HYDROLOGIC ENGINEERING USING THE HEC-HMS MODULE
1 - Theory and Background
This section consists of a teaching module for undergraduate environmental or civil engineering students.
The module is structured around three illustrative examples. The students are asked to determine the
flood potential of a watershed site located in New Jersey. A step-by-step procedure is presented in the
module in which students are directed to determine rainfall runoff, precipitation, interception/infiltration,
unit hydrograph, and flood routing. Three illustrative examples are given in this module. The first example
is the determination of a 100-year flood hydrograph for a New Jersey site. In the second example, the
students are instructed to route the hydrograph determined in the first example through a given
reservoir/spillover structure. In the third example the first two examples are repeated by dividing the same
watershed into multiple watersheds.
A computer program, "Hydrologic Modeling Systems" (HEC-HMS), developed by the U.S. Army Corps of
Engineers is used in all of the three examples. This program is used in engineering practice to determine
the drainage characteristics of both rural and urban watersheds. The use of this program is given in this
module with instruction on how to prepare the input for the three illustrative examples and the output is
interpreted. A listing of the program input and output is also given in the module.
Hydrologic Modeling System (HEC-HMS) is new generation software for precipitation runoff simulation
that will supersede the HEC-1 Flood Hydrograph Package. HEC-HMS was developed by the U.S. Army
Corps of Engineers and is a Windows version of HEC-1 with significant advances in computer science
and hydrologic engineering.
HEC-HMS contains most of the HEC-1 capabilities, such as flow-frequency curve analysis, snow
accumulation and melt.
Hydraulic features of dam capabilities are underway but have not yet been
incorporated. The flood damage analysis will be performed by HEC-FDA software and are not included in
HEC-HMS.
The HEC-HMS computer model has a large number of options, such as multiple basin watersheds, flood
damage analysis, etc.
The Soil Conservation Service (SCS) TR 55 approach to the determination of
interception/infiltration and unit hydrographs will be used (TR 55 (1986)). This approach is commonly used
for urban watersheds by the U.S. Army Corps of Engineers.
MODEL USERS BACKGROUND
A description of the HEC-HMS model and its use is given in site examples. There will be little instruction on
the hydrology to determine the computer model input parameters. The user should have taken a first course
in Water Resource Engineering where hydrologic and hydraulic techniques are discussed. A number of
references (Linsley, et. al. (1992), Viessman, et. al. (1989), Hoggan (1989)) are given at the end of this
module for review purposes.
RAINFALL RUNOFF SIMULATION
Simple mathematical relationships are intended to represent model component functions such as
meteorological, hydrologic and hydraulic processes.
These processes are divided into precipitation,
interception/infiltration, transformation of precipitation excess to sub-basin outflow, addition of base flow and
flood hydrograph routing. The HEC-HMS model has a number of options for these processes. Refer to the
illustrative examples for the use and application of each processes.
PRECIPITATION
The precipitation model used in the illustrative examples is the frequency-based design storm. This is the PH
record in HEC-1 – hypothetical storms. The 100-year storm frequency is used for all the examples on an
exceed probability of 1 percent. The storm size is the same as the basin area. The series type is either
annual or partial. An annual series has been selected. The duration of the maximum intensity is the smallest
time entered in the duration precipitation list (5 min.) and the storm duration is the longest (24 hours). The
values of the precipitation for the 5 and 15 minute, and 1, 2, 3, 6, 12, and 24 hour 100 year storm was
obtained from charts. By entering the chart in references NOAA (1977) and TP 40 (1961) with the latitude of
41°00’ and the longitude of 74°25’ of Stickle Pond, the precipitation in inches can be determined. Figure 1 is
a typical chart showing the 24 hour 100-year storm.
INTERCEPTION/INFILTRATION
In the HEC-HMS computer model, the land surface interception, depression storage, and infiltration are
referred to as loss rates. The Soil Conservation Service soil classification system will be used here. The
SCS has been able to relate the drainage characteristics of soil groups to a curve number, CN. These CN
values are based on experimentation and experience. The SCS provides information on relating soil group
type to the curve number as a function of soil cover, land use and antecedent moisture conditions at the
onset of a storm. Refer to the illustrative examples for the determination of a CN value.
UNIT HYDROGRAPH
The unit hydrograph technique is used in the runoff component of a rain event to transform rainfall excess to
outflow. A unit hydrograph can be directly input into HEC-HMS or a synthetic unit hydrograph can be
computed from user supplied parameters (TP 40 (1961), TR 55 (1986), Army Corps. Eng. (1990, 1998)).
The SCS synthetic dimensionless unit hydrograph method is used in the illustrative examples.
A single
parameter, TLAG, is needed to determine this unit hydrograph. TLAG is equal to the time between the
center of the excess rain and peak of the unit hydrograph where,
TLAG = 0.6 x (Time of Concentration).
An important limitation on the HEC-HMS program is:
∆T < 0.29 x TLAG,
where, ∆T = computation time interval (HMS Control specifications. setup).
