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Example 8

Example 8. Multiple Rain Gages and Weighting in HEC-HMS. Purpose. Illustrate the steps to use multiple rainfall gages on a watershed. Multiple gage inputs to a sub-basin Input weighted by gage weights, in the example determined by Theissen polygons. Learning Objectives.

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Example 8

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  1. Example 8 Multiple Rain Gages and Weighting in HEC-HMS

  2. Purpose • Illustrate the steps to use multiple rainfall gages on a watershed. • Multiple gage inputs to a sub-basin • Input weighted by gage weights, in the example determined by Theissen polygons.

  3. Learning Objectives • Reinforce use of HEC-HMS Graphical User Interface. • Reinforce the concepts of “Projects” as a data-storage paradigm. • Explore different meterological model concepts: • Initial loss and constant rate loss model • SCS Unit Hydrograph transformation model • User-specified hyetograph.

  4. Problem Statement • Simulate the response of the Ash Creek watershed at Highland Road for a 5-year for the May 1978 storm using the two recording rain gages as the input. • Treat the entire watershed as a single sub-basin. • Each gage contribution is proportional to their Theissen weights (determined elsewhere)

  5. Example 8 Multiple Rain Gages • Ash Creek Example 3 • Import basin model • Import meteorological model • Import time series model • Verify run, then save.

  6. Example 8 Multiple Rain Gages • Verify run, then save

  7. Example 8 Multiple Rain Gages • Examine the rainfall source file • Two gages • 14-W • 15-W • Theissen weights are unreported • 14-W = 0.12 • 15-W = 0.88 should work for this example

  8. Example 8 Multiple Rain Gages • Create two gages in Time Series manager • 14W • 15W

  9. Example 8 Multiple Rain Gages • Copy the date from the external file (Excel) into the respective gages. • Pay attention to non-uniform time, use the fill feature as needed Use chart to look for gaps in input

  10. Example 8 Multiple Rain Gages • Once gage data is entered, save. • Meteorological model is next, we need to change the model to allow multiple gages. Select “Gage Weights”

  11. Example 8 Multiple Rain Gages • Select which gages will be applied to Subbasin-1

  12. Example 8 Multiple Rain Gages • Select which gages will be applied to Subbasin-1

  13. Example 8 Multiple Rain Gages • Enter gage weights – in this case depth weights • Time weights are set to 1.0

  14. Example 8 Multiple Rain Gages • Run the model and examine output

  15. Example 8 Summary • Illustrate how to use multiple gage data and assign by weights each gages contribution to rainfall on a sub-basin. • Example can be extended to multiple sub-basins. • Gages need not be physically present on the watershed, but should be close by (same hemisphere?) • More exotic approaches are mentioned in Chapter 11, HEC-HMS user manual. • Weighting methods require external data preparation and weight determination

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