Example 11

Design Storms in HEC-HMS

Purpose

• Illustrate the steps to create a design storm in HEC-HMS. – The example will create a variety of design

storms for a particular Texas location.– Focus on HOW to construct the hyetograph

(for design storms requiring external processing) and the two built-in methods

Learning Objectives

• Generate an input hyetograph design storm using several different methods.– External processed storms

• Generate an SCS and Frequency Storm using HEC HMS– Internal processed storms

• Generate rapid generic HMS models for creating input data (for later export).

Problem Statement

• Generate a 24-hour, 25-year design storm for Harris Co. Texas using– SCS Design Storm Approach and EBDLKUP– 0-4194-3 Empirical Hyetograph

• Generate a 6-hour, 25-year design storm for Harris Co. Texas using– SCS Design Storm Approach and EBDLKUP– 0-4194-3 Empirical Hyetograph

Problem Statement

• Generate a 24-hour, 25-year design storm for Harris Co. Texas using– Frequency Storm and DDF Atlas

Required Tools

• TP-40, HY35, DDF Atlas, or EBDLKUP– This example will use both the DDF Atlas

and EBDLKUP to illustrate use of the two tools, you don’t need both.

• 0-4194-3 Empirical Hyetographs

Precipitation Depth

• Using EBDLKUP– 24 hr, 25 yr Depth = 10.01 inches– 6 hr, 25 yr Depth = 6.75 inches

Rapid HMS Model

• Create a new project– Basin model

• Dummy subbasin• No loss• No UH transform

Rapid HMS Model

• Create a new project– Meterological model

• SCS Storm

Rapid HMS Model

• Meterological model– SCS Storm

• Select Type• Insert Depth

Rapid HMS Model

• Control Specifications– Time Window

• 24 hours for SCS storm

Rapid HMS Model

• Run the model

Rapid HMS Model

• We will want the SCS 24-hour storm for the later work, so lets get a copy from HMS.– Observe that element time series has no rain – storm

is produced directly, but we can convert the 1 sq.mi. discharge into watershed inches/hour in Excel

HEC-HMS Output

• Convert the No-transform hydrograph into the SCS Type 2 storm (AREA=1 sq. mi.)

SCS Type-2 Storm

6-Hour Storm

• Now we will figure out the 6 hour SCS storm.– Idea is to use the most intense part of the

storm.– Use the 6 hours centered on 12:00 of the

storm, rescale these to the correct depth and we have a 6-hour storm.

SCS 6-hour

SCS 6-hour, Unscaled

• Pick the 6-hour period.– Then set remainder to

zero– Compute total depth– Adjust to get the

required total depth of 6.75 inches

SCS 6-hour, Unscaled

• Pick the 6-hour period.– Then set remainder to

zero– Compute total depth– Adjust to get the

required total depth of 6.75 inches

SCS 6-hour, Unscaled

• Pick the 6-hour period.– Then set remainder to

zero– Compute total depth– Adjust to get the

required total depth of 6.75 inches

SCS 6-hour, Scaled

• Pick the 6-hour period.– Then set remainder to

zero– Compute total depth– Adjust to get the

required total depth of 6.75 inches

SCS 6-hour, Scaled

• Cut and past into HMS – Time series data

manager

HEC-HMS Model

• Run the model

HEC-HMS Model

• Summary– SCS 24-hr is “built-in”, specify storm type and depth.– SCS 6-hr is processed externally

• Results– 24 hr, Qp = 9340 cfs, Tp = 11:52 , V= 10.01 in.– 6 hr, Qp = 8905 cfs , Tp = 2:52 , V = 6.75 in– Recall the Qp are not true “runoff” in this example –

they represent “excess precipitation” expressed in units of watershed discharge for a 1 sq. mi. watershed.

Using DDF Atlas

• Repeat the example using the DDF atlas– Need two maps; 25 yr – 24 hr and 25 yr – 6 hr.

• Use DDF atlas to find depths would produce nearly identical results– 25 yr, 24 hr ~ 9-10 inches– 25 yr, 6 hr ~ 6-7 inches depth

• Building an HMS model would be the same for SCS Type 2 storm.