FLOOD ROUTING
Flood movement through river reaches and reservoirs is simulated by flood routing. Most of the flood-routing
methods available in HEC-HMS are based on the continuity equation and some relationship between flow
and storage (or stage). A storage versus output table must be provided for a reservoir/spillway as direct input
for the HEC-HMS model. The modified Puls routing method is used in Illustrative Example 2.
2 - Illustrative Examples: Stickle Pond
Three illustrative examples are given here. The first is a determination of the flood hydrograph for an actual
site, Stickle Pond, in New Jersey.
The second example is to route this hydrograph through a
reservoir/spillway. A multiple watershed is given in the third example. In all cases, the actual site conditions
and the SCS technique will be used to determine the input parameters to the HEC-HMS computer model.
The examples given are common drainage problems in the determination of flood peaks and
reservoir/detention pond water level elevations for urban watersheds.
Stickle Pond (Lake Kinnelon) is located in Morris County, New Jersey at a latitude 41°00’ and longitude
74°25’. Its surface area is 124 acres, which accounts for 7.4% of the total 1683 acres of its watershed. Refer
to Figures 2 and 3.
The region of New Jersey in which the watershed is located is very rural. Few developed areas dot the site.
For a large part, this is due to the nature of the land - mostly steep rocky forest. Clearing trees, removing
boulders, and building on hillsides with the danger of erosion are all a hindrance to development. Also limiting
the development of the area is the presence of a swampy low region in the watershed.
To the north and east of the pond is a medium-duty and light-duty road system. Any development within the
watershed is located in this general area. Running around the lower or southern half of the pond is an
unimproved dirt road.
PROGRAM STRUCTURE
There are a series of steps to run HEC-HMS.
These steps are:
1.
Start a new project; titles,
2.
Basin model data; setup the basin network, watershed area, base flow, loss rates, unit hydrograph,
routing data, etc.,
3.
Precipitation model data; type in values of the precipitation,
4.
Control specifications; starting and ending dates and computation intervals,
5.
Create and execute a run,
6.
View; tables, graphs, and print results,
7.
Exit.
These steps will be described in the Illustrative Examples.
PROGRAM LIMITATIONS AND INSTALLATION
The following is a list of some of the limitations of Version 2.0 HEC-HMS. Up-to-date information on HECHMS execution, features, and limitations can be found on the HEC web site, http://www.hec.usace.army.mil.
The program can also be obtained from this website.
1.
The importing of HEC-1 rainfall data files does not always work.
2.
When printing tables or graphs from within HEC-HMS, output must be directed to a postscript printer.
3.
The currently active screen can be copied to the Windows clipboard by pressing ALT+Print Screen. That
screen can be "posted" into a document.
These limitations should be corrected in later versions of HEC-HMS.
HEC-HMS INPUT PARAMETERS
To compute an SCS synthetic hydrograph for Stickle Pond the following parameters will be determined.
WATERSHED AREA
A topographic map is necessary to delineate a watershed for the study area and calculate its enclosed area.
Generally a detailed topographic map of the site does not exist. Therefore, U.S.G.S. quadrangle maps of the
site must be obtained. These maps may be purchased from map stores in large cities or from the federal
government (Geological Survey).
Two quadrangle maps: Boonton, NJ, and Newfoundland, NJ, are
necessary to determine the contributing watershed for Stickle Pond.
Each quadrangle is identified with a name in the upper right hand corner of the map to help locate the sites.
The contour interval is printed at the bottom of the map. For the Boonton, NJ and Newfoundland, NJ
quadrangles, the contour interval is 20 feet.
Figure 2 is a topographic map of the site. The Stickle Pond watershed is delineated in Figure 3. The area of
this watershed may be determined with use of a planimeter.
HYDROLOGIC SOIL CHARACTERISTICS
The SCS curve number loss rate is used to determine the hydrologic soil characteristics for Stickle Pond
watershed. A soil characteristics map of the site is published in SCS books for each county (Morris County
for Stickle Pond (SCS (1981))). This book may be obtained from the SCS office in the state or county of
interest. The soil characteristics map for Stickle Pond watershed is shown in Figure 4. The name and area of
each of the soils making up this watershed are given in Table 1. A hydrologic classification of each soil may
be determined from reference TR 55 (1986).
Antecedent moisture condition (AMC) II was assumed for this analysis. AMC II should be selected unless
rain records of the site are available. The curve numbers in Table 2 apply for AMC II. For dry conditions
(AMC I) or wet conditions (AMC III), equivalent curve numbers can be computed (Veissman, Lewis and
Knapp (1989)).
PRECIPITATION
There are six methods of precipitation models available.
The gage data is rarely available for small
watersheds and will not be used for the examples given. The frequency-based storm will be used in the
illustrative examples. This hypothetical storm will be automatically distributed according to the specified
depth/duration data. A triangular precipitation distribution is constructed such that the depth specified for any
duration occurs during the central part of the storm.
The required inputs are:
Exceed Probability; storm frequency, the 100-year storm is used for the examples or 1% probability
Storm Area; storm size will be the same as the watershed area.
Series Type; annual or partial.
