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HEC-HMS flood basin Modeling as per CWC Subzone SUH method

HEC-HMS flood basin Modeling as per CWC Subzone SUH method

HEC-HMS flood basin Modeling as per CWC Subzone SUH method

Hi there, I am Hydrologic Engineer from India,

I am new to HEC-HMS i request help. Here in one of our project, we have to calculate design discharge (50 / 100 year return period floods) for many bridges. Central Water Commission of India has divided the country into various meteorological sub zones and published synthetic unit hydrograph parameters, 50, 100 Year rainfall maps and guidelines (including base flow & loss rate and temporal distribution co-effcients%) to calculate flood upto 5000 sq km catchment area. Though we have excel calculation sheets, As we have more no of catchments to do, we decided to adopt HEC-HMS with SCS-Unit Hydrograph method transform method. My issue is
1) with manual(Excel) method we use duration of storm (Td) = time to peak (UH) X 1.1 (suggested in guidelines) with the total depth of storm obtained from the map. In Frequency storm method of HEC-HMS has storm duration options of 3Hr, 6hr, 12hr, 24hr upto 4 days fixed. How can i use it if duration of storm calculated is not of any available options in HMS? Moreover peak intensity position option available in HMS are 25%, 33%, 50%, 67% & 90%. What if my manually calculated value is falling in-between these % values. I tried with nearby options and getting peak flow having difference of more than 25% compared to manually (excel) calculated methods. How to adopt this method efficiently to get results within 5 to 10% difference to manual method(excel)?

2) If i use gauge weight method rainfall in HMS using rainfall provided directly calculated using co-efficients, if the duration of storm is less than base width of UH and if we arrange the hourly increments to match with UG peak ordinates vise versa, should we fill initial hours until start of actual rainfall as zero rainfall? I am getting large% (>25%) of difference in flood peak (Manual VS HMS) in this method too. Kindly help.

I attached the model with this.

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