Watershed Modeling using HEC-HMS and EPA-SWMM ©T. G. Cleveland, Ph.D., P.E. 10 July 2012 Lesson 3

Outline Precipitation Hyetographs Design Storms Precipitation in HEC-HMS Exercise: Minimal HMS Model and Texas DDF

Precipitation Driver of rainfall-runoff models Event Continuous

Hyetographs The plot of depth versus time (or intensity versus time) is a hyetograph.

Hyetographs The plot of depth versus time (or intensity versus time) is a hyetograph.

Design Storms Precipitation pattern defined for use in the design of hydrologic system Serves as an input to the hydrologic system Can by defined by: Hyetograph (time distribution of rainfall) Isohyetal map (spatial distribution of rainfall)

Design Storm Estimates Could use observed data and prepare your own Depth-Duration-Frequency relationship Outside scope of this course. Use existing Depth-Duration-Frequency (DDF) or Intensity-Duration-Frequency (IDF) tools for a study area These produce point estimates! If area on the large side, consider ARF.

Module 11: Design Storms WRI 99-4267 ARF for Texas Design Storms A design storm for a point is the depth of precipitation that has a specified duration and frequency (recurrence interval). The effective depth often is computed by multiplying the design-storm depth by a “depth-area correction factor” or an “areal-reduction factor.”

ARF in Texas Region of Unit Hydrograph applicability

ARF in Texas

Concept of IDF for Design Estimate intensity for 5-yr return period for a 30-minute duration i ~ 2.75 inches/hour

Design Storms for Texas TP-40 - Maps of storm depths for different storm durations and probabilities

Design Storms for Texas HY-35 Maps of storm depths for different storm durations and probabilities TP40, HY35 both have interpolation guidance to construct values between mapped values.

Design Storms for Texas TxDOT spreadsheet that tabulates information in the maps. Beware it is units dependent! http://onlinemanuals.txdot.gov/txdotmanuals/hyd/ebdlkup.xls

Design Storms for Texas Link is good (verified 5 AUG 11) Reports intensity instead of depth. Multiply by time to recover depth. Author added this row, not in on-line version http://onlinemanuals.txdot.gov/txdotmanuals/hyd/ebdlkup.xls

Design Storms (elsewhere) NOAA Atlas-14 On-line collection of interactive maps. Select a location, and the atlas generates DDF information.

Design Storms for Texas What the spreadsheet and the maps represent is a hyperbolic model that relates time and intensity. The values e,b, and d parameterize the model. The value Tc has meaning of averaging time, although usually treated as a time of concentration.

Design Storms for Texas D moves this “knee” LEFT/RIGHT The values e,b, and d parameterize the model. The shaded polygon is a hull that encloses TP-40 and HY-35 for Harris Co., TX (barely visible open circles) The “design equation” curve is the EBDLKUP.xls curve for Harris Co., TX B moves this curve UP/DOWN E changes slope of the curve

Design Storms for Texas D moves this “knee” LEFT/RIGHT Aside: The “blue” cloud is a simulation using the empirical hyetographs and PP1725 for Harris Co. The solid red dots are maximum observed intensity regardless of location (some dots are from Texas) The empirical curves represent an alternative model. B moves this curve UP/DOWN E changes slope of the curve

Design Storms for Texas DDF Atlas Alternative to TP40, HY35, and EBDLKUP. Includes more recent data (20 years) than these other tools Provides guidance for interpolation and extrapolation Works in depth – the native unit in HMS

Rainfall Depth Look up depths by recurrence interval, STORM duration, and location.

Local Information DDF for Austin, TX

Local Information IDF for Houston, TX Most Metropolitan areas in Texas (USA) have similar DDF/IDF charts and tables published. Serve as a basis for Design Storms

Design Precipitation Hyetographs Ultimately are interested in entire hyetographs and not just the depths or average intensities. Techniques for developing design precipitation hyetographs SCS method Triangular hyetograph method Using IDF relationships Empirical Hyetographs (Texas specific) This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

SCS Method SCS (1973) analyzed DDF to develop dimensionless rainfall temporal patterns called type curves for four different regions in the US. SCS type curves are in the form of percentage mass (cumulative) curves based on 24-hr rainfall of the desired frequency. This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

SCS Method SCS (1973) analyzed DDF to develop dimensionless rainfall temporal patterns called type curves for four different regions in the US. SCS type curves are in the form of percentage mass (cumulative) curves based on 24-hr rainfall of the desired frequency. If a single precipitation depth of desired frequency is known, the SCS type curve is rescaled (multiplied by the known number) to get the time distribution. For durations less than 24 hr, the steepest part of the type curve for required duration is used This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