Duration of Max Intensity; smallest input duration
Storm Duration; maximum input duration.
UNIT HYDROGRAPH
The SCS dimensionless unit hydrograph is used in the illustrative examples. The only parameter necessary
to obtain this unit hydrograph is the Time of Concentration (TC), which is determined by techniques given in
TR-55 (1986).
For the Stickle Pond watershed, there are three components of TC: sheet flow, shallow
concentrated flow, and open channel flow. See Table 3 for details in the determination of these values.
EXAMPLE 1: FLOOD HYDROGRAPH
The flood hydrograph for the Stickle Pond watershed is determined in Example 1. As outlined above the
frequency based storm will be used to determine the precipitation data, SCS curve number for the loss
rate, and the SCS unit hydrograph. The determination of these parameters are given in Tables 1, 2 and
3.
EXAMPLE 2: FLOOD HYDROGRAPH ROUTING
The second example is to route a flood hydrograph through a reservoir/spillway complex. The same site,
Stickle Pond, is selected. Water is retained in Stickle Pond by a dam at its southeastern end. Two spillways
are available to prevent overtopping of the dam during periods of excess inflow to the reservoir, see Figure 5.
The main spillway (22.75 feet long) is spanned by a concrete walkway. Three piers support this walkway,
thus subtracting from the spillway length. A secondary spillway (a clear 12 foot long span) is at a higher
elevation, 792.05 feet; than the main spillway at 791.7 feet; so there is a range of water surface elevations
within which overflow occurs only over the main spillway.
The dam is breached (overtopped) when the reservoir water surface reaches an elevation of 792.4 feet. It is
assumed that this constitutes failure of the dam.
The hydrograph determined in Example 1 will be used as the input discharge into the reservoir.
The
additional input parameters to compute the resulting reservoir volume and elevation and the output discharge
due to this flood hydrograph are:
FLOOD ROUTING
A storage versus output table is determined for the Stickle Pond reservoir/spillway complex. The Stickle
Pond spillway is shown in Figure 5. As in most routing procedures, the initial routing computation is for a
water elevation at the crest of the spillway. A volume versus elevation table is determined by calculating
the area of each contour enclosing the pond/reservoir. The first area is the area of the pond. See Table
4 for the determination of the storage volumes.
The spillway output versus elevation, Table 5, is
determined by the use of the weir equation (King and Brater (1963)) for the Stickle Pond spillways. The
output versus elevation table has been calculated for elevations up to 796 ft. At elevation 792.4 feet, the
dam will overtop. Therefore, there is no need to continue the run. In order to avoid error messages the
dam was extended to an imaginary top of 796 feet even though the dam top is 792.4 feet.
EXAMPLE 3: MULTIPLE WATERSHEDS
Subdivision of the area of a large basin may be necessary because of the size and complexity of the physical
system. A basin with streams and/or a diverse topography may be divided into smaller components. In this
section, the model input preparation for a subdivided basin is presented. The Stickle Pond watershed is
divided as shown in Figure 6.
This watershed may be divided into approximately equal sub-watersheds or sub basins. The watershed
parameter for the sub basins are given in Table 6. These parameters have been described in examples 1
and 2. Since Stickle Pond is a small watershed, one precipitation distribution is assumed for both sub basins.
The base flow will be defined here as the flow which results from releases of water from subsurface storage.
The input parameters starting base flow discharge (ST), threshold discharge (QR), and rate of recession
index (RT) are used to model the base flow in the HEC-HMS model. ST represents the initial flow in the river,
RT is equal to the ratio of the recession limb flow to the recession limb flow occurring one hour earlier and QR
indicates the flow at which an exponential recession begins on the receding limb of the computed
hydrograph.
The base flow record is used to input the base flow into the HEC-HMS model. Using the Stickle Pond case,
there are three streams feeding into the reservoir, and hydrograph output is available from Examples 1 and 2
to estimate the base flow input. The parameters selected will be 5 cfs and 8 cfs for the values of ST for the
north and south sub basins, respectively, 0.05 for QR (i.e. 0.05 times the peak runoff, recommended in the
HEC-1 manual) and 0.9 for RT (a typical recession ratio) for both sub basins.
3 - References
1.
Boonton Quadrangle, New Jersey-Morris County, and Newfoundland Quadrangle, New Jersey-Morris
County; U.S. Geological Survey, Washington, D.C.
2. "Five to 60 Minute Precipitation Frequency For the Eastern and Central United States"; NOAA Technical
Memorandum NWS HYRDO-35, 1977.
3. "HEC-1 Flood Hydrograph Package"; U.S. Army Corps of Engineers, Davis, California, 1990.
4. “HEC-HMS Hydrologic Modeling System,” U.S. Army Corps of Engineers, Davis, California, 1998
5. Hoggan, D.; "Computer Assisted Floodplain Hydrology and Hydaulics", McGraw-Hill, New York, 1989.
6. King, H and Brater, E., "Handbook of Hydraulics" McGraw-Hill, New York, 1963.
7. Linsley, R., Franzini, J., Freyberg, D., and Tchobanoglous, G.: “Water Resource Engineering”,
McGraw-Hill, New York, 1992.
