Mapping T&T

Setup the RAS Project

HEC-RAS main window — The main HEC-RAS window

Starting a new RAS project is always exciting, possibly because my mistake counter is at 0. I am using HEC-RAS 6.4.1 since that is the latest version as of right now.

Project Naming

An end user should be able to get a general idea of the project from the file name. Any file (and folder) names) should not contain any spaces or special characters. Make use of underscores or CamelCase instead. Now there is a limit on the number of characters so be wary of names getting cut. I have named this project Caroni.prj

Switch the Units

Since we are not in the US, we need to switch the Units to Metric. Go to Options > Unit System. The main RAS window should now say SI Units in the bottom right corner.

Define the Projection

Before we create any geometry, we need to define the projection. This is done in RAS Mapper, which is HEC-RAS’s built-in GIS. Its not ready to put ESRI out of business, but it is getting more powerful with each iteration of RAS.

I like to create a folder called Projection and stick that in the model folder. In fact, any files that the model references should be located within the model folder. The projection for this project is WGS 1984 UTM Zone 20N since that sounds familiar to me. You can copy the projection file (.prj) from an existing shapefile that is in the correct projection and rename it projection.prj.

Open RAS Mapper and go to Project > Set Projection. Choose the projection file and ensure that it is correct before hitting OK.

HEC-RAS Projection — Define the Projection

Map Layers

Map layers are used for reference to add additional detail when creating geometry or viewing results. Right click on Map Layer > Add Web Imagery. There are several data sources to choose from. I like using Google Hybrid because of the labels. If the imagery does not show up, there may be an issue with the projection.

There is very good documentation for HEC-RAS that I encourage you to explore if you want to learn more. Now that we have a project set up with the basics. The next step is to Create a Terrain.

Delineate the Basin Boundary

We need to delineate the basin boundary if we want to build a 2D model of a watershed. A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel.

The most essential dataset in this process is a Digital Elevation Model (DEM). Luckily I have access to a 2m DEM (I am unable to share). A 2m resolution is definitely not bad although the current industry standard is 1m or better. The more detailed model you want is the higher resolution data you need.

I am using ArcPro for this process. The steps to delineate watershed are as follows:

Fill – This fills in sinks in a surface raster so that we know which direction water will flow when the depressions are filled. Imagine a natural pond or lake. When water fills the area, the water will leave the area at the lowest point of the basin boundary. This helps to find that point.
Flow Direction – This creates a raster that shows which direction water flows from each cell, based on the Fil raster. The D8 method is used, which models flow direction from each cell to its steepest downslope neighbour.
Flow Accumulation – This is raster with a count of all the cells that flow into each cell based on the Flow Direction raster.
Create Pour Points shapefile – A Pour Point is the outlet of a watershed. It is the most downstream point of a watershed, where all cells of a Flow Accumulation Raster flow to. When placing a point, take care to place to point on the drainage line of the basin represented by a line a cells in the fac raster with significantly higher values than the surrounding cells.

Placing a Pour Point. The white line represents the drainage line in the Flow Accumulation raster

For the Caroni watershed I used 3 Pour Points as when the Caroni River reaches the lower part of the watershed near the swamp, the floodwaters and there is no singular outlet for the entire watershed. Well there is the main Caroni River outlet. According to Google Maps, the other two outlet points are at the Madame Espagnol River and the Cunupia River.

Snap Pour Points –This ensures that the Pour Points are placed directly on the Flow Accumulation cell that represents the drainage line of a watershed.
Watershed – The Watershed tool determines the contributing area above the Pour Points based on the Flow Direction raster. Note that multiple Pour Points can be computed at the same time.
Raster to Polygon – Convert the output watershed raster to polygon shapefiles that can be cleaned up and imported into HEC-RAS. Simplify the polygons, inspect and remove self-intersecting loops and sliver polygons.

Now we have a basin boundary! Next up is setting up the HEC-RAS project.

Let’s Create a 2D RAS Model

2D RAS Model of the Caroni River basin near Piarco international Airport

Let’s create a 2D RAS model of the Caroni River basin in Trinidad. Any data presented here is only for educational purposes and is not to be used for commercial or political purposes.

This will be in form of a rough tutorial. If you want more details, find a way to contact me. Each post will focus on a different aspect of the model. I use ArcPro in addition to HEC-RAS to build the model. Possibly in the future I will go over the geoprocessing steps using QGIS or Python and GDAL.

The first step is to delineate the basin boundary.