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*[[Case studies for monitoring to meet TMDL permit requirements]]
*[[Case studies for monitoring to meet TMDL permit requirements]]
The P8 (Program for Predicting Pollution Particle Passage through Pits, Puddles, and Ponds) Urban Catchment Model is a physically-based stormwater quantity/quality model. P8 predicts the generation and transport of stormwater runoff and associated pollutants from urban watersheds and predicts runoff and pollutant removal at user defined stormwater BMPs through the processes of sedimentation, filtration, and infiltration. The model simulates runoff and pollutant transport for a maximum of 250 watersheds, 75 stormwater best management practices (BMPs), 5 particle sizes classes (including soluble fraction), 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, infiltration basins, filtration basins, street sweeping, and user defined “General” devices. Model simulations can be run for a long duration (e.g., 50 years) and require inputs such as continuous, hourly rainfall and daily temperature data, and inputs related to particulate and pollutant loading. An extensive user interface providing interactive operation, spreadsheet-like menus, help screens and high resolution graphics facilitate model use.
P8 is a highly-versatile program suitable for TMDL applications ranging from TMDL development, to demonstrating WLA and permit compliance from individual municipal separate storm systems (MS4s). Download links for the P8 model and related documentation are provided below.
Note: information provided in the following subsections does not reiterate or re-present information readily available in model documentation files. Instead, guidance provided in this document provides engineers and planners with recommendations for development of model inputs, provides guidance for interpreting and summarizing model results, provides supplementary information not included in model documentation, and provides example case studies showing how P8 can be used to demonstrate TMDL compliance.
The P8 model is a relatively complex tool. Using it likely requires some training. The following pages provide guidance and examples useful for learning the P8 model. Training workshops are periodically offered by various entities, including the University of Minnesota.
P8 is a continuous water quality model capable of summarizing runoff and associated total suspended solids (TSS) and total phosphorus (TP) generation, removal, and outflow loading from individual catchments, individual BMPs and pipes (referred to as “devices”), or as a model wide summary. Additionally, because P8 is a physically-based model which continually evaluates the pollutant particle scale distribution (PSD), BMP loading, capacity, and bypass, the model is capable of accurately predicting pollutant removal through BMPs in series as well as predicting runoff and pollutant bypass from undersized BMPs. Due to this flexibility, P8 is capable of providing accurate pollutant removal estimates regardless of BMP network and subwatershed configuration, and is capable of demonstrating compliance to mass-based WLAs (e.g. pounds of TSS per year), concentration-based WLAs (e.g., mg/L of TSS), and areal-loading based WLAs (e.g., pounds of TSS per acre per year) for both TSS and TP.
The following subsections outline data sources and special consideration related to model inputs, model setup, and model initialization. Note: these sections do not represent information readily available in Model documentation, but instead highlight data sources (e.g., spatial datasets), special consideration, and important notes for engineers and planners to consider while generating model inputs.
P8 requires a daily temperature input file and hourly precipitation file to simulate rainfall, runoff, and associated particulate and pollutant mobilization. Although P8 provides a default precipitation and temperature file (“p8_default.pcp” and “p8_default.tem”, respectively), these files should not be used for modeling purposes. These files are provided only as example files of how the precipitation (“.pcp”) and temperature (“.tem”) files should be formatted. It is critical for the engineer or planner to develop rainfall and temperature input files specific to the area modeled.
The National Oceanic and Atmospheric Administration (NOAA) maintains a searchable database that can be used to search for daily temperature average and hourly precipitation data based by city or geographic region (e.g., zip code). Local area airports (e.g., Minneapolis Saint Paul International Airport), local government units, and area watershed management organization (WMOs) and watershed district (WDs) are additional resources that can be used to develop required temperature and precipitation data inputs.
P8 documentation does provide default or suggested inputs for several general case specifications related to general inputs (e.g., start date, stop date, time steps per hour). Below is a brief description of each general input parameter, as well as suggested inputs for each.
