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OpenFlows | Water Infrastructure Wiki Modeling Low Impact Development (LID) Controls
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    Low Impact Development (LID)

       
      Applies To 
       
      Product(s): CivilStorm, SewerCAD, SewerGEMS
      Version(s): V8i
      Environment:  N/A
      Area:  Modeling
      Subarea:  N/A
      Original Author: Terry Foster, Bentley Technical Support Group
       

     

     

     

     

     

     

     

     

    Overview

    The purpose of this technote is to guide the user on entering the data required for Lower Impact Development (LID) Controls..

     Low Impact Development Types

    • Bio-retention cells (rain gardens, green roofs)
    • Rain barrels
    • Porous pavements
    • Vegetative swales
    • Infiltration trenches

    Summary of LID Control Layers

    LID control is represented by a combination of vertical layers, which may include (in top to bottom order):

    • Surface
    • Soil
    • Pavement
    • Storage (with optional underdrain except at rain barrels)

    Defining Low Impact Development Control

    Controls are defined in the LID Controls editor

    • Defined per unit area
    • Basic design can be reused in multiple locations

    Model performs moisture balance

    • How much water moves between layers
    • How much water is stored within each layer

    In a project, you would go to Components > Low Impact Development Controls.  This is where you would enter the different LID Controls for the project.  After these are set up, you can export them to the Engineering Libraries for use in other projects.  


    Note:  The underdrain is only required on a rain barrel control.  If using a rain barrel, you will notice that it has a Drain Delay.  The drain delay is the number of dry weather hours that must elapse before the drain line in a rain barrel is opened (the line is assumed to be closed once rainfall begins).

    After entering the Low Impact Development (LID) Controls, you will add the catchments and LID elements to the network.

    Note:

    • Every LID element is associated with a "Parent" catchment that contributes flow to the LID.
    • The parent catchment outflow hydrograph reflects LID Control(s).
    • The parent catchment may be associated with multiple LID controls acting in parallel.
    • A parent catchment may include a non-LID area.
    • The catchment must have an outflow node, which can be another catchment (useful for modeling LID controls in series).

    Low Impact Development Element Properties

     

    1. Occupies Full Catchment?:  If set to True, the LID Control will occupy the full area of the LID Control's parent catchment.
      Occupied Full Catchment?:  If set to False, the properties will change and prompt for three additional fields.
      Area of Each Unit (ft^2):  Specify the surface area occupied by the LID.
      Percent Impervious Area Treated (%):  The percentage of the impervious portion of the sub-catchment's non-LID area whose runoff is treated by the LID practice.
      Send Outflow to Pervious Area?:  If the outflow from the LID Control is returned onto the sub-catchment's pervious area rather than going to the sub-catchment's outlet, set this to True.  An example where this might apply is a rain barrel whose contents are used to irrigate a lawn area.  This field is ignored if the LID Control property Occupies Full Catchment? is set to True.
    2. Top Width of Overland Flow Surface of Each Unit (ft):  This is a parameter that is only applied to LID processes such as Porous Pavement and Vegetative Swales that use overland flow to convey surface runoff.  Bio-retention Cells and Infiltration Trenches simply spill any excess captured runoff over their berms.
    3. Percent Initially Saturated (%):  This is the degree to which the unit's soil is initially filled with water in a Bio-retention Cell.  Zero saturation corresponds to the wilting point moisture content.  One hundred saturation has the moisture content equal to the porosity.  The storage zone beneath the soil zone of the cell is assumed to be completely dry.  For other types of LID Controls, this corresponds to the degree to which their storage zone is initially filled with water.
    4. Number of Replicate Units:  This is the number of equal size LID units.

    In CivilStorm and SewerGEMS, storm information and catchment input are the same as with the traditional Rational Method (StormCAD) - the user must enter an IDF curve for the model and Tc, area and C factor for each catchment. As discussed above, the difference is that a single storm duration must be entered, which is then used to determine the peak flow and duration of each catchment's Modified Rational runoff hydrograph.

    The duration in this case is stored in the Rainfall Runoff alternative and can be entered by going to Components > Global Storm Events. In this dialog, the Modified Rational storm duration is entered in the "duration" field next to each rainfall runoff alternative. The alternative will of course need to be assigned to the appropriate scenario being analyzed. The Modified Rational hydrograph will then be constructed per the diagram further above, using this duration.


