Cumulative & Secondary Impact (CSI): Stormwater Impact Assessment

The Office of Planning (OP) is pleased to introduce a new guidance the Stormwater Impact Assessment: Connecting primary, secondary and cumulative impacts to Hawaii’s Environmental Review Process.  This unique document provides easy to follow guidance on assessing stormwater impacts in the planning phase of project development.  Developed for reviewers of Environmental Assessments (EAs) and Environmental Impact Statements (EISs), this guidance emphasizes the planning phase of project development when there is flexibility to feasibly incorporate design, construction, or other mitigation strategies to address stormwater impacts.  To ensure that design commitment and mitigation concepts are implemented and enforced, this manual suggests the incorporation of appropriate mitigation strategies.

The Guidance document provides a five-step framework for assessing the primary, secondary and cumulative impacts associated with stormwater.  The appendix includes vital data resources available for stormwater impact assessment; a table of Best Management Practice (BMP) techniques; and a reviewer’s checklist.  Copies of the entire document and the Reviewer’s Checklist are available for download below.

This Guidance document is the result of a three phased project to address the CSI of stormwater management.  The project, outlined in and funded by the Federal CZMA Section 309 Enhancement Area Grant Program, Cumulative and Secondary Impact, for FY 2006-2010 (Section 309 Assessment and Strategy (2006-2010).

Phase One of the project involved the examination of Hawaii’s regulatory structure in which stormwater infiltration and runoff is assessed.  By evaluating EIS documents for their assessment of cumulative impacts it was found that oftentimes, stormwater impacts, and especially secondary and cumulative impacts, receive only cursory mention with limited analysis.  Our research noted the existing state of practice to incorporate EIS stormwater mitigation measures as conditions in by simply asserting the need for compliance with other Federal, State, or County regulations/codes to be enforced later during various project development permits.

In Phase Two of the project a pilot test was performed involving evaluation of past assessment practices and the new assessment method that was proposed in the Phase One final report.  To evaluate these two methods a hypothetical future development in Waiulaula watershed in West Hawaii was applied.  An emphasis on mitigation of post-construction impacts was conducted because existing federal regulations require satisfactory mitigation of impact during the construction period.  The results obtained were compared to results obtained using a state-of-the-art quantitative modeling study.  To assist in the interpretation, existing studies and hypothetical proposed development in the context of state and county general plans were examined.

In Phase Three we refined the methodologies developed in Phases One and Two through collaboration with professional in the field.  Utilizing a focus group of planners, engineers and architects to ensure the guidance methodology is reasonable, implementable and appropriate we developed the step-by-step guidance document, Stormwater Impact Assessment: Connecting primary, secondary and cumulative impacts to Hawaii’s Environmental Review Process.  Through training sessions held in Honolulu, Wailuku, Lihue, Hilo and Kona, we to provided an introduction to the use and implementation of the guidance document for reviewing and commenting on EISs in regards to CSI on project and regional stormwater management to representatives from federal, state and county agencies.

Although the guidance document identifies the rules and regulations that will be enforced, it is not prescriptive in nature to provide site designers maximum flexibility in selecting control practices appropriate for the site.  The guidance document does not impose any legally binding requirements on county, state or federal agencies and does not confer any legal right or impose legal obligations upon any member of the public.

Stormwater Impact Assessment guidance
Appendix C: Reviewers Checklist

Hilo Training Presentation
Kauai Training Presentation
Kona Training Presentation
Maui Training Presentation
Oahu Training Presentation