P8 requires the user specify information regarding characteristics of the five (5) modeled particle size fractions (i.e., “Particle Parameters”), which includes one soluble (dissolved) fraction, as well as pollutant particle composition associated with each particle size (i.e., “Water Quality Components”). These parameters establish how particles are generated by watersheds (e.g., build-up and wash-off from impervious surfaces and event mean concentration loading), how particles are removed by water quality BMPs (e.g., settling velocity and filtration efficiency), and the amount of pollutant associated with each particle class (e.g., 3,850 mg TP per kg of the P10% particle class).
P8 contains a default particle file (“p8_default.p8c”) as well as two particle files based on findings from the EPA’s National Urban Runoff Pollutant study (“nurp50.p8p” and “nurp90.p8p”; Cole et al., 1983). These particle files specify both all “Particle Parameters” and “Water Quality Components” input parameters. The “NURP50” and “NURP90” particle files are based on the 50th and 90th percentile results from the nationwide NURP study, respectively. If sediment and pollutant sampling have not been conducted from the study area, it is recommended that the NURP50 (“nurp50.p8p”) default value be used.
The default filtration efficiency values for the dissolved (P0%) and particulate (P10% through P80%) particles are 90% and 100%, respectively. Based on information and sources presented in the Minnesota Stormwater Manual, it is recommended that the filtration efficiency values shown in the table below be used. The same filtration efficiency values would generally be applicable for modeling other filtration practices (e.g., permeable pavement, tree trench with underdrain, etc.). Since the values shown below are constants that are applied to all modeled BMPs, the Particle Removal Scale Factor can be changed in P8 for individual practices to account for pollutant removals that are expected to differ from the constants.
Recommend filtration efficiency for P8
Link to this table
|Filtration type||Filtration efficiency (% removal)|
|Dissolved (P0%)||Particulate (P10%)||Particulate (P30% through P80%)|
|Sand filters and biofiltration ||0%||25%||100%|
|Iron-enhanced sand filter ||60%||25%||100%|
Evapotranspiration (ET) input parameters are only required if the model contains an “Aquifer” device for computation of baseflow. If using an aquifer device to model baseflow, update monthly ET coefficients based on information specific to the modeled region. If no such devices are modeled, the user can retain the default values in the ET table.
Snowmelt parameters define, based on daily average temperature, when precipitation is delivered as snowfall, when snowmelt begins, and antecedent moisture conditions throughout the growing and non-growing season. Values should be reviewed, but it is recommended that default parameters be retained for most areas in Minnesota.
P8 allows the user to specify input parameters for up to 250 subwatersheds. The program uses the curve number method (USDA, 1986) to model runoff from pervious and indirectly connected impervious surfaces. Directly connected impervious runoff is generated for precipitation amounts larger than the depression storage, with excess precipitation multiplied by the impervious runoff coefficient up to the breakpoint precipitation amount. Impervious runoff pollutant loading is generated from buildup/washoff routines, while pervious runoff pollutant concentrations are simulated from exponential relationship with rainfall precipitation rate. Additionally, the user can specify street sweeping scheduling (e.g., start date, end date, and frequency) and the fraction of impervious area that is “vacuum swept” (i.e., vacuum based street sweeping) versus “not swept”. It is recommended that the engineer or designer use methodology outlined in the USDA TR-55 manual (USDA, 1986) to calculate the pervious curve number for each modeled watershed, which will be weighted with the indirectly connected impervious fraction by the model.
Model results are highly-sensitive to the fraction of directly and indirectly connected impervious area. For this reason, it is recommended that directly and indirectly connected impervious area be estimated based on site-specific information when possible. For large watershed areas where individual review of directly versus indirectly connected impervious area is not feasible, directly connected impervious area can be estimated from land use spatial datasets, total impervious area spatial datasets, and land use-based directly connected impervious area fractions included in P8 documentation.
Water quality BMPs and other hydraulic routing elements in P8 are referred to as “devices”. The following subsection provide background and special considerations related to each device type.
