Wai-Fah Chen.The Civil Engineering Handbook

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Simulation in Hydraulics and Hydrology 38-13

must have a number placed in columns 16 and 17 of the ﬁrst row of the STRUCT statement. This

number, 01, should be the same as the structure number on the RESVOR standard control

statement.

4. The RUNOFF Standard Control statement for area 1 is used to obtain the runoff from the 0.021

square miles. The CN and t

for the developed conditions replace the numbers used in Example 38.1.

5. A RESVOR Standard Control statement is used to have the hydrograph calculated from area 1

routed through the pond structure. This is done by placing “01” (the pond structure number) in

columns 19 and 20. The routed hydrograph is placed in a computer memory location by placing

a number 1 through 7, not already used, in column 23. Therefore, a 2 is placed in column 23.

6. The INCREM and COMPUT Executive Control statements are the same as in Example 38.1. With

these statements, the peak discharge rate and time will be computed, as well as the peak elevation

in the pond. In order for the ﬂow in pond to be analyzed, the last cross section must be speciﬁed

as the pond structure. This is done by placing the cross section number of the culvert in columns

20 and 21 of the COMPUT statement. Again, ENDJOB is the last statement.

Output

The output format is the same as in Example 38.1 because the output options in columns 61–70 are the

same, with the addition of the options chosen for the RESVOR statement. A condensed version of the

output is shown on Fig. 38.5 and Table 38.3.

Summary

As in Example 38.1, the summary table on the last page of the output summarizes all of the information

obtained. This table contains the developed runoff information for area 1 and for peak ﬂow and elevation

through the pond. The pond used to control the runoff from area 1 is checked to see if it is sufﬁcient.

The peak discharge elevation is 783.07 feet for the 10-year storm. This indicates that the pond does not

overtop for the storm event modeled.

Example 38.3

Description

The entire watershed is analyzed to determine the amount of runoff from the 0.3 square miles. This

example incorporates the same area as Example 38.1, but also includes the runoff from the three other

subbasins under existing conditions from the entire watershed. The watershed is subdivided into

4 subbasins as shown on Fig. 38.1. Area 1 has been analyzed with and without the pond in the two

previous examples. There is a 1-acre lake in the middle of the watershed, which takes in the runoff from

TA B L E 38.2 Stage-Storage-

Discharge Relationship for Pond

in Example 38.2

Stage Discharge Storage

780.5

781.0

781.5

782.0

782.5

783.0

783.5

784.0

784.5

785.0

785.5

786.0

786.5

787.0

32.5

0.0

0.004

0.014

0.019

0.046

0.88

1.33

1.67

2.07

2.46

2.85

3.23

3.49

4.52

38-14 The Civil Engineering Handbook, Second Edition

SUMMARY TABLE 1 - SELECTED RESULTS OF STANDARD AND EXECUTIVE CONTROL

INSTRUCTIONS IN THE ORDER PERFORMED (A STAR(*) AFTER THE PEAK DISCHARGE TIME AND RATE (CFS) VALUES INDICATES

A FLAT TOP HYDROGRAPH A QUESTION MARK(?) INDICATES A HYDROGRAPH WITH PEAK AS LAST POINT.)

SECTION/

STRUCTURE

STANDARD

CONTROL

OPERATION

DRAIN.

AREA

RAIN

TABLE

ANTEC

MOIST

COND

MAIN

TIME

INCREM

(HR)

PRECIPITATION

RUNOFF

AMOUNT

(IN)

PEAK DISCHARGE

BEGIN

(HR)

AMOUNT

(IN)

DURATION

(HR)

ELEV.

(FT)

TIME

(HR)

RATE

(CFS)

RATE

(CSM)

ALTERNATE 1 STORM 1

XSECTION 1 RUNOFF 0.02 7 2 0.10 .0 3.23 6.00 2.48 — 3.05 46.63 2220.4

STRUCTURE 1 RESVOR 0.02 7 2 0.10 .0 3.23 6.00 2.47 783.07 3.45 15.72 748.8

FIGURE 38.5 Output from Example 38.2.

Simulation in Hydraulics and Hydrology 38-15

Areas 1, 2, and 3. The discharge from this lake is routed through a channel to the outlet of the watershed.

Table 38.4 shows the stage-storage-discharge relationship for the 1-acre lake.

The hydrologic input data is the same as in the previous examples. This refers to the rainfall data for

the SCS Type II distribution and the rainfall amount for a 10-year recurrence interval in Indianapolis.

