Hello Nikolas,

I guess you are using 2D unsteady modeling but I am not sure if you are going only with hydraulic model or morphodynamic one. For hydraulic model, you need some kinds of upstream and downstream boundary conditions. Further, if you have more than one reach, you may need internal boundary conditions. To solve the Saint-Venant equations with Hec-RAS, you need flow hydrograph as an upstream boundary condition. There is more than one option, however, for downstream boundary condition. And each one has its limitation and accuracy. For most research work, the flow-stage relationships tends to be the best option.

Hello Zeyad,

Thank you for spending some time answering my question. Your advice was really useful. So i should use external boundary (flow hydrograph) for upstream, downstream( Normal Depth) and internal boundaries for points of interest(junctions,reaches). I want to ask you if there is a difference to the way i place the boundary lines. For example if i place the internal boundary for the reach horizontally covering the whole reach, is this ok? Where exactly should i place it(at the start of the reach or middle, if its vertically). Same for the juctions. I would appreciate some more help. Thank you in advanced.

N.P.

Dear Nickolas,

When you use Normal Depth as a downstream boundary condition, you assume that at downstream node, the energy slope (Sf) is equal to ( or in balance with) channel gradient or what we called the channel bed slope (S). In this case, you force model to start with normal depth at downstream reach. This approximation is valid when you have subcritical flow in you system. However, I prefer to use Stag- Flow hydrograph id you data at downstream station of your model.

Regard to your question about the internal boundary, I am not sure which kind of boundary you have there. But for sure there is difference if you mean kind of physical boundaries such as levee or something else. So, please give more deals about the internal boundary of your model. I though you have lateral flow at some point along your study reach but it seems you are referring to something else.

Zeyad

Dear Zeyad,

Thank you again for your answer. As for the first scale of your answer, unfortunately i dont have enough information about the stage of the water so i cant use a stage hydrograph.

You understood well the situation about the internal boundaries. I have two reaches and 3 junctions (lateral flows). I need some help to the way i place the line(internal boundary). For junctions, i assume that i have to place the boundary line vertically to the point. For reaches should i place the boundary line vertically to some random point on the reach or horizontally covering the line of the whole reach as a boundary(so the flow hydrograph will cover the whole reach).

I attached some pictures for a better display.

N.P.

Hello all,

I'm a new user of HEC RAS. But as I'm also studying HEC RAS I will try to solve the in this way. What if we create 2D flow area for the basins (in this question, 3 basins) and add precipitation data for each basin and have only one downstream boundary condition at the outlet (which we can take as a stage hydrograph if the stage versus time data is available). This is just my idea and hope to know your comments.

Dear @ Nickolas,

I looked at the Figures and seems to be interesting to me. For internal boundary conditions of lateral flow, they should be vertically placed ( perpendicular to river reach) at specific node. You can easily determine the location of lateral flow in your model from Ras map tools as I can see attached photos. So, flow should be specified at specific location or node and not horizontally along the reach and at each point. I am sure you have transects along the study reach. So, the location of each of this transect is known. You need only to know at which transect you have lateral flow and then you need to use your internal boundary conditions there where flow as changed.

You may look at P. D. P. O. Peramuna comments and see it works your model objectives. Sometime, separating the basin into sub basins or not depends on the objectives of work. But both cases should work fine.

All the best,

Zeyad

I am really grateful for your answers. Thank you both. I will try to run the model based on your suggestions and make an update.

You are welcomed and best of luck.

Zeyad

Thank you for the comment Dr.Zeyad and hoping to see the update Nickolas

Hi Dr. Zeyad, if you know if in HEC-RAS 6.0 can introduce the infiltration without the rain? I have only the flow hydrograph upstream and donwnstream boundary conditions, and i need that a volume to be lost.

I entered the % impervious and infiltration layer, but this dont work

Thank you