Watershed Analysis

This command has a collection of tools to analyze the runoff of a surface defined by a triangulation or grid surface file. After selecting the surface file of the surface, the program docks a dialog on the left side of the drawing window. While the Watershed Analysis dialog is running, other AutoCAD and Carlson commands are not available. To zoom or pan the drawing view, use the buttons at the top of the dialog, or use the middle button of a wheel-mouse.

Watershed Analysis calculates the flow connections between the triangles and along the edges of the triangulation. The Rainfall amount is used in the processing for figuring the runoff volume to determine when the volume is enough to spillover a local depression in the surface. Besides the Rainfall amount, the runoff coefficients as defined in Define Runoff Layers are also used to calculate the runoff volumes. When the local depression is small enough the runoff will continue through. Otherwise this spot is called a sink for where the runoff stops. The Allow Overflow Along Boundary option applies to watersheds that have runoff that hits the surface border. This option will check whether this border runoff can spillover and merge with the neighboring watersheds along the border.

The Draw Watersheds function draws the watershed areas using the settings under the Draw tab. The back arrow next to the Draw Watersheds button will erase any previous Draw Watershed entities. The Watershed Perimeters option will draw closed polyline perimeters for each watershed area. The Fill Watershed Areas option will solid fill hatch each area using different colors. The Buffer Hatch option will hatch the perimeters of the watershed areas with the specified width instead of hatching in the entire watershed area. The Hatch Structure Areas option will hatch the drainage areas covered by structure inlets defined in the Structures tab. The Sink Locations setting draws a symbol at the low point for each drainage area. The High Point Locations option draws a triangle symbol at the highest point within each watershed. Typically, this high point will be along the watershed boundary polylines that follow the high points along the ridges between the watersheds. The Pond Areas option draws a solid fill hatch in blue for the area covered by the runoff volume of low points. In the example shown, the Fill Watershed Areas and Sink Locations options are active. The Max Flow Lines option draws polylines for the longest flow line within each watershed. These longest flow polylines can be used to calculate the time of concentration. The Group Watershed Entities option will make AutoCAD groups for the set of entities drawn for each watershed.

The Spillover Location option draws symbols at low points within the watershed area that fill up with runoff and spillover on the way to the lowest (sink) location of the watershed. The Setup button allows you to specify criteria for identifying spillover points. These settings include the minimum drainage area, storage volume, drainage volume. 

These settings allow you to filter out small spillover points (ie a pothole) and only draw the significant ones. The Calculate and Draw Options buttons allow you to control what is going to be drawn how, it contains settings for  the symbol, size and layers to use for the entities created by Watershed Analysis and a number of labeling options.

Gate Control

The watershed calculation can be controlled through use of "gates". Gate is placed at the points of likely overflow from one watershed to another. The gate in its natural state is simply an indication that potentially the connection can occur at the point, joining two watershed areas into one if there is too much runoff volume in at least one of the watershed areas to be contained within. The gate can also be forced closed, which indicates that two areas will not be joined regardless the runoff amount. The example of such case is when there are two large ponded areas you want to treat separately in calculations or further design. On flip side, there are instances when you may want two areas to be joined regardless the overflow actually occurring, for example one area is very small. This is accomplished with a gate forced open. There two ways to control gate state: automatically and manually. Here are the settings for automatic control.

If amount of ponded runoff in an area exceeds the maximum pond volume to merge, the downstream gate will be closed automatically. If area is too small or too shallow, its downstream gate will be forced open, joining it with watershed downstream.

To control gates manually, please first draw gate labels, exit Watershed Analysis, then double-click on gate labels in the drawing to control their state. Once done editing the gate state, just re-run the calculation to draw and report the new adjusted watersheds.

Runoff Tracking

The Above Point function reports the watershed data of the current pointer position in real-time as the pointer is moved around. The watershed data is shown in a tooltip next to the pointer position. This data has values for the overall watershed that the position is in including the sink elevation, sink name, drainage area and average slope percent. This data also has values for the watershed above the current point including the drainage area and runoff volume. Plus this data shows the elevation and runoff coefficient at the current point. If the position is picked with the mouse, then the program draws a polyline perimeter for the drainage area above the current point.

The Above Line function is similar to Above Point except that you pick two points and the program draws the watershed for all flow that crosses the line between these two points. For example, you can pick points at the left and right banks of a stream to get the drainage area for that stream above these points.

The Draw Connections function draws lines with arrows between the triangles for how the program has determined their flow connections.

When a triangulation file is processed by Watershed Analysis, some of the flow connection data is stored into the triangulation file to speed up reprocessing. The Re-Process function resets this flow connection data to start the flow calculations from scratch.

The Detail Inspect function reports flow connection data at the pointer position in real-time as the pointer is moved. This data includes the current position triangle number, connecting flow triangle number, sink node number, watershed name, border elevation, ridge elevation, low elevation, downstream sink number, number of source triangles, number of source nodes, current elevation and spillover elevation.

The Watershed Inspect function reports runoff flow data at the pointer position in real-time as the pointer is moved. The runoff data is shown in a tooltip next to the pointer and in the Data tab. This data has values for the overall watershed that the position is in including the sink elevation, sink name, drainage area and average slope percent. This data also has values for the watershed above the current point including the drainage area and runoff volume. Plus this data shows the elevation and runoff coefficient at the current point. When the Hatch Area Being Inspected option is active, the watershed area for the current position is hatched during inspection.

The Watersheds Report function runs the report formatter to choose which of the watershed parameters to report. The Ponds Report function reports the position and depth of each ponding area.

Here are some of the values contained in watershed report:
Rain volume - total volume of the runoff for the area
Holding volume - the maximum volume the watershed can contain near the sink
Ponding volume - the volume of all the ponds within the watershed
Uncontained volume - the difference between amount of runoff and the volume of runoff trapped on the slope or at the sink

Additionally, the properties of the pond at the sink are reported: surface elevation, max depth, volume and area.

Structures

Besides calculating the runoff of the triangulation surface, Watershed Analysis can also process the runoff effects from structures for inlets, storage ponds, culverts and channels. The structures in Watershed Analysis are simply for placement and watershed delineation. These structures do not have design considerations for parameters like pipe size. In the Structure tab, there is a list of the structures to apply with the current surface. The list shows the name, type and drainage area for each structure. The Draw function will draw symbols for each structure. The Inlet structures act as sinks in the watershed and capture all the flow that comes to the inlet point. Each inlet is defined by a single point and a name. The Storage Tank structures also act as sinks and are defined by a single point and name. The Culvert structures route the flow from the culvert inlet to the outlet. The culverts are defined by two points for the inlet and outlet and by a name. The Channel structure is the same as the Culvert except that it can have more than two points to define the flow path. The structure data can be stored to a Watershed Structure File (wst) using the Save button. The Load button can read the structure data from either a wst file or from a sewer network file (.sew).

Pulldown Menu Location: Watershed
Keyboard Command: watershed
Prerequisite: Triangulation File