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These decisions will shape the degree of economic viability and ecosystem equity that may be achieved into the future. Dams were constructed as early as BC to regulate the spatial and temporal variability of water and marked the major episodes of human civilizations in Asia and Europe. Globally, greater seasonal and interannual variability is significantly correlated with lower per capita GDP [ Brown and Lall , ] and higher water storage capacity emerges as a pathway to resilience and economic growth.
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In the United States dams provided a gateway and supporting mechanism to industrialization, urbanization, and agricultural expansion. Army Corps of Engineers , ]. In the Midwest, South, and Southeast, the widespread construction of levee systems by the USACE's implementation of the Flood Control Act encouraged urban and agricultural development on the fertile floodplains along the Mississippi River and in the Floridian wetlands. The passage of the Reclamation Act by Congress in led to the creation of the Bureau of Reclamation and the construction of major dams for irrigation and hydroelectric production in the West, such as the Hoover Dam and Glen Canyon Dam, each over m tall.
This Act was perhaps the most transformative legislation in the history of the western United States enabling urban, energy, and irrigated agricultural development. Department of Agriculture , ]. Subsequently, over the last half of the 20th century demands for irrigation water sourced from reservoirs tripled [ Biemans et al. In regions where surface water resources are not available, groundwater is typically used to supply the deficit [ Ho et al.
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Across the United States, many dams are nearing or have already exceeded the nominal 50 year economic design life planned for government permitted dams Figure 1. While the physical life span of dams is typically greater than 50 years, the physical diminishment of constructed dams and their components results in increased budgets needed for maintenance and repair. Each state's dam safety inspector is, on average, responsible for over dams [ American Society of Civil Engineers , ]. Furthermore, inspection requirements and emergency dam failure plan requirements differ from state to state.
For example, in Alabama regulations for dam safety in design, construction, and ongoing inspections do not exist a house bill for dam safety was introduced in but is yet to progress further. In Texas, a ruling by the Texas Attorney General resulted in limitations to accessing dam hazard information citing homeland security concerns [ Buchele , ] and the ability of citizens to remain informed of proximal dam risks.
In some other states, the qualifications for dam safety inspectors are not specified [ Association of State Dam Safety Officials , ] and many of these dams are only physically inspected on a 10 year schedule. Age of dams in the United States that meet the criteria of 1. Possible or likely loss of human life in the event of dam failure; 2.
Data from Stanford University [ ]. The inability to adequately fund safety inspections and address dam vulnerabilities result in real societal risks in terms of public safety and potential economic losses. During the April floods, Houston residents were evacuated over flooded roadways out of the potential flood zone of two dams, both of which have exceeded their economic design life spans by around 20 years [ Borrello , ]. Alabama, the state with no dam safety laws, is not immune to dam failures either: six families were evacuated after heavy rains in caused the face of a dam to slump [ Association of State Dam Safety Officials , ].
In , a single storm event in South Carolina triggered the failure of over 30 dams. Such an event may be a precursor of future flood destruction under both a changing climate and aging dam infrastructure. Modeling the potential for dam failures or a series of cascading dam failures at a watershed or regional scale is needed across the nation to better inform risks to critical infrastructure that is operationally or physically linked to a dam break e. Encouragingly, some recent efforts toward funding nonfederal dam safety works have been made through direction in the Water Resources Development Act of that would allow nonfederal dam owners to apply for grants to address high hazard dam issues.
Metrics for the level of hazard associated with a dam exist and have been embraced by organizations and agencies at both federal and state levels e.http://maisonducalvet.com/padrenda-lugares-para-conocer-gente.php
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The lack of rigor is reflected in the more frequent occurrence of dam failures amongst privately owned dams [ Costa , ]. This suggests that either the risk metric is perceived as too general for prioritizing funding allocations or there is a serious issue with the increasing potential for dam failures across the country. The peak period of dam design and construction in the United States occurred when there was a limited history and understanding of instrumental hydrologic and climatic data. For example, the Colorado River Compact of , which stipulates water transfers from the upper to lower Colorado today largely regulated through water releases from Glen Canyon Dam, was predominantly based on less than 20 years of instrumental streamflow data.
