016

Mississippi River Basin Model Clean-Up Day

For all the HEC-RAS users in the Mississippi area, please consider participating in this service event.  A great opportunity to network with others interested in hydraulic models and to help with the restoration of the historic Mississippi River Basin Physical Model.

Please contact Ms. Sarah McEwan at friendsofMRBM@gmail.com or visit https://www.facebook.com/FriendsofMississippiRiverBasinModel
http://www.friendsofmrbm.org/