We have several buildings which we are seeking to harvest rainwater to an earth dam for use. Basically we have several gutters and downpipes. The Downpipes will in turn be connected to collection pipes which eventually flow into the earth dam. The water should basically flow into the earth dam through gravity. The objective here is to establish the furthest building we can collect water from and size the collection pipes accordingly.
So far I have done the following:
Am i in the right direction so far? Is WaterGEMS the right modelling solution?
I currently seem to have one problem though. Negative Junction Pressures, with reasonably high velocities, in areas where logically there shouldn't be none. Am i missing something.
Hi Japheth,
When you say that the objective is to determine the furthest building you can collect rain water from, do you mean that you want to determine the capacity of existing pipes, so you can determine how many building can connect to this system without causing an overflow/backup? Or, are you trying to size the pipes and find a balance between the material/excavation cost and the number of buildings that can contribute to this? Or, something else?
The underlying assumption with WaterGEMS is that pipes are flowing full and under pressure. So, if these "downpipes" are flowing partially full, the results will not be accurate and it would be better to use SewerGEMS or CivilStorm. Both of these programs can model unsteady simulations for gravity pipes, and model ponds, pond outlets and pond sizing. See:
You would then be able to model the runoff hydrograph of the contributing buildings (timing of the hydrograph peaks could be of concern, if some buildings are much farther away than others) and view a profile of the pipeline to determine if it has adequate capacity, along with sizing of the pond and pond outlet.
If the "downpipes" are flowing under pressure/full and you want to assume a certain inflow rate, then WaterGEMS can be used with a negative demand as you described. Keep in mind that the negative demand is a flow rate, which is different from a volume (unless perhaps you configured a demand pattern to model the varying inflow to get the desired overall volume after a period of time). In this case you would need to make an assumption on the downstream side, such as using a reservoir or tank to model the hydraulic grade at the pond (earth dam). Whether or not this is an OK approach may come down to what you ultimately need the model to tell you (see further above).
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Answer Verified By: Japheth Osumo
Japheth Osumo said:I wanted to use SewerGEMS but i was not sure how my network would look like. Will the Tees be defined as Manholes? Should this be defined as a Pressure Network or I simply use Conduits and Manholes? Which Numerical Solver is suitable.
If the pipes may sometimes be full and sometimes not, model them as conduits. If the tee junction is an enclosed structure (with no vertical vault), model it as a transition element. See: Pressure Junction vs Transition vs Manhole
The solver will depend on your needs. The Explicit (SWMM) solver tends to be best for dynamic simulations where you need to route flow over time and the GVF-Rational would be suitable if you need to use Constraint-based pipe sizing. See more here: Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM)
Japheth Osumo said:In SewerGems will i be able to check Pressure and Velocities coz these two are basically the bread the key parameter for my design
Velocity is a result field for conduits but for transitions (and manholes), pressure is not a result field (because they are not always flowing under pressure) but you can view the hydraulic grade and if needed, create a formula-derived user data extension to calculate and report the pressure.
Pressure and velocity can be reported with pressure junctions and pressure pipes, but those are used in cases where the pipe is always flowing full.
Japheth Osumo said:The only data that i have is the recorded daily rainfall for several years. Am not sure this is adequate for a Hydrograph.
SewerGEMS can use storm data hyetographs and rain files along with catchment properties (area, Tc, loss method) to calculate a runoff hydrograph based on several available runoff methods. See: Runoff methods supported by each solver
Japheth Osumo said:At the moment i am simply working with an average flow rate which varies depending on the size of the building.
You can also input inflows in SewerGEMS using a fixed flow or user defined hydrograph.
Japheth Osumo said:Main challenge i have had with this approach is some areas have negative pressures, with reasonably high velocities (2- 3 m/s), but logically they shouldn't. Reducing the pipe sizes makes the pressures Okay but then i end up with even higher flow velocities (5- 7 m/s). My thinking is basically the pipe in those sections wasnt flowing full hence the negative pressures in the first place.
If you are using the approach of forcing an inflow with an upstream negative demand, with a downstream known hydraulic grade (reservoir boundary condition), then the negative demands and pipe physical properties will determine the headloss and velocity.
When you reduce the pipe size, you increase the headloss and thus drive the upstream pressure/HGL higher (enough to prevent the negative pressure from occurring). However, this will result in higher velocity as you noticed, and the upstream pressure required to "push" the assumed flow through the smaller pipes may be higher than the upstream storm connection points, suggesting that they would overflow.
The negative pressure with the larger pipe sizes may suggest part-full flow, requiring SewerGEMS. See: Troubleshooting negative pressures at pumps, junctions, & other node elements
Take a look at a profile of the system (physical elevation and hydraulic grade over the distance) to better understand the distribution of headloss.
Okay. That's very detailed.
Jesse Dringoli said:model it as a transition element
So my model basically starts at the Tee where the connection is basically Gutter > Downpipe> Conveyance Pipeline (Tee). Had actually gone through the Pressure Junction Vs Manhole writeup. Issue I had though is the Tee is essentially enclosed structure so didn't make much sense to define it as a manhole. But then again the Transition element does not have Inflow. So I wasnt sure how how to proceed.
Jesse Dringoli said:upstream pressure required to "push" the assumed flow through the smaller pipes may be higher than the upstream storm connection points, suggesting that they would overflow.
I dont understand what you mean by this.
Lastly is there a way to get my model from WaterGEMS into SewerGEMS, without necessarily starting all over. Obviously some parameters will have to be redefined.
Japheth Osumo said:Issue I had though is the Tee is essentially enclosed structure so didn't make much sense to define it as a manhole. But then again the Transition element does not have Inflow. So I wasnt sure how how to proceed.
You can either attach a catchment (representing the runoff characteristics of the roof/gutter/downspout) directly to the transition element, or model it as a manhole with a bolted cover and top set equal to the pipe top.
Japheth Osumo said:Jesse Dringoli said:upstream pressure required to "push" the assumed flow through the smaller pipes may be higher than the upstream storm connection points, suggesting that they would overflow. I dont understand what you mean by this.
Take a look at a profile and it may make more sense. If you have a known hydraulic grade on the downstream side and a known inflow on the upstream side, then the higher the inflow, the higher the hydraulic grade will be at the upstream side, because the additional flow will cause additional pipe headloss. In the real system this can be visualized as an increase in the height of water needed to push the desired flow rate through the pipes (the "head"). That head/HGL might be above the top of the ground/downspout, which would indicate an overflowing/flooding condition would occur and that larger pipes are needed.
Japheth Osumo said:Lastly is there a way to get my model from WaterGEMS into SewerGEMS, without necessarily starting all over. Obviously some parameters will have to be redefined.
You would need to export the WaterGEMS model (via each element Flextable) to Shapefile format, then import into SewerGEMS using ModelBuilder.