Another question that often arises is "what if the forecast storm were to change its course?" Perhaps the storm was initially going to border your area of interest, but "what if" it shifts and the heart of the storm is now centered directly over your watershed? Task 3 will demonstrate how to trim, translate and save precipitation data as well as view the resulting impacts to streamflow within HEC-HMS. 

Copy the Map Window.

Return to HEC-MetVue, create a Copy of the "Radar" Map, and name the copy "Radar – Trimmed – Translated."

  • Right-Click on the "Radar" Map
  • From the "Map Window" option, select "Copy Map Window…"
  • Name the new map "Radar – Trimmed –Translated"

Trim the Precipitation Data.

Use the Polygon tool to draw a boundary around the highest area of storm intensity in the Radar image.

Right-click on the polygon when done drawing

From the Ad-Hoc Polygon Operations, select: Polygon Operations → Delete Points outside polygon 

The TIN values outside of the selected polygon will be removed, leaving only the most intense portion of the storm.

Translate the Precipitation Data.

Use the Translation tool to move the storm over the Bald Eagle watershed

When the Translation tool is selected, use the mouse Click button to move the storm, and pressing the Right-Click button simultaneously also allows for rotating the storm.

Save the Translated Precipitation Grids.

Use the "Save modified TIN reprojection" tool to write out the modified TIN images.

  • Select Save to "DSS: SHG projection stored to DSS"
  • Select "Source TIN" for Target Extents
  • Fill out the DSS Path Parts (A: SHG, B: BALDEAGLE, C: PRECIP, F: NEXRAD-TRANSLATED)
  • Browse to and select the DSS input file for the HMS model:

...\HEC-MetVue_Overview_Workshop\HEC-MetVue_Overview_Start\HEC-HMS\BaldEagle\dss\BaldEagle.precip.2018.09.dss

  • Press OK

Run HEC-HMS (Translated Precipitation).

Return to the HEC-HMS model and compute the "Sep2018 – Translated" Gridded Mod Clark Simulation Run.

  • Open the "BaldEagle" basin model.
  • From the HEC-HMS Compute Selection menu, select the "Run: Sep2018 – Translated"

  • Select the Compute All Elements () icon to compute the Sep2018 - Translated model which utilizes the Translated precipitation grids.

View HEC-HMS Results (Translated Precipitation).

Right-click on the Outlet element, "Lock Haven SNK" → View Results (Sep2018-Translated) → Graph.

You can also select the "Summary Table" option to view quantitative metrics (peak flow, peak timing, volume, etc.) from the simulation.

It is important to note that the translated precipitation hydrograph results will vary depending on how each user uniquely translates and rotates the precipitation grids. The hydrographs above are simply one possible result.

Question: What is the Peak Flow of the Sep2018-Translated event (using the translated & rotated gridded precipitation) at the Bald Eagle watershed outlet? 

The peak flow at Lock Haven SNK will vary for each user depending on the unique way they translate and rotate the gridded precipitation data. In the example above, the peak flow is still about 20,000 cfs for the Sep2018-Translated event when HEC-HMS is run with the translated gridded precipitation data. 

Question: How does the Sep2018-Translated Peak Flow compare to the initial Sep2018 event with the original gridded precipitation data? 

The Sep2018-Translated peak flow is nearly identical to the initial Sep2018 event. However, the event is peaking twice with the translated precipitation. There is also another, smaller peak that occurs before the primary peak with the translated precipitation, whereas the initial Sep2018 precipitation event had only a single peak. Since the user clipped and translated the most intense part of the precipitation event to completely cover the Bald Eagle watershed, the translated precipitation hydrograph is also wider which indicates that the total event volume is greater than the original Sep2018 precipitation event. 

We've now demonstrated just a few of the gridded data manipulation options the HEC-MetVue offers. However, occasionally there are circumstances where an HEC-HMS modeler may prefer to use basin average hyetographs instead of gridded precipitation data.  The next task demonstrates how hyetograph data can be extracted from gridded precipitation data within HEC-MetVue and fed into HEC-HMS. 

Tasks

Part A – HEC-MetVue Orientation:

Part B – HEC-MetVue Data Input:

Part C – HEC-MetVue Precipitation Preprocessing for input into HEC-HMS :