Subdivision Design and Flood Hazard Areas
PAS Report 584
By James Schwab, FAICP, Chad Berginnis, Terri Turner, AICP, Anna Read, AICP, Nicholas Walny
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This report was produced in partnership with the Association of State Floodplain Managers, with funding from FEMA, which makes it available for free to all.
A lot has changed since APA released its first PAS Report on Subdivision Design and Flood Hazard Areas in 1997. Today sustainability, resilience, and climate change are top of mind for planners and floodplain managers.
But for subdivision design, those ideas haven't yet hit home. The results? Untold damage from Hurricanes Sandy and Katrina as well as floods in places like Colorado and Atlanta.
This updated report is out to end the cycle of build-damage-rebuild. Editor James C. Schwab, FAICP, manager of APA's Hazards Planning Center, gives communities sound guidance to bring subdivision design into line with the best of floodplain planning.
Six planning and design principles help put subdivisions on the right footing. Standards for review, inspection, and maintenance cover all types of terrain and infrastructure. And nine concrete recommendations lay out steps to keep subdivisions safe and dry. Readers will get the tools they need to save lives, protect property, and build a better future.
MAKING THE CASE TO BETTER MANAGE FLOOD RISK
Much has changed in the 19 years since the publication of Subdivision Design in Flood Hazard Areas, PAS Report 473 (Morris 1997). Billion-dollar flood disasters have grown in frequency and regularity, and these increases likely will continue well into the future. Hurricanes Katrina and Sandy provide convenient touchstones for truly massive flooding disasters — both exceeded $200 billion in losses. The federal taxpayer now bears a disproportionate share of disaster costs, with federal aid rising precipitously over the past decade. Demographic shifts are placing more people and property in harm's way, as natural hazards are either ignored or downplayed in the development process.
Meanwhile, the concept of resilience has emerged as a focus of planning practice. Significant time, money, and attention are being dedicated to understanding how cities can withstand and rapidly recover from disasters. Far more accurate tools, models, and methods for assessing and defining specific flood risks are now available, which allows local planners and practitioners to go well beyond the limited focus of Federal Emergency Management Agency (FEMA) flood zone maps. Adapting to the multiple cascading risks associated with a changing climate is a primary focus of planners today. However, while the profession as a whole is gradually incorporating these concepts into long-term planning practice, subdivision design practice has changed comparatively little.
The intent of this report is to complement PAS Report 473 by providing an expanded and updated second volume. This report reflects an evolved way of thinking about planning and hazards that allows for a more comprehensive approach to saving lives, protecting property, and building a future that is free from, or has a reduced risk to, flooding hazards.
Defining Flood Risk
A commonly used expression of flood risk is probability multiplied by consequences. The probability element of the flood risk equation is commonly expressed through flood maps.
FEMA's Flood Insurance Rate Maps (FIRMs) are the most commonly used resource for assessing flood risk in planning practice. Their primary purpose is in determining flood insurance rates based on variable levels of mapped risk. FIRMs are also used as a standard reference for local policy makers and regulators. They are used to establish zoning and land use, to plan for the location and protection of infrastructure, and to develop emergency preparedness and evacuation plans. FIRMs are the essential default standard for communities nationwide. Yet they are also quite limited tools for assessing true risk, especially in recently subdivided areas where maps may be far out of date or incomplete, or may only include the most basic risk information.
Floodplains and watersheds are extraordinarily dynamic environments where risk is neither static nor beholden to mapped boundaries. Thus, there are a number of factors related to floodplains that must be taken into account when considering subdivision design and development practices:
Flood risk is not static. Communities must consider how hypothetical development may shape future risk for the existing built and natural environments. Similarly, climate change and sea-level rise data are at a point where the data can and should be used for planning purposes.
There is value to the floodplain and watershed purely as a complex natural system. Green infrastructure, low-impact development, and effective land-use regulations can help protect and preserve the natural and beneficial functions of the floodplain and the wider watershed.
