Western Washington Hydrology Model

WWHM2012 Version 4.2.16 (Dated: 10-10-2018)

Version 4.2.16 addresses two issues described below. The first issue is considered a bug fix. The second issue is considered a model enhancement. According to the version numbering system in use as of October 21, 2015, the digits after the last decimal point are changed for this WWHM update.   

WWHM2012 Bioretention/Bioswale update
The bioretention element has been updated with the following changes:

1. Issue with allowable flow into the subsurface soils from the surface retention has been updated to handle KSAT safety factor.

2. Issue with maximum flow into subsurface section being limited to pre-discharge volumes has been fixed so that the maximum inflow now includes discharge during the timestep.

We have not evaluated these updates to the simple Bioretention routine and have not modified our guidance in the SWMMWW at this time. The user can run the more complex Bioretention to verify the simple Bioretention algorithm results. 

Bioretention Modeling Update Preliminary Findings:
The two above changes have improved the modeling calculations accuracy when the Simple Bioretention option is used. Using the Simple Bioretention option now appears to give identical results to the more complex Bioretention algorithm (the WWHM2012 default). This preliminary finding is based on some initial testing and under the following conditions:

• Native infiltration rate is greater than flow through the amended soil

• Underdrain flow rate is greater than the flow though the amended soil

• Both the native infiltration and the underdrain flow combined is greater than the flow through the amended soil.

The changes have also improved the modeling calculations accuracy when the Simple Bioretention option is used. Using the Simple Bioretention option now gives results within 1 to 4 percent of the complex Bioretention algorithm under the following conditions:

• Flow through the amended soil is greater than native infiltration.

• Flow through the amended soil is greater than flow through the underdrain.

• Flow through the amended soil is greater than the combined native infiltration and flow through the underdrain.

Bioretention modeling using the Simple Bioretention option has always been multiple times faster than using the more complex Bioretention modeling algorithm (the WWHM2012 default). The use of Simple Bioretention may be helpful in scenario modeling due to the apparent improved accuracy and speed. The simple Bioretention algorithm can be used by checking the box next to “Use simple Bioretention” on the Bioretention element screen.