8. "Rainfall Frequency Atlas of the United States, Technical Paper 40"; Department of Commerce;
Washington, D.C., 1961.
9. “Soil Survey of Morris County, New Jersey”; Department of Commerce; Washington, D.C., 1981.
10. "Urban Hydrology for Small Watersheds", Technical Release 55; Department of Commerce; Washington
D.C., 1986
11. Viessman, W., Lewis G., and Knapp, J.; "Introduction to Hydrology"; Harper and Row, New York, 1989.
TABLE 1
SOIL TYPES AND CURVE NUMBERS COMPUTATION SHEET
Soil
Description
Area*
CN
WATER
RsC
RvF
HbC
RsD
RgA
RpC
RiB
RrD
Cm
Ad
PeC
Weighted CN
(Area x CN)
2800
5362.5
2862
1925
2069.25
1480
1242
972
722
635.5
240
140
Stickle Pond, Hoot Owl Lake, New Pond
28
100
Rockaway: Rock Outcrop
71.5
75
Rock Outcrop: Rockaway
31.8
90
Hibernia: Stony Loam
27.5
70
Rockaway: Rock Outcrop
23.25
89
Ridgebury: Very Stony Loam
18.5
80
Rockaway: Very Stony Sandy Loam
17.25
72
Ridgebury: Extremely Stony Loam
12
81
Rockaway: Extremely Stony Sandy Loam
9.5
76
Carisle Muck
7.75
82
Adrian Muck
3
80
Parker-Edneyville:Extremely Stony Sandy
2
70
Loam
Wm
Whitman: Very Stony Loam
1.5
80
120
RsC
Rockaway: Rock Outcrop
1.5
83
124.5
RpC
Rockaway: Very Stony Sandy Loam
0.75
72
54
* Note: Area in arbitrary units
Total =
255.8
Total = 20748.75
(Total Weighted CN)/(Total Area) = CN = 81.11
TABLE 2
RUNOFF CURVE NUMBERS
COVER DESCRIPTION
CURVE NUMBERS
FOR HYDROLOGIC
SOIL GROUP
A
B
C
D
COVER TYPE
HYDROLOGIC
CONDITION
Pasture, grassland, or range-- continuous forage
Poor
Fair
Good
68
49
39
79
69
61
86
79
74
89
84
80
Meadow, continuous grass, protected from grazing and generally
mowed for hay.
Brush--brush-weed-grass mixture with brush the major element.
---
30
58
71
78
Poor
Fair
Good
Poor
Fair
Good
Poor
Fair
Good
48
35
30
57
43
32
45
36
30
67
56
48
73
65
58
66
60
55
77
70
65
82
76
72
77
73
70
83
77
73
86
82
79
83
79
77
----
59
74
82
86
Woods--grass combination (orchard or tree farm).
Woods.
Farmsteads--buildings, lanes, driveways and surrounding lots.
TABLE 3
COMPUTATION SHEET FOR TC, TIME OF CONCENTRATION
Because of the varied shape and topography of the Stickle Pond watershed, three paths of flow were
selected to determine TC, see Figure 2. These three paths are located in the top, middle and bottom of
the watershed. The TR-55 reference 1 is used to determine TC.
There are three components of TC:
1.
.8
.5
Sheet flow TC1 = .007 (nl) / (P 2)
s
.4
Where, n = 0.4 woods, L = 150 (L maximum of 300 feet, a smaller value is recommended),
P2 = 3.25 inches (2 year, 24 hour rain), S = slope of watershed at the divide.
2.
3.
Shallow concentrated flow TC2 = L / 3600v where, v = 16.134s
2/3
Open channel flow TC3 = L / 3600v where, v = (1.49r
s
1/2
1/2
; s = water course slope.
)/n
where n = .03 from site visit, TR-55 recommends .05; s = channel slope;
r = hydraulic radius = .4(TR55).
4.
Path
Total TC = TC1 + TC2 + TC3
S
TC1
L (ft.)
DH(ft.)
S
TC2
L (ft.)
DH(ft.)
S
TC3
Top
.060
.317
1600
80
.050
.123
4400
78.3
.0178 .339
Middle
.0313
.411
4900
125
.026
.523
2000
68.3
.0342 .111
Bottom
.224
.187
3400
272
.080
.207
3200
19.3
.0060 .425
The largest value of the time of travel is usually selected for TC. But because of the number of
assumptions made in selecting the parameters, an average TC will be used.
