Abstract

This guide lays out a six-step watershed-based approach for documenting the costs of flooding, projecting increased flooding and associated costs under future land use and climate conditions, and calculating the long-term benefits and costs of a green infrastructure approach.

To plan successfully, communities need to understand the options for addressing flood-related issues and their associated costs. This guide lays out a six-step watershed-based approach for documenting the costs of flooding, projecting increased flooding and associated costs under future land use and climate conditions, and calculating the long-term benefits and costs of a green infrastructure approach. The guide draws from four case studies (Duluth, Minnesota; Toledo, Ohio; Green Bay, Wisconsin; and He‘eia, Hawaii) to provide key considerations, recommended expertise, practical implementation tips, and lessons learned.

The purpose of this guide is to provide a process that communities can use to assess the costs and benefits of green infrastructure to reduce flooding. The guide takes a step-by-step, watershed-based approach to documenting the costs of flooding; projecting increased flooding and associated costs under future land use and climate conditions; and calculating benefits and costs of reducing flooding with green infrastructure over the long term. The guide is based on the experiences of three communities in the Great Lakes region (Duluth, Minnesota; Toledo, Ohio; and Green Bay, Wisconsin). To ensure transferability to different types of watersheds and communities, the methods were applied to the He‘eia watershed in Hawai‘i. Valuable transferability insights from the He‘eia watershed partners have been incorporated.

This process guide outlines a six-step process, with specific tasks associated with each step:

1. Define the Flooding Problem
2. Assess Flooding Scenarios without Green Infrastructure
3. Identify How a Flood Reduction Target Can Be Met with Green Infrastructure
4. Assess Flooding Scenarios with Green Infrastructure
5. Estimate Benefits and Costs
6. Identify and Communicate the Desired Green Infrastructure Strategy

Key considerations, recommended expertise, and case study examples are highlighted for practical implementation tips and lessons learned. Finally, key resources are provided at the end of the guide.

Communities should use this guide as a framework that can be adapted for their own purposes to inform planning-scale assessments and spark discussion about green infrastructure options to mitigate flooding and provide other watershed benefits. By following the guide’s six steps for developing a green infrastructure strategy, including estimation of associated costs and benefits over a chosen planning horizon, communities can demonstrate the cost-effectiveness of implementing green infrastructure projects. This guide does not compare costs between traditional gray and green infrastructure. It should also be noted that there is always a level of uncertainty associated with the use of models and future flooding predictions; the models and predictive tools used here are intended for planning-scale levels of effort. More detailed analyses are recommended for site-specific green infrastructure design.