Computation Options

Section Name

Control Name

Value

Basin Shapefile


This is a required setting for specifying the file path of the input basin shapefile for the HMR52 PMP computation.


Basin shapefile

This is the name of the shapefile that is used to optimize the HMR52 storm size, location, and rotation. The shapefile should contain only the required subbasin polygons. All of the individual polygons in the shapefile will be automatically aggregated together as part of HEC-MetVue computations.

For a USACE site-specific PMP analysis, the shapefile specified here should only include the subbasins that are upstream of the reservoir. To compute average precipitation for additional basins, add their shapefile(s) to the list of shapefiles in the Project Explorer window for the Map Window. The additional shapefiles will not affect the storm computations, but the computed storm precipitation will be applied to the additional polygons to determine their basin averages.


Browse…

Displays a standard file selection dialog to find a shapefile. When a new shapefile is selected the message shown in the following figure is displayed to make sure it is ok to reset some of the information in the HMR52 Computations Options tab to the defaults for the new shapefile.

Depth-Area-Duration Curve Options


This is a required setting to select the method for determining the Depth-Area-Duration (DAD) curve data to use in the HMR52 PMP computation.  HEC-MetVue has built in digital HMR51 maps from which the DAD data is sampled based on the various options available in this section of the dialog, except for the Custom Curves option.


Use Basin Centroid

HMR52 studies typically use a single set of DAD curve for a study area based on the centroid of the basin. When the HMR52 procedure was released in 1982, this option was the standard due to the amount of time, effort, and complexity involved in using the basin area and its intersection with the multiple area-duration maps.


Use Basin Area

This is the default HEC-MetVue option to use for extracting the depth values from the built-in HMR51 area-duration maps.  As opposed to the Basin Centroid option, this option provides more a reasonable DAD curve estimate, especially for basins with significant elevation ranges. 


New curves for each storm position

This will harvest a new set of DAD curves every time the storm is moved to a new position. This may be advantageous for a large basin in which the DAD relationship is considerably different from one side of the basin to the other. 


Use specified position

Using this option, the location for the DAD curves can be selected to be somewhere other than the basin centroid.


Longitude

The longitude for the manual positioning using the specified option. This must be a negative value for correct positioning within the western hemisphere.


Latitude

The latitude for the manual positioning using the specified option. This must be a positive value for correct positioning within the northern hemisphere.


Define Custom Curves

Select this to input DAD data manually. The area-duration maps from HMR52 have not been updated since 1982 when the original report was published, so this option can be used to input updated DAD curves from more recent analyses or studies.


Populate From Maps

This populates the depth values from the area-duration maps as a starting point. Doing this makes it easy to observe the differences between the manually input curves and the ones based on the input maps.


Area-Duration Table

This is where custom depth values are entered. Any values that are different than what is read from the maps using the entire basin area has a bolded font. Placing the cursor over the cell will show the original basin average computed depth value.

Temporal Hyetograph Distribution Options


These options provide settings for the PMP hyetograph distribution using the default or a user-specified pattern.  These options also provide settings for the disaggregation of the output hyetograph using a user selected interval that is less than 6 hours as well as automatically sampling and applying the 1:6 hour adjustment ratio from the built in digital HMR51 maps.


6 hr. distribution

This is the selection list of the 6-hour hyetograph distribution along with a preview thumbnail of the pattern.


Edit

Used to open the Hyetograph Pattern Editor to define an alternate distribution pattern.


Create shorter intervals

Select this to create hyetographs with intervals shorter than 6 hours.


Interval

This performs computations using the HMR52 rules to define the shorter intervals using the defined 1:6 hour ratio along with the generated depth-duration curves.


Periods to subdivide

This defines the number of 6-hour periods to subdivide to the shorter interval. Typically HMR52 requires that only the four largest 6-hour periods be subdivided. This HMR52 guidance was likely due to the computational effort that was involved in using more periods. Subdividing more periods makes little difference in the results and will increase computation times somewhat as more temporal TIN images must be generated.


1 hr. to 6 hr. ratio: auto

This is the ratio used to adjust the lower portion of the generated depth-duration curves and is usually automatically obtained from the digital maps at the same location used for the DAD curves. It sets a point on the depth-duration curve at the 1 hour mark. For example, if the depth-duration curve had 15 inches of precipitation in the first 6-hour period with a ratio of 0.30, the depth for the first hour would be 4.5 inches.


1 hr. to 6 hr. ratio: use specified value

Select this to manually enter the 1:6 hour ratio.


Specified value

The value for the 1:6 hour ratio. HMR52 guidance suggests to use values between 0.27 and 0.35.

Storm Size Options


These options allow for choosing that HEC-MetVue determine the optimal storm size based on the storm location and orientation or a user-specified storm size is used instead.


Re-compute for each location and rotation

Select this to have the plugin optimize the storm size based on each individual placement and orientation of the storm. This is the default for HMR52 analysis and is the critical part of what the plugin is designed to do. If the storm size and intensity optimization is not a requirement, then a standard, static elliptical storm pattern could be used by HEC-MetVue.


Use specific storm size

Select this to force the HEC-MetVue to generate a specific storm size for all computations. This may be used if a specific location/rotation/storm size computation needs to be reproduced for historic purposes.


Force definition of entire curve

If the storm size is being optimized, the plugin actually needs only a few points on each side of the optimal storm size to compute a valid curve. However, even though the plugin does not require it, the entire curve can be computed through this option if necessary. Note that this will increase computation time significantly.

The plugin produces an output table for the curve as shown in the following example.


Time intervals to use

An HMR52 storm consists of twelve 6-hour periods. Computing the optimal storm size can be laborious, so by convention only the largest four periods were used to determine the optimal storm size. A more accurate optimization computation can be done using all the periods if desired. The difference in computation time is negligible.


Storm area

Select from the standard storm sizes or enter a new one if necessary.