Introduction:
Watershed is the area where the flows from higher point drains all the water running to a common outlet. It combines with other watersheds forming network of stream that all drain into larger body of water, such as lakes and oceans. Topography determines where the water flows.
Problem:
It is essential to determine the pre development flow discharge for a development area, where engineers are required to determine the overland flow time and time of concentration which involving the calculation of the length, slope of a catchment. However, it is not easy to manually determine the location of those catchments and the streamline based on the topography although rough estimation can be performed.
Solution:
In MiTS 2, method to delineate watershed is developed to give users the picture of location/area of catchment and streamlines. TauDEM (Terrain Analysis Using Digital Elevation Models) is used as a tool for the extraction and analysis of hydrologic information from topography.
Result:
Following are several examples of watershed analysis juxtaposing between 3D view and the catchment. The color of catchment won’t reflect any elevation but it is indicating each of the catchments. The streamlines thickness also denoting the amount of flow, the thicker the line, the bigger the stream.
The elevation of the topography can be referred in 3D view, where the blue color shade is the lowest point. In comparison to the watershed analysis, the streamlines are thicken as they approach to the blue area, indicating that the flow has accumulated from the higher spot.
You can try on your project or use the provided project files below as an example.
1.Download Project File
The streamline (right image) thickens as it flows to the left side, matches with the 3D view.
2. Download Project File
The elevation is gradually decreasing from the red shade to the blue in the 3D view. The streamline generated also flows towards the upper part as shown in the 3D.
3. Download Project File
The lowest elevation is located on the right side, hence the flow accumulates towards right area as indicated in the 3D and watershed analysis.
4. Download Project File
As indicated in the 3D view, the highest point is situated at the centre and decreasing in elevation to the sides. Streamlines also thickens to the sides indicating that the streams flows to the sides coming from the centre.
Comparison Between Software Generated Streamline vs Actual Streamline
A comparison between existing drain and watershed analysis was conducted using the software. Given a drawing with existing earth drain with survey points. The drawing and project file can be downloaded as per link.
The location of the existing earth drain is as in the image below, indicated with blue-color line.
Based on the data, you are required to estimate the discharge given the existing drain, which therefore require you to determine the catchment and streamlines.
By using MiTS watershed delineation, streamlines and catchments are generated based on the topography. As you can see, the line thickens as it approaches towards the bottom right, which indicating that the area is the lower land. You can also compare the result with the 3D view to see the condition of the topography.
Based on 3D view, the highest point (red shade) is located at the upper part and gradually decreases towards the bottom right area. Watershed analysis also shows the right behaviour with the minimum thickness at the upstream and accumulated towards the bottom area with increasing thickness.
From watershed analysis, you can also obtain the catchment area and the overland flow. The catchments are indicated with different color of shades indicating each of the catchments and the overland flow is denoted as white color line connecting from a catchment towards the streamline. From the analysis, users are able to estimate the area and length even more accurately, rather than through rough estimation.
If users want to directly use the information from the watershed analysis, you are required to superimpose the watershed image to the keyplan. Users can use the superimposed watershed image as tracing drawing and manually create the catchments following the tracing, or easily import the catchment from the ACAD. The rest such as the streamlines and the overland flow length can be created manually following the tracing. The example of the superimposition result is as image below.
(Project File)
The yellow lines and purple lines indicate the streamlines and catchments generated by computer, whereas the blue line indicates the actual streamline drawn by the surveyors. You can see that what is observed in nature and the computer generation is highly fidel to one another, and hence validating the our computer simulation result.
One can tune the parameter as explained in the blog to obtain more or less fine-details of the streamlines.
Conclusion:
Based on the watershed modelling, the computer can delineate streamlines and catchments to the streamlines with high degree of fidelity. Based on this, users can now sort of scientifically compute the hydrograph and have the feel of where and how the water flows more accurately, instead of just relying on guesswork and hunches to draw the catchments and compute the tc quantities such as streamline lengths and slopes and so on.
You can also read other details pertaining watershed such as on how to use the feature or what approach we use as per link;
Watershed Analysis: Automatic streamline and catchment delineation in MiTS 2;
