Modernizing Stormwater Design Criteria for Today's Multi-Benefit Solutions

Stormwater design criteria have not been substantially updated since they were first established back in the 1950s. Most stormwater design manuals still reference those same criteria and event rainfall data from the 1961 Technical Paper No. 40, Rainfall Frequency Atlas of the United States. In urban areas we continue to apply these standards without thinking what it means or how it impacts our aging systems. We add on stormwater control measures as required as part of the post construction best management practices of the MS4 permit but those criteria are not integrated into the overall stormwater criteria. The results is an antiquated and outdated set of design standards that do not provide the functional value that the public is paying for. It is even more critical in this time when many cities want to improve the resiliency of urban areas in the face of climate change. However, do we have the criteria and tools to meet this need. Do we have the right design requirements that will result in the stormwater management outcomes that we desire. In Lawrence, Kansas these questions are being asked and evaluated using a city-wide watershed approach. 2D watershed models of all 59 watersheds are being developed to first fully understand the existing conditions and then to apply a range of potential rainfall events to determine what is the most cost-effective solutions. This is not a one size fits all approach but a watershed system approach that looks at what is the most optimal functional value in an area to reduce flooding, improve water quality, and protect public safety and property. This city-wide system multiple benefit approach is then being used to develop design criteria and stormwater management policy that will provide the highest value at the lowest cost. This approach is significantly different than blindly applying an assumed level of service design standard that would result in only larger and larger pipes or large underground storage. Using this approach, the City of Lawrence will develop a set of affordable solutions that will provide the greatest benefit across the watershed to maximize their available budget. This is in contrast to the past approach of designing a project to meet an outdated standard that was costly requiring it to be delayed for several years before it could be built leading to a back log of projects that would never be built. In Westport, Missouri, the oldest part of the City of Kansas City, Missouri, stormwater solutions have been designed to maximize the functional value within an available budget. This design was even more challenging because it was within a combined sewer system. A new design approach was therefore needed to address sever and frequent flooding, meet a federal combined sewer consent decree, and improve the business district. Several past studies had attempted to solve the problem using the old, antiquated design standards that resulted in over designed and unaffordable solutions that could not be built. Meanwhile the area still flooded almost every year impacting business, flooding cars, and causing public health issues given that the flood waters were combined sewer. Something had to change and that required a change in the design standards. Instead of meeting a 10-year level of service requirement the design focused on two recent real rainfall events along with a range of design events from the 2- to 10-year return frequency to establish the highest value solution for the budget. The resulting solution did not eliminate flooding but rather limited the flooding withing the public right of way at a depth and duration that was acceptable. This presentation will provide an overview of the current issues with design standards developed in the 1950s, ideas to improve them, and two case studies that will provide real examples of how and why we need to change. As cities strive to meet sustainability goals stormwater management must also be sustainable in the form of providing for adaptive designs that optimize the functional value at the most affordable price. Furthermore, if we start to consider stormwater as a resource rather than a waste our design objectives change from a drainage design standard to a water resource design standard. How will climate change impact us we really don't know but if we only require more and more conservative design that result in larger and larger pipes, we won't be creating more resilient cities we will only be creating larger uncontrolled conveyance system that impact our downstream neighbors. Resiliency means our design need to meet a range of potential future conditions not just one. Rainfall may get more intense and be of higher volume, but it also may be more time between events. We should be striving to create design standards that consider both outcomes so we can create more sustainable and resilient cities.