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Sub-basins of the watershed can be created by adding any number of additional outlet points along the stream network. Besides the sub-basin
boundaries, WMS automatically creates a topologic representation of the watershed which is used for defining models such as HEC-1 and TR-20.
WMS includes a comprehensive interface to the TR-20 hydrologic program used by many hydrologic engineers to model the rainfall-runoff process. The
interface has been created in such a way that models can be built from TINs used to delineate basin boundaries and compute geometric data or by manually constructing a series of outlets and basins to form a
topologic representation of the watershed. When a TIN is used the topologic model is automatically constructed as outlets are added and basins defined. Also, any geometric parameters computed by WMS are supplied to
corresponding TR-20 input fields.
TR-20 parameters are edited from a master dialog. This dialog allows the user access to the several user-friendly dialogs including the channel data
dialog used to specify desired options. WMS allows the user to define the main TR-20 options such as the RUNOFF, REACH, RESVOR, and DIVERT cards. Composite curve numbers and basin times of concentration can be
computed automatically.
Once a TR-20 model has been defined, the TR-20 model checker can be run to try and identify potential problems in the data prior to actually running
the TR-20 model. The model checker provides several hints (although it does not guarantee a successful run or that the answers will be correct) for correcting the data prior to running TR-20.
TR-20 can be launched from within WMS, and after completion, hydrographs can be displayed on the TIN, the topologic model and blown up within the
hydrograph window.
The National Flood Frequency (NFF) Program
was developed by the USGS in cooperation with the Federal Highway Administration (FHA) and the Federal Emergency Management Agency (FEMA). It evaluates regression equations for estimating T-year flood-peak discharges for rural and urban watersheds. As many as 7 multiple regression equations (2-, 5-, 10-, 25-, 50-, 100-, and 500-year) are defined for each of 200 plus flood regions across the US. Methods are also available for estimating a typical flood hydrograph corresponding to a given T-year peak discharge.
The NFF program is composed of two components: (1) Each state's regression equations, standard errors, etc., and (2) a calculation routine for rural
and urban flood characteristics including tabling and graphing capabilities. To use the regression equations, a basin is defined in WMS using the TIN and drainage analysis tools for basin delineation. The NFF main
dialog is then used to define the state and region(s) where the watershed is located. Variables for the regression equations are defined and peak flows computed. NFF has the capability to generate an approximate,
design hydrograph which can be displayed within WMS in the same way hydrographs for HEC-1 and TR-20 are displayed.
An interface to NFF within WMS will provide the following benefits:
1. WMS computes basin geometric parameters from a TIN. Many of the regression equations are functions of geometric parameters computed by WMS such
as area, slope, elevation, basin length etc. Whenever an equation is specified which uses a variable computed by WMS, the value is automatically substituted. The user always has the chance to "override"
the value computed by WMS. Other variables such as mean annual precipitation, basin storage, etc. can be defined prior to performing computations.
2. WMS works in both MS-Windows and UNIX-X-Windows environments. Therefore the interface to NFF within WMS is an intuitive graphical interface
that runs on both types of platforms. The current interface to NFF is strictly DOS-based.
3. The interface can be used with or without terrain data to supply geometric parameters. There are obvious advantages to using digital terrain data
to aid in basin delineation and geometric parameter computations, but in many instances, the terrain data is not available. In such cases WMS can still be used in the same way the NFF program currently works (the
user specifies values for all variables in the specified regression equation(s)), except that it is runs in the Windows environment.
4. WMS has interfaces to other hydrologic modeling programs such as TR-20 and HEC-1 which, when necessary, can be used to compare results with NFF.
The Rational Method
is one of the simplest and best known methods routinely applied in urban hydrology. Peak flows are computed from the simple equation: Q = kCiA where: Q - Peak flow, k - conversion factor, C - Runoff coefficient, i - Rainfall intensity, and A - Catchment area.
With WMS' capability to create TINs from feature arc data, roads, railroads, canals, and other urban features which control runoff are easily
incorporated into the model so that urban catchment areas can be delineated so that the variable A can be automatically computed.