Coastal areas are often notoriously difficult to regulate because of comparably high land value and development demand. Takings challenges often threaten robust regulation from practitioners. Conversely, a more permissive approach toward floodplain development can lead to legal challenges from existing residents, especially if flooding is worsened by new development.
THE PRINCIPLES OF SUBDIVISION DESIGN IN FLOOD HAZARD AREAS
During a 2015 symposium at the American Planning Association (APA) — which included subject matter experts and staff from APA, FEMA, and the Association of State Floodplain Managers — participants developed an overarching vision for subdivision design in flood hazard areas:
Adopt a comprehensive and integrated approach to protecting floodplains and other natural areas and aligning development with community goals, in order to increase community resiliency and reduce flood hazard risks.
Subdivision design in flood hazard areas has become increasingly important due to the high social and physical costs associated with flood damages. Now, more than ever, communities must adapt to the ever-growing threat of humanmade and natural disasters.
These five general principles lay the foundation for mitigating flood hazards within subdivision design:
- Maintain natural and beneficial functions of the floodplain.
- Adopt a No Adverse Impact approach to floodplain management.
- Avoid new development in the floodplain whenever feasible.
- Focus on data-driven decision making, using only the best available data to assess risk and inform decisions.
- Consider future conditions of the floodplain, including development impacts and climate change.
Planning and Design Principles
Together with the general principles, the following six planning and design principles can help communities develop a comprehensive and integrated approach to protecting floodplains by aligning development with community goals:
- Communicate with and inform stakeholders and community members throughout the planning and design process in order to facilitate coordination and community buy-in.
- Apply multiple tools and techniques for structural and nonstructural flood mitigation measures.
- Allow for creativity in design and, where possible, adopt a "watershed-scale approach" to design and an "ecosystem-based approach" to disaster risk reduction.
- Design new infrastructure and adapt existing infrastructure, including stormwater facilities and transportation networks, to be resilient to both high- and low-frequency flooding events.
- Protect open space and incorporate green infrastructure into development patterns.
- Ensure that subdivision and related development regulations include provisions for enforcement personnel.
INTEGRATED COMMUNITY APPROACH
As outlined in Hazard Mitigation: Integrating Best Practices into Planning, PAS Report 560, effective hazard mitigation is best achieved through coordinated plans, complementary goals, and collaborative efforts (Schwab 2010). The following discussion focuses on three key categories of planning actions within which subdivision design for flood hazard areas takes on special importance: (1) comprehensive plans, (2) other types of community plans, and (3) implementation tools.
Comprehensive Planning and Visioning
According to Sustaining Places: Best Practices for Comprehensive Plans, PAS Report 578, the traditional focus of the comprehensive plan has been on the long-term physical development of a local government municipality (Godschalk and Rouse 2015). Comprehensive plans tend to be made up of discrete planning elements such as land use, transportation, and community facilities — though many contemporary plans have expanded in scope and focus to address such topics as long-term global sustainability. Recent advances in both technology and the planning process allow contemporary comprehensive plans to address a wider and more creative range of possibilities, and they are generally less beholden to the generic format of past plans.
Recent innovations include the integration of local hazard mitigation into the comprehensive planning process. This integration can be achieved in various ways. In some instances, the mitigation plan may feature as a planning element within the comprehensive plan, as an addendum, or as an appendix. Deeper integration is possible as well. Analyses and policy recommendations developed for the mitigation plan or hazards element can be tied to specific land-use actions in the comprehensive plan. With regard to subdivision design, high-risk areas of the floodplain (perhaps land in a levee or dam failure zone) may be subject to a stricter regulatory regimen than land elsewhere in a community. Additionally, a focus on hazards should not preclude addressing complementary goals within the floodplain or watershed, such as open space preservation or wildlife conservation.