TC = 0.88 hours and, TLAG = 0.6 x TC = 0.53 hours
TABLE 4
STORAGE CAPACITY
Drainage area: 1682.5 ac
Stickle Pond area: 124ac
Small islands in Pond: 2.8 ac (included in Pond area)
New Pond & Hoot Owl Lake: 18 ac
Area of 800 ft. contour: 204 ac
Assuming linear spread from 791.7 Pond elevation to 800 ft. contour. Therefore,
204 - 124 = 9.639 ac/ft, and the volume =
800 - 791.7
ELEVATION
(ft)
791.7
791.8
791.9
792.0
793.0
794.0
795.0
796.0
∆Z (A 1 + A2)
2
AREA
(ac)
124.0
125.0
125.9
126.9
136.5
146.1
155.8
165.4
VOLUME
(ac-ft)
0.0
12.5
12.5
12.6
131.7
141.3
151.0
160.6
CUM. VOLUME
(ac-ft)
0.0
12.5
25.0
37.6
169.3
310.6
461.6
622.2
Total TC
.779
1.045
.819
TABLE 5
SPILLWAY CAPACITY
Use the weir equation
where
1.5
Q = CL H
Q = discharge over the spillway
L = length of spillway (adjusted for pier-edge effects)
H = water height above the spillway crest
Take C = 3.39 (H:V 5:1) (King and Brader (1963) Table 5.11)
1.
Primary spillway
Crest 791.7 ft. el.; length = 22.75 ft. with 3 piers
L = 22.75 - 0.8(H)
2.
Secondary
Crest 792.05 ft. el.; length = 12 ft.
L = 12 - 0.2H
3.
Reservoir
Elevation
791.7
792.0
793.0
794.0
795.0
796.0
Crest 792.4 ft. el.
Primary
H
[ft.]
0.0
0.3
1.3
2.3
3.3
4.3
Spillway
L
[ft.]
22.75
22.51
21.71
20.91
20.11
19.31
Q
[cfs]
0.00
12.54
109.09
247.26
408.68
583.69
Secondary Spillway
H
L
Q
[ft.]
[ft.]
[cfs]
0.00
12.00 0.00
0.00
12.00 0.00
0.95
11.81 37.07
1.95
11.61 107.17
2.95
11.41 195.98
3.95
11.21 298.33
Total Q
[cfs]
0.00
12.54
146.16
354.43
604.66
882.02
TABLE 6
SUB BASIN CHARACTERISTICS
Sub Basin
North
South
Tc (hours)
0.77
0.9245
Area (Square miles)
0.771
1.859
CN
85.2
79.4
100 yr Precip (in)
7.5
7.5
Figure 1: One-hundred year, 24-hour rainfall
Figure 2: USGS Topographic Map of Stickle Pond
Figure 3: Stickle Pond Watershed & TC Computation Paths
A
5’-7”
5’-7”
A
5’-6”
6 ft. 1 in.
TOP DAM El. 792.4ft.
El. 791.7ft.
5
1
SECTION A - A
Figure 5: Diagram and Cross Section of Primary Spillway
Figure 4: Soil Survey of Stickle Pond Watershed
Figure 6: Stickle Pond Sub basins & TC Computation Paths
4 - Example 1: Flood Hydrograph
A flood hydrograph will be determined for a rural watershed in this example.
1.
Start HEC-HMS by choosing HMS from the Programs item on the Windows Start Menu. The main
HEC-HMS windows will appear on screen (Figure 7).
2.
Using the mouse select New Project from the File pull down menu in the main HEC-HMS window
(Figure 7).
Figure 7: HEC-HMS Main Window
3.
Enter a name for the project and an optional description. The program will automatically choose a
location for the project files, although this may be changed by clicking the Directory Select button.
For this example, the name of the project is Stickle Pond. Press OK to continue, or Cancel to start
over.
4.
The program defaults are selected during installation. This example uses the English system of units.
Verify that the English system is selected by choosing Default Preferences from the Options pull
down menu in the main HEC-HMS window, and selecting English in the Units section. Choose OK to
continue.
5.
Choose Basin Model from the Component pull down menu in the main HEC-HMS windows, then
select New. The New Basin Model dialog box will appear on screen. Press OK to accept the default
name for the basin. Optionally, you may enter your own name and a description in this dialog box
before pressing OK. The HMS Basin Schematic window will appear on screen.
6.
Create a subbasin by clicking on the Subbasin button, and while still holding down the mouse button,
dragging the cursor to the center of the Basin Schematic window and releasing the mouse button. A
subbasin icon will be created in the Basin Schematic window (Figure 8).
Figure 8: Basin Schematic Window
7.
Edit the characteristics of the subbasin by double clicking the subbasin icon just created.
The
Subbasin Editor window will appear (Figure 9). Enter 2.629 sq. mi. as the subbasin area. In the Loss
Rate tab, change the Method to SCS Curve No. Enter 0 for Initial Loss (assuming no loss for this
example) and % Impervious and 81.11 for SCS Curve No.
Figure 9: Subbasin Editor Window
8.
Select the Transform tab in the Subbasin Editor window and change the Method to SCS. Enter 0.53
hours for the SCS Lag (change the Lag time units from the pull down menu). Choose OK to accept
and save the data. You will now be returned to the Basin Schematic window. The subbasin editor
will then prompt you to confirm the changes made to the old Loss Rate method. Click OK and then
continue.
9.
Choose Close from the File pull down menu. You will be prompted to save changes. Accept the data
by choosing Save Changes. You will be returned to the main HEC-HMS window.
