Chimney Hill is known as a peaceful and desirable neighborhood in Waco, Texas. However, there have been several disturbing occurrences for this neighborhood over the past three decades due to flooding. At least two homeowners have reported repeated instances of their homes flooding. These property owners have been engaged in ongoing conversations with the City over the years. During this time, the City of Waco has attempted to address the drainage problem, but the flooding has persisted. The City retained Walker Partners to perform a drainage study to determine the cause of the flooding as well as potential solutions.
Walker Partners began the study by collecting existing field information through historical topographic maps, as-built construction plans, aerial photography, field visits, property owner interviews, and topographic survey. Based on this information, as well as our personal familiarity with the neighborhood, Walker Partners developed drainage area maps and runoff calculations. Next, Walker Partners analyzed the existing storm drain system and streets to determine its drainage capacity. Two critical issues became very clear during this analysis.
First, the drainage area was much larger than originally thought. Past efforts to address the flooding primarily focused on Chimney Hill Drive, assuming that MacArthur Drive (the next street to the north, or upland) handled all of the drainage runoff that came to it from the north. When looking at the drainage associated with Chimney Hill Drive alone (excluding MacArthur Drive), the drainage area is approximately 11 acres. The drainage area associated with MacArthur Drive is approximately 44 acres. Based on our field investigation, it was clear that MacArthur Drive was not handling all of the drainage runoff coming to it from the north, which means that its drainage area is being added to the Chimney Hill Drive drainage area. In other words, Chimney Hill Drive is receiving drainage runoff from 55 acres instead of 11 acres – that’s five times the runoff originally thought!
The second issue that became clear during the drainage analysis was that the existing storm drain and street system has very little drainage capacity. Based on our calculations, MacArthur Drive’s drainage capacity is surpassed in the 2-year storm event. If MacArthur Drive handled all of the drainage, the Chimney Hill Drive drainage capacity is exceeded between the 5 and 10-year storm event; however, since MacArthur Drive’s system is overwhelmed in that it does not handle all of the drainage coming to it, Chimney Hill Drive is receiving much more drainage than it was designed to handle. The overall system is calculated to be exceeded between the 2 and 5-year storm events! On top of this, there are no well-defined drainage swales or channels between homes to handle drainage overflows. In other words, when the storm drain system and street’s drainage capacity is exceeded, flood waters head directly for homes. Two homes have received flooding on multiple occasions leading to restoration work. Once we began talking to other property owners, we learned that others had experienced significant flooding through their yards, leading to property damage. The property owners were weary and on-edge due to these past experiences.
Walker Partners began the solution process with the aim of significantly increasing the City’s drainage system capacity. The first goal was to increase the drainage capacity in MacArthur Drive to handle as much runoff as possible from its associate drainage area, to prevent it from being added to Chimney Hill Drive’s drainage system. To accomplish this, Walker Partners analyzed numerous options. In the end, we recommended a three-pronged approach. First, add drainage inlets to the side streets on the north side of MacArthur Drive to minimize the amount of runoff entering MacArthur Drive. Second, add inlets along MacArthur Drive to increase the amount of runoff that can be collected underground. These first two approaches require a new storm drain trunk line under MacArthur Drive. Third, reconstruct the south curb line of MacArthur Drive, along with its driveways, to create a berm in the front yards, thus containing runoff in MacArthur Drive, rather than allowing it to overflow towards Chimney Hill Drive. Based on our calculations, these solutions will increase the MacArthur Drive system’s drainage capacity to the 100-year storm event!
The second goal in our solution process was to increase the drainage capacity of Chimney Hill Drive. With the MacArthur Drive solution described above, this made the second goal much more achievable. To address Chimney Hill Drive, Walker Partners designed additional curb inlets to increase the amount of drainage runoff that can be collected and conveyed underground. Additionally, Walker Partners identified a route to install larger storm drain pipe between Chimney Hill Drive and E. Landon’s Branch Tributary, the natural outfall for this drainage area. The combination of these solutions will increase the Chimney Hill Drive’s drainage system capacity to the 100-year storm event, also!
Walker Partners strives to provide a unique blend of technical competence with real world understanding. This project was an opportunity to practice both. The technical analysis for the existing and proposed storm drain system was very complex, requiring extensive research and numerous calculation iterations for potential solutions. But the calculations could not be performed in a vacuum – Walker Partners combined its “institutional knowledge” of Waco with its ability to hear property owners’ concerns to understand the real problem. This blend of technical factors and human factors ultimately provided a solution that will restore peace of mind to the property owners of the Chimney Hill neighborhood!
completion date: 2020
location: waco, Tx