Extracting Impervious Surface Data to Support Stormwater Planning and Assessment

The Need for Sustainable Stormwater Infrastructure Funding

As cities grow, there is an associated increase in impervious land, posing a significant threat to stormwater management. Given that impervious surfaces reduce the infiltration, storage, and flow of water below ground, they create high-speed water movement throughout the urban area, often resulting in mass destruction. Currently, in the United States, there is an estimated $9 billion in urban flooding damages annually, and over 600,000 miles of rivers and streams and 13 million acres of lakes, reservoirs and ponds are considered impaired. To solve these challenges, cities need to modernize their stormwater infrastructure.

One of the primary challenges to support the development of a sustainable stormwater program is funding. The US and many other countries around the world recognize the importance of green infrastructure and the proper management of stormwater runoff, yet have failed to adequately fund these initiatives. In fact, the US Congress passed the Clean Water Act (CWA) in 1972 and amended it in 1987 to explicitly address the growing requirement for a program to support stormwater runoff. Recently, the Environmental Protection Agency (EPA) has estimated that $67.2 billion is needed over the next 20 years to sustain stormwater infrastructure and program investments.‍

In recent years, the EPA has brought more attention to this issue by providing some guidance to municipalities, and the federal government enabled access to one-time funding through the American Recovery Program to support stormwater planning. However, there is still no sustainable source of federal funding to support the development of stormwater programs. As of 2019, there were 1,716 stormwater utilities in the United States - 60% of which indicated that their major challenge was a lack of funding for implementation, design, construction and maintenance of stormwater infrastructure.

Stormwater Utility Fees (SUFs) as a Sustainable Funding Solution

The burden on overseeing stormwater management programs lies mostly with individual municipalities. Municipalities must comply with federal mandates to come up with programs that support the CWA, but without the necessary support and resources to address their stormwater challenges. This challenge has given rise to the concept of Stormwater Utility Fees (SUFs). Effectively, these SFUs assign an associated tax to properties based on the impact of water run-off from their properties, calculated based on the amount of impervious surface on that property, or more commonly, averaged based on property classification (residential, commercial, industrial, etc).

Many strategies have been enacted by both municipalities and counties to manage their Stormwater Programs. SUFs can be calculated in a variety of ways, namely through a flat fee structure (Equivalent Residential Units), tiered programs, or precise calculations derived at the property layer. This case study will look at the City of Detroit's utilization of Ecopia’s high accuracy, AI-enabled impervious surface vector data to assess the total area of impervious surfaces on parcels. By analyzing the strategic tactics taken in Detroit, we will uncover clear benefit identification for these programs. This strategy not only enables a more precise metric to assign SUFs, but encourages individuals and corporations to develop more green infrastructure to reduce their annual tax fees.

How the City of Detroit Leveraged Ecopia AI's Maps to Uncover an Average of $5.6M in Annual SUF Discrepancies

The City of Detroit has been at the forefront of modern development as they redevelop their existing landscape to accommodate for a transition to green infrastructure. The city is also under more pressure than other municipalities to manage their stormwater systems as the impact of runoff is augmented by their combined septic systems.‍

In 2019, following a competitive bidding and evaluation process, Ecopia AI was awarded a multi-year contract by the Detroit Water & Sewerage Department (DWSD) to create a planimetric level detail map of all impervious and non-paved compacted surfaces. Although DWSD had previously gone through the process of contracting to create data layers to support their stormwater utility fee calculation, they concluded that the manual heads-up digitization approach was not sufficient in supporting their long-term goals of maintaining a high precision data set year-over-year, as these methods could not meet the data standardization, timeliness, or flexibility required to meet Detroit’s specific accuracy requirements.

By working closely with Ecopia, DWSD was able to satisfy these requirements - empowering them with the data standardization, speed of data creation, and flexibility required. Below we will highlight each of these components.

Data Standardization: Ensuring Consistency At-Scale

Creating an impervious surface database is straightforward in principle. First,  the scope of the project is defined; the features that must be extracted, and the level of detail required to support the project. The challenge then comes with interpreting what is seen in the imagery, a challenge that is amplified when it comes time to update the original dataset. These challenges were so significant for DWSD that their attempts to use updated datasets that were created with a heads-up digitization strategy were unsuccessful - resulting in the conclusion that they need to recreate the data each year.

By leveraging Ecopia's AI-based mapping technology, DWSD was able to guarantee consistency - resulting in uniform data creation and updating. Not only does this guarantee the highest quality product to ensure fair SUFs, but it also enables rapid change detection and updating when new imagery is captured. This is especially critical when the constituents of the City of Detroit are being charged a fee directly based on the data.

“Ecopia’s ability to create a high-accuracy product was critical, but their ability to maintain the dataset is what really differentiates them from our previous efforts. DWSD is able to rely on the interpretation not changing year over year and thereby minimizes staff resources to integrate the product into our workflow. This directly translates into a cost and time savings.” - Dan Rainey, Chief Information Officer, Detroit Water and Sewerage Department

Speed of Updating: Leveraging Data That Represents Reality

DWSD's first efforts at maintaining an annual impervious surface data layer were challenged by the fact it would often take 12-18 months after capturing imagery to receive the new data layer. This delay meant that, by the time that data was integrated, the SUF program was charging fees based on information that was 2 years old. With current rates of development and growth within cities, this rendered DWSD’s “updated” data useless.

Ecopia’s solution enabled access to data within 6 weeks of imagery being received. This guaranteed update schedule ensures that DWSD is always supporting their critical decision-making with the most recent and accurate information.

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Flexibility: Customizing Specifications to Meet SUF Program Needs

Defining what constitutes an "impervious surface", or the unique layers that a municipality requires to support their SUF is not a standardized process. It is critical to understand the existing operations of a department and its specific data requirements. Ecopia works with our clients to ensure that we define specifications according to client requirements, and is available to support post-delivery integration and feedback. We recognize that impervious surface data extraction is highly subjective and, at the end of the day, we want to make sure you are getting exactly what your departments needs.

Outcome: an Optimized & Fair SUF Program

Ecopia started the multi-year contract by deriving a new impervious surface map for the City in 2019 and subsequently updated that data in 2020 and 2021. Over the course of the project, the total impervious surface across Detroit has changed at an average rate of 2% per year. This includes both resurfacing and new construction. Although 2% may sound marginal, this resulted in change to impervious surface area of 2 sq. miles - which equates to an annual discrepancy of nearly $5,600,000 based on the city’s current Stormwater Utility Fee. Identification of those changes is not only critical for planning purposes but making sure that any fee assigned to a residential or commercial property is equitable. This is equally important as a transparency metric and in minimizing the volume of complaints received from constituents.

Following the success of Ecopia’s impervious surface deliveries, the City of Detroit has augmented their product to include natural layers, thereby receiving a unique 25-layer planimetric level detail land cover map. Furthermore, DWSD is already in the process of renewing its contract with Ecopia to ensure it can maintain the same quality of data in the future.

“Ecopia has been a trusted partner to DWSD and we look forward to working with them more in the future. The Ecopia team has supported our unique requirements with systematic attention to detail and their unique ability to extract high-quality geospatial data has been critical to the Stormwater Utility Program and additional ongoing work across the City of Detroit.” - Dan Rainey, Chief Information Officer, Detroit Water and Sewerage Department

To learn more about this solution, or how we are supporting geospatial data creation in other municipalities, get in touch with our team.