The Effect of the Wrong Raingage Interval in H2OMap SWMM and InfoSWMM

Here is an example showing the Runoff, Rainfall and Infiltration losses when you use the wrong interval based on the hyetograph time series.

1.       Figure 1 shows the Rainfall, Runoff and Infiltration for a time interval of 5 minutes which matches the hyetograph interval, the raingage has units of mm and is the total volume over the interval

a.      The volume of rainfall is 20.03 mm as it the total rainfall over the interval

b.      The rainfall, runoff and infiltration graphs are smooth

2.      Figure 2 shows the Rainfall, Runoff and Infiltration for a time interval of 1 minutes which does NOT match the hyetograph interval, the raingage has units of mm and is the total volume over the interval

a.      The volume of rainfall is 20.03 mm as it the total rainfall over the interval

b.      The rainfall, runoff and infiltration graphs are not smooth as there is a gap of no rainfall between minute 1 and 5 for all of the hyetograph time intervals

c.       The peak rainfall is higher as the rainfall is over 1 minute instead of 5 minutes

d.      The peak runoff and infiltration rates are also higher but this difference depends on the roughness, depression storage and infiltration parameters of the catchment

e.      The slower the runoff the more Figure 1 and Figure 2 runoff will look alike even though the rainfall is the spiky versus smooth (Figure 3).

Figure 1.  The Rainfall, Runoff and Infiltration loss for a gage interval of 5 minutes which matches the Time Series Interval

Figure 2.  The Rainfall, Runoff and Infiltration loss for a gage interval of 1 minutes which does not  match the Time Series Interval

Figure 3.  The Rainfall, Runoff and Infiltration loss for a gage interval of 1 minutes which does not  match the Time Series Interval, the Subcatchments Width is smaller than was used in Figure 1 and Figure 2

How are Flooded Time, Surcharged Time and Flooded Volume Calculated in InfoSWMM and H2OMAP SWMM?

How are Flooded Time, Surcharged Time and Flooded Volume Calculated in InfoSWMM and H2OMAP SWMM?

The time, volume and flooded rate shown in the InfoSWMM and H2OMAP SWMM Report File Node Flooding Summary (Figure 2) are calculated as follows (Figure 1):

For All Nodes NOT Outfalls ( this includes Junctions, Storage Nodes, Dividers)

If the New Volume is greater than the Full Volume of the or there is Overflow then at each time step the Time Flooded is increased

If the New Volume is greater than the Full Volume of the or there is Overflow then at each time step the Volume Flooded is increased by the Overflow *Time Step

If the New Volume is greater than the Full Volume of the or there is Overflow AND Surface Ponding is Used then the Ponded Volume is New Volume – Full Volume

If the Node Depth Plus the Node Invert Elevation is above the Node Crown Elevation then at each time step the time surcharged is increased.   The InfoSWMM andH2OMAP SWMM Map Display variables should be FLOOD_VOLM for the No Surface Ponding option (Figure 3) and PONDED_VOL if you are using the Global Surface Ponding Option (Figure 4).

Figure 1.  Levels of Surcharged and Flooding in SWMM 5.

Figure 2.  SWMM 5 Node Flooding Summary or the InfoSWMM and H2OMAP SWMM HTML Report file.

Figure 3.  The Map Display of the Node Flooding using the No Surface Ponding Option should use the Map Display Variable FLOOD_VOLM

Figure 4.  The Map Display of the Node Flooding using the Surface Ponding Option should use the Map Display Variable PONDED_VOL which shows the Maximum Stored Pond Volume.

How is RDII Storage Simulated in SWMM 5?

Subject:  How is RDII Storage Simulated in SWMM 5?

If you are using the SWMM 5 Rainfall Dependent Infiltration and Inflow(RDII)  feature you can also use the RDII storage parameters to change the RDII runoff by simulating the storage in the Sewershed.   The code in RDII.C as implemented by Lew Rossman of the EPA keeps track of used and unused initial abstraction or IA (Figure 1)

When there is rainfall the following actions are taken:

·         The raindepth available to be convoluted by the RDII unit hydrograph method is reduced by unused IA
·         The amount of IA used up is then updated 

When there is no rainfall

·         A portion of the IA already used is recovered using the recovery rate parameter and the variable IAUsed

Figure 1.  The long term effect of the RDII storage on the generated RDII Unit Hydrographs.  IA1, IA2 and IA3 are the Storage values for the short, medium and long term UH's.

How to Subdivide Subcatchments in SWMM 5

Subject:   How to Subdivide Subcatchments in SWMM 5

If you want to subdivide a larger Subcatchment in SWMM 5 and get around the same peak flow then a good suggestion would be to make sure that (Figure 1):

1.   The sum of the new areas equals the original Subcatchment Area and
2.   The sum of the total Width values equals the original Subcatchment Width on the one Subcatchment
3.   The infiltration, percent imperviousness, roughness and depression storage should be the same. 

Figure 1.  Subdividing a Subcatchment

🌧️ Rules for NRCS Unit Hydrographs in InfoSWMM 📈

Subject: 🌧️ Rules for NRCS Unit Hydrographs in InfoSWMM 📈

When applying NRCS (Natural Resources Conservation Service) Unit Hydrographs for hydrological modeling in InfoSWMM adhere to the following rules or guidelines to ensure accurate hydrological simulations:

1. Curve Number (CN) Source 🗂️: Obtain the Curve Number (CN) from the NRCS_CN column in the Subcatchment Table, representing runoff potential based on soil type, land use, and treatment practices.

2. Time of Concentration (TC) ⏱️: The Time of Concentration, critical for hydrograph development, is sourced from the TC column within the Subcatchment Table.

