WinTR-20 Project Formulation Hydrology Computer Program How does WinTR-20 compute results ? Presented by: WinTR-20 Development Team WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

WinTR-20 Computation Differences Runoff hydrographs Time of Concentration Adjusting curve numbers Adding hydrographs Diverting or dividing hydrographs Structure Routing Reach Routing Smoothing NOAA Atlas 14 and NRCC data These are differences from the WinTR-20 DOS versions of 1980’s and 1992. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Runoff Hydrographs WinTR-20 will compute runoff hydrographs according to NEH Part 630 Chapter 16 Hydrographs (2007). The time increment used to develop the hydrograph is computed internally in WinTR-20. However, the user may set a time increment for output (interpolated values). The next few slides explain how the internal computational time increment is determined. The user may select an output time interval in the Global Output window. However, the discharge values at the user-specified time increment are interpolated from the hydrograph developed at the internal computational time increment. In this case it is good to know what the internal time increment is because if the selected output time interval is much larger, then the hydrograph shape and sometimes the hydrograph peak are not representative of the shape of the computed hydrograph. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Runoff Hydrographs (cont) To see the internal time increment in WinTR-20, leave the Print Time Increment blank on the Global Output window. WinTR-20 Computation Methods June 2015

Runoff Hydrographs(continued) The table above is the dimensionless unit hydrograph (DUH) with PRF 484. The peak of the DUH is at point number 11. Delta t = 0.6 Tc / ((point number – 1) – 0.5) Delta t = 0.063 Tc PRF is Peak Rate Factor. The dimensionless unit hydrograph coordinates start at 0.0, peak at 1.0 and end at 0.0. Delta t is the time increment that WinTR-20 uses to develop the hydrograph for a particular sub-area. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Runoff Hydrographs (cont) The table above the DUH for PRF 350. The peak of the DUH is at point number 6. Delta t = 0.6 Tc / ((point number – 1) – 0.5) Delta t = 0.133 Tc WinTR-20 Computation Methods June 2015

Calculating Time of Concentration (Tc) Besides direct entry of a Tc value, WinTR-20 has two options for computing Tc. Velocity Method for Tc from WinTR-55 computer program. Uses sheet flow, shallow concentrated flow, and channel flow segments. Lag Method for Tc calculation from EFH-2 computer program. Uses curve number, watershed length, and watershed slope. The Lag Method for computing Tc is new with WinTR-20 version 3.10. It is not included in prior WinTR-20 versions. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Velocity method for Tc WinTR-20 Computation Methods June 2015 The 2-yr 24-hour rainfall may be entered in Storm Analysis, Principal Spillway, or Stability Freeboard data windows. If not entered in one of these 3 windows, it is blank on the Time of Concentration – Velocity Method window with a message appearing that it is missing. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Calculating Sheet Flow Travel Time The reference for this equation is NEH Part 630 Hydrology Chapter 15, Time of Concentration (2010). WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Calculating Shallow Con-centrated Flow Travel Time The reference for this equation is NEH Part 630 Hydrology Chapter 15, Time of Concentration (2010). The coefficients in the equations for Unpaved and Paved surfaces are based on specific values of Manning n and hydraulic radius, R. Unpaved n = 0.05 and R = 0.4 feet. Paved n = 0.025 and R = 0.2 feet. Tt = Length (feet) / V (feet/sec) / 3600 (sec/hr) WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Calculating Channel Flow Travel Time or Channel Velocity may be entered directly in data window. Tt = Length (feet) / V (feet/sec) / 3600 (sec/hr) WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Lag Method for Tc WinTR-20 Computation Methods June 2015 The Lag Method for computing Tc is new with WinTR-20 version 3.10. It is not included in prior WinTR-20 versions. The curve number must be either entered in the Sub-area window or computed by Land Use Details data in order to have the Time of Concentration value shown in this window. The user has the choice to either enter average watershed slope in percent or enter a contour length and contour interval which are used to compute average watershed slope. There is a maximum limit on size of sub-area where the Lag Method is applicable. The limit is 2000 acres or 3.125 square miles. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Lag Method for Tc WinTR-20 Computation Methods June 2015 There are limits on the input variables in this equation. They are: flow length 200 to 26,000 feet, average watershed land slope 0.1 to 64%, and CN 30 to 100. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Viewing Tc calculations To open this window, select Verification: from the WinTR-20 Main window. Select Sub-Area Tc Computations. Tc calculations for each sub-area will be placed in the .dbg output file. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Viewing Tc calculations After running WinTR-20, a debug output file is created. Click view and Debug File to display it. From the View pull-down menu select Debug File. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Adjusting curve numbers Two methods have been added to WinTR-20 to adjust the curve number of a sub-area. One allows entry of an ARC value for a sub-area (used in Texas and Kansas). The second allows the user to reduce the curve number by a specific value for a sub-area (used in Oklahoma). These two methods are used to adjust curve number to calculate peak discharges for design of certain conservation practices. Note that these methods are used in Texas, Kansas and Oklahoma. These methods are included in WinTR-20 version 3.10 but not in prior versions. Each NRCS state office has published guidelines and maps for the use of these procedures. The method of adjustment for ARC may be useful for other purposes such as model calibration to a historical storm. Previously, the only possible adjustment of ARC was to select ARC 1, 2, or 3 in the Storm Analysis data window. To use these CN adjustment procedures, ARC must be 2 in Storm Analysis data window. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Adjust curve number by ARC ARC may be any value from 1.0 to 3.0 Original CN tables are based on ARC 2 For example, adjust CN 80 from ARC 2 to ARC 1.6 If CN is 80 for ARC 2, then CN is 63 for ARC 1 Use a linear interpolation to calculate CN 73.2 for ARC 1.6 NEH Part 630 Hydrology Chapter 10 has a Table 10.1 with CN’s for ARC 1, 2, and 3. This is used as a basis for interpolating CN’s for any value of ARC between and including 1 and 3. This adjustment applies to CN entered directly for a sub-area and if the CN is calculated from the Land Use Details data window. Since this CN adjustment is entered with Sub-Area data, if there are multiple sub-areas, it must be entered for each one individually (or left blank). WinTR-20 will not give an error if the values are different. If what values are different? WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Reduce curve number by specific value CN may be reduced by a number from 1 to 14. For example, if the CN for ARC 2 is 78 and reduced by 8, WinTR-20 will use a CN of 70 for this sub-area. This adjustment applies to CN entered directly for a sub-area and if the CN is calculated from the Land Use Details data window. Since this CN reduction is entered with Sub-Area data, if there are multiple sub-areas, it must be entered for each one individually (or left blank). WinTR-20 will not give an error if the values are different. Same question as last slide? WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Adding Hydrographs Two hydrographs may be combined or added such as where two streams join at a junction. If the time intervals are the same, the discharges are added directly. If time intervals are different (as they generally are) WinTR-20 will combine hydrographs using the following method. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Adding Hydrographs (cont) For example, let us add hydrographs A and B. Hydrograph A has a time interval of 0.1 hour and Hydrograph B has an interval of 0.33 hour. Hydrograph A is interpolated to a time interval of 0.33 hour and added to Hydrograph B. Hydrograph B is interpolated to an interval of 0.1 hour and added to Hydrograph A. WinTR-20 Computation Methods June 2015