The Rational Method main dialog includes the capability to generate Intensity Duration Frequency (IDF) curves from HYDRO-35 maps (eastern US), NOAA
Atlas maps (western US), or user-specified data of rainfall intensities. A kinematic wave equation, used by the Federal Highways Administration (FHA) design handbook, can be used to estimate the time of
concentration from the catchment length, slope, and a Manning's roughness coefficient.
The conversion of rural land to urban land usually increases erosion and the discharge and volume of storm runoff in a watershed. It also causes
other problems that affect soil and water. As part of programs established to alleviate these problems, engineers increasingly must assess the probable effects of urban development as well as design and implement
measures that will minimize its adverse effects.
Technical Release 55 (TR-55) presents simplified procedures for estimating runoff and peak discharges in small watersheds. In selecting the
appropriate procedure, consider the scope and complexity of the problem, the available data, and the acceptable level of error. While this TR gives special emphasis to urban and urbanizing watersheds, the procedures
apply to any small watershed in which certain limitations are met.
Developed by the USGS and EPA, HSPF simulates hydrologic and water-quality processes on land surfaces, streams, and impoundments. HSPF is generally used
to perform a watershed-based analysis of the effects of land use, reservoir operations, point and nonpoint source treatment alternatives, flow diversions, etc. It is accepted by the EPA as a tool for the development
of TMDLs in the United States.
Digital Terrain Modeling and Mapping in WMS
Watershed models can be created in WMS from triangulated irregular networks (TINs), Digital Elevation Models (DEMs), or GIS vector data. WMS is
compatible with ARC/INFO and ArcView data and includes many utilities for both importing and exporting. Major features of WMS include:
- Watershed model development from imported ARC/INFO or ArcView shape files.
- Delineates watershed and sub-basin boundaries from TINs or GRIDs.
- Computes geometric parameters in a fraction of the time required from traditional methods.
- Complete Interfaces to HEC-1, TR-20, Rational Method, and NFF.
- Composite curve number generation from GIS vector or grid data.
- Automatic computations of time of concentration or lag time from computed geometric parameters.
- Flood plain delineation.
- Use of TIFF images as backdrops for on-screen digitizing or to enhance final presentation graphics.
- Hydrologic/hydraulic calculators for detention basin, curb and gutter, wire, and improved channels.
Overview of WMS
WMS merges information obtained from terrain models and GIS with industry-standard lumped parameter hydrologic analysis models such as HEC-1 and
TR-20. Terrain models can obtain geometric attributes such as area, slope and runoff distances. Many display options are provided to aid in modeling and understanding the drainage characteristics of terrain surfaces.
The distinguishing difference between WMS and other applications designed for setting up hydrologic models like HEC-1 and TR-20 is its unique
ability to take advantage of digital terrain for hydrologic data development. WMS uses three primary data sources for model development:
1. Geographic Information Systems (GIS) Data
2. Digital Elevation Models (DEMs)
3. Triangulated Irregular Networks (TINs)
Guidelines for Using Geographic Information Systems (GIS) Data
Watershed and sub-basin boundaries may already be known and stored as part of a GIS or CAD database, or it may be straight- forward to trace an
existing map to define streams and basins. With WMS, properly structured hydrologic models can be created automatically from points, lines, and polygons.
Since this data is often already developed and stored in a GIS, importing from ARC/INFO and ArcView, or DXF files is easily done.
The following are the basic steps required to create watershed models from scratch using GIS data.
1. Obtain a Map of Already-Developed GIS or CAD Data or Digitize a TIFF Image of a Map On-Screen
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Import the digitized map as a CAD or
GIS file, or use "heads up" or "on-screen" digitizing inside WMS. In order to do heads up digitizing, you will need either: (1) digital elevation (DEM) data that can be contoured by WMS or ( 2) a
scanned tiff image that can be read into WMS and used as a background map.
2. Construct Feature Object Topology
- A point layer representing the watershed outlet and any sub-basin outlet or confluence points
- A line layer representing a stream network
- A polygon layer representing watershed boundaries
3. Define the Hydrologic Mode
Once the watershed model representation has been created, data defining a specific hydrologic model can
be entered. Since WMS allows for all hydrologic modeling input to be defined separate from any digital terrain or spatial data, it is not required that the watershed model developed with feature objects be to scale.
Area and length parameters can be calculated or simply be manually defined using the model interface.
Topologic model automatically constructed from feature objects.
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