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  • Sometimes it is necessary to determine the mean annual flow or runoff for a given stream. When published flow records are available they are the best source of information. Minor streams which do not have any gaging records available can be estimated by the following procedure:

    Metric Units and English Units

    Mean Annual Runoff


  • Flood reports have been developed for many rivers in Washington State. Most of these reports, and the ones that are most readily accessible, have been developed by the Federal Emergency Management Agency (FEMA). Other reports have been developed by the United States Army Corps of Engineers and by some local agencies.

    These reports are a good source of flow information since they were developed to analyze the flows during flooding conditions of a particular river or stream. The types of calculations used by the agency conducting the analysis are more complex than the rational method or USGS regression equations and because of this are more accurate. The increased time required to perform these complex calculations is not justified for the typical structure that WSDOT is designing; however, if the analysis has already been performed by another agency, then it is in WSDOTs best interest to use this information. Flood study data should never be used in place of published flow records.

    The OSC Hydraulics Branch maintains a complete set of FEMA reports and also has several Corps of Engineers flood reports. Regional Environmental Offices should be contacted for local agency reports.

  • While measured flows provide the best data for design purposes, it is not practical to gage all rivers and streams in the state. A set of equations have been developed by the USGS to calculate flows for drainage basins that do not have a streamflow gage. The equations were developed by performing a regression analysis on streamflow gage records to determine which drainage basin parameters are most influential in determining peak runoff rates.

    The equations break the state up into nine unique hydrologic regions. A map of the regions can be found in Appendix 2-2. The various hydrologic regions require different input variables so the designer should determine which set of equations will be used before gathering data for the analysis. Appendix 2-2 also contains precipitation information that is required input for many of the equations. Other input parameters such as total area of the drainage basin, percent of the drainage basin that is in forest cover, and percent of the drainage basin that is in lakes, swamps, or ponds will need to be determined by the designer through use of site maps, aerial photographs, and site inspections.

    The equations are listed in Figures 2-7.1 through 2-7.9. Each figure contains one set of equations for a hydrologic region of the state. Each figure also lists the statistical accuracy of the individual equations and describes the required input parameters for the equations and their limits of usage. The designer should be careful not to use data that is outside of the limits specified for the equations since the accuracy of the equations is unknown beyond these points.

    The designer must be aware of the limitations of these equations. They were developed for natural basins so any drainage basin that has been urbanized should not be analyzed with this method. Also any river that has a dam and reservoir in it should not be analyzed with these equations. Finally, the designer must keep in mind that due to the simple nature of these equations and the broad range of each hydrologic region, the results of the equations contain a fairly wide confidence interval, represented as the standard error.

    The standard error is a statistical representation of the accuracy of the equations. Each equation is based on many rivers and the final result represents the mean of all the flow values for the given set of basin characteristics. The standard error shows how far out one standard deviation is for the flow that was just calculated. For a bellshaped curve in statistical analysis, 68 percent of all the samples are contained within the limits set by one standard deviation above the mean value and one standard deviation below the mean value. It can also be viewed as indicating that 50 percent of all the samples are equal to or less than the flow calculated with the equation and 84 percent of all samples are equal to or less than one standard deviation above the flow just calculated.

    The equations were developed with data ranging through the 1992 water year. They represent updates to the USGS regression equations developed for Washington State in 1973 and the designer should disregard the previous version of the equations.

    The equations are only presented in English units. While WSDOT is in the process of converting to metric units, the equations were developed by the USGS which is not actively converting to metric units. In the interest of keeping the equations in their original form, no metric conversion was performed for this manual. To obtain metric flow data, the designer should input the necessary English units data into the appropriate regression equation and then multiply the results by 0.02832 to convert the final answer to cubic meters per second.

    The OSC Hydraulics Branch has a computer program available for distribution that does the calculations for these equations.

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  • When available, published flow records provide the most accurate data for designing culverts and bridge openings. This is because the values are based on actual measured flows and not calculated flows. The streamflows are measured at a gaging site for several years. A statistical analysis (typically Log Pearson Type III) is then performed on the measured flows to predict the recurrence intervals.

    The USGS maintains a large majority of the gaging sites throughout Washington State. A list of all of the USGS gages that have adequate data to develop the recurrence intervals and their corresponding flows is provided in Appendix 2-1. In addition to these values, the OSC Hydraulics Branch maintains records of daily flows and peak flows for all of the current USGS gages. Also, average daily flow values for all current and discontinued USGS gages are available through the Internet on the USGS homepage (note that these are average daily values and not peak values).

    Some local agencies also maintain streamflow gages. Typically, these are on smaller streams than the USGS gages. While the data obtained from these gages is usually of high enough quality to use for design purposes, the data is not always readily available. If the designer thinks that there is a possibility that a local agency has flow records for a particular stream then the engineering department of the local agency should be contacted. The OSC Hydraulics Branch does not maintain a list of active local agency streamflow gages.

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  • When designing flood control structures and some stormwater treatment facilities, the designer must know more than just the peak flow that will occur. Along with the peak flow, the volume of runoff must be calculated as well as the relationship between time and the rate of runoff. The only way to accomplish this is to use a method of analysis that incorporates a hydrograph. A hydrograph is a graphical representation of flow versus time.

    Of the several commonly accepted hydrograph methods, the Santa Barbara Urban Hydrograph (SBUH) method is the best suited for the types of projects that WSDOT designs. It was developed to calculate flows from small to medium sized urban basins using input data that is readily available and equations that are easily understood. While not all WSDOT projects are in urban basins, it is typically the paved surfaces (similar to urban areas) that generate the majority of the total flow.

    The SBUH method is computationally intensive. Calculations for even a single drainage area would take hours if done by hand. Because of this, the only practical way to perform an analysis is to use a computer application. The equations used are simple enough to be incorporated into a spreadsheet which would provide accurate calculations; however, it is highly recommended that one of the commercially available computer programs that includes the SBUH method be used. The advantage of using commercial software is the overall consistency of input and output formats and the reliability obtained from being tested in several different design circumstances.

    There are several commercially available computer programs that include the SBUH method. Each of these programs have certain features that make them unique from other programs but the primary calculations are performed the same way. Because of this, nearly any commercially available computer program that includes the SBUH method is acceptable for designing WSDOT projects.

    Site licenses for the computer program WaterWorksTM have been purchased by each WSDOT region and by the Washington State Ferries Division so this program and associated manual are available to all WSDOT designers. The OSC Hydraulics Branch encourages the use of WaterWorksTM whenever performing an SBUH method analysis. The OSC Hydraulics Branch is available to lend technical assistance on using WaterWorksTM.

    The SBUH method only calculates flow that will occur from surface runoff and thus is not accurate for large drainage basins where ground water flow can be a major contributor to the total flow. As a result, the SBUH method is most accurate for drainage basins smaller than 40 hectares (100 acres) and should never be used for drainage basins larger than 400 hectares (1,000 acres).

    Chapter 3 of the WSDOT Highway Runoff Manual discusses the details of performing an analysis using the SBUH method. The Highway Runoff Manual also includes information on flood control structures and stormwater treatment facilities which will be used in conjunction with almost all SBUH method analyses.

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