SCS Method If a single precipitation depth of desired frequency is known, the SCS type curve is rescaled (multiplied by the known number) to get the time distribution. This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

SCS Method For durations less than 24 hr, the steepest part of the type curve for required duration is used (i.e. 6-hour as shown) HEC-HMS has SCS built-in, but does not rescale time – storm must be 24-hours (or analyst rescales external to the program) 1.0 0.0 This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

SCS type curves for Texas (II&III) SCS 24-Hour Rainfall Distributions T (hrs) Fraction of 24-hr rainfall Type II Type III 0.0 0.000 11.5 0.283 0.298 1.0 0.011 0.010 11.8 0.357 0.339 2.0 0.022 0.020 12.0 0.663 0.500 3.0 0.034 0.031 12.5 0.735 0.702 4.0 0.048 0.043 13.0 0.772 0.751 5.0 0.063 0.057 13.5 0.799 0.785 6.0 0.080 0.072 14.0 0.820 0.811 7.0 0.098 0.089 15.0 0.854 8.0 0.120 0.115 16.0 0.880 0.886 8.5 0.133 0.130 17.0 0.903 0.910 9.0 0.147 0.148 18.0 0.922 0.928 9.5 0.163 0.167 19.0 0.938 0.943 9.8 0.172 0.178 20.0 0.952 0.957 10.0 0.181 0.189 21.0 0.964 0.969 10.5 0.204 0.216 22.0 0.976 0.981 11.0 0.235 0.250 23.0 0.988 0.991 24.0 1.000 Not much difference in the two curves in dimensionless space! This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

SCS Method Steps Given Td and frequency/T, find the design hyetograph Compute P/i (from DDF/IDF curves or equations) Pick a SCS type curve based on the location If Td = 24 hour, multiply (rescale) the type curve with P to get the design mass curve If Td is less than 24 hr, pick the steepest part of the type curve for rescaling Get the incremental precipitation from the rescaled mass curve to develop the design hyetograph This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

Example 9 – SCS Method Find - rainfall hyetograph for a 25-year, 24-hour duration SCS Type-III storm in Harris County using a one-hour time increment a = 81, b = 7.7, c = 0.724 (from Tx-DOT hydraulic manual) Find Cumulative fraction - interpolate SCS table Cumulative rainfall = product of cumulative fraction * total 24-hour rainfall (10.01 in) Incremental rainfall = difference between current and preceding cumulative rainfall This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

SCS – Example (Cont.) If a hyetograph for less than 24 needs to be prepared, pick time intervals that include the steepest part of the type curve (to capture peak rainfall). For 3-hr pick 11 to 13, 6-hr pick 9 to 14 and so on. This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

Triangular Hyetograph Method Time Rainfall intensity, i h ta tb Td Td: hyetograph base length = precipitation duration ta: time before the peak r: storm advancement coefficient = ta/Td tb: recession time = Td – ta = (1-r)Td Given Td and frequency/T, find the design hyetograph Compute P/i (from DDF/IDF curves or equations) Use above equations to get ta, tb, Td and h (r is available for various locations) This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

Triangular hyetograph - example Find - rainfall hyetograph for a 25-year, 6-hour duration in Harris County. Use storm advancement coefficient of 0.5. a = 81, b = 7.7, c = 0.724 (from Tx-DOT hydraulic manual) 3 hr 3 hr Rainfall intensity, in/hr 2.24 6 hr Time This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

Alternating block method Given Td and T/frequency, develop a hyetograph in Dt increments Using T, find i for Dt, 2Dt, 3Dt,…nDt using the IDF curve for the specified location Using i compute P for Dt, 2Dt, 3Dt,…nDt. This gives cumulative P. Compute incremental precipitation from cumulative P. Pick the highest incremental precipitation (maximum block) and place it in the middle of the hyetograph. Pick the second highest block and place it to the right of the maximum block, pick the third highest block and place it to the left of the maximum block, pick the fourth highest block and place it to the right of the maximum block (after second block), and so on until the last block. This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

Example: Alternating Block Method Find: Design precipitation hyetograph for a 2-hour storm (in 10 minute increments) in Denver with a 10-year return period 10-minute This slide adapted from: www.ce.utexas.edu/prof/maidment/GradHydro2010/.../DesignStorms.ppt

Empirical Hyetograph Dimensionless Hyetograph is parameterized to generate an input hyetograph that is 3 hours long and produces the 5-year depth. For this example, will use the median (50th percentile) curve Rescale Depth Average Intensity Rescale Time

Tabular values in the report. This column scales TIME This column scales DEPTH

Dimensional Hyetograph

Dimensional Hyetograph Use interpolation to generate uniformly spaced cumulative depths.

