The History of Drywells in Arizona
A Vital Solution for Monsoon Flooding
To understand how drywells became an essential part of Arizona’s stormwater infrastructure, we have to go back to the 1970s—a period of rapid growth for Phoenix and the surrounding Valley. The city’s population swelled by nearly 50% during this decade, reaching 2.6 million residents by 1979. While this growth was beneficial, it presented major challenges, especially when combined with Arizona’s annual monsoon season, which brings severe storms and heavy rainfall in a short time.
Phoenix was not prepared for how new buildings and developments would interact with the intense seasonal flooding. The city first experienced a catastrophic weather event during the Labor Day Flood of 1970, when 11 inches of rain fell in a single day, causing widespread damage. This marked the beginning of a series of flood events throughout the 1970s, as urbanization strained the natural stormwater drainage systems.
Local governments responded by introducing ordinances requiring stormwater to be retained onsite for new developments. A critical part of these regulations was the 36-hour rule, which mandated that any standing water must be drained within 36 hours to prevent mosquito breeding, reduce odor, and protect landscaping. However, initial solutions, such as surface retention basins, encountered problems due to the Valley’s unique soil composition. Arizona’s clay and caliche soils are impermeable, meaning water remained in basins well beyond the 36-hour limit.
The Evolution of Drywell Design
Early drywells were essentially deep holes filled with rocks, designed to aid in drainage by allowing water to seep underground. While this worked better than surface basins, these initial designs failed to account for key factors like soil layers and pollution treatment. Furthermore, they couldn’t be easily maintained, meaning drywells would eventually clog or stop working altogether.
Engineers and municipalities recognized the need for a more effective solution. They hypothesized that if they could drill past the impermeable soil layers, they could reach permeable soils, allowing for faster drainage. In addition, they incorporated a deep sump into the design, which served as a pretreatment mechanism. This sump allowed silts and fines, which are heavier than water, to settle at the bottom, keeping them out of the drainage field. This pretreatment process was a substantial improvement for drywell designs.
With the development of these enhanced drywells, Arizona found great success in managing stormwater during monsoon season. Drywells not only helped meet the 36-hour rule but also offered an efficient way to filter stormwater and recharge the Valley’s precious aquifer, turning a flooding problem into a water conservation opportunity.
Modern Drywell Innovations
Over time, drywell designs have continued to evolve. While the basic elements remain—a deep sump, screened inlets, and consideration for permeable soils—today’s drywells are far more efficient. For example, modern drywells often include a chamber with an overflow pipe that extends deep into the ground, sometimes reaching up to 120 feet to access permeable soil layers. This ensures that water is quickly absorbed, even during heavy storm events.
Another innovation was the introduction of a shield around the inlet screen, which helped prevent debris and pollutants from entering the drainage field, improving the system’s overall efficiency. In the most recent PureWell design, the shield has been removed, and the inlet screen is positioned below the flow line. This adjustment keeps floating debris out of the drainage field while ensuring the screen remains visible for easy inspection and maintenance. While a clogged screen is a potential issue in all designs, having the screen visible allows for quicker identification and resolution if drainage problems occur.
Additionally, modern drywells now feature oil absorbent pillows, which float on the water surface and absorb oils and hydrocarbons, providing an extra layer of pretreatment to protect the drainage field from pollution. According to the EPA, up to 30% of stormwater pollution is from runoff, which collects oils, trash, and debris on its way to disposal. These pretreatment innovations significantly reduce the environmental impact of stormwater runoff.
Drywell designs have also expanded to include secondary chambers, allowing for more water to be disposed of at a faster rate. Perhaps one of the most significant advances is that today’s drywells are designed for maintenance. Hydro Vac trucks can be used to clean out the accumulated dirt, sludge, and debris, greatly extending the life of the drywell system.
Conclusion
From its humble beginnings as a simple hole in the ground, the drywell has evolved into a sophisticated piece of stormwater infrastructure. As Phoenix and the greater Southwest continue to grow, drywells play an increasingly important role in both flood control and water conservation. Today’s drywell systems not only help manage stormwater but also treat and filter it, contributing to the preservation of our natural resources.