![manning manning](https://image1.slideserve.com/3215654/manning-s-n-value-l.jpg)
Also dont forget about the cross section linear interpolation tool. A way to check would be as you alluded to, is see if there are any quantifiable differences in results when you add more cross sections. However in regards to the number of cross sections required it really depends on the nature of the channel a straight channel without obstructions requires fairly few cross sections. I dont know many details in regards to FEMA specs. He wrote a small blurb on cross sections awhile back: My questions is can i change existing FEMA models for my pre-condition and compare them to the originally model and if the model shows no impact, Then i have met the FEMA conditions? Also is there a standard amount of feet I must maintain between cross sections? It is up to the engineer best judgement to add or minus as many cross sections as they feel necessary to accurately create a model. Per FEMA manaul there is no set amount of cross sections that a modeler needs to include. My questions is regarding moving cross-sections and the amount of distance that I can keep between each cross sections. I need to modify the HEC-RAS model to accurately reflect the conditions at the proposed project. I have a situation where the model that I am currently working in doesn't properly represent the real world. I have a question regarding the location of cross-sections in my model. But a very solid and important point to make nonetheless! Thanks!
![manning manning](https://d2vlcm61l7u1fs.cloudfront.net/media%2F424%2F42454b84-60f1-4c19-baf5-20578a589a0d%2Fphpbc6XDh.png)
I agree with you completely, however, in my experience natural river channels that fit this description of being "narrow" are not very common. As you've already discovered, calibration is the key! That's a good way to approach it, but I would still check both (with and without compositing) to see which provides more sensible results for you. I believe this all this is consistent with your recommendations above, but felt that elaborating on the fact that there are cases where the roughness break is not necessarily the conveyance break may be helpful to new HEC-RAS users. This is similar to the "law of the wall" adjustment common to many flume studies.
![manning manning](https://www.cesdb.com/images/preview/hec-ras-thumb.jpg)
![manning manning](https://www.researchgate.net/publication/272879823/figure/fig2/AS:612905665974274@1523139506972/Observed-and-simulated-flow-hydrograph-at-Barmul-calibration-for-flood-year-2003-using_Q320.jpg)
Placing the bank stations at the "bottom" of the physical bank may overestimate the conveyance capacity of the main channel, as only bed friction (not additional friction caused by rough banks) will be accounted for in the hydraulic calculations for the main channel. This allows a composite Manning's n to be used for the main channel that accounts for the flow resistance imparted by the banks on the main channel. Well done! One clarifying comment on delineating bank stations based on conveyance: for narrow channels, rough banks may affect the conveyance capacity of the main channel such that the bank stations should still be placed at the top of bank / break in grade. Field investigation should be conducted either. I also suggest to run the model with bankfull discharge to cross check the bank stations. Civil3D is great for topography and computing volume, but not for hydraulic analysis 2-6Īlso one of the reasons why I don't like the automatic tool of Civil3D for extracting XSections it is easy and fast, but missess several details and even overlaps XSections.
#MANNING'S N FOR HEC RAS MANUAL#
In this case it is better to use "CompositeManning's n for the Main Channel" Reference Manual pag. In case of steep slope the results is not correct, if the banks are vertical you complitely lose the roughness on the banks. To push the banks to the toe of the banks is correct only if the slope is mild. I think we should pay attention in case like figure 2, with a lot of thick vegetation down the banks of the channel. I've been fortunate, in most cases, to have some high water staking to calibrate to, but sometimes I just have to designate roughness values based on experience of past models. What are your thoughts on putting the bank stations more at a grade break and then selecting to not composite the channel n values? I've also done this when a channel has thick vegetation on the sides of the low flow channel. I usually do as you say, trying to put bank stations at conveyance changes, but have questioned this when I placed them low in the channel and not at a grade break. Thanks for the write-up on bank stations. I'd be thrilled to have some posts by you up here.
#MANNING'S N FOR HEC RAS SERIES#
Any time you feel like writing something up for a "Back to the Basics" series or anything else RAS-related, please send it over. Thanks Vince! I agree with you completely. I'd love to see a whole "Back to Basics" series of posts, which are a lot more user friendly than my standard advice of "read the manual". I've seen a lot of these types of basic questions recently as well, likely as a results of the booming popularity of RAS outside of the US.