Apply direct and indirect impervious fractions

[ali.rabiei@hatch.com]

Hi everyone,

I have a RORB model which contain “direct and indirect impervious fractions’ by turning on the functionality inside the GUI.

I want to run this RORB model using Storminjectore. My question is how can I be sure that the correct loss values will be used for DCIA, ICIA and PA? Does the loss calculation in Storminjector consider adjustment of IL and CL as a result of “indirect impervious fraction”?

Thanks,
Ali

[Benson Liu]

Hi,

This question was answered privately but I will post the answer here for other people to see.

Storm Injector is not part of RORB and is created and maintained by others. Therefore, I can’t say for sure how Storm injector handles the loss calculations when dealing with indirect impervious fractions. My understanding is that Storm Injector uses the command line version of RORB. The command line version of RORB by itself is unable to handle the direct and indirect impervious fractions functionality that is available in normal RORB.

Kind regards,

Benson

[dineshdcr]

Hi Ben,

I have a RORB model with DCIA, and ICIA turned on. The event rainfall was an average of 18.4mm across the catchment. The following scenarios were run to identify the DCIA and ICIA functionality.

Scenario 1
IL=100mm, ROC=0 for pervious (No runoff from pervious areas)
IL=0mm, ROC=1.0 for DCIA (some runoff from DCIA)
IL=100mm ROC=0 for ICIA (No runoff from ICIA)

The calculated hydrograph shows no discharge. This means no discharge from DCIA. Why?

Scenario 2
IL=100mm, ROC=0 for pervious (No runoff from pervious areas)
IL=0mm, ROC=1.0 for DCIA (some runoff from DCIA)
IL=30mm ROC=0 for ICIA (No runoff from ICIA)

The calculated hydrograph shows some discharge though IL in ICIA is more than the Avg. Rainfall. Why?

Thank you,
Kind Regards,
Dinesh Ratnayake

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[Benson Liu]

Hi Dinesh,

What RORB does is that it averages the losses from the different types of surfaces over the subarea and then applies the calculated average loss based on the proportion of the subarea that is DCIA, ICIA and pervious area. RORB does not calculate them separately.

This works in the same way as the original pervious and impervious fractions functionality that was already in RORB. The new DCIA and ICIA just allows the user to have a third surface and to specify the initial and continuing losses instead of using the default 0.9 runoff coefficient for the impervious areas.

This is explained in more detail for the original impervious/pervious functionality in the RORB manual Section 3.4.2.

Kind regards,

Benson

[dineshdcr]

Hi Benson,

Thank you for your explanation.

As per the ARR2016 loss chapter, the most urban runoff comes from the DCIA; therefore, all runoff after the loss should be treated as surface runoff and go directly to the runoff at each node. Generally, the runoff from the indirectly connected area (after loss) is added to the pervious area before losses are taken out. Hence, what is the best way to assign losses to these three areas in RORB when obtaining a best-calculated hydrograph for an urban catchment?

Regards,
Dinesh

[Benson Liu]

Hi Dinesh,

I can’t say for sure what is the ‘best’ way to model this as it is dependent on your catchment but one option is to separate out each surface into its own subarea. This way RORB will not average the losses as there is only one surface per subarea but this may be a lot of work.

Another way is to reduce your initial loss for the frequent and short duration events such that you do not get 0 rainfall after losses are accounted for but you would need to think about how you reduce your losses and if it is reasonable to do so.

Kind regards,

Benson

[bjorn.bryant]

Hi All,

I have received an old RORB model file (cat) which contain “Total Impervious Area” values. I want to run the existing RORB model with ARR19 rainfall data and include direct and indirect impervious fractions.

When I adjust the TIA to EIA/ICA values in the data tab, all of a sudden I receive a data error in my catg file (Data error: possibly character data where numerical value expected, or illegal number). I am a little perplexed as to why this error does not show up when running with TIA values only but spits out an error when using EIA/ICA values.

I have attached the catg file just in case.

thank you in advance.

[roloko3965]

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