The limited hydrologic record was collected during the wettest decade in the 20th century and excluded data from an anomalously dry period prior to [ Hundley , ; Advisory Committee on Water Information Open Water Data Initiative , ]. A major national question exists as to whether existing dams are able to meet their design objectives over a full range of probable hydrologic variability given that paleoclimate records show the occurrence of catastrophic droughts and floods larger than any event considered in the design scope of existing dams [ Cook et al.
A recent evolution of stochastic models that consider such features is starting to inform operational aspects [ Kwon et al. Despite the use of large dams, including Glen Canyon Dam, which allow for management and equitable distribution of water between upper and lower Colorado basin states in addition to hydropower, flood control, and recreational services , there have been ongoing calls to remove Glen Canyon Dam [ Joint Hearing on the Sierra Club's Proposal to Drain Lake Powell or Reduce its Water Storage Capability , ; Lustgarten , ].
To illustrate our point, we evaluate the performance of the Colorado River Compact's distribution of water between the upper and lower basins to complement existing Bureau of Reclamation [ ] studies. We consider the presence and the absence of Glen Canyon Dam and Lake Powell using a paleoclimate perspective. Lee's Ferry streamflow delineates streamflow between the upper and lower basins and was developed by Woodhouse et al. From this data, we developed stochastic simulations using wavelet auto regressive models [ Kwon et al.
The Colorado River Compact stipulates a minimum delivery of 75 million acre feet of water over a 10 year period from the upper to lower basin. An average of 7. With Lake Powell, reservoir mass balances are computed annually, and spills occur if the reservoir capacity is exceeded in a given year. This time series is composed of 0 values in years when the streamflow meets the target release and a negative value for years in which the target demand is not met. An analysis of the frequency spectrum of shortages reveals recurrence intervals greater than 20 years similar to findings in Bureau of Reclamation [ ].
The most severe hydrologic shortages have a recurrence interval of 60—80 years irrespective of whether or not the dam is in place see Figure 2. Including the dam, which has a storage volume of around twice the mean annual streamflow of the Upper Colorado River [ Woodhouse et al. The peak periodicity for severe shortages remains at 60—80 years even with storage in Lake Powell.
The wavelet analysis was performed on simulated time series of failure occurrences determined using simulations of paleoclimate streamflow at Lee's Ferry by Woodhouse et al. The primary utility of a dam such as Glen Canyon is the ability to meet administratively defined water allocation requirements.
An even longer reconstruction of Lee's Ferry streamflow from AD to not included in the analysis here highlights an even drier period in the 12th and 13th centuries [ Meko et al. This drought is theorized to be a contributing factor to the disappearance of the Ancestral Pueblo civilization that previously populated the Four Corners region [ Cordell et al. Consequently, even if the dam were not removed, one needs to think of financial, social, and ecological risk management strategies to mitigate the impacts of catastrophic adverse effects associated with extreme hydrologic events.
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In addition to the consideration of past hydroclimatic variability informed by both instrumental and paleoclimate records, projections of future water availability across the United States show that changes in water supply should be expected. For example, Christensen et al. This is in addition to increases in evapotranspiration [ Walter et al. Flood risks will be further exacerbated through land use changes that increase runoff peaking and volume [ Kousky and Kunreuther , ; Ceylan and Devineni , ] leading to higher sedimentation rates [ Kondolf , ].
Projected increases in temperature would also enhance eutrophication resulting in anoxic conditions in reservoirs [ Paerl et al. As understanding and detection of hydroclimatic variability and change improves with longer observations and subsequent analysis, we have come to the realization that many dams, particularly older dams designed with limited climate data, omitted extreme climate scenarios that are not or are no longer considered to be remote events.
