Five-Step Structure Enables the User to Go ‘Beyond the Guidebook’
Two rainfall-runoff simulation models have been merged to create a tool that integrates the site with the stream and watershed.
The web-based Water Balance Model for British Columbia decision support tool was launched in 2003 as an extension of Stormwater Planning: A Guidebook for British Columbia to promote rainfall capture, where rain falls. In 2007, the Water Balance Model was rebuilt on a new platform to integrate the powerful continuous hydrologic simulation capabilities of QUALHYMO (QUALity HYdrologic MOdel), while providing a standardized presentation of calculation results..
To enable the user to progressively go Beyond the Guidebook, the Water Balance Model powered by QUALHYMO comprises five steps:
Configure Surface Enhancements
Configure Source Controls with Baseflows
Configure Off-Site Storage
The first two steps are consistent with the capabilities of the original Water Balance Model as a planning-level tool. The last three steps represent the value-added capabilities resulting from integration of the two tools. The five steps are described briefly as follows.
Step 1 – Configure Catchments
The catchment is the area that concerns the user. It can be as small as a single lot or site, it can encompass a larger development, or it can represent a watershed.
Step 2 – Configure Surface Enhancements
Several rainwater runoff volume reduction methods (or `Low Impact Development` facilities) can be assessed using altered catchment parameters describing the surface conditions of the catchment. Low impact development facilities aimed at capturing rainfall and reducing runoff consist of augmented or enhanced surface conditions that include:
- increased top soil depth
- soil porosity or moisture holding capacity
- surface infiltration rates
- vegetation and ground cover
- surface roughness
Similarly, alterations of the surface conditions such as increased imperviousness can also be analyzed using these techniques.
Imperviousness is a surface enhancement (or alteration), whether positive or negative. One possible method of mitigating impacts is to reduce the value, hence an alteration in imperviousness can be a positive enhancement. A reduction can be as simple as reducing the area or by introducing pervious pavements.
Step 3 – Configure Source Controls with Baseflows
Source control systems affect the surface runoff and come into play after the Surface Enhancements are filled and are producing surface runoff. These runoff control or Low Impact Development (LID) facilities provide discharge control or reduce surface runoff volumes following the calculations of catchment hydrology.
These systems typically include a storage volume and can include infiltration to ground. The infiltration will be in addition to the surface infiltration calculated for the catchment. Source controls falling into this include:
some forms of green roof
most bio-filtration swales
The key to this type of “LID analysis” is reducing surface runoff after it occurs by providing a storage volume.
Step 4 – Configure Off-Site Storage
Off-site storage is in the form of detention ponds that function to limit discharges to downstream drainage systems or streams. These systems are typically placed at a neighbourhood, or regional level. They are not applied at a site level.
Step 5 – Configure Stream
Defining a typical, or a critical, stream section allows a determination of the energy available to cause stream erosion.