Conference Agenda

Session 08B: Stormwater: Modeling
Monday, 09/Sep/2019:
1:15pm - 2:45pm

Session Chair: Richard Boyle, Clean Water Services;
Location: E147-148

1:15pm - 2:00pm

The Right GSI in the Right Place - Leveraging Technology For Basin Scale GSI Implementation

Dustin Atchison1, Alice Lancaster2, Kelsey Hu1

1Jacobs, United States of America; 2Herrera Environmental Consultants, United States of America; ,

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.

2:00pm - 2:45pm

Water You Waiting For: Using 1D/2D Modeling to Address Nuisance Urban Flooding

Ryan Retzlaff

Brown and Caldwell, United States of America;

Localized urban nuisance flooding is often not well understood, and the cause or contributing factors can be challenging to determine with traditional information and modeling methods. Understanding the timing, extent, depth, and velocity of flood events are all important characteristics to account for when planning for flooding, assessing and designing system improvements, and determining the root cause of the flooding and the nature of rainfall events that result in flooding. One-dimensional (1D) models fail to provide a comprehensive assessment of flood event characteristics, and Federal Emergency Management Agency (FEMA) flood mapping is often inaccurate, regionally focused, and with poor resolution resulting in difficulty capturing localized flooding areas. When attempting to understand street-scale areas that are flood prone, a more comprehensive and locally focused flood model is needed.

Two recent studies for communities in the Portland Metro area by Brown and Caldwell employed one-dimensional/two-dimensional (1D/2D) models and have been able to bridge the gap of information and provide a clear picture of localized nuisance flooding on smaller, urban systems. These studies have provided information to identify the source of flooding, the extent of flooding for nuisance rain events and potential solutions. This information was previously not available to the municipality attempting to address the flood events. Utilizing modern modeling techniques have allowed Brown and Caldwell to provide clients with the necessary hydraulic details and visual aids to reflect the depth and velocity of flooding and surcharge, allowing informed decisions to be made to manage the stormwater collection and conveyance system.