Model Set-up and Result Analysis of HEC-RAS Model

HEC-RAS is a freeware developed by US Army Corp of Engineers. It can be used to build 1D/2D hydraulic modelling. In this article I am going to show you on how to build 1D model and analyze the results.

Model Set-Up

1D River Model has been developed based on the collected data as shown in Figure below:

Figure above shows the measured point data at the site. As you can see, the measured points still appear. However, the data is good enough especially in the river lines (see the green color). From this data, we extracted the sectional cross area of the river every 25 meter interval and slope for the longitudinal profile. And also we need this data to put as background in HEC-RAS model as we are going to digitize the background image. After you add background image, you are going to find something like this:

To make it easier when you start to digitize the river, I’ll suggest to divide the pictures based on your interested area. For my case, I divided into: river, center line, cross-section, layout plan in different colors as you can see above. For your information, HEC-RAS can support for figures as shape files, CAD files, TIF files, etc. You can choose whatever data you have. However, it is important to make sure that your background figures have been correctly projected.  This is important when you want to move your data and result to the other software such GIS, then it is going to be much easier.

After you have put the background pictures, the next step is digitizing the river and input the cross-section data as follows:

Steps:

1. Add a new Cross Section from Options panel and then enter the river station name.
2.  Fill in the station and elevation values for each stations.
3. Data for Reach Length, Manning’s Values and Main Channel Bank stations are also need to be filled in.

After we have done with geometric data, then we can continue to fill the flow data. It depends on model that we set-up, for my case, I’ll simulate in unsteady flow. Open Edit panel and click Unsteady Flow Data...

Basically it will show you what kind of data that would you input on the upstream and downstream boundaries. There are several options there such as stage hydrograph, flow hydrograph, stage/flow hydrograph, rating curve or normal depth. For my case study, the input for the upstream boundary was determined by hydrological modelling using HEC-HMS. The result is flow hydrograph in meter cubic per second. For the input of the downstream boundary is normal depth which generated by friction slope. After this you can start running the model. Of course you still need to setup your time step and data management stuff.

Model Results

There are some options that can be used to show the results (see View panel). Let’s open DSS data …  and check the result. As my interest is the capacity of river in catering the rainfall design of 100 year return period therefore I’ll check the maximum elevation of the river whether it is beyond the existing river bank or not. 

You can see the animated result as shown above. The important part is the data that showing the maximum elevation at the river. It can been seen for 100 year flood design, it is higher that river bank even higher than reserve levels. I have also made simulation for 2 year flood design and it seems the river was originally designed for 2 year flood. Therefore improvement of the existing river is necessary as it makes flooding when the rainfall is higher than 2 year flood design statistically.