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Hydrologic, hydraulic, and water quality models all have different purposes and will provide different information. Table 8.1 summarizes some of the commonly used modeling software and modeling techniques and the main purpose for which they were developed. The table shows the relative levels of complexity of necessary input data, indicates whether the model can complete a continuous analysis or is event based, and lists whether the model is in the public domain. For hydraulic models, Table 8.1 indicates whether unsteady flow calculations can be conducted. For water quality models, the table indicates whether the model is a receiving waters model, a loading model, or a BMP analysis model.

The selection of a stormwater modeling tool is based on the modeling objectives and on the available resources. When evaluating the modeling objectives, the modeler should consider:

- The type of information desired from the modeling effort.
- The specific conditions to be modeled.
- The required level of accuracy and reliability of the model.
- The further use of the model and model results.

For example, estimating peak runoff rates is a different problem than estimating the peak elevation of a water body and could require the use of a different model. A model able to estimate phosphorus loading from a network of detention ponds may not be able to model the phosphorus loading from an infiltration pond.

When evaluating the resources available, the modeler should consider:

- The general limitations of modeling which include imperfect approximations of natural processes, uncertainty and variability in results, and uncertainty and error in the input parameters.
- Availability of existing models used for site analysis.
- Familiarity with the specific model.
- Modeling expertise available.

Hydrologic models are used to estimate runoff volumes, peak flows, and the temporal distribution of runoff at a particular location resulting from a given precipitation record or event. Essentially, hydrologic models are used to predict how the site topography, soil characteristics, and land cover will cause runoff either to flow relatively unhindered through the system to a point of interest, or to be delayed or retained somewhere upstream. Many hydrologic models also include relatively simple procedures to route runoff hydrographs through storage areas or channels, and to combine hydrographs from multiple watersheds.

The rational method is a simple calculation of peak flow based on drainage area, rainfall intensity, and a non-dimensional runoff coefficient. The peak flow is calculated as the rainfall intensity in inches per hour multiplied by the runoff coefficient and the drainage area in acres. The peak flow, Q, is calculated in cfs as Q = CiA where C is the runoff coefficient, i is the rainfall intensity, and A is the drainage area. The conversion factor of 1.008 is necessary to convert acre-inches per hour to cfs, but this is typically not used. This method is best used only for simple approximations of peak flow from small watersheds.

HEC-1 is a rainfall-runoff model developed by the U.S. Army Corps of Engineers. HEC-1 is a single storm event, lumped parameter model that includes several options for modeling rainfall, losses, unit hydrographs, and stream routing. The 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 precipitationrunoff process within a portion of the basin. 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 stream flow hydrographs at the desired locations in the river basin. The upgraded version of this model is HEC-HMS.

HEC-HMS is a rainfall-runoff model developed by the U.S. Army Corps of Engineers to compute runoff hydrographs for a network of watersheds. The model evaluates infiltration losses, transforms precipitation into runoff hydrographs, and routes hydrographs through open channel routing. A variety of calculation methods can be selected including SCS curve number or Green and Ampt infiltration, Clark, Snyder or SCS unit hydrograph methods, and Muskingum, Puls, or lag routing methods. Precipitation inputs can be evaluated using a number of historical or synthetic methods and one evapotranspiration method.

Natural Resources Conservation Service Technical Release No. 20 (TR-20): Computer Program for Project Formulation Hydrology was developed by the hydrology branch of the U.S.D.A. Soil Conservation Service in 1964. TR-20 is a single-event rainfall-runoff model that is typically used with a design storm for rainfall input.

There is no provision for recovery of initial abstraction or infiltration during periods of no rainfall within an event. The program computes runoff hydrographs, routes flows through channel reaches and reservoirs, and combines hydrographs at confluences of the watershed stream system. Runoff hydrographs are computed using the SCS runoff equation and the SCS dimensionless unit hydrograph. A rainfall-runoff analysis can be performed on as many as 200 subwatersheds or reaches and 99 structures in any one continuous run. TR-20 does not provide for losses of runoff in the transmission of the flood hydrograph due to seepage or other causes of flood water loss.