Another storm, the 100-year 12-hour event, was used to demonstrate how easy it is to add additional

storms.

Calculations

Table 38.5 contains the information from the computation of the CN and t

values. The longest t

is for

drainage area 4. The total time of concentration for the entire watershed is 1.05 hours.

Computer Input

The input ﬁle from Example 38.1 is edited to include the information for the additional subbasins. A

reach is added by using the tabular control XSECTN and the corresponding codes. The STRUCT table

is changed to reﬂect the 1-acre lake. The Standard Control statements, RUNOFF and ADDHYD, are

needed for the additional subbasins. The area, CN, and t

are described on each RUNOFF statement.

COMPUT and ENDCMP statements are used to add the 100-year storm event values. The ADDHYD

statement is used to combine two hydrographs computed from the respective RUNOFF statement. The

ADDHYD is used because there are only two input hydrographs entries for each RUNOFF statement.

The input is shown in Fig. 38.6.

For example, the hydrographs from areas 1, 2, and 3 are combined and routed through the 1-acre lake

before adding runoff from area 4. Therefore, two ADDHYD statements are used to combine the hydro-

graphs computed from the pond in area 1 and the runoff from area 2. Another ADDHYD statement is

used to combine the previously obtained hydrographs with the hydrograph computed from area 3. The

1-acre pond in area 3 is then described by a RESVOR statement. This statement refers to STRUCT 02

for the stage-storage-discharge relationship. The runoff from all three subbasins is routed through the

TABLE 38.3 Summary of Example 38.2

Location

Peak

Discharge

(cfs)

Runoff

Amount

(in.)

Water Surface

Elevation (ft)

Time to

Peak (hr)

Area 1 46.63 2.48 — 3.05

Pond 15.72 2.47 783.07 3.45

TA B L E 38.4 Stage-Storage-

Discharge Relationship for the

1-Acre Lake in Example 38.3

Stage Discharge Storage

773.0

773.5

774.0

774.5

775.0

775.5

776.0

776.5

777.0

777.5

778.0

0.0

3.0

5.0

7.0

9.0

11.0

13.0

19.0

116.0

325.0

656.0

0.0

0.34

0.86

1.33

1.87

2.41

3.03

3.56

4.37

5.17

5.96

38-16 The Civil Engineering Handbook, Second Edition

pond by placing a “1,” the combined hydrograph number, in column 19 of the RESVOR statement. The

discharge from the pond is then routed through a channel to the outlet of the watershed. The routing is

described by a REACH statement which refers to XSECT 006. The XSECT is the stage-discharge and area

relationship for the 2050-foot channel. The relationship is computed by using the channel cross section

shown in Fig. 38.7. The RUNOFF statement is then used to describe the runoff from area 4. Finally, the

hydrograph from the REACH and area 4 runoff are added together with the ADDHYD statement.

The Executive Control statements are the same as those used in the previous two example problems,

except the last cross section number used must be placed in columns 20 and 21 and the additional

COMPUT and ENDCMP. The COMPUT statements require the input of the initial and ﬁnal cross

sections that are to be analyzed. Because the ADDHYD was the last Standard Control statement, its cross

section number (columns 14 and 15) is placed in columns 20 and 21 of the COMPUT statement.

Output

The type and amount of output can be controlled by input options on the JOB record and by the output

options on the Standard Control records. All of the output options on the Standard Control statements

were left blank because only a summary table is desired. To accomplish this, SUMMARY is placed in

columns 51–57 of the JOB statement. The output is shown in Fig. 38.8.

Summary

From the summary table of the output shown in Fig. 38.8, it is determined that, for a 10-year storm, the

peak discharge is 233 cfs, which occurs after 3.71 hours. For the 100-year 12-hour storm, the peak

discharge is 361 cfs with a time to peak at 6.46 hours. Output from each subbasin can be inspected to

determine the amount it contributes to the overall runoff from the watershed. The outﬂow from drainage

areas 1, 2, and 3 are routed through the 1-acre pond and then through a channel. The reach had very

little effect on the time to peak or the discharge. The channel would have to be much wider and longer

for any attenuation to be realized. The results are also summarized below in Table 38.6.