The recent record low reservoir water levels at both Lake Mead and Lake Powell, behind the Hoover and Glen Canyon dams respectively, exemplify the risks associated with prolonged droughts with return periods of more than 20 years see right graph of Figure 2 and the inability to meet legal flow requirements. Both paleoclimate and future climate scenarios suggest that administrative and legal structures should be reformed to reflect and adapt to existing and future hydrological conditions.
Kolesar, a business school professor and, as a private citizen, an avid fisherman developed an optimization model for the timing and volume of releases from the Delaware reservoirs that maximize fisheries benefits while meeting water demands with no increase in drought exposure. Working with a number of fishing and ecological interests, he was instrumental in the Delaware River Basin Commission adopting a flexible streamflow management program using his modeling as a tool for making operational changes in water releases.
The success of this assessment and implementation signals a significant change in the way ecological and water supply goals can be achieved using dams. Although the Delaware River Basin reservoirs are operated largely for seasonal storage, the need to have a management strategy for the financial, social, and ecological impacts remains given the risk of exposure to severe [ Namias , ] and sustained [ Devineni et al. A number of states, such as Michigan, Massachusetts, Connecticut, and Colorado, have implemented river basin programs [ Kendy et al.
Although floodplains are, by definition, at risk of flooding, these areas also offer amenity values e.
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In addition, many individuals, who are least financially capable of rebuilding, live in the most dangerous flood zones and are denied adequate emergency assistance when floods eventuate [ Barry , ; Gladwell , ]. Efforts to raise flood insurance rates in the aftermath of Hurricane Katrina in , and other recent flooding events such as Hurricane Sandy in , were intended to send a price signal that reflected the true cost of locating in a flood plain.
These efforts have seen Congressional resistance and have had little success so far. In addition to subsidies for flood insurance, the provision and maintenance of flood control infrastructure continues to encourage development in the flood plain. The Natomas subdivision in Sacramento, CA, is a case in point.
Record flooding in and [ National Research Council et al. The occurrence of a flood event similar to the flood with an estimated 1 in — year ARI [ Porter et al. Putting Natomas in the context of such a scenario is downright scary, and speaks to the human tendency to discount low probability, high impact events [ Kousky and Kunreuther , ]. We are now conscious of the environmental impacts caused by dams. These include fragmentation of water ways [ Graf , ], obstructing movements of keystone fish species or rearing habitats and resulting impacts that propagate through the watershed [ Bednarek , ], trapping sediment and altering river beds and banks [ Kondolf , ; Wisser et al.
Louis et al. There are now multiple environmental coalitions and advocacy groups emphasizing river restoration ecology and recommending direct intervention. Historically, the social and economic benefits of dams were perceived to be high and took precedence over environmental degradation, the protection of downstream water supplies [ Lawson , ; Pitt , ], and indigenous communities, which have often been displaced without adequate compensation [ Babbitt , ; Cernea , ].
Federal agencies and watershed commissions now address some concerns through the Secretary's Indian Water Rights Office to facilitate settlements of Native American water rights claims [ Department of the Interior , ] and addressing climate change and environmental streamflow requirements through more flexible water release policies. In summary, a discussion as to whether or not to renew or remove dams in the face of age related structural decline and an unfavorable climate immediately takes on larger social dimensions.
The social aspect of dams requires an examination of the variety of interventions, ranging from structural to financial to nonstructural, and the notion of acceptable risk for society and for individuals. There have been strong calls to remove dams to restore riverine systems, such as calls to remove Glen Canyon and the Snake River dams but the question remains: Are we prepared to live without some of these dams?
The converging issues of growing populations [ Colby and Ortman , ], evolving demands for food, energy, and water, aging dams, and reduced water storage capacity through decommissioning and sedimentation highlights the pressing need for a national water assessment and a subsequent national water plan.