Technical Release 55 (TR-55; *Urban Hydrology for Small Watersheds*) was developed by the U.S.D.A. Soil Conservation Service, now the Natural Resources Conservation Service (NRCS), in 1975 as a simplified procedure to calculate storm runoff volume, peak rate of discharge, hydrographs and storage volumes. In 1998, Technical Release 55 and the computer software were revised to what is now called WinTR-55. The changes in this revised version of TR-55 include: upgraded source code to Visual Basic, changed philosophy of data input, development of a Windows interface and output post-processor, enhanced hydrograph-generation capability of the software and flood routing hydrographs through stream reaches and reservoirs.

WinTR-55 is a single-event rainfall-runoff small watershed hydrologic model. The model is an input/output interface which runs WinTR-20 in the background to generate, route and add hydrographs. The WinTR-55 generates hydrographs from both urban and agricultural areas at selected points along the stream system. Hydrographs are routed downstream through channels and/or reservoirs. Multiple sub-areas can be modeled within the watershed. A rainfall-runoff analysis can be performed on up to ten sub-areas and up to ten reaches. The total drainage area modeled cannot exceed 25 square miles.

HEC-RAS is a river hydraulics model developed by the U.S. Army Corps of Engineers to compute one-dimensional water surface profiles for steady or unsteady flow. Computation of steady flow water surface profiles is intended for flood plain studies and floodway encroachment evaluations. HEC-RAS uses the solution of the one-dimensional energy equation with energy losses evaluated for friction and contraction and expansion losses in order to compute water surface profiles. In areas with rapidly varied water surface profiles, HEC-RAS uses the solution of the momentum equation. Unsteady flow simulation can evaluate subcritical flow regimes as well as mixed flow regimes including supercritical, hydraulic jumps, and draw downs. Sediment transport calculation capability will be added in future versions of the model. The HEC-RAS program is available to the public from the U.S. Army Corps of Engineers.

HEC-2 is a rainfall-runoff model developed by the U.S. Army Corps of Engineers to compute steady-state water surface elevation profiles in natural and constructed channels. HEC-2 uses the standard step method for water surface profile calculations assuming that flow is one- dimensional, gradually varied steady flow. Subcritical and supercritical flow profiles may be evaluated.

The water surface profile through structures such as bridges, culverts, weirs and other types of structures can be computed. The upgraded version of this model is HEC-RAS. The HEC-2 program is available to the public and can be downloaded from the U.S. Army Corps of Engineers Web site at: http://www.hec.usace.army.mil/software/legacysoftware/hec1/ hec1-download.htm.

WSPRO is a model for water surface profile computations developed by the U.S. Geological Survey. The model evaluates one-dimensional water surface profiles for systems with gradually varied, steady flow. The open channel calculations are conducted using backwater techniques and energy balancing methods. Single opening bridges use the orifice flow equation and flow through culverts is computed using a regression equation at the inlet and an energy balance at the outlet. The WSPRO program is available to the public and can be downloaded from the U.S. Geological Survey Web site at: http://water.usgs.gov/software/wspro.html

CulvertMaster is a hydraulic analysis program for culvert design. The model uses the U.S. Federal Highway Administration Hydraulic Design of Highway Culverts methodology to provide estimates for headwater elevation, hydraulic grade lines, discharge, and culvert sizing. Rainfall and watershed analysis using the SCS Method or Rational Method can be incorporated if the peak flow rate is not known. CulvertMaster is a proprietary model that can be obtained from Haestad Methods, Bentley Systems, Inc. http://www.haestad.com/software/culvertmaster/

FlowMaster is a hydraulic analysis program used for the design and analysis of open channels, pressure pipes, inlets, gutters, weirs, and orifices. Mannings, Hasen-Williams, Kutter, Darcy- Weisbach, or Colebrook-White equations are used in the calculations. FlowMaster is a proprietary model that can be obtained from Haestad Methods, Bentley Systems, Inc. http://www.haestad.com/software/flowmaster/