TABLE 38.5 Curve Numbers and Times of Concentration for Example 38.3

Drainage Area Area (acres) Land Use CN Area * CN

Drainage Area 2

Total

Residential

Commercial

Open Area

Composite curve number

3096

1034

1554

5684

Drainage Area 3

Total

N/A

Residential

Commercial

Open Area

Composite curve number

1634

658

2292

Drainage Area 4

Total

Residential

Commercial

Open Area

Composite curve number

2408

3478

1184

7070

Simulation in Hydraulics and Hydrology 38-17

JOB TR-20 SUMMARY NO PLOTS

TITLE EXAMPLE 3 - ENTIRE WATERSHED

TITLE PROPOSD CONDITIONS

5RAINFL 7 0.05

800.01 0.03 0.04 0.06

8 0.08 0.10 0.13 0.17 0.22

8 0.64 0.78 0.84 0.87 0.90

8 0.92 0.94 0.96 0.98 0.99

8 1.0 1.0 1.0 1.0 1.0

9 ENDTBL

2 XSECTN 6 1.0 770.0

8 759.0 0.0 0.0

8 760.0 15.09 5.0

8 761.0 51.53 12.0

8 762.0 110.34 21.0

8 763.0 194.31 32.0

8 764.0 306.33 45.0

8 765.0 372.72 71.0

8 766.0 670.69 121.0

8 767.0 1208.35 195.0

8 768.0 2034.72 293.0

8 769.0 2751.61 455.0

8 770.0 4579.98 723.0

9 ENDTBL

3 STRUCT 1

8 773.0 0.0 0

8 773.5 3.0 0.34

8 774.0 5.0 0.86

8 774.5 7.0 1.33

8 775.0 9.0 1.87

8 775.5 11.0 2.41

8 776.0 13.0 3.03

8 776.5 19.0 3.56

8 777.0 116.0 4.37

8 777.5 325.0 5.17

8 778.0 656.0 5.96

9 ENDTBL

6RUNOFF 1 1 1 0.021 93 0.24 AREA 1

6RUNOFF 1 2 2 0.106 84 0.96 AREA 2

6RUNOFF 1 3 3 0.041 88 0.68 AREA 3

6 ADDHYD 4 4 1 2 4

6 ADDHYD 4 5 3 4 1

6 RESVOR 2 01 1 2 LAKE

6 REACH 3 6 2 3 2050 REACH

6RUNOFF 1 7 4 0.127 87 1.01 AREA 4

6 ADDHYD 4 8 3 4 5

ENDATA

7 INCREM 6 0.1

7 COMPUT 7 1 8 0.00 3.23 6 7 21110-YR

ENDCMP 1

7 COMPUT 7 1 8 0.00 5.24 12 7 211100-YR

ENDCMP 1

ENDJOB 2

FIGURE 38.6 Input from Example 38.3.

38-18 The Civil Engineering Handbook, Second Edition

38.4 The HEC-HMS Model

Uses of HEC-HMS

HEC-HMS is a precipitation-runoff simulation model developed by the U.S. Army Corps of Engineers

Hydrologic Engineering Center (HEC) that is a “new generation” software to supercede the HEC-1 Flood

Hydrograph Package. Similar to HEC-1, HEC-HMS can be used: (1) to estimate unit hydrographs, loss

rates, and streamﬂow routing parameters from measured data and (2) to simulate streamﬂow from

historical or design rainfall data. Several new capabilities are available in HEC-HMS that were not

available in HEC-1: (1) continuous hydrograph simulation over long periods of time and (2) distributed

runoff computation using a grid cell depiction of the watershed. Capabilities for snow accumulation and

melt, ﬂow-frequency curve analysis, reservoir spillway structures, and dam breach are under development

for HEC-HMS but not yet incorporated.

HEC-HMS consists of a graphical user interface, integrated hydrologic analysis components, data

storage and management capabilities, and graphics and reporting facilities. The program features a

completely integrated work environment including a database, data entry utilities, computation engine,

FIGURE 38.7 Cross section of reach through drainage area 4.

TA B L E 38.6

Summary of Results for Example 38.3

Drainage

Area

6-Hour Storm Event 12-Hour Storm Event

Time of

Peak

(hours)

Peak

Discharge

(cfs)

Time of

Peak

(hours)

Peak

Discharge

(cfs)

Reach

Outlet

3.05

3.60

3.37

3.73

3.63

3.71

46.64

80.62

46.00

127.71

107.55

232.79

5.97

6.43

6.22

6.47

6.46

46.93

125.86

63.91

201.82

159.03

360.85

Simulation in Hydraulics and Hydrology 38-19

SUMMARY TABLE 1 - SELECTED RESULTS OF STANDARD AND EXECUTIVE CONTROL

INSTRUCTIONS IN THE ORDER PERFORMED (A STAR(*) AFTER THE PEAK DISCHARGE TIME AND RATE (CFS)

VA L UES INDICATES A FLAT TOP HYDROGRAPH A QUESTION MARK(?) INDICATES A HYDROGRAPH WITH PEAK AS LAST POINT.)