    Note: do not confuse the "duration" here with the simulation duration. If you enter 24 hours, your Modified Rational hydrographs will extend to 24 hours and the peak will be based on the intensity on the IDF curve at 24 hours.

    How do I decide what duration to enter?

    The storm duration you enter is based on your engineering judgment. If unsure, you may want to check with your local reviewer to see if they require the use of a specific duration with the Modified Rational method. You may find that the Modified Rational method isn't an accepted hydrograph method and that something else like the Unit Hydrograph may be better suited for analysis (as opposed to design) in CivilStorm or SewerGEMS.

    A trial and error process could also be used, to find which duration is best for your modeling case. Shorter durations may yield higher peak flows, but longer durations will yield higher overall volume. For instance, take a look at the below graph of runoff from a single Modified Rational method catchment along with overflow at a downstream node.

    As you can see, the duration that resulted in the highest overflow volume was 12 minutes, not 10. There is a balance between hydrograph volume and peak flow.

    I am required to use the Rational Method but I also need to quantify overflow, so I chose CivilStorm/SewerGEMS

    As per above, you will need to be careful to recognize the differences between the traditional Rational Method that your reviewer is likely referring to and the Modified Rational method, which is not quite comparable. The Rational Method in StormCAD is typically used to size pipes for proposed systems to prevent flooding, whereas the Modified Rational method used by CivilStorm and SewerGEMS is a hydrograph routing method more often used for analysis of existing systems experiencing flooding.

    If you have a license, you might still consider using StormCAD for this analysis. For example if you're trying to identify areas with flooding problems and then resize the pipes using the standard Rational Method, you will still be able to do both. StormCAD cannot quantify overflow and will reset the HGL to the rim elevation for flooded nodes, but you may find that knowing these locations where flooding is a problem may be good enough. In this case, it may then be appropriate to use the automated design tool along with the inherent Rational Method to size the pipes.

    If you find that your reviewer accepts other methods such as the Unit Hydrograph for analysis of flooding (as opposed to design/pipe sizing), you could continue using CivilStorm/SewerGEMS with a change to the catchment's runoff method. Keep in mind though that the Rational/Modified Rational method and Unit hydrograph methods are not comparable, so different peak flows are generally expected. Ultimately it will be up to you as the engineer to decide how to proceed.

    PondPack Implementation

    Implementation of the Modified Rational method in PondPack is mostly the same, with one key difference: it automatically computes the "critical" storm duration for you, using an iterative process. This critical duration only applies to a single catchment and is based on the assumption that you are designing a pond which reduces outflow down to a predevelopment "target" flow. It is more common to use the Modified Rational method in PondPack for a single pond and catchment for this purpose, as opposed to using it with CivilStorm or SewerGEMS with a storm drainage network.

    In contrast to CivilStorm or SewerGEMS where there may or may not be a pond, PondPack assumes that you will be designing a pond, with the criteria being that you wish the peak outflow from the pond to match that of the conditions before development. This is a typical requirement which is meant to reduce the impact of development on downstream hydrology.

    So, PondPack requires the same Rational Method input (Tc, Area and C) plus the predevelopment peak flow. It then constructs a series of Modified Rational hydrographs per the same process discussed above, overlaying an approximated pond outflow hydrograph. Two options are available for approximating this pond outflow hydrograph: Method I and Method T. These methods are shown in the below illustration. Method I assumes a linear outflow hydrograph, going from zero flow at time zero to the predevelopment peak where it meets the falling limb of the hydrograph. Method T's approximated outflow hydrograph follows a shape similar to the trapezoidal runoff hydrograph. For each storm duration tested, PondPack finds the estimated storage volume, which is the shaded area above the estimated outflow hydrograph. This volume represents how large the pond will need to be in order to store the runoff hydrograph while still keeping the peak outflow below the predevelopment target (with the assumption that the pond you design will have an outflow hydrograph shaped like either the "I" or "T" method). The "critical" storm duration is the one that results in the highest storage volume (shaded area). This duration will be longer than the Tc, but typically less than an hour.

    PondPack stores information on this iterative process, which can be found in Report Builder, in the report entitled "Modified Rational Storm Calculations":

    In the example above, a storm duration of 0.383 hours resulted in the highest required storage volume of 0.155 ac-ft, and was selected as the critical storm duration. The critical Storm duration is then used to generate the postdevelopment runoff hydrograph for that catchment. (peak flow and duration). As mentioned in previous sections, this duration is normally greater than the catchment's Tc, so it will result in a peak flow less than the traditional Rational Method. From the illustrations above, it is clear that a storm duration equal to the Tc would result in a high peak flow, but a smaller volume. Basically volume becomes more of a concerned with pond design, whereas peak flow is more a concern with pipe sizing (such as with StormCAD, which uses the traditional Rational Method.)

