Rainwater Harvesting

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:

  1. Modelled the network in WaterGEMS
  2. Calculated the volume of rainfall that will be collected at each downpipe. The Volume is then added to the junction as a negative value to the downpipe. FYI the Junction is basically at the Tee (Connection between downpipe and Collector)

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.

Parents
  • 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 Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

    Answer Verified By: Japheth Osumo 

    1. 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?
      1. This is a new system without existing pipes. Basically am trying to come up with a new network, determine the sizes of pipes and which buildings can i collect their water from by gravity.
    2. So, if these "downpipes" are flowing partially full, the results will not be accurate and it would be better to use SewerGEMS or CivilStorm.
      1. My model begins at the Tee between the downpipe and the Conveyance pipes. Am not modelling the downpipes themselves. The assumption here is that at the Tee the flow is better though obviously it might not also be full.
      2. 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.
      3. In SewerGems will i be able to check Pressure and Velocities coz these two are basically the bread the key parameter for my design
    3. 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.
      1. The only data that i have is the recorded daily rainfall for several years. Am not sure this is adequate for a Hydrograph.
      2. At the moment i am simply working with an average flow rate which varies depending on the size of the building.
    4. 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).
      1. This is precisely what i have so far. But there was barely any reference on the same so i was wondering whether i was even on the correct path hence why i asked.
      2. I dont have much data with respect to time. Just daily records for several years
      3. On the downstream side i have a reservoir that at the moment is basically where everything flows into.
      4. 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.
  • 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.

    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.

    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.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • It means setting the rim elevation field in the manhole properties equal to the top of the actual enclosed structure. Meaning, if this represents a pipe junction with no actual vertical structure, then the top of the "junction" is the top of the pipe. If you must use a manhole (instead of the suggested transition element) then you would set the rim elevation (top of the structure) to the elevation of the top of the pipe, and choose the bolted cover option so that surcharging occurs if the HGL is above the rim.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • Have tried the two options, with GVF Rational, and need some clarifications.

    1. Use of Transition Element: I have already calculated the flows at all the drop pipes. This requires me to define catchments for like 1,000 drop pipes. Now since i already know the flows do i really need to draw the catchment areas accurately or i can simply have random shapes. I get a Global Storm event is not selected. I already have calculated the expected flows in Excel. Cant i simply use these without defining storm events
    2. Use of Manholes: There's Ground, Rim and Invert Elevations. Assuming my pipe will basically follow the ground surface (Invert= Ground Surface). What is the best way of populating these..
  • Now since i already know the flows do i really need to draw the catchment areas accurately or i can simply have random shapes

    Yes, you can hold the CTRL key and left click in the drawing to start drawing the default shape. Move your cursor and then click again to place. Select "None" as the runoff method in the catchment properties (or global edit in the catchment Flextable), then enter your fixed flow in the "Inflow (wet collection)" as a fixed flow. You can also use the Inflow Control Center (Components > Loading) to initialize and enter a fixed flow for all catchments, if needed.

    Note that the Implicit and Explicit solver perform an unsteady simulation where the element of time is involved. This means that if you enter a constant flow, it will be the same flow value for all timesteps. So, you would want to make sure that the flow values you calculated are acceptable to assume as a constant flow rather than a hydrograph that rises up to a peak for a short period of time and then drops back down. Consider the amount of water volume from a constant flow and the impact it has on the hydraulics of an unsteady simulation.

    Use of Manholes: There's Ground, Rim and Invert Elevations. Assuming my pipe will basically follow the ground surface (Invert= Ground Surface). What is the best way of populating these..

    If you are modeling an enclosed junction of pipes (no vertical vault structure) and the pipes are resting on the ground, then the manhole invert and ground elevation would be equal to the ground and the rim elevation would be equal to the pipe top/crown.

    To quickly populate the ground, you could use Terrain Extractor, and then copy/paste the ground elevation values to the invert elevation, using the Flextable: Copy and paste (import or export) tabular or Flextable data

    If all pipes are the same diameter, you could also paste the ground elevation to the rim elevation field, then perform a global edit to globally add the pipe diameter/rise value. If there are varying conduit diameters, you would need to manually add the diameter of the adjacent conduit to the invert to get the rim elevation. You could perhaps use annotations to help with this.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

Reply
  • Now since i already know the flows do i really need to draw the catchment areas accurately or i can simply have random shapes

    Yes, you can hold the CTRL key and left click in the drawing to start drawing the default shape. Move your cursor and then click again to place. Select "None" as the runoff method in the catchment properties (or global edit in the catchment Flextable), then enter your fixed flow in the "Inflow (wet collection)" as a fixed flow. You can also use the Inflow Control Center (Components > Loading) to initialize and enter a fixed flow for all catchments, if needed.

    Note that the Implicit and Explicit solver perform an unsteady simulation where the element of time is involved. This means that if you enter a constant flow, it will be the same flow value for all timesteps. So, you would want to make sure that the flow values you calculated are acceptable to assume as a constant flow rather than a hydrograph that rises up to a peak for a short period of time and then drops back down. Consider the amount of water volume from a constant flow and the impact it has on the hydraulics of an unsteady simulation.

    Use of Manholes: There's Ground, Rim and Invert Elevations. Assuming my pipe will basically follow the ground surface (Invert= Ground Surface). What is the best way of populating these..

    If you are modeling an enclosed junction of pipes (no vertical vault structure) and the pipes are resting on the ground, then the manhole invert and ground elevation would be equal to the ground and the rim elevation would be equal to the pipe top/crown.

    To quickly populate the ground, you could use Terrain Extractor, and then copy/paste the ground elevation values to the invert elevation, using the Flextable: Copy and paste (import or export) tabular or Flextable data

    If all pipes are the same diameter, you could also paste the ground elevation to the rim elevation field, then perform a global edit to globally add the pipe diameter/rise value. If there are varying conduit diameters, you would need to manually add the diameter of the adjacent conduit to the invert to get the rim elevation. You could perhaps use annotations to help with this.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

Children