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Section 7: Selection of the Appropriate Method for Calculating Runoff

The designer is expected to select an appropriate hydrologic analysis method for each project, seeking assistance from the DHE or DES-HYD and other resources, as needed. TxDOT has no standard method, realizing that methods used must satisfy the requirements of individual studies.

To select the appropriate method, the TxDOT designer should consider, at a minimum, the following:

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  • Information required for design or evaluation and where that information is needed. For example, if the TxDOT project requires designing a culvert, the rational method, which computes peak only, may be adequate. However, if the TxDOT project is affected by or will affect behavior of a detention or retention pond, a runoff hydrograph will be required for the evaluation.
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  • Data available to develop the required hydrologic information. For example, the designer must determine if flow records are available from a stream gauge at or near the location of interest. If not, frequency analysis to find the design flow is not possible, nor is proper calibration of a conceptual model that will compute a hydrograph.
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  • Conditions in the watershed that may limit applicability of alternative models. For example, regression equations for Texas were estimated for watersheds with less than 10 percent impervious cover. If the watershed upstream of the point of interest has more impervious cover, the equations are not applicable. Similarly, if ponds, lakes, and depressions in the watershed will affect runoff by storing water, the rational equation will not be appropriate, as it does not simulate behavior of these features.

Methods acceptable for estimating peak discharges and runoff hydrographs for TxDOT design and evaluation include, but are not limited to the following:

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  • Statistical Analysis of Stream Gauge Data. This empirical method calibrates a probability model with peak annual discharge observations. The probability model relates design flow magnitude to frequency directly, without explicit consideration of rainfall or watershed properties or processes. The method is particularly useful where records in excess of 20 years of stream gauge data are available at or on the same stream near the drainage facility site.
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  • Omega EM Regression Equations. This empirical method relies on application of equations, previously developed through extensive statistical analysis, to predict the peak discharge for a specified frequency (TxDOT 0-5521-1). The equations relate the peak to watershed properties, including watershed area, mean annual precipitation, and main channel slope. This method is useful if streamflow data are not available at or near the project site, or other methods are judged inappropriate. TxDOT designers may use Omega EM regression equations for validation and verification of results from other methods, or for computation of flows for limited detail evaluation of impacts of TxDOT designs on off-system facilities. Omega EM regression equations are reliable beyond about 10 sq. mi. drainage area. A comparison method should be used for drainage areas below 10 sq. mi., and must be used for drainage areas below about 5 sq. mi. This method should not be used for drainage areas less than 1 sq. mi. Discretion may be used on off-system bridges and culverts. As the design of these crossings is typically "hydraulically same or slightly better," the importance of having an exact flowrate is of lesser importance than on-system crossings. At the engineer's discretion, the use of a comparison method may be disregarded.
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  • Rational Method. This simple conceptual method estimates peak runoff rate for a selected frequency. It is appropriate for urban and rural watersheds less than 200 acres (80 hectares) in which natural or man-made storage is minor. It relies on an assumption that the design flow of a specified frequency is caused by rainfall of the same frequency. This method is best suited to the design of urban storm drain systems, small side ditches and median ditches, and driveway pipes.
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  • Hydrograph Method. This conceptual method (actually, a set of methods and models) relies on a mathematical representation of the critical processes by which rainfall on a watershed is transformed to runoff from the watershed. The method is used with a design rainfall hyetograph, which specifies the time distribution of rainfall over a watershed. The method computes a runoff hydrograph, which shows how runoff varies with time; from that, the peak flow, time of peak, and corresponding volume can be found.

Figure 4-2 is a flowchart to aid the designer in selecting an appropriate hydrologic method from among these. The designer must ensure that the conditions in the watershed conform to the limitations of the selected hydrologic method, as described in detail in the sections that follow.

The TxDOT designer is not limited to using only the methods shown here. If none of the methods is judged appropriate, the designer may use an alternative method, with the approval of the DHE or DES. In every case, the rationale for selecting the method must be presented as a component of the design report.

The TxDOT designer should:

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  • Identify and apply alternative methods, recognizing that these will yield different results.
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  • Compare the results from several methods and the historical performance of the site.
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  • Use the discharge that best reflects local project conditions. Averaging of results of several methods is not recommended.
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  • Document the reasons for selection of the methods and the historical performance of the site.

Hydrologic method selection chart (click in image to see full-size image) Anchor: #GLKLFGJIgrtop

Figure 4-2. Hydrologic method selection chart

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