MAIN

TIME

INCREM

(HR)

SECTION/

STRUCTURE

STANDARD

CONTROL

OPERATION

DRAIN.

AREA

RAIN

TABLE

ANTEC

MOIST

COND

PRECIPITATION

RUNOFF

AMOUNT

(IN)

PEAK DISCHARGE

BEGIN

(HR)

AMOUNT

(IN)

DURATION

(HR)

ELEV.

(FT)

TIME

(HR)

RATE

(CFS)

RATE

(CSM)

ALTERNATE 1 STORM 1

XSECTION 1 RUNOFF 0.02 7 2 0.1 0 3.23 6 2.48 3.05 46.63 2220.4

XSECTION 2 RUNOFF 0.11 7 2 0.1 0 3.23 6 1.71 3.6 80.62 760.6

XSECTION 3 RUNOFF 0.04 7 2 0.1 0 3.23 6 2.02 3.37 46 1121.9

XSECTION 4 ADDHYD 0.13 7 2 0.1 0 3.23 6 1.83 3.5 90.94 716

XSECTION 5 ADDHYD 0.17 7 2 0.1 0 3.23 6 1.88 3.43 136.16 810.5

STRUCTURE 1 RESVOR 0.17 7 2 0.1 0 3.23 6 1.87 777.04 3.61 130.97 779.6

XSECTION 6 REACH 0.17 7 2 0.1 0 3.23 6 1.88 762.21 3.73 127.71 760.1

XSECTION 7 RUNOFF 0.13 7 2 0.1 0 3.23 6 1.94 3.63 107.55 846.9

XSECTION 8 ADDHYD 0.30 7 2 0.1 0 3.23 6 1.91 3.71 232.79 789.1

ALTERNATE 1 STORM 2

XSECTION 1 RUNOFF 0.02 7 2 0.1 0 5.24 12 4.43 5.97 46.89 2232.7

XSECTION 2 RUNOFF 0.11 7 2 0.1 0 5.24 12 3.49 6.43 125.86 1187.4

XSECTION 3 RUNOFF 0.04 7 2 0.1 0 5.24 12 3.89 6.22 63.91 1558.8

XSECTION 4 ADDHYD 0.13 7 2 0.1 0 5.24 12 3.65 6.4 143.16 1127.3

XSECTION 5 ADDHYD 0.17 7 2 0.1 0 5.24 12 3.71 6.3 202.98 1208.2

STRUCTURE 1 RESVOR 0.17 7 2 0.1 0 5.24 12 3.7 777.21 6.34 202.33 1204.3

XSECTION 6 REACH 0.17 7 2 0.1 0 5.24 12 3.71 763.07 6.47 201.82 1201.3

XSECTION 7 RUNOFF 0.13 7 2 0.1 0 5.24 12 3.79 6.46 159.03 1252.2

XSECTION 8 ADDHYD 0.30 7 2 0.1 0 5.24 12 3.74 6.46 360.85 1223.2

FIGURE 38.8 Output from Example 38.3.

38-20 The Civil Engineering Handbook, Second Edition

and results reporting tools. A graphical user interface allows the user seamless movement between

different parts of the program. The HEC Data Storage System (DSS) [HEC, 1993] allows the transfer of

data between HEC programs. The data are identiﬁed by unique labels called PATHNAMES, which are

speciﬁed when the data are created or retrieved. For example, a hydrograph computed by HEC-HMS

can be labeled and stored in DSS for later retrieval as input to HEC-RAS. The DSS program has several

utility programs for manipulating data. HEC-HMS is an easier tool to develop a hydrologic model for

ﬁrst time users because of its graphical user interface. Those familiar with and who routinely use HEC-1

may be frustrated with the set-up and complexity of running design storm events with user deﬁned

cumulative rainfall distributions. The more complex the hydrologic model is, the more liable the program

is to crashing. It is anticipated that these problems will be improved upon with each new release of

HEC-HMS. The graphics and reporting facilities is a very valuable tool that saves time and makes the

presentation of results easy to produce.

Summary of HEC-HMS Input Structure

A project serves as a container for the different parts that together form the complete watershed model

in HEC-HMS. The three components required for a hydrologic simulation are: (1) Basin Model,

(2) Meteorologic Model, and (3) Control Speciﬁcations. A project ﬁle may contain more than one of

these three components and is useful for running scenario analysis.

