SSA in place of HEC RAS

[Anonymous]

I have been using SSA (and formerly Stormnet) for quite some time.  However, this has been mainly for onsite sizing of storm drains, detention, etc.

I have a new project where I need to supply a 'flood study' for the effect of a culvert we are installing across a stream.  To me I can accurately model this in SSA with the irregular channels, junctions, etc.  I have calculated the peak 100 year flow using Andersons method and plan on adding this into an upstream node as an 'External Inflow'.

The local reviewer prefers HEC RAS due to his familiarity with it, of course.  Having not used HEC RAS before I have been reading the manual and it seems to me that SSA would just as accurately model the flood.  BUT I was hoping someone would be able to comment on the following:

1.  I assume that to find the effect of the peak 100 year flow, HEC RAS would use a Steady State Flow analysis with this peak flow.  I believe SSA would accomplish this same task using my calculated peak Q as an 'External Inflow' in the most upstream node (note, I am only modeling a few hundred feet of channel.)

2.  It is easy enough to plot HGL profiles, but I can't think of a way where I would be able to plot/show the flood depth of water in my irregular channel cross sections I would input.  I will instead print out the defined cross section from the dialogue box and manually draw in the water elevation.  It would just be nice if it was automatic.

3.  Does anyone have any other general input as to why I may want to use HEC RAS instead of SSA?  It just seems like a time sink for me to get up to speed on HEC RAS if I can accomplish the same in SSA.

Thanks for any feedback.

[Matt.Anderson]

JMiller -

You have an interesting quandry. 

• I am interesting the Anderson method...(for obvious reasons! )  Is this in the Virginia area and the Dan Anderson method? (love google!)

• Take a look at River Analysis, which is HEC-RAS on Civil 3D-roids! Check it out here: http://labs.autodesk.com/utilities/civil3d_river/

1. Yes.  SSA external flow value would be equivalent to the Steady Flow value.
2. If you use River Analysis, or Sample Lines/ALignment; this could provide you the cross-section irregualar channel data for either HEC-RAS or SSA.  I would agree that there is a need in getting SSA results into a plan/flooding form.
3. As for the difference between SSA and HEC-RAS;  the input data is nearly identical between the irregular channel of SSA and that needed for HEC_RAS.   The calculations are going to be very close, if not identical for an equally setup model.  HEC-RAS will allow you to model a more complex crossing that contains ineffective flow areas around the culvert.

Hope that helps!

[Anonymous]

Thanks for the feedback Matt.

A couple thoughts:

• I will add being able to plot x-sections of links with peak water depth to the wishlist.  I would think it would be relatively simple to accomplish.
• Yes, the Anderson method is a Virginia DOT method used for larger watersheds and handy for computing peak flows.  See Chapter 6 of the VDOT Drainage Manual if interested in incorporating into SSA

http://www.extranet.vdot.state.va.us/locdes/electronic%20pubs/2002%20Drainage%20Manual/pdf/drain-man...

[Anonymous]

If you are doing river/stream/open channel analysis, Hec-Ras is far better than SSA.

In Hec-Ras you can assign xsec in the node thereby giving you xsec that varries between nodes.

While in SSA you can only assign xsec in its conveyance link which is uniform between nodes.

Hope Autodesk solves this problem.

[Matt.Anderson]

Which problem is it that you want solved?

HEC-RAS defines a channel at a point and describes the distance downstream to which that channel applies.  

SSA defines a conveyance link with a distance between nodes, and applies a cross-section to that lenght.

[Anonymous]

yes your right.

But what happens if you have several cross-sections.

What I mean is in SSA you cannot assign various xsecs at nodes to make it a real representative of the actual stream or river.

All SSA can do is to have a uniform xsec along the length.

[Anonymous]

instead of tapering, it will be a recession at  the nodes.