10. Select Meterelogical Model from the Component pull down menu in the main HEC-HMS windows,
then select New. The New Precipitation Model dialog box will appear on screen. Press OK to accept
the default name for the model. Optionally, you may enter your own name and a description in this
dialog box before pressing OK.
11. Next you will be prompted to confirm your Meterelogical Model selected from the subbasin distribution
window. This can be done by clicking OK in the pop up window. A Meterelogical window will now
appear. Select Frequency Storm from the Method pull down menu, then choose OK to continue. The
Precipitation Model data window will appear on screen (Figure 10). The elements of this window will
be substantially different for each of the different precipitation model methods (for example, Standard
Project Storm – Eastern US and User-specified Gage Weighting).
12. The storm used in this example has an Exceedence Probability of 1% (100-year storm), a storm area
equal to the basin area (2.629 sq. mi.), a 5 minute duration of maximum intensity and a storm
duration of 24 hours. For storms with an exceedence probability of less than 10%, the Series Type
may not be selected. For an exceedence probability of 10% or more, the user may select Annual or
Partial Duration series. The storm precipitation at the 5 and 15 minute and 1, 2, 3, 6, 12 and 24 hour
intervals are 0.77, 1.61, 3.05, 3.75, 4.3, 5.2, 6.7 and 7.5 inches, respectively. Enter this data as
shown in Figure 10.
Precipitation cannot be entered for time periods less then the duration of
maximum intensity or greater than the storm duration. For example, if the storm duration is 12 hours
with a maximum intensity of 15 minutes, it would not be possible to enter data for 5 minutes or 24
hours or greater. Choose OK to accept and save the data. You will be returned to the main HECHMS window.
13. Choose Control Specifications from the Component pull down menu in the main HEC-HMS windows,
then select New.
The New Control Specifications dialog box will appear on screen. Press OK to
accept the default name for the model. Optionally, you may enter your own name and a description
in this dialog box before pressing OK.
14. The Control Specifications Setup dialog box will appear on screen (Figure 11). Enter the starting and
ending dates and times and a computation time interval. For this example, the starting date and time
is 1 Jun 1998, 00:00 and the ending date and time is 2 Jun 1998, 00:00. The computation interval
(∆T, Section 1) is 6 minutes. After entering the data, choose OK to continue. The computation time
interval must divide the time interval between the starting time and date and the ending time and date
by an integral number, or an error will result during computation and the run will stop. For example,
the 24 hour time interval used in this example is evenly divided by computation time intervals of 5
minutes (288 intervals), 6 minutes (240 intervals), 10 minutes (144 intervals), etc., but not by 7
minutes (205.7 intervals).
Figure 10: Completed Precipitation Data Window
Figure 11: Completed Control Specifications Dialog Box
15. You will be returned to the main HEC-HMS window. Save the project by selecting Save Project from
the File pull down menu.
16. To execute a run, double click on Basin 1 in the main window. The Basin Schematic window of
Figure 8 will appear. Choose the components to be analyzed by selecting
Run Manager from the
Simulate pull down menu. The Run Manager dialog box will appear on screen (Figure 12). Click on
Run Configuration from the Simulate pull down menu and the Run Configuration dialog box will
appear (Figure 13). Choose Basin 1, Precip 1 and Control 1 from the Basin ID, Precip ID and Control
ID lists. The program automatically assigns a Run ID, however the user can optionally change the
Run ID. For this example, the default value of Run 1 will be kept. Click Apply and Close to accept
the components and return to the Simulation Manager dialog box (Figure 12). For this example, there
is only one basin model, one precipitation model and one control specification and the program
automatically selects these as defaults.
Therefore, this step is performed automatically by the
program and is not actually necessary. However, the program is capable of storing more than one
basin model, precipitation model or control specification per project. The user can mix and match
among the different models, however each run MUST have one basin model, one precipitation model
and one set of control specifications selected.
17. Execute the run by clicking on the Compute button (Figure 12). The program will then perform the
analysis. The HMS Compute dialog box will appear, informing you of the progress of the run. When
complete, it will display “Compute Successful.” You can view a log of any errors, warnings and notes
by pressing View Log. Press Close to continue. You will be returned to the Simulation Manager
dialog box. Press Close again to be returned to the Basin Schematic window.
Figure 12: Simulation Manager Dialog Box
Figure 13: HMS Run Configuration Dialog Box
18. To view the computation results, click and hold the right mouse button while the cursor is positioned
over an element in the Basin Schematic window (for this example, there is only one element,
Subbasin 1). A pop-up menu will appear on screen. While still holding the right mouse button, move
the cursor down to View Results on the pop-up menu. A new pop-up menu will appear (Figure 14).
When this menu appears, release the right mouse button and move the cursor on to the new menu.
19. Select Graph from the pop up menu to view the hydrograph for the subbasin. The hydrograph will
appear on screen (Figure 15). The hydrograph may be printed by clicking the Print button. When
done viewing the hydrograph, press Close to return to the Basin Schematic window.
20. Repeat Step 18, then select Summary Table on the pop-up menu. A summary of the run results will
appear (Figure 16). Click on Print to print out the results and Close to return to the Basin Schematic
Window.
