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Soil/Land use Curve Number-Antecedent Mosisture condition

Hi

I am sending a pdf file which has a Soil/Land use Curve Number that are a little bit different with the library into the software.

1-Please let me know how can I add a new library based on those

2-In the attached file there is a graph for Antecedent Mosisture condition.

3-where can I find the time to peak calculated in the software?

I will send a design example based on that, could you please show me how can I model that example into the SewerGems?

 

Thanks

Hamid

Parents
  • Please let me know have you get the attached files or not?
  • Hello Hamid,

    Please let us know the names of the files associate with this.

    Thank you.

    Regards,
    Scott
  • Hi
    MTO Drainage Management Manual.pdf & design example.pdf
    this one is urgent
    thanks a lot
  • Hello Hamid,

    For the engineering libraries, the steps to update or add to the library can be found here:

    http://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/modifying-and-adding-entries-to-engineering-libraries

    For the antecedent moisture item, I'm not sure what you are asking (if anything). There is limited information in the Help documentation on that, though it seems to be related to the Horton method. You can find more information in the Help topic "Unit Hydrograph Runoff Method."

    for the time to peak, you can often find this in the element properties. The exact name of the field will be related to the field you are interested in. However, you can use the Property Search feature found in the properties grid in SewerGEMS V8i SELECTseries 3 and later to help with this. You can search for keywords such as "Time" and have the properties grid filter on that phrase. The following link has information on using the search function:

    http://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/19205.how-to-search-for-key-words-or-phrases-in-the-properties-grid

    For the design example, what part of that are you having an issue with? I will be happy to help with specific issues, but at this point there doesn't appear to be a model started, so I cannot be certain where the issue is.

    Regards,
    Scott
  • if you take a look to the design example it calculate CN composite and then by using of Design chart 1.10 indicate AMC III .
    let say CN composite is 65 , from design chart 1.10 it says I should intersect with AMC III and get the 81 as new composite CN.
    Actually this is standard of ministry of transportation in Canada.
    Thanks

    Hamid
  • I put some part of the guideline from Ministry of Transportation-Canada:

    SCS CN Procedure:

    The SCS method uses a parameter called the curve number (CN). CN is a measure of a watershed's hydrologic response potential.

    The CN is related to the land use and the hydrologic soil groups, A,B,C, and D, with A being for low runoff potential soils, and D being for high runoff potential soils. The higher the CN, the higher the runoff potential. CN is given in tables in "Modern Sewer Design" or the "National Engineering Handbook". In this procedure, there are also three levels of antecedent moisture conditions (AMC). AMC I is when the soils are dry. AMC II is the average case. AMC III is used to model saturated soil conditions. AMC III conditions are assumed when modeling the final 12 hours of the Hurricane Hazel event. AMCII conditions are assumed when modeling for the Timmins Storm event. The CN is modified according to the antecedent moisture conditions.

    S is related to the curve number CN by:

    S=25400/CN- 254

    In the CN procedure, the initial abstraction Ia is calculated by 0.2 S.

    For small rainfall events, the runoff volumes may be underestimated as the Ia value can be high for some CN values. Therefore in OTTHYMO the Ia value can be directly specified (i.e. 1.5 mm) as a more realistic estimate. The corresponding modified CN that result in the same runoff volume are called CN*. Charts can be plotted to compare CN and CN*. For different values of Ia, there would be different charts. Where available, use the calibrated CN’s used in watershed plans, sub-watershed plans or master drainage studies. Sources for all modeling approaches must be provided for the selection / calculation of Curve Numbers, Runoff Coefficients, Initial Abstraction, Time of Concentration, Overland Flow Lengths, Manning Roughness Coefficients, Infiltration Rates, Orifice and Weir Coefficients. 

    Composit CN from the following Design Chart  S=25400/CN- 254 Ia=0.2S Q=(P-Ia)2/(P-Ia+S) modified Ia, Manually change to 5 Modified S Modified AMCIII Modified CN
    CN AMCIII S Ia Q Ia* S* AMC*III CN*
    83 93 19.1 3.8 67.4 5.0 17.4 93.6 84

    Not to mention, in Canada companies are using VisualOTTHYMO for the hydrological modelling and recently InfoWORKS ICM is coming to the market.

    I have planing to present SewerGEMS to my co-workers to convince them it can do whatever they want, I will wonder if you say: I can't enter Ia manually!Because they want to model all the stuff automatically. 

    Thanks in advance and looking forward to hearing from you.

    Regards

    Hamid  

Reply
  • I put some part of the guideline from Ministry of Transportation-Canada:

    SCS CN Procedure:

    The SCS method uses a parameter called the curve number (CN). CN is a measure of a watershed's hydrologic response potential.