Pond devices are used primarily to model wet and dry pond BMPs. The pond device type requires that the user input information related to dimensions of the BMP (e.g., permanent pool volume, flood stage area, etc.) and routing (e.g., downstream device for normal outflow, flood outflow, etc.). In addition to modeled wet and dry pond BMPs, “Pond” devices can also be used to model infiltration and filtration BMPs by assigning an infiltration rate to the permanent pool and/or flood pool. This strategy is useful for modeling infiltration devices that have a normal outlet (e.g., a pipe or orifice outlet).
Infiltration basins are used to model simple storage features which infiltrate a fixed volume of water and have no defined outlet (e.g., a residential raingarden).
“Swale” devices are used to model infiltration and filtration swales. Special considerations related to “swale” devices are outlined in the “Infiltration Basin” section.
“General” devices are used to model complex or unique BMP types that cannot be modeled using the specific BMP types discussed above (e.g., a wet pond with a two-stage outlet). “General” devices allow the user to input unique normal, spillway, and infiltration rating curves. Because “General” devices can be used to model a variety of BMP types, the special considerations discussed above should be reviewed.
A “pipe” device is used solely for routing volume through the network. The only inputs required for a pipe device are time of concentration (ToC) and normal outflow device.
A “Splitter” device splits flow to two (2) different outflow devices. Note: the function of a “Splitter” differs from modeling a “General” device, which allows the user to more-control over the rating curves used to split flow to the two receiving devices. The “Splitter” tracks the estimated water surface, and routes flow to, one downstream device until it exceeds the threshold elevation, above which, it then routes flow to a second specified device.
An “Aquifer” device provides storage and discharge of percolation from watersheds and infiltration devices. Evapotranspiration losses are modeled from aquifers, so it is critical for the engineer or designer to review evapotranspiration parameters when modeling an “Aquifer” device.
P8 provides a wide variety of model outputs which can be used by engineers and planners to evaluate and demonstrate WLA compliance. Model outputs and results can be reviewed in the following formats.
The following subsections provide guidance on how to interpret and check the quality of model outputs.
The figure at the right, generated from P8 model documentation, provides a summary of the fifteen (15) mass balance inputs and output used by P8 to track pollutant loading and removal at modeled devices. These are summarized in the following table.
P8 mass balance terms
Link to this table
|Mass balance term||Description|
|Terms related to inflow|
|(01) Watershed Inflow||Inflow from watershed(s) assigned to the device|
|(02) Device Inflow||Inflow from upstream devices|
|(09) Total Inflow||Total Inflow = (01) + (02)|
|Terms related to sedimentation, infiltration, and filtration|
|(03) Infiltration||Outflow passing through bottom/sides of device|
|(05) Filtration Removal||Pollutant removed through filtration through media|
|(04) Exfiltration||Exfiltration = (03) - (05)|
|(08) Sedimentation + Decay||Pollutant removed via sedimentation and/or decay|
|(13) Total Trapped||Total Trapped = (05) + (08)|
|Terms related to outflow|
|(06) Normal Outflow||Outflow passing thru the primary outlet|
|(07) Spillway Outflow||Outflow passing thru the secondary outlet|
|(10) Surface Outflow||Surface Outflow = (06) + (07)|
|(11) Groundwater Outflow||Groundwater Outflow = (03) - (05) = (04)|
|(12) Total Outflow||Total Outflow = (10) + (11)|
|Terms related to mass balance and error|
|(14) Storage Increase||Increase in storage volume (or mass) from one time step (or event) to the next|
|(15) Mass Balance Check||Error term in mass-balance equations. Error = (12) - (09)|
A notable omission from the mass balance terms is a term summarizing total pollutant removal. The “Load Reduction %” presented in Output Explorer is calculated as the difference between “(09) Total Inflow” and “(10) Surface Outflow”. Although this calculation is correct for many varieties of BMPs, it is not correct for filtration BMPs (correct load reduction calculations shown here). For this reason, it is critical that the engineer or planner determine which forms of potential load reduction apply to each device and calculate removal from mass balance terms. The table below provides a summary of how to calculate load reduction at sedimentation, infiltration, and filtration BMPs. Additionally, the table provides a summary of how to calculate WLA reporting terms commonly required by TSS and TP TMDLs.
