WMS Models

WMS Models

The Watershed Modeling System supports several industry-standard, numerical models to compute peak flow, hydrographs, water quality, water surface elevations, and other hydrologic or hydraulic parameters. Each model is supported through the Hydrologic Modeling Module or the River Module with a completely integrated interface for parameter input, job control, and output review. A model checker is also featured with each model; this checker guides you to correct errors or omissions in model input data. The models available for use with WMS are described below – each model is included with the WMS installation (model executable files and documentation) and is fully linked with the WMS software.

Click on a link to the right to learn more about a specific model.

xpswmm

xpswmm is a comprehensive software package for modeling stormwater, sanitary and river systems. xpswmm is used by scientists, engineers and managers to develop link-node (1D) and spatially distributed hydraulic models (2D). It simulates natural rainfall-runoff processes and the performance of engineered systems that manage our water resources.  See the xpswmm page for more details.

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HEC-1

HEC-1, developed by the Hydrologic Engineering Center in Davis, California, has long been one of the industry-standard programs for hydrologic analysis. It is a single storm event, lumped parameter model, but includes several different options for modeling rainfall, losses, unit hydrographs, and stream routing. The HEC-1 interface contained within WMS makes it simple to enter and manage input data and display analysis results.

The HEC-1 model is designed to simulate the surface runoff response of a river basin to precipitation by representing the basin as an interconnected system of hydrologic and hydraulic components. Each component models an aspect of the precipitation-runoff process within a portion of the basin, commonly referred to as a subbasin. A component may represent a surface runoff entity, a stream channel, or a reservoir. Representation of a component requires a set of parameters which specify the particular characteristics of the component and mathematical relations which describe the physical processes. The result of the modeling process is the computation of streamflow hydrographs at desired locations in the river basin.

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HEC-HMS

HEC-HMS is the next generation hydrologic model developed by the Hydrologic Engineering Center. Its intent is to replace HEC-1, and it features many of the same modeling capabilities as HEC-1 and additional capabilities such as the MODClark quasi-distributed (2D) hydrologic model. WMS has a fully-featured interface to the MODClark hydrologic model and includes documentation and step-by-step tutorials showing how to setup MODClark and other types of HMS models.

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TR-20

The TR-20 Interface Module provides a complete graphical interface to the U.S. Soil Conservation Service TR-20 hydrologic analysis model.

The TR-20 computer program assists the engineer in hydrologic evaluation of flood events for use in analysis of water resource projects. The program is a physically-based event model which computes direct runoff resulting from any synthetic or natural rainstorm. There is no provision for recovery of initial abstraction or infiltration during periods of no rainfall within an event.

The program develops flood hydrographs from runoff and routes the flow through stream channels and reservoirs. Routed bydrographs are combined with those from tributaries. Procedures for hydrograph separation by branching or diversion of flow and for adding baseflow are provided. The program uses procedures described in the SCS National Engineering Handbook, Section 4, Hydrology (NM-4) except for the reach flood routing procedure. The reach routing is described in Hydrology Note 2.

Peak discharges, their times of occurrence, water surface elevations and duration of flows can be computed at any desired cross section or structure. Complete discharge hydrographs, as well as discharge hydrograph elevations, can be obtained if requested. The program provides for the analysis of up to nine different rainstorm distributions over a watershed under various combinations of land treatment, floodwater retarding structures, diversions, and channel modifications. Such analysis can be performed on as many as 200 subwatersheds or reaches and 99 structures in any one continuous run.

The program was originally developed by the Hydrology Branch of the Soil Conservation Service (SCS) in cooperation with the Hydrology Laboratory, Agricultural Research Service (ARS), through a contract with C-E-I-R, Inc. Numerous modifications and additions have been made since by the SCS.

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TR-55

The TR-55 Interface provides a complete graphical interface to the U.S. Soil Conservation Service TR-55 hydrologic analysis model. All modeling parameters and input data are entered through interactive graphics and easy-to-use dialog boxes, enabling the user to easily define the TR-55 hydrologic routing model.

TR-55 is perhaps the most widely used approach to hydrology in the United States. TR-55 provides a number of techniques that are useful for modeling small watersheds. TR-55 utilizes the SCS runoff equation to predict the peak rate of runoff as well as the total volume. TR-55 also provides a simplified “tabular method” for the generation of complete runoff hydrographs. The tabular method is a simplified technique based on calculations performed with TR-20.

