	Applies To
	Product(s):	SewerGEMS, CivilStorm
	Version(s):	08.11.XX.XX and higher

Problem

How do you enter the RTK information into a SewerGEMS or CivilStorm model for either the SWMM or the Implicit solver?

Background

The RTK method is used to convert rainfall into RDII inflow in subsurface piping, by generating triangular hydrographs based on sewer flow monitoring and precipitation data. One each for rapid inflow, intermediate infiltration and inflow, and long-term infiltration.

The shape of each of the three triangles is represented by three parameters:

R: the fraction of the rainfall over the watershed that enters the sanitary sewer system.

T: the time to peak in hours.

K: the ratio of the time to recession to the time to peak.

The sum of the three R-values equals the total fraction of rainfall over the sewershed that enters the sanitary sewer system:

R = R1 + R2 + R3

where R1, R2, R3 = fraction of rainfall entering the sewer system from the fast, intermediate, and slow components.

The total R value provides an indication of the total amount of rainfall produced inflow and infiltration that enters the sanitary sewer system. This rate varies with such factors as the specific soil conditions and topography. The total R is less than 0.01 for sanitary sewers in good condition. Typical values range from 0.02 to 0.04 for sanitary sewer systems. Values of total R as high as 0.2 have been found in sanitary sewer systems that are in very poor condition.

This procedure provides nine parameters that can be adjusted to estimate observed wastewater flows from rainfall (R, T, and K for each of the three unit hydrographs). The fitting of unit hydrographs can be time consuming, depending on the number of flow meters and the number of events. Bennett et al. (1999) reported that it took approximately 100 hours to determine the coefficients for five basins using this approach.

As shown below, the three hydrographs are summed to produce the total RDII hydrograph.

The first triangle represents rapid infiltration and inflow. This is the rainfall that more or less immediately enters the sewer during the rainfall event. This typically produces high peak flows in the sanitary sewer system.
The second triangle represents intermediate-term infiltration and inflow response.
The third triangle represents long-term infiltration response.

These parameters represent the recession curve of the observed wastewater flows. It is not always necessary to develop three hydrographs. For example, in a basin where rapid inflow represents almost all RDII, it may be possible to adequately model wet weather flow with only a single hydrograph. Examples of values for the R, T, and K parameters used in two studies are presented below:

See more in chapter 7.6 of Wastewater Collection Systems Modeling and Design

Solution

RTK information is loaded differently depending on whether you are using the SWMM numerical solver or the Implicit numerical solver.

With the Explicit (SWMM) solver, RTK inflows are loaded at manholes and SWMM RTK sets are used to configure RTK parameters.

With the Implicit solver, RTK inflows are loaded at catchment elements and separate Implicit RTK sets are used.

Explicit Solver (SWMM RTK - manhole-based)

Here are the steps to use RTK loads with manholes and the SWMM dynamic solver:

1) Go to Components > Runoff > SWMM RTK Unit Hydrographs

2) Create a new entry (see red box in the screen shot below) by clicking the new icon and then enter your RTK data in the table (see blue box in the screen shot below)

NOTE: the values seen in the screenshot below were arbitrarily entered and not intended to represent real values for RTK.

R: the fraction of the rainfall over the watershed that enters the sanitary sewer system.
T: the time to peak in hours.
K: the ratio of the time to recession to the time to peak.
Dmax: The maximum depth of initial abstraction available (in rain depth units).
Drec: The rate at which any utilized initial abstraction is made available again (in rain depth units per day).
Do: The amount of initial abstraction that has already been utilized at the start of the simulation (in rain depth units).

3) Open the properties of your manholes by double clicking on them and set the property for "Apply SWMM RTK Unit Hydrograph Set?" to "True".

4) Enter the Sewershed Area value, which is the area of the sewershed that contributes rainfall dependent infiltration to the node.

5) Choose the SWMM RTK Unit hydrograph set that you created in step 2.

Implicit Solver (Bentley RTK - catchment-based)

Here are the steps to use RTK loads with catchments and the Implicit dynamic solver:

1) Go to Components > Runoff > RTK Tables

2) Create a new entry by clicking the new icon and then enter your RTK data in the table:

3) Open the properties of your catchments, set the "Runoff Method" to "Unit Hydrograph", select "RTK Unit Hydrograph" from the "Unit Hydrograph" dropdown, then select your RTK set. Global edit in the catchment flextable if you need to change multiple catchments. Use a new Hydrology alternative if you want to keep this configuration separate from other scenarios.

4) Enter the Sewershed Area value in the catchment area field. This is the area of the sewershed that contributes rainfall dependent infiltration.

5) For the "Outflow element" catchment property, select the node which will receive the I&I inflow from the RTK catchment (manhole, catchbasin or cross section, typically).

Troubleshooting and Calculation Options

Problem: My Implicit (catchment-based) RTK results changed after I upgraded to a version higher than 24.00.02.21

Solution: In previous versions, a flow tolerance check was used during unit hydrograph convolution which set the hydrograph flow ordinates below 0.000001 cfs to zero. This was found to cause small differences in results when compared to using the SWMM (manhole-based) RTK method with the same area and storm (reference # 1725058). This impacted mostly small catchment areas but can lead to differences in results when upgrading to version 24.00.03.23 or higher.

Although the new results are technically more accurate, since users often calibrate RTK parameters, it is an undesirable change and therefore a new calculation option was introduced in a patch to version 24.00.03.23, and is included in version 24.00.03.27 and higher.

This new calculation option is called "Use Minimum Flow Tolerance?" and is found near the bottom of the calculation options, in the section "Hydrology (Other)". If set to TRUE, then if using the Bentley implementation of the RTK method (“RTK Tables”), a flow tolerance check is applied during unit hydrograph convolution that sets hydrograph flow ordinates below 0.000001 cfs to zero. This option was created to maintain results consistency with older versions of the application. For new models, leave this option set to FALSE. Note that this option does not apply to SWMM RTK Unit Hydrographs.

So, if you would like to maintain previously-calibrated results for existing models, set this option to "True". For all new models (before RTK calibration), keep it as the default (False) to ensure that the fix is applied (no flow tolerance), so that Implicit RTK results will be comparable to SWMM RTK results if a comparison is performed in the future.

Please contact Technical Support if you need the patch for version 24.00.03.23 (if you cannot upgrade to 24.00.03.27).