Analyzing Flow direction in Geometric Networks in ArcGIS

Question

I have a task in ArcGIS which involves using Geometric Network and right now I am experimenting with Set Flow direction function from Utility Network Analyst toolbar in ArcMap. I've read section Flow direction in a geometric network from the help but there is some unclear for me things left.

I want to clearly understand how this function is working in ArcGIS. So, I've:

1. Created two feature classes in common feature dataset: one polyline pipelines and second - point water_obj (there will be junctions, sources and consumers).
2. Created Geometric Network, added there these two feature classes. For water_obj I have stated possibility to set Source/Sink.
3. Established connectivity rules:
   • pipelines can connect to water_obj and set it to default.
   • pipelines can connect to other pipelines via water_obj.

Result of my connectivity rules looks like:

Then goes my testing...

STEP1

Trying to draw some example edges and click Set Flow direction button:

Having Indeterminate flow and this is predictable.

STEP2

Change one junction to Sink:

This is also OK and corresponds to documentation.

STEP3

Change one more junction to Sink:

This one actually I can't understand.

STEP4

Trying to add source:

Nothing changed.

QUESTION

Finally I need to create a utility network with many consumers (connected houses to water net).

How can I correctly create it in order to be able to use Set Flow direction? I am trying to avoid custom programming of flow direction if it will be possible.

Can anybody point me into right way?

PS the final network should look like:

Answer 1

First, let me say that I share your frustration with the inability to set flow direction where there is a split with multiple sinks. As far as I can see, you have set up your dataset, layers and network properly.

The explanation that I received is that when there are 2 or more sinks on a common junction, each sink potentially has the ability to draw flow from all the other junctions, and ArcGIS cannot determine which sink is going to be more dominant, based on elevation or some other input value. Here is what ArcGIS actually sees.

Since Sink 1 could draw flow from the Source and Sink 2, and Sink 2 could draw flow from the Source and Sink 1, ArcGIS sees the flow potentially going in both directions simultaneously, so it marks them as indeterminate.

If I read your question correctly, you are connecting houses to a water network. If by this you mean a water supply network, then you are probably more likely to see a network like the following:

A water system tends to be looped throughout neighborhoods to ensure consistent water supply if and when one section has to be shut off for maintenance. It also helps to ensure that water does not sit for too long in one part of the network, and thus degrade in quality, flavor, etc. One of the characteristics of a looped system like this is that the flow direction is variable. Usage at one or more sinks changes the water pressure in different parts of the network, which then causes the water flow direction to shift, moving water toward the lowest pressure. This help document discusses Flow direction in a geometric network.

The graphic that you are showing looks more like a sewer collection system, except in reverse. In a sewer system, there are multiple sources, forming a tree, that all feed into branches, which merge into a trunk, and usually end at a single sink. The problem that arises is when there are multiple sinks, for example, when there are split manholes that divert flow to an overflow pipe in case of heavy flow. In cases like this situation described above occurs.

This issue is the main reason that the people I know who do water and wastewater hydraulic modeling do not use the Network Analyst for setting up utility networks. They usually use modeling software that works as an extension to ArcGIS. Some examples are InfoWater or InfoSWMM by Innovyze, formerly MWH Soft. These set flow direction based on digitized direction and attributes of the network itself, like invert elevations.

One place you might check out is the ESRI Water Utilities Group. They have developed some customized tools to help manage water utilities in ArcGIS. I don't know if they have solved the flow direction problem, but they may have some tools to help you set it manually.

I leave you with the final question though, of why do you want to set flow direction for a water supply network?

Hope this helps.