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MODRAT

MODRAT is a modified rational method computer program developed by the Los Angeles County Department of Public Works (LACDPW) to compute runoff rates under a variety of conditions common to the area of Los Angeles, California. The successor to F0601, MODRAT contains all the features of the F0601 as well as updated capabilities for watershed modeling in the Los Angeles area. MODRAT may be used to find flow rates for any watershed with any combination of existing or proposed channels and drains. Further, the watershed may be undeveloped, partially developed, or completely developed. The model will compute runoff rates for a 50-year, 25-year, or 10-year frequency design storm (developed by LACDPW), as well as any other storm which can be represented by a rainfall mass curve. Given any combination of the above variables, MODRAT will compute a hydrograph for each subarea and mainline collection point in the watershed.

Modifications to the Rational Method in MODRAT

As a method of urban hydrology, the rational method falls short in several ways. First, the method does not produce a hydrograph, only a single flow rate. Second, the rational method does not account for changing (time dependent) conditions such as soil condition or rainfall intensity. Finally, results are not very accurate for large areas. Due to these problems, MODRAT contains the following modifications:

Rainfall intensity, i, is a variable dependent on rainfall frequency, storm time, and time of concentration. The variation of i is represented by a temporal distribution curve (rainfall mass curve). C, the runoff coefficient, varies with soil type, rainfall intensity, and imperviousness. The time variation of C and i allow the flow, Q, to vary with time, thus producing a hydrograph. The area under the hydrograph represents the total volume of flow from a watershed, a variable which the rational method does not provide. Hydrographs may be computed for a number of subareas, for each lateral to the main channel, and for each collection point on the main channel. These hydrographs are routed and combined as computation progresses downstream.

The above modifications to the rational method allowed for the computation of storm hydrographs for any size watershed. With such improvements, the modified rational method (MODRAT) has been adopted by LACDPW as the preferred method of hydrologic analysis.

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CE-QUAL-W2

CE-QUAL-W2 is a two-dimensional, longitudinal/vertical, hydrodynamic and water quality model. Because the model assumes lateral homogeneity, it is best suited for relatively long and narrow waterbodies exhibiting longitudinal and vertical water quality gradients. The model has been applied to rivers, lakes, reservoirs, and estuaries.

Model capabilities:

Hydrodynamic. The model predicts water surface elevations, velocities, and temperatures. Temperature is included in the hydrodynamic calculations because of its effect on water density. Water quality. The water quality algorithms incorporate 21 constituents in addition to temperature including nutrient/phytoplankton/dissolved oxygen (DO) interactions during anoxic conditions. Any combination of constituents can be simulated. The effects of salinity or total dissolved solids/salinity on density and thus hydrodynamics are included only if they are simulated in the water quality module. The water quality algorithm is modular allowing constituents to be easily added as additional subroutines.

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NATIONAL FLOOD FREQUENCY (NFF)

WMS includes an interface to the National Flood Frequency Program (NFF). The NFF program is a compilation of all the current (as of September 2003) statewide and metropolitan area regression equations. The regression equations are a result of years of effort by the United States Geological Survey (USGS) to develop regional regression equations for estimating flood magnitude and frequency of ungaged watersheds. The USGS, in cooperation with the Federal Highway Administration and the Federal Emergency Management Agency compiled all the regression equations into a single database file. This database file is the basis of the NFF program, which can be used to guide the user through the input required to compute peak flows for different frequencies using the database of state by state regression equations.

The NFF interface in WMS provides a windows based, graphical user interface to the same database of regression equations. The entire program is run from a single dialog. Further, if a digital terrain model is available for the study area, all of the geometric parameters required for the regression equations are automatically supplied as the individual equations are specified. These parameters include area, slope, elevation, distances, and others.

The NFF equations are useful for estimating a peak flood discharge and typical flood hydrograph for a given recurrence interval of an unregulated rural or urban watershed. These techniques should be useful to engineers and hydrologists for planning and design purposes. A statewide summary, along with other technical information can be found in the USGS Water-Resources Investigations Report 94-4002.

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RATIONAL METHOD

WMS includes an interface to the rational method which can be used for computing peak flows on small urban and rural watersheds. The interface includes the capability to combine runoff from multiple basins. Two different methods for determining peak flows/hydrographs at downstream confluences are available.