3. Infiltration Model Selection 💧: Select the Infiltration Model according to the CN Infiltration Model Column in the Subcatchment Database Table to ensure consistency in infiltration calculations.

4. CN Consistency Across Tables 🔄: The CN value in the Soil Database Table must match the CN in the Subcatchment Database Table for uniform hydrological parameters.

5. Depression Storage and Initial Abstraction 🕳️➡️💧: If Depression Storage is zero in the Subcatchment Database Table, Initial Abstraction (IA) in inches will be internally calculated using the formula $��=0.2\times (1000\mathrm{/}��-10)$.

6. Initial Abstraction Calculation ✏️: Clearly state that Initial Abstraction in US units is $��=0.2\times (1000\mathrm{/}��-10)$, indicating the use of American measurement standards.

By following these emoji-highlighted guidelines 📝, hydrological modeling using NRCS Unit Hydrographs within InfoSWMM will be precise and reliable, leading to better water management outcomes. 💦🏙️

Convolution of the RDII UH from R, T and K in SWMM 5

Subject:  Convolution of the RDII UH from R, T and K in SWMM 5

The convolution uses the value of R and the Time Base to estimate the amount of Infiltration and Inflow in the Sewer Network.  The short, medium and long term UH's are estimated at each Wet Hydrology time step to make a smooth hydrograph out of the R, T and  K parameters of the Rainfall Dependent Infiltration and Infiltration Method (Figure 1).  The three UH's can be displaced as well if you use the RTK storage parameters (Figure 2)

Figure 1.  The short, medium and long UH's are convoluted in SWMM 5 from the Rainfall Time Series.

Figure 2.   The Initial Abstraction Depth can be used to shift the generated UH in time or reduce the peak flow and total volumes.

c.

Water Quality Treatment Removal Variables in SWMM5

Subject:    Water Quality Treatment Removal Variables in SWMM5

The treatment variables for Water Quality in a SWMM 5 storage unit (Figure 1) can be either: 

1.       A Process Variable

a.       HRT or Hydraulic Residence Time

b.      DT or Time Step

c.       FLOW or The Current Inflow

d.      DEPTH or the Mean Flow over the Time Step

e.      AREA or the Mean Area over the Time Step

2.       Pollutant Concentration
3.       Pollutant Removal based on the Removal of Other Pollutants 

With a Wide Range of Treatment Functions (Figure 2).

Figure 1.  SWMM 5 Treatment Variable Names in the Treatment Equation

Figure 2.   Treatment Functions in SWMM 5

Three Flow Divider Link Example in SWMM 5

Subject:  Three Flow Divider Link Example in SWMM 5

You can have more than 2 downstream OUTLET Type links in the SWMM 5 dynamic wave solution.  Each link, Under5, Over5 and ReturnFlow is an OUTLET Link with a rating curve depth/flow table.  Depending on the depth in the storage node DIVIDER, the flow is computed from the table for links Under5, Over5 and ReturnFlow.

Average Residence time in InfoSWMM and H2OMAP SWMM

Subject:  Average Residence time in InfoSWMM and H2OMAP SWMM

Here is one way to estimate the residence time:

1.       Plot the System Outflow and Storage in the InfoSWMM Report Manager

2.       Click on the Report Button and copy the Outflow and Storage Time Series

3.       Paste to  Excel

4.       Calculate the Residence time as Storage / Outflow and Graph

5.       You will have an understanding of the residence time in your network

6.       If you have a dry weather flow then a hot start file will give a better estimate at the start of the simulation 

Use the SWMM 5 Scatter Graph to show the Pump Curve used during the Simulation

Subject:  Use the SWMM 5 Scatter Graph to show the Pump Curve used during the Simulation

You can use a scatter graph to show the relationship between the pump during the simulation and the Storage Depth.   If the pump is on the curve based on the pump summary table then the scatter graph should  look like the pump curve.  The pump summary table in the  SWMM 5 RPT also shows you the time off the pump curve low and high. 

Detention Pond Infiltration and Evaporation Losses in SWMM 5

Subject:  Detention Pond Infiltration and Evaporation Losses 

You can also add a storage pond infiltration and surface evaporation losses to the pond.  The surface evaporation is added to theinfiltration (computed from the green ampt parameters); a storage volume summary listing the average and maximum volume and the percent loss from the combined infiltration and evaporation from the ponds.  The pond infiltration loss during a time step is basd on the areal weighed average depth, the Green Ampt infiltration and the Area of the pond.

Detention Basin Basics in SWMM 5

Subject:  Detention Basin Basics in SWMM 5

What are the basic elements of a detention pond in SWMM 5?  They are common in our backyards and cities and just require a few basic elements to model.  Here is a model in SWMM 5.0.022 that even has a fountain in the real pond – which we not model for now.   The components of the model are:

1.   An inlet to the pond with a simple time series – a subcatchment can be added to it in a more complicated model but for now we will just have a triangular time series,

2.   A pipe to simulate the flow into the pond from the inlet,

3.   A Storage Node to simulate the Pond that consists of a tabular area curve to estimate the depth and area relationship,

4.   A Storage Node to simulate the Outlet Box of the Pond

5.   Two Small Rectangular Orifices to simulate the low flow outflow from the pond at an elevation less than the weir

6.   A large rectangular orifice to simulate the normal inflow to the Box

7.   A rectangular weir to simulate the flow into the box when the pond water surface elevation is above the box

8.   The outlet of the Box is a circular link with a Free outfall as the downstream boundary condition

9.   The flow graph in the image shows the flow into the box starts from the two small orifices, next from the large orifice and finally from the top of the box or the weir.