Combined Sewer System Design With SewerGEMS

Hello Sergio,

The workflow you mentioned should work for your modeling requirements. However, first try to understand what kind of system you actually want to model. If this is an existing system you are modeling then it should be as close as possible to the actual system, but if its a new system then you would have to "design" the system and see if your design is appropriate for the conditions on your site.

Sergio Aoun said:

Is that the exact methodology before computing the DESIGN? and NOT analysis...

I guess this question is on similar lines as to when a system is to be "designed" and when is it "analyzed". As explained above if you are modeling a network which exists on the field then an "analysis" will help you understand the key parameters of the system such as flow, velocity etc. For this you need actual site data such as elevations, pipe slopes, material, roughness values etc. to model the system as close to the real network as possible. In cases where you might not have data or are working on a new network, "designing" is the way to go. Based on the constraints you provide and the initial data such as ground elevations, pipes you plan to use (sizes and material); the software will "design" the system for the intended flows.

If you are looking to design your system see this video which explains how SewerGEMS can help you in this.

Let me know if this helps.

Hello, again, Yashodhan!
First of all, I want to thank you for your kind assistance; it means a lot really;

Yes, regarding my model, I am designing the whole system from scratch; knowing only the ground elevation of the terrain model, I suggested a default invert elevation for each manhole, but some of my concerns were:

1)Is it CORRECT in SewerGEMS to place 2 catch basins on the sides of each manhole and connect them to the manhole by laterals? Is there anything special I should know when modeling this, or should I completely proceed forward?

2)Is it necessary to connect catch basins with gutters? Assuming the inlet efficiency is 100%

3)I have already specified the design constraints that I want within the model, but I want to ask you if there is a specific way to set the maximum depth of the manhole that I need below the ground elevation is 4m?

4)When I draw the catchment areas that are draining onto each catch basin, is the rainfall data that I defined automatically applied?

Waiting to hear from you,

Kind Regards

Hey Sergio,

Are you using the GVF-Rational Solver for design and analysis?

Sergio Aoun said:

1)Is it CORRECT in SewerGEMS to place 2 catch basins on the sides of each manhole and connect them to the manhole by laterals? Is there anything special I should know when modeling this, or should I completely proceed forward?

What are these two catch-basins representing? Are you planning to capture flow from two different sides of a road or something? You can have multiple catchments draining into a catch-basin or a manhole too. Are you using catch-basins only to capture flow from the catchments? If you are not planning on designing the catch-basins (road side curb / inlet / grate etc.) then you could directly have the catchments flow to your manholes.

Sergio Aoun said:

2)Is it necessary to connect catch basins with gutters? Assuming the inlet efficiency is 100%

Gutters capture the overflow from catch-basins. Usually, when designing an existing catch-basin, the parameters of the inlet are fixed and based on these parameters the inlet has some capture efficiency. So, if an inlet has a capture efficiency of 70% the balance flow (of 30%) is the overflow which will be captured by the gutters and transferred downstream. If your catch-basin has a capture efficiency of 100% or "full capture" then no flow will be diverted to the gutters so you can choose not to have them.

Sergio Aoun said:

3)I have already specified the design constraints that I want within the model, but I want to ask you if there is a specific way to set the maximum depth of the manhole that I need below the ground elevation is 4m?

You can limit the minimum drop in the manholes to restrict the maximum depth of manholes. However, its best to manually tweak the invert elevations after design and re-analyze to see if the results are acceptable. 

Sergio Aoun said:

4)When I draw the catchment areas that are draining onto each catch basin, is the rainfall data that I defined automatically applied?

The rainfall data you provide is used to calculate the rainfall intensity (mm/hr, inch/hr etc.) which is used to calculate runoff from the catchment using the catchment area as input and the runoff coefficient based on the method you choose for runoff analysis. The flow (runoff) from the catchment is based on these factors.

You can find more information in the Help section of SewerGEMS and I recommend you to go through the content Sushma shared for learning.

That is very helpful, Yashodhan! Thank you so much; however, to clarify a small point;

1)The 2 catch basins represent 2 inlets that each capture 100% of the storm runoff from a specific catchment adjacent to each other; my goal is to have 1 inlet of each side of every manhole (flowing through gravity into a free outfall), the inlets (Catch basins) capture 100% of the storm runoff, each inlet has a catchment drawn for it, the inlets capture the runoff, drain them into the manhole by laterals, the manholes are already subject to sanitary loads, just making sure that following this process and using the GVF Convex solver is the right methodology to follow when designing a combined sewer system...

Sergio Aoun said:

That is very helpful, Yashodhan! Thank you so much; however, to clarify a small point;

1)The 2 catch basins represent 2 inlets that each capture 100% of the storm runoff from a specific catchment adjacent to each other; my goal is to have 1 inlet of each side of every manhole (flowing through gravity into a free outfall), the inlets (Catch basins) capture 100% of the storm runoff, each inlet has a catchment drawn for it, the inlets capture the runoff, drain them into the manhole by laterals, the manholes are already subject to sanitary loads, just making sure that following this process and using the GVF Convex solver is the right methodology to follow when designing a combined sewer system...

Using catchbasins connected to a manhole, with catchments assigned to the catchbasins would indeed model the system as you describe - catchment flow will approach the inlet opening, be captured (ensure that the inlet type is set to full capture or the inlet location set to in-sag if you want to assume full capture) and enter the subsurface network along with sanitary loads. This will work with the GVF-Convex solver and you can use automated design if desired, to changed pipe sizes and element inverts to meet design criteria. Note that for the Implicit solver, laterals are not supported. You also have the option of modeling the lateral pipe as a conduit (so, connecting the catchbasins to the manhole via conduits).

Reviewing the training course will also help give you some confidence in the modeling approaches used.

Sergio Aoun said:

That is very helpful, Yashodhan! Thank you so much; however, to clarify a small point;

1)The 2 catch basins represent 2 inlets that each capture 100% of the storm runoff from a specific catchment adjacent to each other; my goal is to have 1 inlet of each side of every manhole (flowing through gravity into a free outfall), the inlets (Catch basins) capture 100% of the storm runoff, each inlet has a catchment drawn for it, the inlets capture the runoff, drain them into the manhole by laterals, the manholes are already subject to sanitary loads, just making sure that following this process and using the GVF Convex solver is the right methodology to follow when designing a combined sewer system...

Using catchbasins connected to a manhole, with catchments assigned to the catchbasins would indeed model the system as you describe - catchment flow will approach the inlet opening, be captured (ensure that the inlet type is set to full capture or the inlet location set to in-sag if you want to assume full capture) and enter the subsurface network along with sanitary loads. This will work with the GVF-Convex solver and you can use automated design if desired, to changed pipe sizes and element inverts to meet design criteria. Note that for the Implicit solver, laterals are not supported. You also have the option of modeling the lateral pipe as a conduit (so, connecting the catchbasins to the manhole via conduits).

Reviewing the training course will also help give you some confidence in the modeling approaches used.