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  • July 6, 2018 at 1:12 am #6947
    rattlesnake200591
    Participant

    Hi everyone,
    i’m setting up a simulation of a river reach with a combined 1/2 D model using Hec -Ras 5.0.5; This is my first combined model so i need an advise from you guys.

    The lateral structures which i’m gonna place right on the river banks as a weir has a limit lenght extent ? Is it better to split the lateral structures in shorter pieces? The lenght of the river reach i’m modelling it’s more or less 35 km. Can i insert a single lateral structure per bank ? I tried to insert it and seemed to be ok, but it was only a try. I’m still setting up the geometry so i still haven’t run the simulation.

    Thank you everyone in advance

    July 6, 2018 at 6:41 pm #11486
    cameron
    Participant

    Break it up into smaller pieces. It If you have a single lateral structure, editing the weir data will take forever as it will take a long time to load data.

    July 6, 2018 at 8:12 pm #11487
    Scott Miller
    Participant

    If the lateral structures serve only to make the 1D/2D connection, then the “weir” profile will follow the terrain, right? Get this from the terrain profile button and transfer it to the embankment station/elevation table. Add a couple millimeters. The station/elevation table will take up to 500 station/elevation pairs. Consider how many elevation grid cells your lateral structures will cross over their lengths.

    Put the ends of the lateral structures on high ground, or the 2D flow area may leak between the ends of lateral structures (someone verify this?). Any bumps on the 35 km profiles may do. If there are reaches of high ground that won’t get flow, skip em. No need for a lateral structure where the water wont cross.

    It may help to keep the lateral structures about the same length. If you need to iterate 1D/2D flow (for stablity), the minimum flow tolerance ought to apply +/- equally to all lateral structures. Or figure which reaches will have the most 1D/2D flow, and make those shorter.

    Take a look at the manual, or this Chris Goodell post and comments, regarding weir coefficients:
    http://hecrasmodel.blogspot.com/2016/07/weir-equations-in-hec-ras.html

    July 7, 2018 at 3:28 pm #11488
    rattlesnake200591
    Participant

    Tnak you for your advices. So since my terrain has a ground resolution of 1 m i should split the lateral structure in pieces of maximum 500 m right ? But i’m not getting why adding some milimeters to the lateral structure elevation profile. Having the lateral structure lifted does it give more stability ?

    Since we’re talking about this i have another qyestion. My DTM isn’t repesentative of the channel, because the Lidar scanner took the water surface, not the channel bottom. To overcome this issue i asked to the client all the cross sections he was in possess of to integrate the DTM, so basically after building the model in RAS Mapper i edited the cross sections obtained from the Terrain with the ones in my posses. Obviously this lead to discrepancies. The question is this, should i make sure that the point in wich i’m gonna place the bank it is placed below the terrain ?

    sezione_27

    zoom

    If i was to insert the banks in the point marked in the previous image should i have edit its elevation to make it lower than the therrain?

    I hope i was clear enough in exposing my question

    July 9, 2018 at 1:57 am #11489
    Scott Miller
    Participant

    It would work to limit your lateral structure lengths to 500 m, but they can be longer since you’re not likely to have a lateral structure completely orthogonal to the elevation grid. I would expect the limit to be 2^0.5 * 500 m, or about 707 m, going entirely diagonal on the grid. But it seems I’ve made lateral structures longer than that limit. Make a center line and take a look at the station/elevation table. If it’s more than 500 rows, shorten it. Or you can edit out lines, but that tends to be time consuming.

    As for adding a couple of millimeters… the elevation profile for an embankment is point to point, jagged. The tailwater 2D flow area elevations are discrete, or stepped, with a single elevation for each mesh cell. It is likely that the jagged line will cut the corners of several steps. The embankment must be at least has high as the tailwater terrain elevation. The geometry preprocessor will indicate where the problems are. Adding a little clearance reduces the number of corrections needed on the profile. There may still be a couple of station/elevation rows to raise.

    Use your professional judgement regarding how to incorporate cross section survey. It’ll be up to you to defend.

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