Other Planning Tools and Linkages
Most communities have a combination of plans aside from the comprehensive plan, and each is an opportunity to align planning with subdivision policy and hazard mitigation. Consideration of future conditions is vital to an effective integrated community approach to subdivision design in flood hazard areas. Quantifying future development potential, the loss of open space buffers, and the significantly higher flood elevations associated with sea-level rise can be extraordinarily useful in various special purpose plans of a community.
Functional plans such as parks plans, transit plans, or water supply plans introduce some difficulties when integrating hazard mitigation plan elements, as they are often produced by regional agencies or special planning districts. These policies can have a significant bearing on subdivision design that is outside of the ability of the community planner to influence, especially if subdivision design approval comes from the special planning district and not the municipality.
Area plans may be extraordinarily well suited for integration with hazard mitigation for subdivision design. These plans have a narrower geographic focus than comprehensive plans, which allows for a far greater level of detail with respect to local flood hazards.
Policy drivers in the form of federal and state laws effectively drive local governments to implement certain measures. These drivers can be leveraged to overcome tepid political will and develop more stringent regulations (for instance, maintaining wetland or open space buffers as a condition of meeting federal Clean Water Act standards) for subdivision planning and development.
Regional plans are useful in establishing a larger context for watershed management and floodplain management that goes well beyond the boundaries of a single subdivision. Regional plans are a step removed from comprehensive plans, and they may not have the force of municipal policy that a comprehensive plan does. However, they can create the needed context for connectivity between communities within the same watershed.
Implementation tools are the most important link between the various plans a community may develop and good subdivision design. Implementation can happen through code enforcement, public investment, creation of new regulations, programmatic efforts, or local incentives for property owners, developers, and investors. Capital improvements policy within a planned subdivision, the encouraging or requiring of conservation development (clustering residential development while preserving natural areas), and the benefits conferred by active participation in the Community Rating System all can function as effective implementation tools.
SUBDIVISIONS: ROLE AND PROCESS
A key to implementing comprehensive subdivision standards for flood loss reduction and preservation of floodplain functions is to have a comprehensive and integrated approach to subdivision review, competent inspections, and ongoing maintenance of any flood loss reduction infrastructure. The subdivision development review and approval process should be consistent with all local plans and standards and informed by the full array of flood hazard data, resulting in resilient design and lessened damage.
Effectively reducing flood risk requires sound planning principles and consistent flood reduction strategies on a dayto-day basis. This includes developing staff floodplain management capability; identifying and gathering all relevant flood hazard information; ensuring consistency of subdivision design across the spectrum of local plans, programs, and policies; proactive communication with developers, partner agencies, and local elected officials; and communication with local owners associations about a variety of flood risk management topics, especially in cases where infrastructure is managed and maintained by the association.
The subdivision design review process is the primary avenue for ensuring that the proposed development is consistent with flood risk management principles and any local, regional, or national plans and policies. Each step in the process can help to incorporate flood risk management into the final subdivision design proposal:
- Due diligence/pre-sketch plan meeting stage: This is an early opportunity to influence design and inform the applicant of the community's development regulations, including its flood loss reductions standards.
- Sketch plan stage: This is an opportunity to more effectively discuss feasibility and plan elements such as general layout, topography, existing conditions, known utilities, and existing storm drainage features.
- Preliminary plan: This should include applicable engineering design elements, including precise hydrologic calculations and analyses, the location of flood hazard areas, subdivision entry and exit points, and preliminary construction drawings for any infrastructure that may be necessary.
- Commencement of construction: A final plat will be submitted to the planning department and should be used to convey vital flood risk information, recordation of flood hazard areas, base flood elevations, or physical monumentation of the flood boundary (if required by the city).
- Proper inspection and oversight: After approval, these activities are important to ensure that floodplains are not degraded, flood risk is not increased, and applicable regulations have been followed. Periodic inspection by the community and the design professional is important to ensure compliance, and oversight is especially important to ensure subdivision stormwater facilities are in proper working condition.