    The CN is related to the land use and the hydrologic soil groups, A,B,C, and D, with A being for low runoff potential soils, and D being for high runoff potential soils. The higher the CN, the higher the runoff potential. CN is given in tables in "Modern Sewer Design" or the "National Engineering Handbook". In this procedure, there are also three levels of antecedent moisture conditions (AMC). AMC I is when the soils are dry. AMC II is the average case. AMC III is used to model saturated soil conditions. AMC III conditions are assumed when modeling the final 12 hours of the Hurricane Hazel event. AMCII conditions are assumed when modeling for the Timmins Storm event. The CN is modified according to the antecedent moisture conditions.

    S is related to the curve number CN by:

    S=25400/CN- 254

    In the CN procedure, the initial abstraction Ia is calculated by 0.2 S.

    For small rainfall events, the runoff volumes may be underestimated as the Ia value can be high for some CN values. Therefore in OTTHYMO the Ia value can be directly specified (i.e. 1.5 mm) as a more realistic estimate. The corresponding modified CN that result in the same runoff volume are called CN*. Charts can be plotted to compare CN and CN*. For different values of Ia, there would be different charts. Where available, use the calibrated CN’s used in watershed plans, sub-watershed plans or master drainage studies. Sources for all modeling approaches must be provided for the selection / calculation of Curve Numbers, Runoff Coefficients, Initial Abstraction, Time of Concentration, Overland Flow Lengths, Manning Roughness Coefficients, Infiltration Rates, Orifice and Weir Coefficients. 

    Composit CN from the following Design Chart  S=25400/CN- 254 Ia=0.2S Q=(P-Ia)2/(P-Ia+S) modified Ia, Manually change to 5 Modified S Modified AMCIII Modified CN
    CN AMCIII S Ia Q Ia* S* AMC*III CN*
    83 93 19.1 3.8 67.4 5.0 17.4 93.6 84

    Not to mention, in Canada companies are using VisualOTTHYMO for the hydrological modelling and recently InfoWORKS ICM is coming to the market.

    I have planing to present SewerGEMS to my co-workers to convince them it can do whatever they want, I will wonder if you say: I can't enter Ia manually!Because they want to model all the stuff automatically. 

    Thanks in advance and looking forward to hearing from you.

    Regards

    Hamid  

Children
  • Hello Hamid,

    Thank you for the information.

    At this time, the initial abstraction (Ia=0.2S) is hardcoded into the program. You can find informaion on this at the following link: http://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/12459.initial-abstraction-solution-500000087100

    The reason for this is that most users simply use the CN values without the adjustment. However, I have logged an enhancement with the product managers to look into adding a method to adjust the initial abstraction to allow for the use of the CN* value that you mention. The reference number for that is 177520. If this is added to SewerGEMS, it will be in a future release of the product.

    At this time, I would recommend using including the CN* values as an entry in the engineering libraries. This would only need to be done once. Once completed, the new CN* library would be available for all future projects. It is also possible to share engineering libraries between machines, using the steps in the following support solution: http://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/19223.transferring-custom-engineering-library-files-from-one-computer-to-another.

    Please let me know if you have any additional questions on this.

    Regards,
    Scott

    Answer Verified By: Sushma Choure 

  • Hi
    The issue is I should back and forth to the design chart.
    let me to give you an example:
    1-there is a catchbasin with area=5.57 ha with 7% impervious connected, so composite will be CN = 83,
    now, I should go to the design chart and read the AMCIII (see the above table) as the new CN , the calculate S and Ia and Q from formula and based on the AMCIII .
    2-assume a number for Ia*
    3-calculate S* from I*
    4-calculate AMCIII* from formula
    5-again go to the chart and read the CN*


    note that, I have the table which is converted graph design , but that is more than 100 raw and is difficult to create a library of that!
    Please let me know how SewerGems can handle the process that I need
  • Hello Hamid,

    As noted earlier, we have created an enhancement request to see if the ability to add a manual value for Ia so that an updated CN value can be included in the model calculation. Given the workflow that you are going through, at this time, you would either need to calculate the CN* externally (using a spreadsheet, other application, manual hand calculations, etc.) and then enter it in the catchment, or use the original CN value for the land type.

    As stated in the Help documentation: "The universal default for the initial abstraction is given by the equation Ia = 0.2S." While there may be local hydrology methods that that can used, this value is not adjusted very often. A future version of SewerGEMS may allow for the ability to adjust the initial abstraction, and give some flexibility to the user. For more information on the SCS CN method used by SewerGEMS, please see the following Help topic: "SCS CN Runoff Equation".

    Note that the Horton loss method allows you to enter a value for initial abstraction. You could look into using that method instead of SCS CN loss method to see if it is feasible for your modeling purposes, but it does not use a CN.

    Regards,
    Scott