21. Repeat Step 18, selecting Time Series Table on the pop-up menu. A table detailing the precipitation,
loss, excess, direct flow due to precipitation, baseflow (0 for this example, see Example 3 for a run
with baseflow) and total flow for each interval appears (Figure 17).
results and Close to return to the Basin Schematic Window.
Click on Print to print out the
Figure 14: Basin Schematic Window with pop-up menus
Figure 15: Output Hydrograph
Figure 16: Summary Table
Figure 17: Time Series Table
The peak discharge is 4378.5 cfs, occurring 12.6 hours after the start of the storm as given in the
Summary of Results Table (Figure 16). The runoff hydrograph is given in Figure 15 with a graph of
the precipitation in gray and loss in inches in black. The difference between the precipitation and loss
is the excess runoff in inches. A portion of a table of precipitation, loss, excess runoff, direct Q,
baseflow (no baseflow input for Example 1) and total Q (the sum of the direct Q and baseflow values
are plotted in the hydrograph, Figure 15) is given in Figure 17 for 6 minute intervals. The remaining
part of this table is basically the same (Figure 17).
5 - Example 2: Flood Hydrograph with Routing
In Example 2, the flood hydrograph from Example 1 will be routed through a reservoir spillway structure.
There will be two methods shown in example 2. The first is to assume Example 1 has been run. The
second assumes no data has been entered for the basin.
METHOD 1
1.
In the main HEC-HMS window, choose Open Project from the File menu and open the project
created in Example 1: Flood Hydrograph. The Open Project dialog box will appear. Click on the
project created in Example 1, then click on the Open button. Double click on Basin 1 under Basin
Model to open the basin model. The Basin Schematic window will appear. Choose Save Basin
Model As from the File pull down menu. Enter Basin 2 in the New Basin Model field and press OK.
The Basin Schematic window will appear on screen, set to Basin 2, which is an exact copy of Basin 1.
2.
In the Basin Schematic window, select Basin Model Attributes from the File pull down menu. The
Basin Model Attributes dialog box will appear on screen. If the Channel Routing field in Default
Methods is not set Modified Puls, click the down arrow next to the field and select Modified Puls.
Press OK to be returned to the Basin Schematic window.
3.
In the Basin Schematic window, click on Reservoir button and, while still holding the mouse button,
drag the cursor to a point below Subbasin-1, and release. A reservoir icon will be created in the
Basin Schematic window.
Figure 18: Basin Schematic Window with Routing
4.
Click on the Subbasin-1 icon. Then click and hold the right mouse button while the cursor is
positioned over the Subbasin-1 icon. A pop up menu will appear on screen. While still holding the
right mouse button, move the cursor down the menu to Connect Downstream, then release the
button. The cursor will become a set of crosshairs. Click on Reservoir-1. A line will be drawn
between Subbasin 1 and Reservoir-1. By this procedure, the direction of flow will be from Subbasin–
1 to the reservoir (Figure 18).
5.
The characteristics of the reservoir are edited by double clicking the Reservoir-1 icon. The Reservoir
Routing window will appear on screen (Figure 19). Under Input Options, select Elevation-StorageOutflow. You will be prompted as to whether you want to change the table type. Press OK when
prompted.
6.
Enter the elevation, storage and outflow data as indicated in Tables 4 and 5 (Figure 19).
Figure 19: Completed Reservoir Routing Window
7.
Press OK. You will be returned to the Basin Schematic window.
8.
Select Run Manager from the Simulate pull down menu. Next, click on Run Configuration from the
Simulate pull down menu and the HMS Run Configuration dialog box will appear on screen (Figure
20). Choose Basin 2, Precip 1 and Control 1 from the Basin ID, Precip ID and Control ID lists. Click
Add and then Close to be returned to the Simulation Manager. Click on Run 2 under Run ID. Click
on the Compute button to complete the run.
9.
Display the results for Subbasin-1 by clicking on Subbasin-1 and then following steps 18 through 21
from Example 1: Flood Hydrograph.
10. To review results, repeat step 9 for Reservoir-1.
Figure 20: HMS Run Configuration dialog box
METHOD 2
1.
Repeat steps 1 through 12 from Example 1.
2.
Repeat steps 3 through 8 from Method 1 in this example.
3.
To review results, repeat steps 18 through 21 from Example 1.
4.
To review results, click on the Reservoir-1 icon in the Schematic window and repeat steps 18
through 21 from Example 1.
The inflow hydrograph is the same as Example 1 since all the input variables are the same. The peak
outflow is 599.72 cfs and peaks at 14.7 hours into the storm, or 2.1 hours after the peak inflow (Figure
21). The elevation in the pond peaked at 794.98 feet (Figure 21). The time of the maximum storage was
14.7 hours, the same time as the maximum outflow. Since the calculated maximum elevation (794.98
feet) is greater than the elevation of the top of the dam (792.4 feet), the dam will overtop. The inflow and
outflow hydrographs are given at the bottom of Figure 22 and the reservoir surface water elevation and
storage volume at the top of Figure 22. A portion of the table with the reservoir storage, elevation, inflow
and outflow every 6 minutes is given in Figure 23.