[Matt.Anderson]

HEC-RAS does NOT vary the channel horizontally from one cross-section to the next.  It does have a tool to allow you to interpolate between sections, but to be fair to SSA/EPA-SWMM, you would need to add those interpolated sections to SSA to be comparing apple to apples.

Within HEC-RAS, the modeling of the floodplains horizontal variations between cross-sections is accepted practice since the real world isn't all straight lines.  The profile values between sections is an interpolation in HEC-RAS.

In SSAs profile view - based on the time series and the channel - those are calculated values - up to 50 slices per link.  

The selection of the appropriate locations with HEC-RAS is critical, as well as the correct selection and division of the channel for SSA.

Each tool has a use.   FEMA does allow the use of EPA-SWMM for floodplain calculations.  EPA-SWMM is the engine within SSA.  There is one caveat - SWMM can't perform floodway analysis.

[Anonymous]

Hi Matthew,

Thanks for all the clarifications.

I learned that Autodesk is coming up with Project River Analysis Extension for AutoCAD Civil 3D 2012.

It is now on beta testing and free to download until the end of this year.

Have you tried this module.

Feedback from one of Autodesk Resellers in Australia said that they got less people interested in it.

They are waiting for more feedbacks until 31 December 2011.

If this does not improve before the cut off date, they will not release it.

Kind regards,

Romeo

[Matt.Anderson]

RCruz -

Yes, I am familiar with Project RIver Analysis. River Analysis comes purchased technology, ie it was the BOSS RiverCAD.

The Project is a Labs preview and very public, and not a beta.  Beta's are not public information and can't be talked about.  

Since it is a preview on Labs, there is an expiration.  If you noticed, the AutoCAD Civil 3D Sub-assembly composer just graduated from the Labs and is now a subscription benefit.

River Analysis recently was publicized in ENR recently.

I am not sure what you mean "they got less people intersted in it" - less people compared to what? 

[RickCarle-PE]

Matt, I see that River Analysis has been extended to July 2013, but it does not appear to install unless C3D 2012 has been installed.  I'm using C3D 2013.  Will River Analysis work with 2013? 

Also, I've read the discussions comparing SSA to HEC-RAS.  I need to model a stream that has culverts and bridges on it and we need to be able to set ineffective flow areas.  Can SSA do this or should we stick with HEC-RAS?

[Matt.Anderson]

Rick -

River and Flood Analysis 2014 is the next step after the Labs release.  Labs was 2012 only.

If you have ineffective flow area, and bridges, you will likely need to stick to a backwater calculation solution like HEC-RAS Steady State.  SSA does not have a specific "bridge" element, nor ineffective flow areas, but it is rather similair to Unsteady HEC-RAS.

[shahriar.abdsharifabadi]

I know it's been a long time since the conversation, but I would like to ask a similar question. I have a piped sewer network that daylights into an irregular open channel. Based on what was mentioned here: SSA does not have ineffective flow areas, hence cannot perform tailwater conditions due to water backing up behind culverts or naturally formed check-dams along the river length. HEC-RAS or RFA can perform more realistic channel hydraulics, but I cannot apply the water profile elevations to the outfall of my piped network to carry the effects through my structures.

Is there any way to couple the two models? i.e. applying the Qp for different events to my RAS model and obtaining WSE's and applying that as tailwater condition to my outfall in SSA. 

Please bear in mind I am fairly new to using SSA. 

[dkhooE3MAL]

Hi there,

SSA performs hydrodynamic modelling and will compute headwater levels for backing up behind culverts or via backing up behind a check dam, it depends on how you model it.  I.e, For modelling a simple culvert, allow a junction US with the invert level being the culvert US invert level and the DS node being a outfall.  SSA will automatically compute the upstream headwater level for you.  You also can manually input a tailwater level into the outfall node if you are subject to downstream tailwater.

You can repeat this process for a series of culverts by adding in a series of nodes and culvert links before finally outfalling somewhere.  Not sure if this answers your question but hopefully it helps.