Tuesday, May 23, 2017

Wormhole Island - "What's the Best Shape for a Wormhole Culvert?"

Written by Krey Price  |  Director, Surface Water Solutions
Copyright © The RAS Solution 2017.  All rights reserved
A recent question was posted on the HEC-RAS blog regarding the optimal shape of the SA/2D Area Connection alignment for a wormhole culvert – in particular, whether a “Z” shape or “S” shape would be preferable. My apologies in advance for the drawn-out response, but I’ve had this question come up a number of times in class and thought I’d post some of my whiteboard sketches along with some random thoughts on the topic:
“Z” or “S”?
If you draw a “Z” shape, the order in which the vertices are entered will determine the direction of flow (always oriented from left to right looking downstream in HEC-RAS). The following image shows four different ways to draw a “Z”-shaped connection along with the associated orientation of flow that will be assumed in HEC-RAS. In the case of a wormhole culvert, flow could enter the “wormhole” at any of the green arrows (or at any point along each of the adjacent faces) and exit along any of the faces indicated by the red arrows. Wormhole culvert inlets and outlets typically wouldn’t be located along the diagonal segment of the “Z”, but directional arrows are shown along those segments to illustrate how the orientation of flow is preserved along the entire shape:

Read more here:

Tuesday, May 16, 2017

Putting Wormhole Culverts to the Test

Written by Krey Price  |  Director, Surface Water Solutions
Copyright © The RAS Solution 2017.  All rights reserved. 

The original post about “Wormhole Culverts” received thousands of hits, and many HEC-RAS users are now applying this method regularly in their models; but how valid are the hydraulics over the full range of open channel flow, pressure flow, and weir flow? Given the amount of use they are getting, it’s high time to put wormhole culverts to the test!

This test run assesses wormhole culverts against other approaches for modelling hydraulic structures in 1D and 2D model reaches.

The results show very similar water surface profiles between the various methods. The wormhole method provides the ability to correctly display terrain data for roadways and bridge decks in viewing plan and profile results.

While coupled 1D-2D reaches would still be required for detailed bridge designs in HEC-RAS, wormhole culverts appear to be a viable means of accounting for bridges and culverts with substantial terrain detail between the inlet and outlet that is subject to 2-dimensional flood flows.

Read more about the model setup and results here.

Monday, May 8, 2017

European HEC Software Workshop - London July 25-27 2017

JBA Consulting will be hosting another HEC Software Workshop this summer.

Back for 2017

This is the second software workshop in Europe dedicated to the HEC hydraulic and hydrological modelling software – HEC-RAS, HEC-HMS, HEC-DSS and HEC-ResSim.

We are working with the team at Hydrologic Engineering Center plus other leading users of the HEC software bringing you the chance to meet, learn, explore and discover the HEC software which is available for free.
This event will be packed with key-note presentations, master-classes, case studies and time to network with fellow modellers and researchers. The workshops and case studies will feature the latest thinking from some of the leading experts in this area, giving you practical solutions to take away with you. Offering you the flexibility of three ways to attend:
  • One day workshop
  • Two day training course
  • Three day event
There will be particular emphasis on HEC-HMS and the two day training course will focus on its use in flood forecasting, routing methods and rainfall-runoff approaches.

Check the following link for more information:

Monday, May 1, 2017

Back to the Basics: Bank Station Placement - Part 2

Written by Martin J. Teal, P.E., P.H., D.WRE  |  Vice President, WEST Consultants 
Copyright © The RAS Solution 2017.  All rights reserved. 

Expanding upon Chris’ discussion of where to place bank stations, what should you do about high terrain somewhere in the middle of your cross section?  Here is an example:

How should we treat the left overbank?  It’s hard to tell if the high area next the left bank is isolated (i.e., it would be an island if the water surface were to get to elevation 370 or so) or if it is a continuous feature (such as a levee) that would prevent flow from accessing the left overbank until it is overtopped.  Looking at this another way, is the lower ground of the left overbank a continuous flow path or is it an isolated low spot (for example, a mining pit)? Aerial photography can often help determine the situation; here is the overhead view for our example:

The area in question is vegetated (the terrain goes up steeply when it gets to the storage yard on the bottom of the photograph) but it is hard to tell if the high point in the terrain would be constraining flow or if the low area is a potential flow path.  Looking at the cross sections upstream and downstream of the one in question will often provide answers, but does not help in this particular example.  In this case, the best course of action would be to go out to the river and see for yourself, then imagine how the water would behave.  Depending on your conclusion, there are several ways that this can be modeled.

1.  Isolated high spot.  If flow can simply go around the high spot in this particular cross section then we probably don’t need any further adjustments. You may get a “divided flow” warning in the output that signifies that the program detected dry ground with water on either side, but no action is needed to address the warning in this case. Assuming that the computed water surface elevation is high enough, this solution will also allow flow in the left overbank.

2.  Isolated low spot in overbank.  You could model this as per #1 above but in that case you should check flow distribution between the channel and overbanks up and downstream of this cross section for reasonable transitions (see earlier blog post from May 20, 2009).  Or, if you think that the low area should only store but not convey water you could set an ineffective flow limit as shown below.

3.  Continuous high ground.  If the high ground is really a ridge that would prevent the water from accessing the lower ground in the left overbank, it should be modeled as a levee. However, in this case another decision needs to be made depending on what happens after the levee is overtopped. Will the water be conveyed on the land side, or will it just pond?  If the latter you may need to add an ineffective flow limit at or just to the left of the levee.

4.  Something in between.  Regardless of whether the high or low features of the cross section are continuous, water is able to access the left overbank.  Natural streams often have “backswamp” areas behind either human-made or natural levees that flood and store water but do not really convey much flow downstream. If the left overbank in our example is like this, we could model it by using the ineffective flow limit as per #2 above.  However, ineffective flow means zero conveyance.  If we expect some water to move in the overbank, albeit very slowly, you may want to allow a small non-zero conveyance.  A few sharp-eyed readers may have noticed that we are using a Manning’s roughness coefficient of 0.3 in the left overbank. Using this value allows a small amount of conveyance in that overbank without zeroing it out completely.