Design Storms in HEC-HMS Example 9 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

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

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 Results 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.

Rainfall Depth 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

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 – 9.5 inches 0 – 6.5 inches Or 0 – 6 hours Or 0 – 24 hours

We saw this same chart in example 2 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

Interpolation in Excel

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 10.01 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.