In contrast to the prescriptive approach to subdivision standards in PAS Report 473, this report categorizes recommended standards based on five considerations for all subdivisions. This approach is intended to give the practicing planner a menu of standards that can be used individually or collectively to decrease the risk of flood impacts to subdivisions and to minimize the impact of subdivisions on the floodplain.
Natural and Human-Made Geographic Features
Flooding can result from any number of natural and human-made features. Flooding of rivers and streams and along coastlines is familiar. However, other geographic features are also susceptible to flooding: gulches that are dry most of the year can become raging torrents during heavy rainfalls in the Southwest, alluvial fans can have unpredictable and undefined flow areas, and shallower lakes with large surface areas can have wind-driven flooding due to storms or frontal systems. Additionally, a number of human-made features can result in flooding. One of the most common flooding events is urban stormwater flooding in older areas of cities.
Recommended standards for natural and human-made geographic features include the following:
- Map waterbodies without identified floodplains (e.g., ditches, ponds, lakes), lower lot or area minimum thresholds to trigger more detailed flood studies, and perform future flood conditions analyses (for both land use and hydrology).
- Protect, inventory, and restore riparian areas. Maximize riparian buffers.
- Identify dam failure areas on preliminary plans and final plats. Perform an impact analysis of any proposed development in the dam failure inundation zone.
- Identify levee protection areas on subdivision plans and plats. Require maintenance easements, buffers, and setbacks along the side of the levee facing the subdivision.
- Protect alluvial fans by prohibiting newly created lots and prohibiting improvements to existing structures using fill.
Layout and Design
Depending on the size of the subdivision, the stormwater created by the development itself can result in increased flood risk to on-site buildings and infrastructure as well as offsite impacts. For practicing planners, a persuasive argument to elected officials is that new development must not create conditions for future problems for those property owners, nor should it result in higher flood risks for existing residents of the community.
Recommended standards for layout and design include the following:
- Ensure that conservation subdivisions protect and preserve natural features. Prohibit the creation of new lots in the floodplain, or require that new lots have adequate buildable areas above the 100-year flood elevation.
- Perform an impact analysis and mitigation by prohibiting the subdivision of land that is unsuitable for development due to flooding, poor drainage, or other conditions that may endanger health, life, or property. Require evaluation of communitywide impacts. Adopt a No Adverse Impact standard.
- Require use restrictions prohibiting the platting of land for uses that may increase dangers to health, life, or property. Prohibit new lots in the floodplain without a natural grade elevation above the regulatory floodplain. Prohibit the use of fill. Do not allow critical facilities in the floodplain extent or the flood of record extent, whichever is greater. Do not locate land reserved for schools or fire stations in the floodplain.
- Require new private or public streets in the subdivision to access an existing "dry" road during the 100-year flood and/or be constructed above the 100-year floodplain.
Where flood risk is present, infrastructure should be carefully considered and protected accordingly. A great deal of infrastructure may be considered "critical" in that is it is needed during a flood emergency, and additional standards and safeguards should be applied to such facilities.
Recommended standards for infrastructure include the following:
- Require local road systems, including culverts and bridges, to be built to a 100-year storm elevation.
- Do not exempt utilities from flood protection standards. When possible, require utility easements to be located outside the floodplain or build redundancy into utilities that cannot be located outside the floodplain. Require transmission lines containing toxic or flammable materials to be buried to at least a depth below the calculated maximum depth of scour for a 100year flood.
- Require that all stormwater and flood protection infrastructure owned by an owners association be turned over to the local government for maintenance. Require a study to identify ongoing maintenance costs of all stormwater and flood protection infrastructure, which incorporates reasonable life cycles and sea-level rise projections. Require the developer to identify annual maintenance costs of stormwater and flood protection infrastructure and fund the maintenance of such facilities until the owners association is established.
While subdivision plats may not ever be viewed by subsequent buyers of lots, they nonetheless can serve an important function in providing information on flood risk. However, thought should be put into how this information is conveyed so that it is not implied that a flood hazard never changes (e.g., not putting a flood elevation on a plat unless it is also accompanied by an explanation that flood risk can change over time).