HydroCAD is a computer aided design program for modeling the hydrology and hydraulics of stormwater runoff. Runoff hydrographs are computed using the SCS runoff equation and the SCS dimensionless unit hydrograph. For the hydrologic computations, there is no provision for recovery of initial abstraction or infiltration during periods of no rainfall within an event. The program computes runoff hydrographs, routes flows through channel reaches and reservoirs, and combines hydrographs at confluences of the watershed stream system. HydroCAD has the ability to simulate backwater conditions by allowing the user to define the backwater elevation prior to simulating a rainfall event. HydroCAD is a proprietary model and can be obtained from HydroCAD Software Solutions LLC. http://www.hydrocad.net/.

PondPack is a program for modeling and design of the hydrology and hydraulics of storm water runoff and pond networks. Rainfall analyses can be conducted using a number of synthetic or historic storm events using methods such as SCS rainfall distributions, intensity-duration-frequency curves, or recorded rainfall data. Infiltration and runoff can be computed using the SCS curve number method or the Green and Ampt or Horton infiltration methods. Hydrographs are computed using the SCS Method or the Rational Method. Channel routing is conducted using the Muskingun, translation, or Modified Puls methods. Outlet calculations can be performed for outlets such as weirs, culverts, orifices, and risers. The program can assist in the determination of pond sizes. PondPack is a proprietary model that can be obtained from Haestad Methods, Bentley Systems, Inc. http://www.haestad.com/software/pondpack/default.asp

The Storm Water Management Model (SWMM) was originally developed for the Environmental Protection Agency (EPA) in 1971 by Metcalf and Eddy, Inc., Water Resources Engineers, Inc. and the University of Florida. SWMM is a dynamic rainfall-runoff and water quality simulation model, primarily but not exclusively for urban areas, for single-event or long-term (continuous) simulation.

The Storm Water Management Model (SWMM) is a comprehensive computer model for analysis of quantity and quality problems associated with urban runoff. Both single-event and continuous simulation can be performed on catchments having storm sewers, or combined sewers and natural drainage, for prediction of flows, stages and pollutant concentrations. Extran Block solves complete dynamic flow routing equations (St. Venant equations) for accurate simulation of backwater, looped connections, surcharging, and pressure flow. A modeler can simulate all aspects of the urban hydrologic and quality cycles, including rainfall, snow melt, surface and subsurface runoff, flow routing through drainage network, storage and treatment. Statistical analyses can be performed on long-term precipitation data and on output from continuous simulation. SWMM can be used for planning and design. Planning mode is used for an overall assessment of urban runoff problem or proposed abatement options. The SWMM program is available to the public and can be downloaded from the U.S. Environmental Protection Agency’s website at: http://www.epa.gov/ceampubl/swater/swmm/index.htm The proprietary shells, XP-SWMM and PC-SWMM, provide the basic computations of EPASWMM with a graphic user interface, additional tools, and some additional computational capabilities. XP-SWMM is available on the XP Software company Web site: http://www.xpsoftware. com/products/xpswmm.htm. PC-SWMM is available on the Computational Hydraulics International Web site: http://www.computationalhydraulics.com/Software/PCSWMM/

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Water quality models are used to evaluate the effectiveness of a BMP, simulate water quality conditions in a lake, stream, or wetland, and to estimate the loadings to water bodies. Often the goal is to evaluate how some external factor (such as a change in land use or land cover, the use of best management practices, or a change in lake internal loading) will affect water quality. Parameters that are frequently modeled include total phosphorus, total suspended solids, and dissolved oxygen.

