Custom unit hydrograph

[b22smith]

Can I create my own custom unit hydrograph or do you have to create it? The City of Charleston, SC requires us to use the "ICPR 323" shape factor unit hydrograph. I think this is based on a Georgia DOT guideline. Data for the unit hydrograph is noted below that HydroCAD created for us based on the ICPR 323 unit hydrograph.

Unit Hydrograph Intensity vs. Time

Time ICPR 323
(Tp) (intensity)
-----------------
0.00 0.00000
0.10 0.02864
0.20 0.06873
0.30 0.12027
0.40 0.18328
0.50 0.25773
0.60 0.35510
0.70 0.45246
0.80 0.54410
0.90 0.56701
1.00 0.57274
1.10 0.56701
1.20 0.54983
1.30 0.51546
1.40 0.47537
1.50 0.43528
1.60 0.40092
1.70 0.36655
1.80 0.33792
1.90 0.30928
2.00 0.28637
2.10 0.26346
2.20 0.24055
2.30 0.22337
2.40 0.21191
2.50 0.19473
2.60 0.18328
2.70 0.17182
2.80 0.16037
2.90 0.15464
3.00 0.12600
3.10 0.11455
3.20 0.10309
3.30 0.09164
3.40 0.08018
3.50 0.07446
3.60 0.06300
3.70 0.05155
3.80 0.04009
3.90 0.02291
4.00 0.01718
4.10 0.01145
4.20 0.00573
4.30 0.00000
4.40 0.00000

[Chris Maeder]

Actually, this is already contained within StormNET.

First, make certain you are using the "SCS TR-20" subbasin hydrology method. Select Input | Project Options in order to change the hydrology method.

Next, select Input | Dimensionless Unit Hydrograph. From the Dimensionless Unit Hydrograph dialog, then select the "Georgia-323" unit hydrograph drop down list. You should have the same unit hydrograph as you provided.

Also, StormNET allows you to define your own dimensionless unit hydrograph. You can either enter the "Peak Rate Factor" and StormNET will automatically construct the dimensionless unit hydrograph, or you can enter the ordinates yourself directly. Then, if you want to save it for later re-use in other projects, select the [Save...] button to save the dimensionless unit hydrograph out to a file to use on the next project.

[b22smith]

Thanks. I set the Georgia-323 unit hydrograph as directed. Does the software automatically use this for the "Rain Gage" now? I had the 2, 10, 25, 50, and 100 year storm events for SCS Type III for Charlestson, SC set previously for the Rain Gage. I'm not sure if I have to import or load the 323 UH or what?

[Chris Maeder]

The dimensionless unit hydrograph is one of several watershed related parameters incorporated into NRCS (SCS) hydrologic modeling procedures. The unit hydrograph influences the shape of the runoff hydrograph generated by the model, particularly the peak rate of discharge. It does not affect the volume of runoff (which is determined by curve number).

A unit hydrograph’s shape varies by watershed based on many factors, including:

+ Watershed size, slope, and length
+ Geomorphic and geologic characteristics
+ Amount of storage
+ Degree of urbanization

The default SCS unit hydrograph represents an average condition for much of the country. It is generally sufficiently accurate for the hydrologic design of SCS conservation practices. Detailed studies, however, have been conducted in some watersheds and regions to develop more representative dimensionless unit hydrographs. StormNET allows the user to define these unique unit hydrographs, as well as provides several standard unit hydrographs used in different parts of the country.

The Dimensionless Unit Hydrographs dialog is used to specify the peak rate factor that defines the shape of the dimensionless unit hydrographs. These dimensionless unit hydrographs are used in both the SCS TR-20 and SCS TR-55 hydrology computations. The selected peak rate factor and corresponding unit hydrograph is applied to all subbasins.

Select INPUT | DIMENSIONLESS UNIT HYDROGRAPH to display the Dimensionless Unit Hydrographs dialog. Note that the Dimensionless Unit Hydrographs dialog is only available if the SCS TR-20 or SCS TR-55 hydrology method is selected from the SUBBASIN HYDROLOGY METHOD drop down list in the Project Options dialog, General tab.

[b22smith]

Thanks, looking for simple answer. I figured out that once I set the 323 hydrograph then StormNET uses it for calculations after that point.