Figure 21: Summary Table
Figure 22: Hydrograph for Reservoir-1
Figure 23: Time series table
6 - Example 3: Multiple Watersheds
The same watershed given in Example 2 will be divided to create multiple watersheds discharging into a
reservoir. This flow will also be routed over the same spillway given in example 2.
1.
In the main HEC-HMS window, choose Open Project from the File menu and open the project
created in Example 1: Flood Hydrograph. Choose Basin Model from the Edit pull down menu in the
main HEC-HMS windows, then select New. The New Basin Model dialog box will appear on screen.
Press OK to accept the default name for the new basin model, which should be Basin 3. The HMS
Basin Schematic window will appear on screen.
2.
Create the first subbasin by clicking on the Subbasin button, and while still holding down the mouse
button, dragging the cursor near the center of the Basin Schematic window and releasing the mouse
button. A subbasin icon, named Subbasin-1, will be created in the Basin Schematic window.
3.
Repeat step 2, placing Subbasin-2 to the right of Subbasin-1.
4.
Create a junction by clicking on the Junction button, and while still holding down the mouse button,
dragging the cursor below the two subbasins and releasing the button.
5.
Create a reservoir by clicking on the Reservoir button, and while still holding down the mouse button,
dragging the cursor below Junction-1 and releasing the button.
6.
Click on the Subbasin-1 icon. Then click and hold the right mouse button while the cursor is
positioned over the Subbasin-1 icon. A pop up menu will appear on screen. While still holding the
right mouse button, move the cursor down the menu to Connect Downstream, then release the
button. The cursor will become a set of crosshairs. Click on Junction-1. A line will be drawn
between Subbasin 1 and Junction-1.
7.
Repeat step 6, connecting Subbasin-2 to Junction-1.
8.
Repeat step 6, connection Junction-1 to Reservoir-1. The Basin Schematic window should look like
that pictured in Figure 24 (the names of the basins will be changed in steps 9 and 12). Using this
procedure, the flow direction goes from the subbasins to the juncture to the reservoir. The reason to
have a juncture in this example is to have a separate record of the combined flows. The junction can
be eliminated by directing the flow from the subbasins to the reservoir.
9.
Double click on Subbasin-1. The Subbasin Editor window will appear. Change the name of
Subbasin-1 to North. Enter 0.771 sq. mi. as the subbasin area. In the Loss Rate tab, change the
Method to SCS Curve No. Enter 0 for Initial Loss and % Impervious and 85.2 for SCS Curve No. You
will now be returned to the Basin Schematic window. The Subbasin Editor will then prompt you to
confirm the changes made to the old Loss Rate method. Click OK and then continue.
10. Select the Transform tab in the Subbasin Editor window and change the Method to SCS. Enter 0.462
hours for the SCS Lag (change the Lag time units from the pull down menu).
11. Select the Baseflow tab from the Parameters tab and change the Method to Recession if it is not
already selected. Enter the Initial Q of 5 cfs, Recession Ratio of 0.9 and Threshold Q of 0.05 cfs in
the appropriate fields as shown in Figure 25. Choose OK to accept and save the data. You will be
returned to the Basin Schematic window.
12. Repeat steps 9, 10 and 11 for Subbasin-2. Change the name of Subbasin-2 to South. The basin
area for the South basin is 1.859 sq. mi., the Initial Loss and % Impervious is 0, the SCS Curve No. is
79.4, the SCS Lag is 0.5547 hours, and for the baseflow, the Initial Q is 8 cfs, the Recession Ratio is
0.9 and the Threshold Q is 0.05 cfs.
Figure 24: Basin Schematic Window with Multiple Watersheds
Figure 25: Baseflow Tab of Subbasin Editor
13. Repeat steps 5 through 7 from Example 2 for the reservoir.
14. Select Run Manager from the Simulate pull down menu. Click on Run Configuration from the
Simulate menu and the HMS Run Configuration dialog box will appear on screen (Figure 26).
Choose Basin 3, Precip 1 and Control 1 from the Basin ID, Precip ID and Control ID lists. Click Apply
and then Close to be returned to the Simulation Manager. Click on Run 3 under Run ID. Click on the
Compute button to complete the run.
Figure 26: HMS Run Configuration dialog box
15. Display the results for the North basin by clicking on North and then following steps 18 through 21 (to
view results) from Example 1: Flood Hydrograph.
16. To review results, repeat step 15 for the South basin, Junction-1 and Reservoir-1.
Figure 27 shows the three inflow hydrographs, North basin, South basin and their combined flows.
The outflow hydrograph and storage elevation is not shown because it is basically the same as in
Example 2. The only difference in the format of the output in Example 3 is the time series table at the
juncture (Figure 28), showing the inflow for the North and South basins with the combined flow. Note,
the inflows include the baseflow for each basin.
Figure 27: Outflow Hydrographs
Figure 28: Time series table at juncture