Planning Module Modeling Calculations

The Planning Module uses the same water quality engine as is used to analyze existing verified BMPs in the Inventory Module. 

The Planning Module calculates and reports modeling results (water quality and quantity) for projects based on BMP type, location, drainage area, and modeling attributes entered by users. It also leverages various reference layers and datasets to develop a full set of model inputs necessary to describe the performance of BMPs. These model inputs are then used to calculate long-term runoff reduction, volume treated, and pollutant load removed. 

What do model outputs represent?

The modeling results represent the benefits of each user-defined project, considering the current state of the watershed accounting for existing verified BMPs, land uses, flow generation rates, and pollutant generation rates. The results are based on how the current system of BMPs would perform over a long-term average condition accounting for the effect of both wet and dry years. The results are not for a given storm, for a given month, or a given year. 

Is there any interaction between planned projects? What if two projects are treating the same area?

There is no interaction between different planned projects. Each project calculation assumes that other planned projects do not exist. A user is free to explore two projects that address the same area. Planned project calculations do account for existing verfied BMPs, such that a project benefit would be reduced if part of the drainage area is already treated. 

How do the modeling calculations work?

The Modeling Module consists of two primary components: A watershed database and a modeling engine. Inputs differ somewhat for North Orange County and South Orange County. The diagram below is applicable to South Orange County. 

 

Watershed Database

The watershed database defines the modeled BMP assets and the geospatial information associated with these assets. This is the same for South and North OC. 

Modeling Parameters: For each BMP or WQMP site, the user enters a limited suite of modeling parameters. These are the most important attributes that define the long-term performance of the BMP, such as volume, footprint, and treatment rate. These modeling parameters are part of the BMP detail pages. 

Delineations: For a BMP to be modeled, the user must define a delineation of the area that drains to the BMP.  Delineations can either be distributed (freeform) or can be traced through the Regional Subbasin network to define centralized delineations. The user can also connect BMPs to upstream BMPs to define treatment trains. 

GIS services: A master table of watershed characteristics and routing information is created by intersecting the BMP delineations and the Regional Subbasin polygons with a layer of hydrologic response units (HRUs) maintained by OC Survey. 

HRUs: https://ocgis.com/arcpub/rest/services/Environmental_Resources/Hydrologic_Response_Unit/FeatureServer 

Regional subbasins: https://ocgis.com/arcpub/rest/services/Environmental_Resources/RegionalSubbasins/FeatureServer

Modeling Engine

The modeling engine is a network solver that interprets the Watershed Database and consults several reference datasets to compute long-term hydrologic and pollutant load performance. Key supporting datasets include:

Stormwater Runoff Volume:  In South OC, unit area estimates for each HRU are based on the Loading Simulation Program in C++ (LSPC) model that was developed and calibrated as part of the Flow Ecology Special Study. https://www.southocwqip.org/pages/flow-ecology-study. In North OC, unit area estimates for each HRU are derived from calibrated SWMM models developed by the OC Stormwater Program.

Stormwater Runoff Concentrations: Representative stormwater runoff concentrations for each land use type are based on local wet weather outfall monitoring data and prior regional land use monitoring studies. In South OC, these concentrations are derived directly from monitoring data. In North OC, water quality is modeled in SWMM, and the long-term results of this model are used to form representative concentrations for use in the Planning Module.  

BMP Stormwater Capture and Retention Performance: The modeling engine uses BMP hydrologic performance curves (aka, nomographs) to determine the long-term fraction of runoff retained and treated at each BMP node in the computational network.  These curves are based on a large number of continuous simulation models. The same method is used in North OC and South OC. 

Dry Weather Runoff Flowrate: Dry weather runoff generation rates are based on monitoring studies performed in North OC and South OC. As a model input, the Modeling Module uses the average flow generation rate from these studies, expressed as a flowrate per developed acre. There was not an appreciable difference between North OC and South OC studies.

Dry Weather Runoff Concentration. For South OC, dry weather runoff concentrations are based on dry weather outfall monitoring data available in the SOC WMA between 2018 and 2020. For North OC, dry weather runoff concentrations were also based on monitoring data. 

Dry weather BMP hydrologic performance.  Based on the user-entered modeling parameters for each BMP, the modeling engine determines the dry weather loss rate and treatment rate provided by the BMP. A steady-state flow routing operation determines the dry weather runoff volume reduced or treated in each BMP and the remaining flow and load present at each calculation node. The same method is used in North OC and South OC. 

BMP Water Quality Treatment Performance. The modeling engine uses influent-versus-effluent concentration curves  to define the water quality treatment performance of each major BMP type. These curves are based on monitoring studies in the International Stormwater BMP Database. In addition, the modeling engine utilizes custom curves for dry weather treatment facilities (ozone and ultraviolet disinfection systems) based on available monitoring data from these systems in the WMA.

Is more documentation available?

Yes! There is a technical report that documents the methodology and inputs used to model BMP performance in OC Stormwater Tools. It is available at: https://ocgov.app.box.com/v/SouthOCRegionalClearinghouse/folder/154813812051

Please file a support request if you have more specific questions.