Recommended platting standards include the following:
- Require flood hazard information on plats and plans. This includes 100-year and 500-year flood elevations and boundaries, specific references to FIRM panels, and relevant information about elevation and flood insurance requirements.
- Permit density bonuses when coupled with restrictive covenants and easements. Require conservation and drainage easements in floodplain communities where lots may not be developed.
- Require physical monumentation of floodplain boundaries.
Perhaps nowhere have the science and techniques evolved more over the last 20 years than in the area of watershed management, especially for flooding. Powerful new models can now precisely show causation of flood events due to improper or undersized stormwater features. New techniques such as low-impact development and green infrastructure can result in more stormwater being held and infiltrated on site. With even a few small changes, a community's subdivision standards can significantly promote these better practices.
Recommended standards for watershed management include the following:
- Require green infrastructure and low-impact development techniques for stormwater management and design. Require submittal of a stormwater control plan. Require post-development peak storm flows and runoff for the 100-year storm to be no higher than was the case prior to development. Require retention and detention facilities based on the 24-hour, 100-year storm.
- Identify conservation land priorities. Within a certain distance of a desired habitat protection area, prepare a habitat assessment. Protect existing ecosystems by implementing a riparian buffer based on habitat protection, and prohibit or minimize clearing, grading, and filling in these areas.
THE ROAD AHEAD
Change is a virtual certainty. Planners and floodplain managers must be prepared to help shape the future in both practice and policy terms. The following are nine specific issues related to the flood risk mitigation measures needed to ensure better public safety in the coming decades.
Incorporate climate change considerations into planning standards for land use and development. Climate science will continue to advance. Data at the regional level are now allowing scientists to provide impacts within certain ranges that can inform land-use policy in areas like floodplains. Within the lifecycle of buildings and infrastructure, it is now possible to build in a margin of safety with regard to likely precipitation, the likelihood of extreme coastal events, and the general heights of floodwaters. Therefore, access to and the availability of data are crucial to planners.
Improve technology and visualization tools for subdivision design. Many communities want and need assistance in using and deploying advanced scenario planning and design visualization tools. It is important that cutting-edge tools be mainstreamed into planning practice in the context of resilience. Better visual and mental representations of the outcomes of decisions will help ensure that developments affected by flooding will be better designed in the face of environmental hazards.
Expand the use of future-conditions analysis to include subdivision standards. Analysis of future conditions based on projected land-use development and flood risk is possible today. These analyses can assess future buildout under existing land-use ordinances and policies as it relates to flood threats. Relatively static data, such as FEMA's flood maps, are useful as general signifiers of risk, but more dynamic analysis is necessary if communities are to comprehensively plan for future development and flood hazards.
Strengthen attention to local planning capacity for floodplain management and subdivision design. Funding and resources in many communities, especially rural communities, is tight. State, regional, and metropolitan area technical assistance and policy guidance are crucial to strengthening local capacity.
Develop best practices and tools for local government to use green infrastructure and No Adverse Impact strategies to improve subdivision design in flood hazard areas. Green infrastructure has emerged as a specific area of interest for local planners, in part to mitigate the impacts of riverine and urban flooding. The recent surge of interest in both green infrastructure and No Adverse Impact principles will hopefully lead to mainstreaming of these approaches in local planning practice. This is likely to be an increasingly fertile area of investigation in the coming years.
Educate and inform stakeholders in the subdivision design approval process. The subdivision design process should not escape notice as FEMA adjusts to a broader, more deliberate engagement with communities. Outreach, tools development, and planning are crucial in this effort. As more infrastructure maintenance and ownership shifts to local owners associations, direct outreach is vital to ensuring sound subdivision flood risk management.
