Tuesday, 2 July 2013

Sewer Network Capacity Analysis Part 2

This post follows on from the previous one on drawing setup

Create Pipe Network
After the initial setup it is time to create your pipe network. There are two main options.

1. Create Pipe Network from Objects. This is usually a better choice for proposed networks where you may have drawn the initial pipe layout using a series of connected polylines. For a survey like this each pipe run will typically be a single polyline. When you use this command it will allow you only to select one object so you would end up with multiple (many, many multiple) pipe networks that would need to be merged together at the end into a complete network. This is a pain and it is best avoided.

2. Pipe Network Creation Tools. This opens up the pipes toolbar and will allow creation of multiple parts. You select the pipe and structure type and size and then start tracing over the survey polylines. This is a better option.When you are prompted, give your network a name and set the parts list. Do not set a reference surface as we do not want elements to try and correct themselves based on any surface information later.

What to Include in your Existing Network
Initially you may plan on recreating all elements of the surveyed network using C3D pipes. This is most likely unnecessary and will probably cause you problems.  See below:
Based on the surface slope arrows and also the layout of the kerbs and gullies I can determine the catchment for a particular structure (Blue Polyline). This structure has 6 gully connections all of which feed from the same catchment. The total flow for the catchment is assigned to the structure and it is not necessary to subdivide the catchment into flow for each gully. Therefore there is no benefit to modelling gully connections as C3D pipes. This will reduce the number of pipes and structures in your network.

Drafting the Pipe Network
The plan here is to use the pipes toolbar to trace over the main elements in your network and recreate the pipes and structures. We will forget about invert levels and cover levels when laying out the network and return to these afterwards.
Use the pipes and structures command above, start to layout your network by clicking at each structure location. The default drafting direction will be downstream  so try to work this way - otherwise you may get warnings when you try to analyse in SSA later.

Editing Invert & Cover Levels
Once you have recreated the main elements of your network we can then inspect the survey elements to determine the actual invert & cover levels of the pipes and structures and apply these to our network elements.

There are two main ways of getting this info from the survey elements:

  1. Use the 'ID' command (type at command line) and click on the ends of the pipe polylines to get invert levels.
  2. Use the text at the structures to determine cover and invert.
Using ID for each polyline is the most thorough and accurate approach. An (quicker and less accurate) alternative is to use the invert level of the structures as the invert level of the incoming and outgoing pipes - and assume they are they same. This is a lot quicker and may not make that much difference to the overall capacity analysis. Whatever approach you decide to use you will need to make note of the invert levels of the pipes and structures and then apply these to the C3D elements.

The best way to manually add this info is to select a structure and click on Structure Properties on the ribbon. On the Connected Parts tab you can edit the invert levels of the incoming and outgoing pipes first.

Next move to the Part Properties tab and set the Automatic Surface Adjustment to False - you can then enter the cover level (Rim elevation). Set the Control Sump Depth By property to Elevation and enter the structure invert level (Sump Elevation).

Click Apply and ensure that the values you enter hold. If they change then check to make sure that the structure invert level isn't higher then the lowest connected pipe invert. If this is ok then maybe look at the type of structure you are using. Sometimes there will be minimum dimensions for the structure elements and these may affect the manually entered invert levels - an you may need to use a different structure type.

This process is repeated for the entire network. It is important to ensure that you have the network as correct as possible at this stage before you export to SSA or you will be presented with a series of errors when you run the analysis.

Common things to look out for are that the correct number of pipes are connected to a structure and that they are sloping the correct way.
In the connected parts tab you can identify what pipes are coming into a structure (should be IN and have positive slope) and those which are going out (should be OUT and have negative slope. If these look wrong then check invert levels before exporting.

Another common issue at this stage is that a pipe is not connected to a structure. It may look like it is connected in plan but if it is not listed on the Connected Parts tab then it is not connected. You can fix this by selecting the pipe in plan and choose Connect Part on the ribbon and select the structure to connect.

If a pipe or structure is the incorrect type or size you can select it, right click and choose swap part and choose the correct one.

Next Post covers how to create catchments, and export to SSA.

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