The Source Loading and Management Model is a water quality model developed by John Voorhees and Robert Pitt for evaluation of nonpoint pollution in urban areas. The model is based on field observations of infiltration practices, wet detention ponds, porous pavement, street sweeping and other source area and outfall control practices. The focus of the model is on small storm hydrology and particulate washoff. Local data files for input into SLAMM may be obtained from the U.S. Geological Survey at their Web site http://wi.water.usgs.gov/slamm/. The SLAMM model may be obtained from PV & Associates at their Web site: http://www.winslamm.com/

P8, Program for Predicting Polluting Particle Passage through Pits, Puddles & Ponds, is a physically-based model developed by William Walker to predict the generation and transport of stormwater runoff pollutants in urban watersheds. The model simulates runoff and pollutant transport for a maximum of 24 watersheds, 24 stormwater best management practices (BMPs), 5 particle size classes, and 10 water quality components. The model simulates pollutant transport and removal in a variety of BMPs including swales, buffer strips, detention ponds (dry, wet and extended), flow splitters, and infiltration basins (offline and online). Model simulations are driven by a continuous hourly rainfall time series. P8 has been designed to require a minimum of site-specific data, which are expressed in terminology familiar to most engineers and planners. An extensive user interface providing interactive operation, spreadsheet-like menus, help screens and high resolution graphics facilitate model use. A copy of P8 may be obtained (free of charge) from William W. Walker at the following Web site http://wwwalker.net/.

The Better Assessment Science Integrating Point and Nonpoint Sources (BASINS) model is a multipurpose environmental analysis system developed by the U.S. Environmental Protection Agency’s (EPA’s) Office of Water. The model was originally introduced in 1996 and has had subsequent releases in 1998 and 2001. BASINS allows for the assessment of large amounts of point and non-point source data in a format that is easy to use and understand. BASINS incorporates a number of model interfaces that it uses to assess water quality at selected stream sites or throughout the watershed. These model interfaces include:

- QUAL2E: A water quality and eutrophication model
- WinHSPF: A watershed scale model for estimating in-stream concentrations resulting

from loadings from point and non-point sources

- SWAT: A physical based, watershed scale model that was developed to predict the impacts

of land management practices on water, sediment and agricultural chemical yields in large complex watersheds with varying soils, land uses and management conditions over long periods of time.

- PLOAD: A pollutant loading model.

The EPA’s Office of Science and Technology provides technical support to users of the BASINS system.

The PONDNET model (Walker, 1987) is an empirical model developed to evaluate flow and phosphorous routing in Pond Networks. The following input parameters are defined by the user in evaluating the water quality performance of a pond: watershed area (acres), runoff coefficient, pond surface area (acres), pond mean depth (feet), period length (years), period precipitation (inches) and phosphorous concentrations (ppb). The spreadsheet is designed so that the phosphorous removal of multiple ponds in series can be evaluated.

The Wisconsin Lake Modeling Suite (WiLMS) is a screening level land use management/lake water quality evaluation tool developed by the Wisconsin Department of Natural Resources. It is a spreadsheet of thirteen lake model equations used to predict the total phosphorus (TP) concentration in a lake. TP loads can be entered either as point sources or by entering export coefficients for land uses. WiLMS can be downloaded for free at the following Wisconsin DNR Web site:

Bathtub is an empirical model of reservoir eutrophication developed by the U.S. Army Corps of Engineers. Single basins can be modeled, in addition to a network of basins that interact with one another. The model uses steady-state water and nutrient balance calculations in a spatially segmented hydraulic network, which accounts for advective and diffusive transport and nutrient sedimentation.

WASP, Water Quality Analysis Simulation Program, is a model developed by the U.S. EPA to evaluate the fate and transport of contaminants in surface waters such as lakes and ponds. The model evaluates advection, dispersion, mass loading, and boundary exchange in one, two, or three dimensions. A variety of pollutants can be modeled with this program including nutrients, dissolved oxygen, BOD, algae, organic chemicals, metals, pathogens, and temperature. The WASP program can be downloaded from the U.S. EPA Web site:

SWMM-Based Programs SWMM is a hydraulic and hydrologic modeling system that also has a water quality component. Please see the full description above for more details on the model.