Increase professional development of city staff on floodplain management and its relationship to good subdivision design and plan review. Floodplain management principles and their relationship to good subdivision design must be more deeply integrated into the planner's educational and training regimen. It is the role of APA, its chapters, and other organizations to make such training available. Additionally, local governments must take the initiative to highlight and encourage these opportunities. Increase the focus on hazard management to broaden the view of impacts from development. Good floodplain management is about how we manage the landscape. It is vital to develop a more comprehensive understanding of how development in the floodplain affects the watershed and the wider built environment. Without a more holistic understanding of the topography and of our communities, we are doomed to repeat mistakes of the past.
Incorporate the review of subdivision standards in local and regional hazards plans. The role of land use in reducing flood hazards is undeniable. A review of the role played by subdivision design standards and the subdivision approval process within the hazard mitigation plan is long overdue. Pre-disaster and post-disaster resiliency plans are a prime opportunity to incorporate lessons learned as they relate to subdivision design. Ultimately it is essential that these plans, along with the comprehensive plan, work together to ensure a stronger, safer, and more resilient community.
Ultimately, most of these goals work toward a common theme in APA Hazards Planning Center's work: the integration of resilience and hazard mitigation throughout the planning process. It is best to make various plans work in concert toward this end. The precise issues may vary from one community to another. What matters is the willingness to use the available tools as best they apply and to keep in mind an adage from Planning for Post-Disaster Recovery: Next Generation, PAS Report 576: We cannot know when a community will encounter its moment of truth, but procrastination is not an option.
About the Authors
James C. Schwab, FAICP (Editor) served as the project manager and principal investigator. He is the manager of the Hazards Planning Center at the American Planning Association (APA) and co-editor of Zoning Practice. He previously edited the PAS Reports Planning for Post-Disaster Recovery: Next Generation and Hazard Mitigation: Integrating Best Practices into Planning.
Chad Berginnis, CFM, is executive director of the Association of State Floodplain Managers (ASFPM). Since 1993, his work has focused on floodplain management, hazard mitigation, and land-use planning at the state and local levels and in the private sector. He worked with the Ohio Floodplain Management Program and as Ohio's state hazard mitigation officer; as a local planning director in Petty County, Ohio; and as the national practice leader in hazard mitigation for Michael Baker Jr. Inc.
Terri L Turner, AICP, CFM, is the development services administrator/floodplain manager/hazard mitigation specialist for the Planning and Development Department in Augusta, Georgia. She is the outgoing ASFPM Region 4 director and the ASFPM No Adverse Impact Committee co-chair, and she was recently appointed to the ASFPM Foundation as an associate.
Anna Read, AICP, is senior program development and research associate at APA. She conducts applied research within the National Centers for Planning and works on educational programs for practicing planners. Prior to joining APA, she worked on regional broadband planning efforts for the State of Missouri and as a project manager for the International City/County Management Association's Center for Sustainable Communities, where her work focused on smart growth and rural communities.
Nicholas A. Walny is an intern for the APA Hazards Planning Center. He provides research assistance for projects related to hazard mitigation, post-disaster recovery, and climate adaptation. Previously, he worked as a student planner in the City of Oxford's Town Planning Department in the United Kingdom.
Table of Contents
Chapter 1. Making the Case to Better Manage Flood Risk
Era of the Billion-Dollar Disaster
The Rise of Resilience, Climate Change, and Adaptation
Changes in Technology
Improvements in Defining Flood Risk
Recognizing the Benefits of Protecting the Floodplain Resource
Legal Issues for Subdivisions and Floodplain Management Regulations
Structure of the Report
Chapter 2. The Principles of Subdivision Design in Flood Hazard Areas
Planning and Design Principles
Chapter 3. Integrated Community Approach
Comprehensive Planning and Visioning
Other Planning Tools and Linkages
Chapter 4. Subdivisions: Role and Process
Routine Activities of the Planning Department
Chapter 5. Subdivision Standards
Natural and Human-Made Geographic Features
Layout and Design
Chapter 6. The Road Ahead
Appendix A: Sample Workshop for Homeowners Associations