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.

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  • 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.
  • A workaround i found is in the link below. Though not exactly what i was looking for but it is better than the alternative.

    communities.bentley.com/.../assigning-catchment-outflow-node-and-tc-automatically-in-bulk

  • 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.

    I get the following Error, 

    The Selected Runoff Method None is not supported. Only the Rational Method is supported by this Solver.

    I wanted to use GVF Rational, as my analysis is more or less Steady State and not Dynamic/ EPS. I have already calculated specific Base Flows at each downpipe.

    What i simply need is to size the network to convey these flows.

  • For the Rational solver you would need to use manholes or catchbasins to directly enter your flows. For a manhole at an upstrea-most point, you can use the Known Flow field or the inflow collection in the manhole properties. For a catchbasin there are a few more options as seen here, though it cannot be modeled with a bolted cover.

    Per my previous replies you can configure the manhole with a bolted cover to allow surcharging (if expected) and set the rim elevation equal to the pipe top if it represents an enclosed junction.


    Regards,

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

  • 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.

    With Invert Elevation and Pipe Diameter Values, shouldn't the Rim Elevation be calculated by the Software? Not sure why these need to be defined separately.

  • The pipe top elevation is indeed calculated by the software automatically but we are referring to the manhole node element.

    The rim elevation of a structure (manhole or catchbasin) is typically at or near ground elevation, whereas the incoming and/or outgoing pipe are below the ground some distance below the rim. My comments were regarding the use of the manhole element instead of a transition element (so that you can enter inflow in the node properties) to represent a below-surface pipe junction.

    Since you can have multiple pipes connected to a manhole at potentially different elevations and diameters/sizes, it would not be possible to compute a single pipe top elevation in the manhole properties. If both of the adjacent pipes happen to be at the same invert and with the same diameter/size, you can infer the manhole rim elevation from the pipe crown (top) elevation and enter it as the rim elevation. Or, if you want to assume that all of these nodes are able to freely surcharge, you could simply add a small distance to the manhole invert elevation to set the rim for all of these manholes, and use the bolted cover option. They would then be able to surcharge if the hydraulic grade is above the rim elevation. This could be accomplished for example by using the manhole Flextable to copy/paste the invert elevations of just these manholes (representing the transition/enclosed junction) to the rim elevation, then perform a global edit on the rim elevation to globally add that small distance.


    Regards,

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

Reply
  • The pipe top elevation is indeed calculated by the software automatically but we are referring to the manhole node element.

    The rim elevation of a structure (manhole or catchbasin) is typically at or near ground elevation, whereas the incoming and/or outgoing pipe are below the ground some distance below the rim. My comments were regarding the use of the manhole element instead of a transition element (so that you can enter inflow in the node properties) to represent a below-surface pipe junction.

    Since you can have multiple pipes connected to a manhole at potentially different elevations and diameters/sizes, it would not be possible to compute a single pipe top elevation in the manhole properties. If both of the adjacent pipes happen to be at the same invert and with the same diameter/size, you can infer the manhole rim elevation from the pipe crown (top) elevation and enter it as the rim elevation. Or, if you want to assume that all of these nodes are able to freely surcharge, you could simply add a small distance to the manhole invert elevation to set the rim for all of these manholes, and use the bolted cover option. They would then be able to surcharge if the hydraulic grade is above the rim elevation. This could be accomplished for example by using the manhole Flextable to copy/paste the invert elevations of just these manholes (representing the transition/enclosed junction) to the rim elevation, then perform a global edit on the rim elevation to globally add that small distance.


    Regards,

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

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