As a senior designer at a civil engineering firm, I have been challenged with many stormwater management designs for new facilities and retrofitting of existing facilities with current best management practices. To provide consistent results to local and state agencies during the review process, I have organized an analytic process using free software and standard mathematical calculations to assess the effect of a potential dam failure on the downstream channel. The sudden release of stored runoff can have significant impacts on the community, including damage to property, roads, and even potential loss of life should someone be in the danger reach when a breach wave is released downstream. The analysis process is intended for the most common stormwater management facilities—small pond designs receiving less than 1 square mile, or 640 acres, of drainage. Often these facilities have minimal safety concerns as compared to lakes and reservoirs; however, every dam design must evaluate the possibility of a dam failure. According to the National Inventory of Dams, there are more than 87,000 registered dams across the United States. Other studies such as the National Land Cover Database (NLCD) suggest a total of more than 8 million natural and manmade impoundments.
For local municipalities to meet the requirements of the federal and state government agencies for water quality and to better control stormwater runoff, many are retrofitting drainage basins with improved structural stormwater management practices. This includes improving existing facility construction to current treatment criteria or the construction of a new facility to treat stormwater to the maximum extent possible. In certain circumstances, ponds built in the 1980s and 1990s are in excellent condition but do not provide the best management practices for today’s stormwater management criteria. Designed to reduce peak runoff and leave little impact to water quality, these facilities were typically built as part of development projects. Many ponds were constructed with no downstream concerns. With the construction of additional roads, homes, and businesses within the potential breach wave, these communities may increase the danger classification for the pond. When a facility is constructed within a drainage basin without any current attenuation systems, there may be a dramatic reduction of peak runoff during design storms; however, the design may have to be reduced to limit the potential damage resulting from a dam failure.
Many different studies, technical reports, and software have been developed for all aspects of stormwater management, several of which specifically discuss dam breaches. Without detailed failure information, a dam break analysis is largely based on assumptions and scenarios. An analysis should be conservative in nature, but not encompass unrealistic situations. Engineers and review authorities must use professional judgment to adequately protect the public, yet not become overbearing to stormwater management design.
Designers are tasked with following each jurisdiction’s requirements to meet the minimum criteria for stormwater management facility design and construction. With this in mind, it is difficult to prepare a “one and done” design process for analyzing dam failures. However, the approach described in this article provides basic guidance and can be adjusted to meet your review authority’s approval standards.
General Overview of Analysis
Long before preparing a formal dam breach analysis, a designer must look at the ramifications of impounding water within a watershed. Engineers should examine the potential downstream impacts and danger analysis points prior to getting deep into the intricacies of design. This is the pre-design phase. Once the best point of discharge is determined, the design moves into the design phase where the watershed hydrology and facility hydraulics are determined. This is the meat and potatoes section of most engineers’ work. Following the design phase, the designer moves to the analysis phase. Depending on the results of the failure analysis, the designer may be forced to make changes to the facility design to reduce downstream concerns. The fourth and final phase of dam break consideration is the post-construction phase, which begins upon construction of the dam and continues throughout the life of the facility (Figure 1).
It is important for designers, engineers, approval authorities, and property owners to understand the importance of dam failure analysis and how it cannot be successful when it becomes an afterthought. The perception of a dam break should not be based on one opinion of how unlikely a failure would be to occur but on how much damage could occur if the unthinkable happened.
Incorporating Standard Design Software. Throughout the United States there are thousands of review authorities. Many accept commercial design software, while nearly all accept free standard software such as TR-20, TR-55, and HY-8. TR-55 and TR-20 are products of the USDA Natural Resources Conservation Service (NRCS) currently available in Windows format and known as WinTR-20 and WinTR-55. TR-55 is a tool for modeling urban watersheds that calculates storm runoff volumes and peak rates of discharge. TR-20 is a watershed runoff and routing model, using rainfall data to generate hydrographs by passing flow-through tributaries, reservoirs (areas of stored runoff), and structures. The HY-8 is provided by the US Department of Transportation Federal Highway Administration (FHWA) and is a hydraulic analysis program for evaluating the efficiency of culverts.
Compatibility between software systems is not necessarily direct, but can they be utilized in combination to assist in accurately modeling drainage systems. TR-55 is used to generate time of concentration (Tc) and cumulative surface runoff curve number (CN) data used to input drainage areas in the TR-20 platform. HY-8’s analysis of a culvert, in combination with headwater storage volume calculations upstream of the pipe, can be entered into TR-20 as a structure to determine reservoir storage elevation and discharge rate for breach analysis.
Using a combination of accepted software platforms provides the engineer with the opportunity to accurately and consistently model the breach and non-breach conditions. The review authority has the ability to recreate design information and verify that it is acceptable within the regulations.
Factors of Safety. Engineering and stormwater management design is conservative by nature. While we attempt to predict worst-case scenarios, it is impossible to accurately model all storm and runoff conditions. The typical length of service for a small pond averages 25–50 years. When a facility is nearing the end of its life expectancy, it is likely to be considered for retrofit and upgrade to current standards, allowing it to sustain another 25–50 years. It is unrealistic to design every facility to handle a 200-year or even 500-year storm event, because the likelihood of these events occurring while these facilities are in good working order is minimal.
It is important to remember that there is always the possibility for a catastrophic dam failure causing damage to the downstream ecosystem, drainage structures, personal property, and even the possibility of losing human lives. Several sources suggest that overtopping of the dam is the most common cause for dam failure because facilities are rarely designed to safely convey the probable maximum flood (PMF), followed by seepage or piping failures. Other factors including differential settlement, ice, structural damage, and vandalism or sabotage can lead to failure, while in some cases, the failure cause cannot be determined. As with designing a facility to withstand any hydraulic condition, it is impossible to analyze every failure condition. Common analysis conditions include sunny day, non-storm-event failure, where water detained in a facility is suddenly released downstream, and storm event failures including the 100-year storm, a brim-up, and the PMF events.
Considering reasonable worst-case scenario conditions throughout the analysis provides the engineer with a level of confidence that the expectations within the report would stand up in a court of law should a dam fail and the engineer be held responsible for not anticipating the level of danger associated with a dam. A tool published by Franklin F. Snyder in 1964, “Hydrology of Spillway Design,” can help determine the danger reach length. The nomograph uses dam height and valley width to determine approximately how fardownstream until the breach wave would be at 1 foot in depth or less (Figure 2). This distance can then be multiplied by the pond’s storage volume at the time of the failure.