• Use these values instead in the empirical hyetograph approach

Rainfall Depth

Generate a Hyetograph

• Dimensionless Hyetograph is parameterized to generate an input hyetograph that is 6 or 24 hours long and produces the 25-year depth.– For this

example, will use the median (50th percentile) curve

0 – 6 hours Or0 – 24 hours

0 –

6.5

inch

es

Or

0 –

9.5

in

ches

• We won’t actually use the graph, instead use the tabular values in the report.– This column scales TIME– This column scales

DEPTH

• We saw this same chart in example 2

Dimensional Hyetograph

Dimensional Hydrograph

• Use interpolation to generate uniformly spaced in time cumulative depths.

• This example will use the HMS fill feature

Input Hyetograph

• Cut-paste-fill to create the hyetograph

• Considerable time required (will illustrate “live”)

Empirical 24-hr, 25-yr

• Cut-paste-fill to create the hyetograph

Data Preparation

• Discovered in this example that using the dimensionless hyetograph requires a tedious cut-paste-fill process to put the data into the uniform spaced time series structure.– Need a better way, that is some kind of

interpolator that will take non-uniform spaced paired data and produce uniform spaced data.

Interpolation in Excel

• Use Excel to interpolate by use of INDEX and MATCH functions. – Takes a bit of programming, but will make

empirical hyetographs easier to manage and will save time.

Interpolation in Excel

• Copy the dimensionalized hyetograph to a different worksheet (as values).– Use MATCH and INDEX to locate the nearest values

in the dimensional TIME and DEPTH to the arbitrary TIME

– Equation to interpolate depth is

)(

))(( edinterpolatedinterpolat

lowhi

lowlowhi

TT

TTDDD

Interpolation in Excel

)(

))(( edinterpolatedinterpolat

lowhi

lowlowhi

TT

TTDDD

6-hr, 25 yr Empirical

• Now that we have an interpolator, we can prepare a six hour storm with less data entry effort in HMS.– Depth ~ 6-7 inches, lets use 7– Duration is 6 hours

• Back to the Excel sheet (we already built)

6-hr, 25 yr Empirical

Change these values as appropriate

Copy to the interpolate sheet

6-hr, 25 yr Empirical

Change these values as appropriate

6-hr, 25 yr Empirical

Copy the interpolated series into HEC-HMS

Copied the interpolated depths here

Frequency Storm

• HEC-HMS has a “frequency” storm option built-in to the meterological manager.

• It requires a set of depths for different times in a storm (kind of like the empirical hyetograph).

• It is a way to directly enter DDF values into HMS without the interpolation issues.

• Will illustrate with the 24-hour Harris County example.

Frequency Storm

• From the DDF atlas we will need a series of depths

Frequency Storm

• From the DDF atlas we will need a series of depths

Read these from the Atlas Maps pp 47-54

Frequency Storm

• Run the model

Comparison of Results

• Several different design storms– SCS, Empirical Hyetograph, Frequency

Storms

• Different durations– Compare the 24-hour

• Anticipate different results because storm “shapes” are different.

• Anticipate about same total depths

Comparison of Results

Design Storm Model

Total Depth IPeak Tpeak

SCS-3 +EBDLKUP 10.01 3723 12:00

DDF+Empirical 9.49 2219 ~ 00:30

DDF+Frequency 9.00 4356 12:05

Summary

• Illustrated a 24-hour SCS storm parameterized using EBDLKUP

• Illustrated how to “export” that storm from HMS and convert into a 6-hour storm

• Illustrated how to use the DDF Atlas and Empirical Hyetograph to generate 24-hour and 6-hour storms.

• Illustrated the Frequency storm parameterized by the DDF Atlas

Summary

• Storm depths similar (anticipated result)

• Time of peak intensity different for Empirical Hyetograph – Anticipated– empirical are front-loaded storms– SCS and Frequency are “balanced” about the

½ storm duration

Summary

• As an aside, the choice of 1-minute time steps was dumb – but this example was about storms and not how well the hypothetical 1 sq. mi. converted those storms into excess.