Each run of the model combines a basin model, meteorologic model, and control speciﬁcations. The

user should be cautious, as some of these components may not be compatible with each other, depending

on methods and time intervals chosen. Runs can be re-executed at any time to update results when data

in a component is changed. The Run Manager is used to manage and execute runs, proportionally adjust

ﬂow or precipitation results, and save or start the basin model in differing states.

Basin Models

The physical representation of watershed or basins and rivers is conﬁgured in the basin model. Hydrologic

elements are connected in a dendritic network to simulate runoff processes. Available elements are:

(1) subbasin, (2) reach, (3) junction, (4) reservoir, (5) diversion, (6) source, and (7) sink. Computation

proceeds from upstream elements in a downstream direction. Hydrologic elements are added to the

model by dragging the appropriate icon from the element palette to the schematic in the Basin Model

screen. Elements are connected to each other within the stream network from upstream to downstream

elements. Elements may also be duplicated and deleted within the Basin Model Screen.

When developing a precipitation-runoff model using HEC-HMS, boundaries of the basin are initially

identiﬁed. Most often, the basin is subdivided into smaller subbasins depending on the study objectives,

drainage pattern, and other factors. Points where runoff information is needed are identiﬁed. The model

can be structured to produce hydrographs at any desired location. As different areas of a large basin may

have different hydrologic response characteristics, it is important to select an appropriate computational

time interval and subdivide the watershed so that lumped parameters provide a reasonable depiction of

the subbasins.

There are several methods that may be used in HEC-HMS to compute surface runoff. These are based

on: (1) initial and constant excess precipitation, (2) SCS curve number, (3) gridded SCS curve number,

(4) and Green and Ampt. The one-layer deﬁcit and constant model can be used for simple continuous

modeling. The ﬁve-layer soil moisture accounting model can be used for continuous modeling of complex

inﬁltration and evapotranspiration environments.

Several methods are included for transforming excess precipitation into surface runoff. Unit

hydrograph methods include the Clark, Snyder, and SCS technique. User-speciﬁed unit hydrograph

ordinates can also be used. The modiﬁed Clark method, ModClark, is a linear quasi-distributed unit

hydrograph method that can be used with gridded precipitation data. An implementation of the kinematic

wave method with multiple planes and channels is also included.

A variety of hydrologic routing methods are included for simulating ﬂow in open channels. Routing

with no attenuation can be modeled with the lag method. The traditional Muskingum method is included.

Simulation in Hydraulics and Hydrology 38-21

The modiﬁed Puls method can be used to model a reach as a series of cascading level pools with a user-

speciﬁed storage-outﬂow relationship. Channels with trapezoidal, rectangular, triangular, or circular cross

sections can be modeled with the kinematic wave or Muskingum-Cunge method and an 8-point cross

section.

Meteorologic Model

Meteorologic data analysis is performed by the meteorologic model and includes precipitation and

evapotranspiration. Seven different historical and synthetic precipitation methods are included: (1) User

Hyetograph, (2) User Gage Weighting, (3) Inverse-Distance Gage Weighting, (4) Gridded Precipitation,

(5) Frequency Storm (6) Standard Project Storm – Eastern US, and (7) SCS Hypothetical Storm. These

methods are summarized in Table 38.7. One evapotranspiration method is included in the model.

The four historical storm methods and the Gage Weights and User-Speciﬁed Hyetograph Methods for

Synthetic Precipitation require entry of time-series precipitation data into the program. Time series data

is stored in a project at a gage. Gage data has to be entered only once. The gages are owned by the project

and can be shared by multiple basin or meteorologic models. The gage data is also stored in the DSS ﬁle

created for the project and may be accessed by other projects.

An example of implementing time-series precipitation gage data in HEC-HMS would be using the

Illinois State Water Survey (ISWS) Bulletin 70 rainfall depths with the Huff Rainfall Distributions. The

Huff Rainfall Distribution for a 24-hour design storm event falling over an area of less than 10 square

miles does not vary with different design frequency events. For this reason, the Huff 3

Quartile Distri-

bution may be entered as a precipitation gage using the cumulative storm percent precipitation data from

0–1 over the 24 hour design storm event duration. However, the Huff Rainfall Distributions are given

in cumulative storm percent distributions rather than cumulative time series distributions that HEC-

HMS accepts for precipitation gage data. Therefore, the cumulative storm percent distributions must be

converted to time series precipitation distributions before being entered into HEC-HMS. The Huff

Distributions in this example is speciﬁcally for northeastern Illinois (Huff and Angel).

