Released nearly a decade ago, the National Oceanic and Atmospheric Administration (NOAA) Atlas 14’s precipitation frequency predictions left watersheds and cities reeling from the significant increases in estimated design storm event depths and resulting flooding impacts within their jurisdictions. Many watersheds and cities are just now starting to get a handle on what the shift in design precipitation depths means—identification of additional flood-prone areas, challenging infrastructure upgrades, and changes in expectations around the flood protection level of service in some areas. And impacts from climate change will only continue.

In response to these larger storm events, many watershed organizations have been striving to holistically understand their watersheds and stormwater conveyance systems.

Our own research supports these predictions. Over the last few years, Barr’s experts have reviewed climate models, also known as general circulation models (GCMs), using three different downscaling methods to help our watershed clients interpret impacts at a localized level. While there is variability between these models, many climate models indicate increasing trends in precipitation intensity and frequency into the future.

In the next few years, NOAA plans to release Atlas 15 data that will update the precipitation frequency standard while accounting for climate change, along with developing precipitation frequency estimates for the contiguous U.S. and its territories. This data will be critical to help inform future flood management design and analysis. In response to these larger storm events, many watershed organizations have been striving to holistically understand their watersheds and stormwater conveyance systems. These efforts take a watershed-wide look to better estimate flood risk, identify critical areas within the system, and inform solutions to help mitigate flood risk and provide resiliency.

Managing flood risk with a watershed-wide evaluation

Especially in fully developed watersheds, the rise in event precipitation depths and intensity, along with expected future climate changes, have resulted in a need for increased storage, conveyance, and creative solutions to manage increased flood risk.

Regardless of the level of flood-risk awareness, the most successful approach to mitigating flood risk is through a systematic watershed approach.

There is a wide spectrum of flood-risk understanding among watersheds and cities. Some organizations are still developing system-wide hydrologic and hydraulic models. Others are using complete models to make informed water management decisions and conduct more detailed planning and feasibility studies. Meanwhile, some watershed organizations and cities are already starting to implement flood-risk reduction and resiliency projects to better manage a changing climate.

Regardless of the level of flood-risk awareness, the most successful approach to mitigating flood risk is through a systematic watershed approach. A holistic view of the entire watershed will help with making informed decisions for design and prioritization of resources. Watershed organizations are well positioned to understand this view, evaluating anticipated impacts and working with watershed communities to develop and implement plans.

Understanding climate change’s local impact on flood risk

Many of our watershed clients have completed, or are in the process of completing, watershed-wide hydrologic and hydraulic modeling to understand flood risk across the watershed. With this modeling tool, watershed organizations are leveraging an improved understanding of flood risk to begin identifying which projects are necessary to most effectively mitigate the heightened risk associated with increasing precipitation. Although there is uncertainty in future climate projections, hydrologic and hydraulic modeling can help us understand the system’s sensitivity to increased precipitation and changes within the watershed.
 

Developing a holistic understanding

Barr has been helping numerous watershed organizations develop this holistic understanding to better manage increased precipitation and reduce flood risk:

• Mississippi Watershed Management Organization (MWMO): The MWMO saw a rare opportunity to manage and treat urban stormwater runoff on a district scale in a 300-acre formerly industrial, rapidly redeveloping neighborhood. Barr’s design features two large biofiltration basins that collect, treat, and convey runoff to a 207,000-gallon underground cistern for more treatment. The stormwater receives ultraviolet treatment due to its use in the community garden. The system was sized to manage a 100-year storm event and treat the first flush of runoff before it enters the Mississippi River. The community system reduces landowner costs, implements more effective and environmentally friendly stormwater treatment, and provides public greenspace.

• Nine Mile Creek Watershed District (NMCWD): Utilizing existing hydrologic and hydraulic models, Barr is currently assisting the NMCWD with evaluating flood resiliency within the Nine Mile Creek corridor. The study aims to understand where flood storage can be optimized or better utilized along the existing creek system. Barr has also been helping to address the district’s resiliency needs by assisting with review and consideration of the organization’s current watershed policies and partnership approach.

• Valley Branch Watershed District (VBWD): Barr recently completed both groundwater and surface water modeling on numerous landlocked basins within the VBWD, after historic water levels in 2019 and 2020 impacted homes and other infrastructure. The modeling accomplished numerous goals, including quantifying groundwater-surface water level interactions at each basin, evaluating the flood-risk potential for dwellings on the basins, understanding the impact of changes in land use on anticipated water levels as well as sensitivity to climate change and increased precipitation, and developing and assessing high-water management alternatives and solutions.