    What if this is an existing system where the pre/post conditions are the same, or what if I don't care about the predevelopment peak flow?

    The underlying assumption with the iterative process that PondPack uses to find the critical storm duration is that you are designing a pond to attenuate the peak flow down to some target value. If you want to simply analyze an existing system without having to enter predevelopment information, you should select "true" for "User Defined TD?" in the Modified Rational catchment's properties, then enter the desired storm duration. This bypasses PondPack's iterative process of determining the critical storm duration, which would otherwise fail if the Pre and post information were the same. Remember that the storm duration you enter here determines the peak flow and duration of the runoff hydrograph.

    What if I have more than one catchment?

    If you have multiple catchments in your system, the Modified Rational method may not be the right runoff method for you. Since PondPack will automatically calculate the critical storm duration, differences in the catchments would likely result in different critical storm durations being chosen for each one. This would be like saying that different storms were occurring at the same time over the same site.

    Aside from accepting this inconsistency or choosing another runoff method, the only other option you have in this case would be to select "true" for "User defined Td" for each catchment, then manually set them all to have the same storm duration.

    I changed the predevelopment information and my postdevelopment hydrograph changed. This doesn't make sense!

    One might be tempted to wonder why a change to the predevelopment target flow has an effect on the postdevelopment runoff hydrograph. Intuitively, what happens in the predevelopment conditions should not affect what happens in the postdevelpment conditions. However if you were to change the target peak flow for a Modified Rational catchment, you will likely see a different runoff hydrograph on your model (different peak flow/duration).

    The reason lies in the iterative process that PondPack uses to calculate the critical storm duration. As mentioned further above, this process looks at the required storage volume for a range of storm durations and selects the duration that has the highest. The required storage volume is the area above the estimated pond outflow hydrograph, which is based on the target peak flow. So, the required storage volumes will change with a change to the target peak flow, and thus the critical storm duration may be different. The different duration would then result in a different postdevelopment runoff hydrograph.

    In other words, by changing the target peak flow, you're changing the way the pond outflow will need to be designed, which changes the storage volume you'll need for your pond, which means the storm that results in this critical storage volume may be different. If you wish to override the automatic calculation of the critical storm duration, you will need to force the storm duration by using the "User defined TD" option.

    How do I use the Modified Rational Method with PondMaker?

    When using the Modified Rational method, the pond design steps in PondMaker are slightly different. Since both the predevelopment and postdevelopment information is entered in the catchment, you will not have separate pre and post scenarios.

    In the first step (Design Scenario Configuration), you will need to choose "Modified Rational catchment" for the "Target Flow/Volume source", then select the Modified Rational catchment for the "Target element". Computing step 1 will then populate both the pre and postdevelopment information in the worksheet. To replicate the estimated storage volume used by method I, choose “linear” for the option under the Estimated Storage step of PondMaker.

    What if some of the watershed is not being detained in the postdevelopment conditions?

    To account for a bypass/undetained flow with the Modified Rational method:

    1) Perform a Q=CIA calculation, using only the area of the watershed that is not being detained in the postdeveloped condition

    2) Subtract this peak flow value from your predeveloped peak flow.

    3) Use the resulting value as a user defined target flow in the subarea, instead of the predeveloped. By doing this, a Modified Rational hydrograph can be generated, since the post developed peak will be greater than the predeveloped peak.

    Conclusion

    In summary, results from the Modified Rational Method are not necessarily comparable to those from the traditional  Rational Method due to the introduction of the element of time. Although it may be useful for pond design in PondPack, you may find that the Modified Rational Method may not be appropriate for hydrograph routing in a storm drainage network (in SewerGEMS or CivilStorm) and that another method such as SCS Unit Hydrograph is sometimes a better choice.

    The peak flow from the Rational Method in StormCAD is based on each catchment’s time of concentration. In SewerGEMS and CivilStorm, it is based on a single storm duration that you must enter under Components > Global Storm Events. In PondPack, a critical storm duration is calculated automatically.

    See Also

    Product TechNotes and FAQs

    Haestad Methods Product Tech Notes And FAQs

     

     

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