Report

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 – 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. 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 • Dimensionless Hyetograph is parameterized to generate an input hyetograph that is 6 or 24 hours long and produces the 25-year depth. 0 – 6.5 inches Or 0 – 9.5 inches Generate a Hyetograph – For this example, will use the median (50th percentile) curve 0 – 6 hours Or 0 – 24 hours • 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 Dinterpolated ( Dhi Dlow )(Tinterpolated Tlow ) (Thi Tlow ) Interpolation in Excel Dinterpolated ( Dhi Dlow )(Tinterpolated Tlow ) (Thi Tlow ) 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.