• Ramsey-Washington Metro Watershed District (RWMWD): Barr helped RWMWD complete a resiliency study to identify what it will take to manage or reduce flood risk with greater precipitation. We are also assisting with numerous feasibility studies for flood-risk reduction projects and concepts.

Creating a framework for project opportunities and prioritization

The ability to make these decisions on a watershed scale becomes even more critical to understand how changes within the larger watershed result in upstream or downstream impacts. The holistic approach helps to identify opportunities, allowing watersheds and cities to plan for projects that result in a more resilient system and to develop a comprehensive plan for capital improvement projects that might include:

• Increasing storage, conveyance, and diversion systems

• Developing automated systems to optimize the use of existing storage within the watershed

• Defining or establishing emergency overflow routes

• Maintaining existing systems to keep them functioning as intended

• Leveraging park lands and open space to maximize storage and performance while also providing multiple other benefits

• Considering the need for voluntary acquisition of flood prone properties when an engineering solution cannot adequately address flood-risk concerns

• Taking advantage of opportunities to install infrastructure in advance of future projects. (For example, Barr worked with RWMWD, the Minnesota Department of Transportation, and Metro Transit on the installation of increased pipe conveyance under a highway in preparation for a future flood mitigation project.)

While flood-risk management may be driving much of this conversation, it is important to note that there are other opportunities to create a more resilient system through operations and maintenance, water quality improvements, and restoration/habitat/ecosystem function.

Making the most of partnerships and policies

Even if watershed organizations are leading the charge, local partnerships are often needed to make these projects happen. Watershed organizations coordinate with public partners—such as cities, counties, and state departments of transportation—to coordinate projects to maximize flood-risk reduction benefits while minimizing disruption to the community.

Barr helped the Valley Branch Watershed District obtain a DNR Flood Hazard Mitigation Grant and helped secure $570,000 in assistance from the U.S. Army Corps of Engineers through its Planning Assistance to States program.

These plans and studies can support the need for projects, be used to leverage multiple funding sources, and potentially pursue state funding, like the Minnesota DNR's Flood Hazard Mitigation Grant Assistance Program.

Most existing policies don’t explicitly detail what should be done to address climate resiliency. However, the understanding gained from watershed-wide assessments can be used to facilitate conversations about current policies, alternatives to current policies and approaches to managing flood risk, and how to account for climate change in project design moving forward. Policy guideline efforts can also be used to define when a watershed organization should be at the table for implementation rather than being involved solely for local drainage and flooding efforts.

Getting started

Despite the lingering unknowns of climate change and its local impact on watersheds, those impacted should consider taking steps now to address the threat of increased flood risk. Being aware of how increased precipitation, and even drought, might impact a project should be a factor in design and decision-making.

Barr has worked with numerous watershed organizations for more than five decades, helping to restore and protect water resources. To learn more about how a watershed-wide evaluation can help with your flood-risk management strategies, contact our team.

About the authors

Janna Kieffer, vice president and senior water resources engineer, has 22 years of experience in water resources management. She specializes in hydrologic and hydraulic modeling, water quality modeling, flood-risk reduction planning, and infrastructure vulnerability assessment. Janna also completes water management plans, lake management studies, engineering feasibility studies, and stormwater analysis for low-impact development.

Brandon Barnes, vice president and senior water resources engineer, has over a decade of experience providing a wide range of water-resource management services. A highly regarded modeler, his project work includes coincidental-frequency analyses, water-balance calculations, digital flood-insurance-rate maps (DFIRMs), letters of map revision (LOMRs), and one- and two-dimensional hydrologic and hydraulic modeling for stormwater, storm sewer, flood-protection, floodplain, drainage, and watershed-improvement projects.

John Hanson, vice president and senior civil engineer, has over three decades of water resources experience and has been serving watershed districts since joining Barr. He is currently the engineer for the Valley Branch Watershed District in Minnesota. John’s work focuses on water resources management, hydrologic and hydraulic modeling, and flood-relief and water-quality improvement studies and subsequent implementation projects. He also assists stakeholder groups and facilitates public policy discussions.