Redefining Resilience: Expanding the Approach to Stormwater Management

October 16, 2023
Civil Engineering & Water Resources

How ISWMM’s guidelines are setting the standards in sustainable stormwater management and water resources conservation in Iowa.

Before the turn of the millennia, stormwater management practices were intended to move runoff away from infrastructure and limit localized flooding in the most cost-effective way. Roofs, paved surfaces, pipes and flumes quickly shed runoff; turf-covered dry detention basins offer simple construction, easy maintenance, and can mitigate flooding for very large rain events. Though these traditional methods are useful in certain circumstances, they aren’t optimized to treat the most frequent rain events and often lead to waterway erosion and poor water quality downstream; however, stormwater engineering solutions are evolving in new and effective ways, growing in both availability and functionality. Modern green infrastructure concepts for stormwater management provide water quality enhancements, sustain community green space and reduce waterway erosion, while continuing to address traditional flood reduction goals.

This evolution of stormwater management – from limited intentions to more sustainable and comprehensive solutions – is proving crucial in our efforts to address the multifaceted challenges of waterway degradation, frequent flooding and community enhancement. This shift also reflects widespread momentum towards comprehensive sustainability and resiliency in the field of water resources engineering itself and is apparent in the continuous development of industry resources, including, for example, the Iowa Stormwater Management Manual (ISWMM). The ISWMM is a critical resource developed to cater to the specific characteristics and landscapes of Iowa; it serves as a tool to facilitate the design of practices not just for flood risk reduction but for the creation of valuable community green space that yields water quality enhancement and downstream erosion control. The practices and principles in the ISWMM are not just accepted in theory – they define standards for design in many communities across Iowa through local ordinances that reference the ISWMM. To fulfill a universal expectation of community safety, the design guidelines for the infrastructure we build and pay for must be optimized by professionals with deep knowledge of natural resources and engineering practices. Here, we explore how the ISWMM was developed and the important impact its implementation can have on local landscapes.

Developing Stormwater Management Best Practices

The ISWMM was originally issued in 2009 and is continually updated by a multidisciplinary team that includes engineers, landscape architects, conservationists, scientists, urban developers, municipality staff and others from across the state. RDG Planning & Design has been a key author for the updates and creation of 18 chapters throughout 12 years, with another 11 currently in progress. We plan to continue to contribute as further advancements in stormwater management necessitate additional chapters and revisions in the future. The ISWMM is only one example of modern stormwater management guidelines, and it’s by no means the only or the most rigorous example in the nation; however, the ISWMM is uniquely curated for Iowan landscapes with reference to and review of national trends. The overarching goal of the ISWMM is to provide guidance on proven stormwater best management practices to expand the functionality of infrastructure beyond flood reduction to provide water quality improvements and limit waterway erosion. There are also important references to maintenance, safety and aesthetic considerations. Its particular emphasis on stormwater and exploration of specific design examples separates the ISWMM from other Iowan design guidelines.

ISWMM guidance has been applied to countless stormwater projects on both public and private lands. One prominent example is the Gray’s Station Stormwater Wetland, in the heart of one of Des Moines’ newest neighborhoods. For the project, the City of Des Moines, Hubbell, Civil Design Advantage and RDG built upon the iconic Gray’s Lake and the future ICON Water Trails site to realize a community with water recreation as the centerpiece. The result of these efforts is a lush stormwater maze born from existing dry detention basins. The 8-acre, ¾ mile-long stormwater wetland was engineered using tools and guidance from the ISWMM (section 9.08) that significantly improved the stormwater management infrastructure for a 200-acre urban watershed. Today, this community amenity not only yields water quality improvements for the Raccoon River but also provides opportunities for recreation and wildlife habitat in an urban setting. The busy and green corridor is teeming with life and will continue to mature over time with care from the community. This project is only one example of what is possible when ISWMM principles are applied to build a multi-functional infrastructure that can create more meaning for a project.

Local Interpretation vs. State-Level Standardization

The ISWMM serves as a critical reference for municipalities in developing stormwater ordinances to ensure community safety and infrastructure resilience, but its adoption is often partial, reflecting the unique needs and circumstances of different Iowan communities, especially smaller ones with limited pre-existing ordinances. If communities grow without stormwater management requirements, however, they increase their risk of flash floods, waterway erosion and property damage – especially along small urban streams. While 29 larger communities have integrated aspects of ISWMM into their city codes as of 2022, the variability in adoption levels across municipalities leads to unique requirements and challenges in coordination and interpretation for development, emphasizing the need for design professionals and developers to understand deeply the communities they serve and for municipalities to consider what ordinances may or may not affect development in their communities.

During Iowa’s 2023 legislative session, Senate File 455 was introduced. This bill would have established state-wide limits on city and county enforcement of stormwater management standards, effectively revoking the power of locally elected officials and requiring many communities to curtail their stormwater management standards. The proposed legislation would increase allowable runoff rates for larger rain events and some versions of the bill could have prevented standards for the smallest but most frequently occurring rain events (rain events of 2.75” or less make up more than 98% of all rainfall events in Iowa and contribute to most of the water quality and channel erosion issues observed in urban areas). This type of legislation could prevent multifunctional practices outlined in the ISWMM from being required in new developments and further exacerbate the existing water quality, flooding and waterway erosion issues for many communities. Similar versions of this bill have surfaced and failed in previous legislative sessions, and ultimately SF 455 failed again in 2023, signaling that many members of the Iowa legislature believe stormwater management decisions should be made at the local level to meet local needs.

The need for sustainable and resilient systems has only increased over time, underscoring how important it is for local governments to have the freedom to adopt standards like ISWMM that ensure the health, safety and welfare of their communities. The benefits of multifunctional stormwater infrastructure will only be realized further as case studies like Gray’s Station are recognized. As more governing bodies consider adopting the ISWMM into their ordinances, site designers need to stay in step and offer professional expertise in modern stormwater practices and continue to develop the collective knowledge and perceived value of green infrastructure among their colleagues and communities.

Written by Will Gaskins, Water Resource Designer; Greg Pierce, Water Resource Engineer