OCST offers users two options for expressing and modeling the structural treatment BMPs that exist on WQMP sites in the OCST inventory:

“Detailed” modeling approach: Each BMP has distinct modeling parameters and delineations. Each BMP is analyzed individually. The BMP results are then summed to the WQMP level.

“Simplified” modeling approach: Each simplified BMP has user-entered performance information. The user enters the percentage of the site treated by each simplified BMP, and the modeling calculations are simpler. There are not individual drainage delineations to each BMP.

The table below provides a quick comparison between these two methods.

Additional Information on the Detailed Modeling Approach

“Detailed” modeling approach in which the volume capture and load reduction performance benefits are considered to be the sum of the benefits of the associated detailed BMPs found in the Jurisdiction’s BMP inventory.

This association is made when entering BMP details for the structural facility. In OCST a distributed or centralized BMP can be associated with one WQMP site, and each WQMP site may have many associated BMPs. For a WQMP to use the “Detailed” modeling approach, the associated BMP’s and their delineations must be fully verified and the facility must be fully parameterized for modeling. Guidance on the required modeling parameters for each BMP type can be found here (link to docs). OCST users can consult Modeling Attributes table to review the verified and parameterized status of their facilities here.

When BMPs are set up for detailed modeling, each BMP has it’s own detail page which allows for review of individual modeling results and allows for tracking of various details about the BMP, including field visits, and other data.  If these features are desired by the user, then the ’Detailed’ method is the recommended way for OCST users to inventory BMPs associated with WQMPs.

Additional Information on the Simplified Modeling Approach

When a more efficient approach is desired and individual BMP-level details are less important, OCST allows users to select the “Simplified” modeling approach for WQMP sites. With this method the user may enter a list of BMPs and specify the following simplified modeling parameters that are commonly available in WQMP documentation. Simplified BMP entries are not added to the BMP inventory, instead all of their data is stored along with the WQMP entry.

Each simplified BMP list entry includes the following fields:

Name – This could be the ID of a specific individual facility or a group-name to represent multiple facilities of the same type.

Type – Select from the list of facility types supported by OCST is here.

# of Individual BMPs – 1 if this is an individual facility, but could be many if this is a group of facilities that share the same type. This helps track the total facility count while avoiding the need for many entries of the same type.

% of Site Treated – Percent of site treated by this entry (0-100). The sum of these entries must be <= 100%.

% Wet Weather Captured – The percent of long-term runoff volume that the BMP is designed to treat. If sized for the DCV, then can assume this is 80% capture (which is considered equivalent to single-event DCV sizing). If it is sized for hydromodification, it might be higher. If a very old WQMP with smaller sizing, then may be less than 80% (but 80% is likely safe for most).

% Wet Weather Retained – The percent of long-term runoff volume that the BMP is designed to retain. If no infiltration, then zero. If there is full or partial infiltration or harvesting, then enter an estimate. This must be <= the Wet Weather % Captured value.

Dry Weather Flow Override – Optionally indicate if this facility or group eliminates all dry weather flow. By default OCST will use the facility type and the % performance to estimate the dry weather flow managed.

Notes – Optionally include notes about this facility or group.

Additional Information on Load Reduction Calculations

The pollutant load reduction calculation approach is similar for both the “Detailed” and “Simplified” modeling approaches. The load into a BMP is a product of the volume and the average annual concentration of the land surfaces assigned to it by either a detailed delineation or as a % of the WQMP site area. In the “Detailed” modeling approach BMPs, the long-term capture is determined based on nomographs that relate BMP design attributes like volume, drawdown time, and design flowrate, to pre-computed long-term hydrologic performance results modeled via EPA SWMM 5. For the simplified approach, the long term performance is provided by the user directly. BMP pollutant load reduction is then calculated in the same fashion for both methods: as a function of the BMP’s volume capture performance, its type, and the influent concentration. The overall load reduction calculation is the sum of:

Load Reduction via Runoff Volume Reduction. Based on the hydrologic performance calculations, a certain portion of volume may be retained or reduced by a BMP. The associated pollutant load is also removed.

Load Reduction via Treatment and Discharge. The water quality of the treated discharge varies as a function of the BMP type and the influent concentration. OCST calculates the treatment effectiveness by referencing a set of influent-versus-effluent concentration curves. These define the best estimate of average effluent quality based on the average influent quality. The BMP effectiveness curves are specific to each BMP type and pollutant, and were developed based on monitoring studies in the International Stormwater BMP Database.

For more information on the load reduction approach, please refer to OCST documentation here.