Using P8 to calculate load reduction and WLA reporting terms
Link to this table
|Reporting term||Infiltration/sedimentation BMPs||Filtration BMPs|
|Pollutant Removal and Outflow Concentration Calculations|
|Applicable P8 Removal Terms||(03) Infiltration; (08) Sedimentation + Decay||(05) Filtration Removal; (08) Sedimentation + Decay|
|Pollutant removal||(09) Total Inflow - (10) Surface Outflow||(09) Total Inflow -(12) Total Outflow|
|Outflow Loading** (for outflow concentration summary)||Surface outflow||Total outflow|
|Watershed and Areal Loading Calculations|
|Directly contributing watershed area to a device (acres)||Sum of all watersheds routing directly to BMP device|
|Cumulative contributing watershed area to a device (acres)||Directly contributing watershed area + Directly contributing watershed area of all upstream devices|
|Watershed pollutant areal loading at device (pre-treatment) (lbs/acre/year)||[(01) Watershed Inflow (lbs)] / [Directly Contributing Watershed Area (acres)] / [Simulation Duration (years)]|
|Pollutant areal loading from device (post-treatment) (lbs/acre/year)||[Outflow Loading** (lbs)] / [Cumulative Contributing Watershed Area (acres)] / [Simulation Duration (years)]|
|Watershed pollutant areal loading model wide (pre-treatment) (lbs/acre/year)||[Sum of (01) Watershed Inflow at all devices (lbs)] / [Total Model Watershed Area (acres)] / [Simulation Duration (years)]|
|Model Wide Outflow*** (lbs)||[Sum of (01) Watershed Inflow at all devices (lbs)] - [Sum of (Pollutant Removal*) at all devices (lbs)]|
|Pollutant areal loading model wide (post-treatment) (lbs/acre/year)||[Model Wide Outflow*** (lbs)] / [Total Model Watershed Area (acres)] / [Simulation Duration (years)]|
The engineer or designer should perform a thorough review of all model inputs and outputs. Model inputs should be generated using best available datasets, including record drawings, development data, bathymetric surveys, and best-available spatial land use, land cover, and soil databases. Upon model completion, results should be reviewed to ensure devices were routed correctly, subwatersheds were routed correctly, and that pollutant removal and areal loading results are within typical ranges based on BMP and land use type, respectively. A general model result QAQC list and description of P8 outputs to review is provided here. Additionally, a literature review of typical average annual event mean concentration (EMC) values is provided here, and typical TSS and TP removal values for various BMP types is provided here.
General P8 result QAQC list
Link to this table
|QA/QC review item||Related P8 output1|
|Review device routing||List → Inputs → Network|
|Review watershed routing||List → Inputs → Case (Watershed Data)|
|Review total watershed area is correct||List → Inputs → Case (Watershed Data)|
|Confirm daily precipitation total are correct||List → Time Series → Daily Precip|
|Compare watershed pollutant concentration to tabled values||"Explore" tab; Report: Conc ppm; Term: 01 watershed inflow|
|Compare TSS/TP removal rate to typical tabled values||Calculate removal using guidance in tabled values and mass balance terms from: List → Mass Balance → By Device and Variable|
1For majority of variables, there are several methods which can be used to export results from P8. Refer to model documentation
TSS and TP EMC literature values for TMDL modeling
Link to this table
|Reference||Average annual EMC (mg/L)|
|Residential (Pitt, 2011; NSQD, 2011/ Region 1)||135||0.4|
|Minnesota Stormwater Manual – Commercial||120-160||0.15-0.35|
|Minnesota Stormwater Manual – Industrial||130-170||0.15-0.35|
|Minnesota Stormwater Manual – Residential||100-170||0.2-0.6|
|Minnesota Stormwater Manual – Freeway/ Transportation||115-155||0.3-0.5|
|Nationally Pooled Urban EMCs (Lin, 2003)||54.5-78.4||0.266-0.315|
Typical BMP TSS and TP removal rates from Minnesota Stormwater Manual
Link to this table
|BMP type||Typical pollutant removal rate (%)|
|Iron enhanced sand filter||85||77|
|Constructed wet ponds||84||50|