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hec hms module - Drexel University · PDF fileHYDROLOGIC ENGINEERING USING THE HEC -HMS MODULE 1 - Theory and Background This section consists of a teaching module for undergraduate

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  • 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

    4100 and the longitude of 7425 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 4100 and longitude

    7425. 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 HEC-

    HMS 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

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