Using the User-Speciﬁed Hyetograph precipitation method, this precipitation gage may be assigned

to the subbasins in the model. When running the simulation, the precipitation may be weighted in the

Run Manager to correspond to different design frequency events. The weighted precipitation is the depth

of rainfall for a given design storm event.

Control Speciﬁcations

The control speciﬁcations set the starting date and time of a run as well as the ending date and time.

The time interval, also called the computation step, is also included.

TABLE 38.7 HEC-HMS Meteorologic Model Precipitation Methods

Precipitation Method Explanation

Historical Precipitation User-speciﬁed hyetograph Precipitation data analyzed outside of the program

Gage weights Uses Precipitation data from rainfall gages

Inverse-distance gage weighting Precipitation data from rainfall gages can be used to

proceed when missing data is encountered

Gridded precipitation method Uses radar rainfall data

Synthetic Precipitation Frequency storm Uses statistical data from technical sources to produce

balanced storm with given excedence probability

Standard project storm Implements regulations for precipitation when estimating

the standard project ﬂood

SCS hypothetical storm Implements the primary precipitation distributions for

design analysis using Natural Resource Conservation

Service (NRCS) criteria

User-speciﬁed hyetograph Used with a synthetic hyetograph resulting from analysis

outside the program

38-22 The Civil Engineering Handbook, Second Edition

Preparation of Input Data

Preparation of input data can be divided into the following requirements and functions for using the

SCS Methodology in HEC-HMS:

1. Prepare a schematic drawing that conveniently identiﬁes the locations, drainage areas, curve numbers

(CN), lag times (t

), and reach lengths for the watershed. It should display all alternate structural

systems together with the routing and evaluation reaches through which they are to be analyzed.

2. List the tabular data to support the requirements of the Basin Model. This may consist of structure

data, stream cross section data, source or sink data and diversion data.

3. Establish the Control Speciﬁcations based on the time duration of the storm event to be simulated.

4. Develop a Meteorological Model to be used in the watershed analysis. This may require importing

synthetic or historical rainfall data or developing synthetic rainfall distributions within HEC-HMS.

The starting and ending time and date of the Meteorological Model, as well as the time interval,

should be set in conjunction with the Control Speciﬁcations.

Calculations

For a large watershed it may be necessary to divide the watershed into subbasins. Each subbasin is

determined by ﬁnding the different outlet points or design points within the watershed, then ﬁnding the

area contributing to those points.

1. Area. The area of each subbasin in square miles (mi

2. Curve Number. The curve number (CN) for each subbasin.

3. Lag Time. The lag time (t

) is speciﬁed for each subbasin. Lag time (t

) is 0.6 times the time of

concentration (t

The program is currently only set up to use English units.

HEC-GeoHMS Applications

HEC-GeoHMS has been developed as a geospatial hydrology tool kit for engineers and hydrologists with

limited GIS experience. It combines the functionality of ArcInfo programs into a package that is easy to

use with a specialized interface.

The program allows users to visualize spatial information, document watershed characteristics, perform

spatial analysis, delineate subbasins and streams, construct inputs to hydrologic models, and assist with

report preparation. Working with HEC-GeoHMS through its interfaces, menus, tools, buttons, and con-

text-sensitive online help, in a windows environment, allows the user to effectively create hydrologic inputs

that can be used directly with HEC-HMS. It is intended that these hydrologic inputs provide the user

with an initial HEC-HMS model. The user can estimate hydrologic parameters from stream and watershed

characteristics, gaged precipitation, and streamﬂow data. The user has full control in HEC-HMS to modify

the hydrologic elements and their connectivity to more accurately represent ﬁeld conditions.

HEC-HMS Example

A 60-acre basin in the Chicagoland area is modeled to determine the adequacy of a stormwater detention

basin to which the area drains. This analysis will use SCS methodologies to determine the runoff

hydrograph characteristics with the Illinois State Water Survey (ISWS) Bulletin 70 rainfall depths and

the Huff rainfall distribution for the 100-year 24-hour design storm event. The 60-acre basin is composed

of 40 acres of residential area and 20 acres of commercial area.

Hydrologic Input Data

The hydrologic parameters of the two subbasins within the 60-acre basin are given in Table 38.8.

The stormwater detention basin and control structure were sized based on the DuPage County, Illinois

stormwater detention requirements. The runoff from the developed area must be released at 0.1 cfs per

acre. The elevation-storage-discharge relationship for the detention basin is given in Table 38.9.