Traditionally a time of concentration is determined at a downstream confluence by determining the longest combination of time of concentration and routing travel time. Given a time of concentration for the outlet, a rainfall intensity can be determined from a rainfall-intensity-duration curve and a peak flow computed. The hydrograph for the confluence is then determined in the same manner they are determined for sub-basins; by using the peak flow, time of concentration, and a dimensionless hydrograph.

Alternatively, hydrographs for the sub-basins can be computed and then routed (lagged) and combined by summing at the confluence points. When using this method detention basins may be defined at confluence points in order to determine the effect of storage on the computations. All of the computations for peak flows, hydrographs, and routing are done within WMS.

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HSPF

Developed by the USGS and EPA, the Hydrologic Simulation Program – FORTRAN (HSPF) simulates hydrologic and waterquality 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.

HSPF simulates for extended periods of time the hydrologic, and associated water quality, processes on pervious and impervious land surfaces and in streams and well-mixed impoundments. HSPF uses continuous rainfall and other meteorologic records to compute streamflow hydrographs and pollutographs. HSPF simulates interception soil moisture, surface runoff, interflow, base flow, snowpack depth and water content, snowmelt, evapotranspiration, ground-water recharge, dissolved oxygen, biochemical oxygen demand (BOD), temperature, pesticides, conservatives, fecal coliforms, sediment detachment and transport, sediment routing by particle size, channel routing, reservoir routing, constituent routing, pH, ammonia, nitrite-nitrate, organic nitrogen, orthophosphate, organic phosphorus, phytoplankton, and zooplankton. Program can simulate one or many pervious or impervious unit areas discharging to one or many river reaches or reservoirs. Frequency-duration analysis can be done for any time series. Any time step from 1 minute to 1 day that divides equally into 1 day can be used. Any period from a few minutes to hundreds of years may be simulated. HSPF is generally used to assess the effects of land-use change, reservoir operations, point or nonpoint source treatment alternatives, flow diversions, etc. Programs, available separately, support data preprocessing and postprocessing for statistical and graphical analysis of data saved to the Watershed Data Management (WDM) file.

The model contains hundreds of process algorithms developed from theory, laboratory experiments, and empirical relations from instrumented watersheds.

The model was developed in the early 1960’s as the Stanford Watershed Model. In the 1970’s, water-quality processes were added. Development of a Fortran version incorporating several related models using software engineering design and development concepts was funded by the Athens, Ga., Research Lab of EPA in the late 1970’s. In the 1980’s, preprocessing and postprocessing software, algorithm enhancements, and use of the USGS WDM system were developed jointly by the USGS and EPA.

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HEC-RAS

The HEC-RAS system will ultimately contain three one-dimensional hydraulic analysis components for: (1) steady flow water surface profile computations; (2) unsteady flow simulation; and (3) movable boundary sediment transport computations.

Currently steady and unsteady flow are available and sediment transport is under development. A key element is that all three components will use a common geometric data representation and common geometric and hydraulic computation routines. In addition to the three hydraulic analysis components, the system contains several hydraulic design features that can be invoked once the basic water surface profiles are computed, including bridge scour computations, uniform flow computations, stable channel design, and sediment transport capacity.

The current version of HEC-RAS supports steady and unsteady flow water surface profile calculations. New features and additional capabilities will be added in future releases.

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GSSHA Gridded Surface Subsurface Hydrologic Analysis

Developed with the US Army Corps of Engineers Engineering Research and Development Center (USACE ERDC), the GSSHA model is a significant reformulation and enhancement of the CASC2D model. The CASC2D runoff model began with a two-dimensional overland flow routing algorithm developed and written in APL (A Programming Language) by Professor P.Y. Julien at Colorado State University. The overland flow routing module was converted from APL to FORTRAN by Dr. Bahram Saghafian, then at Colorado State University, with the addition of Green & Ampt infiltration and explicit diffusive-wave channel routing. The FORTRAN code was reformulated, significantly enhanced, and re-written in the C programming language by Dr. Bahram Saghafian at the U.S. Army Construction Engineering Research Laboratories (CERL). Implicit channel routing was added to CASC2D by Fred L. Ogden, formerly at Colorado State University, now Associate Professor, Department of Civil and Environmental Engineering, University of Connecticut, Storrs, Connecticut. The GSSHA model has been integrated into the WMS software with direction from the USACE ERDC who continue to sponsor the model and interface.

