SCS Hydrographs

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SCS Hydrographs

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Hydrology Studio uses the SCS Unit Hydrograph Method for calculating SCS runoff hydrographs. This method is the same approach as used in TR-20. There are basically three steps involved:

 

1.Computing the SCS Unit Hydrograph

2.Computing an excess precipitation hyetograph from the design storm

3.Computing the final hydrograph using the concept of convolution

 

 

SCS Unit Hydrograph

A unit hydrograph is a hydrograph resulting from 1 inch of rainfall excess on a watershed over a given time interval. It is not the final runoff hydrograph but reflects the watershed characteristics. Once a unit hydrograph of a particular watershed is known, any design storm can be applied to it for computing the final runoff hydrograph. Many practicing civil engineers use the SCS 24-hour storms but keep in mind that any storm of any duration can be used with the unit hydrograph method. The Bulletin 71 Huff and the built-in Synthetic distributions are good examples and have gained popularity over the years.

 

The unit hydrograph is constructed using the following methodology:

 

The peak discharge for the unit graph is computed as:

 

 EqUnitQpB

 

Where:

 Qp = peak outflow (cfs)

 484 = SCS Shape Factor

 A  = area (sq. miles)

 Q  = total excess precipitation (1 inch)

 Tp = time to peak (hrs)

 

The shape factor is a user-definable variable. The default value is set to 484 and creates a unit hydrograph that has 3/8 of its area under its rising limb. This factor is higher in mountainous watersheds, for example, 600, while in flat, sandy areas, will be lower, around 300. The Delmarva peninsula in Delaware uses 284.

 

The Time to Peak, Tp, and the Time Base, Tb, are what determines the characteristics of the unit hydrograph. These values are computed as follows:

 

 EqUnitTb

         

Where:

 Tp = time to peak (hrs)

 Tc = time of concentration (hrs)

 D  = time interval (hrs)

 Tc = 1.67 x Lag Time (L)

 

 

 EqUnitLag

 

Where:        

 L = lag time (hrs)

 l = hydraulic length (ft)

 S = (1000 / CN) - 10

 Y = basin slope (%)

 CN = SCS curve number

 

 

Time Base = 2.67Tp

 

Where:

 Tb = time base (hrs)

 Tp = time to peak (hrs)

 

 

It should be noted that the program will adjust the Time to Peak so that it coincides with the current Time Interval.

 

 

Excess Precipitation Hydrograph

An excess precipitation hyetograph (design storm) is needed in order to calculate the direct runoff hydrograph. Hydrology Studio offers several built-in design storms including those which you can customize. Most of which are the SCS 24-hr and 6-hr standard distributions. But other options include the IDF-based Synthetic Storms, Huff and as many as ten custom storms that you input directly. See also Design Storms.

 

SCS 24-Hour Distributions

This hydrology software provides the full library of SCS 24-hr as well as the 6-hour standard dimensionless distributions. The incremental rainfall amounts for the 24-hour storms are computed from a polynomial equation which uses coefficients that vary throughout the storm. The equation is of the form:

 

EqSCSPoly

 

Where:

 Pt = fraction of 24-hour precipitation

 T = elapsed time (hrs)

 C0 = coefficient

 C1 = coefficient

 C2 = coefficient

 C3 = coefficient

 

The list of coefficients for each distribution may be obtained from the NRCS.

 

Synthetic Storms

This option can actually produce an infinite number of design storm hyetographs but for practical reasons, Hydrology Studio limits them to 1, 2, 3, 6, 12 and 24-hour durations. The program uses the rainfall IDF curves to compute depth increments over the time intervals. From this the design storm is constructed by placing the maximum depth increment near the center of the storm and arranging the other increments in a symmetrical alternating form. This is the same method used by the SCS years ago to construct their 24-hour storms.

 

There's much debate about the usefulness of a 24-hour storm applied to a small urban-like site with Tc's as low as 15 minutes. The Synthetic Storm offers a good solution in that it can be matched to the site, that is, its total duration can be specified so that it better fits the computed Tc. For example, if Tc is 20 minutes, you could specify a one-hour storm rather than deal with a 24-hr storm. The one-hour storm lasts long enough so that the entire drainage area contributes to flow to the most downstream point. Going beyond Tc only adds unnecessary volume and calculation resources.

 

 

The Synthetic storm is an excellent alternative to 24-hour distributions

The Synthetic storm is an excellent alternative to 24-hour distributions

 

 

Regardless of which distribution you are using, including the SCS, Synthetic, Huff and the Custom storms directly input, the precipitation increments are converted to excess precipitation. This is where the Curve Number comes in and determines how much of the actual rain is converted into runoff or excess. The following equation is used:

 

 EqSCSQp

 

Where:

 Q = excess volume of precipitation (in)

 P = accumulated precipitation (in)

 S = potential maximum retention

         = (1000 / CN) - 10

 CN = SCS curve number

 

The computed volumes are then converted to excess increments used for the final excess precipitation hyetograph.

 

 

Computing the Final Hydrograph

Hydrology Studio computes SCS Method runoff hydrographs by convoluting a rainfall hyetograph through a unit hydrograph. This method is known as linear superpositioning, and means that each ordinate of the rainfall hyetograph is multiplied by each ordinate of the unit hydrograph, thus creating a series of smaller hydrographs. These hydrographs are then summed to form the final runoff hydrograph.