The King County Wastewater Treatment Division (WTD) is currently planning implementation of green stormwater infrastructure (GSI) retrofits to help reduce combined sewer overflows (CSOs). To support this effort, innovative tools were developed to allow decision makers to evaluate a wide range of GSI solutions within the right-of-way in the University CSO Basin.
The ArcGIS Collector was developed to efficiently populate a database of potential sites with factors that affect feasibility such as infiltration, area delineation, available planter width, trees, utilities, road slope, etc. The database was initially populated using available GIS databases, supplemented in the field using tablets, then formatted as an input to the GSI Cost-Performance Model.
The GSI Cost-Performance model allows comparison of various GSI retrofit options given variability and uncertainty in site conditions and infiltration potential. Based on block-scale user inputs, the Excel-based model assesses feasibility, CSO reduction performance, and cost for each option. It includes various combinations of treatment (multiple bioretention geometries and proprietary technologies) with a range of infiltration approaches (to shallow soil and to deeper soil horizons using pit drains, drilled drains and deep wells). In the University GSI project, this tool was leveraged to evaluate BMPs on over 1,100 potential sites to develop a basin wide database of GSI opportunities.
The BMP Modeler provides a user interface to the data produced by the CPT to guide alternatives development and evaluation. The ArcGIS based webmap utilizes Multi Objective Decision Analysis (MODA) calculations to answer two programmatic questions; “which BMPs should be installed on which block?”, and “which blocks should be constructed for effective CSO volume removal?”. The University GSI project used this tool to explore various scenarios for prioritizing and optimizing cost, risk, and co-benefits with a defensible and replicable methodology that can be applied on future projects.
This presentation will include a description of each tool and the resulting programmatic siting and design guidance.