Adding Hydrographs (continued) Whichever hydrograph has the higher peak is saved for further operations downstream. The hydrograph for further operations downstream is characterized by its discharges and time interval. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Diverting or dividing hydrographs There are two methods of splitting a hydrograph in WinTR-20. Split based on two cross section rating curves. Divert an inflow hydrograph above a specific discharge. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Divide based on two rating curves The split is based on the elevation and discharge data for two cross section rating curves. For example, at an elevation of 100 feet, the discharge at cross section A is 600 cfs and the discharge at cross section B is 400 cfs. At an inflow discharge of 1000 cfs, 600 cfs goes to cross section A and 400 cfs goes to cross section B. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Divert above a specific discharge In this example, all flow above 500 cfs is diverted from the inflow hydrograph (red). The outflow hydrograph at the downstream end of the reach is shown in green. When the red inflow hydrograph is not visible, the green outflow hydrograph is the same and plots directly over the inflow hydrograph. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Structure Routing The recently released Chapter 17 (2014) describes the method and gives an example. The storage indication method (described in NEH Part 630 Hydrology Chapter 17) is used to route the inflow hydrograph through the structure. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Maximum number of Hydrograph Points The maximum number is 100,000. This allows WinTR-20 to save hydrographs at extremely short time intervals. Sometimes with structure routings with very low release rates, this 100,000 point limit can be exceeded. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Channel Reach Routing The Muskingum-Cunge method is described in NEH Part 630 Chapter 17. Examples are provided. WinTR-20 training module 8 presents the Muskingum-Cunge method of routing. The recently released Chapter 17 (2014) describes the method and gives examples. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Smoothing NOAA 14 and NRCC Data When importing NOAA 14 or NRCC rainfall data there is an option to smooth the data. The 60-minute and 24-hour precipitation values are not changed. Precipitation for other durations may be adjusted. Smoothing the data provides for a smooth rainfall distribution and relatively smooth runoff hydrograph. Other training presentations cover how to import NOAA or NRCC data and how WinTR-20 develops rainfall distributions. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Smoothing NOAA 14 and NRCC Data This file is developed whenever the “Smooth” option is selected when importing NOAA 14 or NRCC rainfall data. Return periods from 1-year to 500-years are included in this output file. Import and select Smooth option. From the View pull-down menu select Smoothed NOAA/NRCC File. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Smoothing NOAA 14 and NRCC Data This example is for the 25-year return period NOAA 14 data at College Park, MD. “Precip” is the original NOAA 14 data. “Inc_Int” is the incremental intensity for the original NOAA 14 data. The “Sm” refers to smoothed data. This table is developed whenever the “Smooth” option is selected when importing NOAA 14 or NRCC rainfall data. Return periods from 1-year to 500-years are included in this output file (only the 25-year is shown here). The Incremental Intensity is defined in the next slide. The Precip_dif line shows the difference between the original and smoothed precipitation values. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Smoothing NOAA 14 and NRCC Data Incremental Intensity is the difference in precipitation divided by the difference in duration converted to inches / hour. At 5-minute duration, WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Smoothing NOAA 14 and NRCC Data At 30 minute duration, WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Smoothing NOAA 14 and NRCC Data Incremental Intensity is the difference in precipitation divided by the difference in duration converted to inches / hour. At 5-minute duration, At 30 minute duration, WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Smoothing NOAA 14 and NRCC Data This plot is for the 25-year return period at College Park, MD. Smoothed Incremental Intensity. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

Smoothing NOAA 14 and NRCC Data This plot is for the 25-year return period at College Park, MD. It shows the 60-minute and 24-hour values are not changed. Values for other durations may be adjusted. Smoothed precipitation values. WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

The End WinTR-20 Computation Methods June 2015 WinTR-20 Computation Differences March 2009

WinTR-20 Computation Methods June 2015