The principal purpose of the GSSHA model is to correctly identify and realistically simulate the important hydrologic processes in watersheds. The model is intended to simulate different types of runoff production and determine the controlling physical processes in watersheds, i.e. infiltration excess, saturated source areas, and groundwater discharge. Development of the model was directed by the following requirements:

Model must be capable of explicitly calculating flows, stream depths, and soil moistures in a variety of hydrologic regimes and conditions including non-Hortonian watersheds; Formulation must account for sub-surface effects on stream flow; Numerical algorithms must be robust; Model must conserve mass; Model must be capable of being extended to contaminant transport problems; Simulation times must be short enough to allow real-time predictions for use at DoD training facilities; Model must be supported by the standard DoD graphical user interface (GUI) WMS; Source code must be available to the U.S. Army without restrictions or limitations on modification or publication of results.

Despite the fact that GSSHA is derived from CASC2D, the formulation of GSSHA is fundamentally different from CASC2D in terms of the way the model updates individual processes in time. The formulation of CASC2D version 1.18b used a short time-step event loop, nested within an hourly evapo-transpiration (ET) loop. In CASC2D simulations, when Hortonian runoff production ceases, the short time-step event loop is bypassed, effectively disabling all hydrodynamics other than evapo-transpiration from soil water. This formulation is not well suited for simulation of non-Hortonian watersheds, because the hydrodynamics of flows in the saturated and unsaturated zones cease to be event based. Rather, these processes continue independent from the occurrence of rainfall. Furthermore, with groundwater discharges to streams, channel hydrodynamics are also required to run continuously. To accommodate the inclusion of continuous processes, such as saturated and unsaturated groundwater flow, the nested-loop formulation of the CASC2D model (Figure 2), was discarded. The GSSHA formulation uses only one main loop (Figure 3). In GSSHA, each process has its own time step and an associated update time. During each time step the update time of each process selected by the user is checked against the current model time. When they coincide, the process is updated, and updated information from that process is transferred to dependent processes. The update time or time step of dependent processes may be modified as part of the process update. In other words, any process in the model can effectively change the time step of any other process. This formulation permits the efficient simultaneous simulation of processes that have dissimilar response times, such as overland flow, evapo-transpiration, and lateral groundwater flow. This scheme also allows a more integrated solution of processes coupled through boundary conditions and flux exchanges. Finally, the scheme has allowed considerable increases in computational efficiency through the use of larger time steps when rates of change allow.

The GSSHA model is also fundamentally different from the CASC2D model because it extends the applicability of the model to non-Hortonian basins. The CASC2D formulation assumes that once water infiltrates into the soil, it either drains vertically or is removed by evapo-transpiration. Soil water or groundwater is not considered in the context of non-Hortonian runoff production. The GSSHA formulation can simulate non-Hortonian runoff production.

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SMPDBK

The Simplified Dam-Break (SMPDBK) was developed by the National Weather Service (NWS) for predicting downstream flooding produced by a dam failure. This program is still capable of producing the information necessary to estimate flooded areas resulting from dam-break floodwaters while substantially reducing the amount of time, data, and expertise required to run a simulation of the more sophisticated unsteady NWS DAMBRK, or now called FLDWAV. The SMPDBK method is useful for situations where reconnaissance level results are adequate, and when data and time available to prepare the simulation are sparse. Unlike the more sophisticated versions of DAMBRK and FLDWAV, the SMPDBK method does not account for backwater effects created by natural channel constrictions of those due to such obstacles as downstream dams or bridge embankments.

The input required for a SMPDBK model is a stream centerline, cross sections, and information regarding the storage and failure of the dam being modeled. WMS saves the model data to a properly formatted input file for SMPDBK and then launches the executable. The executable is the same version distributed by the NWS. When a model successfully runs WMS will automatically read the results and create a water surface elevation data set that can be used for automated floodplain delineation.

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EPA-SWMM

The EPA Storm Water Management Model (SWMM) is a dynamic rainfall-runoff simulation model used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. The runoff component of SWMM operates on a collection of subcatchment areas that receive precipitation and generate runoff and pollutant loads. The routing portion of SWMM transports this runoff through a system of pipes, channels, storage/treatment devices, pumps, and regulators. SWMM tracks the quantity and quality of runoff generated within each subcatchment, and the flow rate, flow depth, and quality of water in each pipe and channel during a simulation period comprised of multiple time steps.