Other Design Storms Frequency Design Storm Enter a frequency (probability) Enter intensity “duration” (lengths of pulses) Enter storm “duration” Enter accumulated depths at different portions of the storm (dimensional hyetograph) Enter storm area (HMS uses this value for its own ARF computations)

Summary - I Design storms are precipitation depths for a location for a given storm duration and a given probability. DDF Atlas EBDLKUP.xls, TP40, HY35 Design hyetographs are the time-redistribution of these depths. SCS Triangular Empirical

Summary - II Intensities are average intensities that produce to observed depth. DDF, IDF curves convey same information. Depth is the natural (and measured) variable. Area Reduction Factors may be appropriate for larger watersheds represented by point gages. Theissen weights are for spatial distribution of gages ARFs are computed externally and applied to the time series before areal weighting.

HEC-HMS Overview History HMS is a complex and sophisticated tool Evolved from HEC-1 as part of “new-generation” software circa 1990 Integrated user interface to speed up data input and enhance output interpretation HMS is a complex and sophisticated tool Intended to be used by a knowledgeable and skilled operator Knowledge and skill increase with use

HEC-1 (Predecessor to HEC-HMS) Separate (individual) programs in 1967 (L. R. Beard) Unified into a single program in 1973 Revised in 1981: kinematic wave PC full version in 1988 Revised 1991: Extended memory support Final release 1998 32 years development until final release

HEC-HMS Evolved from HEC-1 Project begun in 1990 HEC-HMS “released” in 1998 Current version is 3.5 21 years of development to date. Include the HEC-1 period and have nearly 50 years of development – The program “engine” is mature!

HEC-HMS Purpose Replacement for HEC-1 Foundation for future hydrologic software Improved interface (GUI), graphics, and reporting. Newer hydrologic computation methods imbedded Integration of hydrologic capabilities

Rainfall-Runoff Process Precipitation Meterology, Climate Watershed Losses Transformation Storage Routing Runoff Fraction of precipitation signal remaining after losses

Land Surface and Vegetation HEC-HMS Hydrologic Cycle Components in HEC-HMS (circa 2008) Snowfall Evapo- transpiration Rainfall, P(t) Snowpack Snowmelt Infiltration Loss Land Surface and Vegetation Runoff Runoff Percolation Loss Channels Reservoirs Discharge, Q(t)

HEC-HMS Conceptualizes precipitation, watershed interaction, and runoff into major elements Basin and sub-basin description Supply how the system components are interconnected Loss model Supply how rainfall is converted into excess rainfall Transformation model Supply how the excess rainfall is redistributed in time and moved to the outlet

HEC-HMS Conceptualizes precipitation, watershed interaction, and runoff into major elements Meterological model Raingage specifications and assignment to different sub-basins Time-series models Supply input hyetographs Supply observed hydrographs Simulation control Supply instructions of what, when, how to simulate

HEC-HMS Precipitation Abstractions Routing Fraction of precipitation that does not contribute to runoff (and ultimately discharge) Routing Watershed routing Stream (Channel) routing Reservoir (Storage) routing

HEC-HMS Data management All files arranged in a “Project” Graphical User Interface (GUI) Multiple input files Multiple output files Time-series in HEC-DSS All files arranged in a “Project” Paths to individual files Can e-mail entire project folders and have them run elsewhere

HEC-HMS, IO Files project-name.hms basin-model-name.basin List of models, descriptions, project default methods basin-model-name.basin Basin model data, including connectivity meterologic-model-name.met Meterologic model data

HEC-HMS, IO Files control-specifications-name.control run-name.log Run log; messages, warnings, etc. during a run. project-name.run List of runs, includes recent execution time.

HEC-HMS Introduction to HMS Minimal Model Rainfall models

Exercise Exercise 3 Interpret Texas DDF Atlas Explore HEC-HMS Minimal Model

Hydrologic Models Require engineering judgment Experience helps Results can be difficult to interpret Require accurate input data Judgment here too, some data have marginal influence on results, other data are vital. Require quality control procedures

Documenting Your Work This exercise is a bit more complex than filling out a table (although that is part of the exercise). This exercise you will complete the table and hand in a report with relevant screen captures of the HMS model and the written component for the last problem.