Big Creek Lake Dam: Spillway Design Precipitation for a Vital, Non-Regulated Dam In Alabama

Abstract Only - Big Creek Lake Dam was constructed in 1952 for the Mobile Area Water & Sewer System (MAWSS) to serve as water supply for the City of Mobile, Alabama and industrial users. The dam embankment is approximately 5,000 feet in length with a maximum height of 75 feet providing storage of approximately 69,000 acre-feet at normal pool. Releases from the reservoir are controlled by seven (7) 31' (w) x 11'(h) tainter gates that discharge to an approximately 1,000 feet long concrete spillway. The dam was classified in the 1978 Army Corps of Engineers Phase 1 report as a large - high hazard dam. Malcolm Pirnie (MPI) was retained by MAWSS to evaluate the current condition of the dam and appurtenant structures and to address short-term remedial repairs. As part of the work, a detailed hydrologic analysis was conducted of the contributory watershed and a detailed hydraulic analysis was conducted for the existing headworks gates and spillway. An important question in evaluatng an existing dam is whether the current embankment, spillway and appurtenant structures meet applicable regulatory criteria. The State of Alabama, however, does not currently have a dam safety program or regulatory legislation in place. So, that leads to the question of what are the approriate design criteria for a large - high hazard dam along the Gulf Coast in the State of Alabama. The safe passage of the full Probable Maximum Precipitation (PMP) event is used as the current standard of care in many states for spillway design criteria for large-high hazard dams such as Big Creek Lake Dam. The PMP rainfall event is defined by the National Weather Service (NWS) as, "Theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of the year." The current methodology for determining the PMP rainfall in the Central and Eastern United States is based upon the NWS Hydrometerological Report (HMR) 52, which was published in 1982. However Big Creek Lake Dam, as many of the nation's large dams, were constructed prior to 1982 and were likely designed based upon lesser precipitation depths such as those provided by application of HMR-33. A practical question is whether application of meteorological methodologies developed subsequent to the design and construction of a dam are appropriate for assessment of an existing dam's compliance with current dam safety regulations. An HMR-52 analysis was conducted for the Big Creek Lake Dam watershed and the resultant 72-hour PMP rainfall event was approximately 51 inches in depth. While engineers can readily take this rainfall depth and determine what improvements (if any) are required to meet spillway and freeboard requirements based on the HMR-52 rainfall event, they are typically not funding or financing the requisite improvements. Owners of dams, the ones paying to maintain and improve dams, can have a hard time believing that they need to design to a rainfall event of that magnitude when peak recorded rainfall events over the history of the dam are substantially below the PMP rainfall depth and especially when there are not current laws in place stipulating the use of the PMP as a design criteria. To address these concerns, an assessment of the applicability of the PMP rainfall was conducted for the Big Creek Lake Dam. First, a review of dam safety regulations and design criterign for states within the Central and Eastern United States was conducted. This allowed for a regulatory perspective of what future dam safety regulations for the State of Alabama would likely be based upon. Secondly, a review of historic storm events of durations between 6-hours and 72-hours near Mobile, Alabama and the Atlantic Gulf Coast region in general was conducted. This allowed for a realworld comparison of recorded rainfall events with the computed PMP rainfall event. As documented by this past years tropical storm activitey, this region of the United States is vulnerable to tropical depressions and cyclonic storm events that can produce tremendous precipitation depths in addition to the high velocity winds and associated storm surge. Lastly, input from a meteorologist local to Mobile, Alabama was solicited to obtain an unbiased opinion on the feasability of a rainfall event on the order of magnitude of the computed PMP rainfall event based upon the physiographic location. We believe that the analyses conducted for Big Creek Lake Dam are not only useful for dams within the State of Alabama, but are also applicable for dams throughout the country.