Safety of Italian dams in the face of flood hazard

Highlights

• We used regional analysis to assess extreme floods at 450 Italian dams site.

• We estimated return period of spillways design discharge R_S.

• One third of the investigated dams displays high flood hazard R_S < 1000 years.

• Dams catching larger catchments (>1000 km²) may display larger flood hazard.

• Our results provide priority for retrofitting of hazardous dams of Italy.

Abstract

Most rivers in Italy are segmented by dams that need rehabilitation because of (1) safety requirements by increasingly risk-averse societies, (2) changes in the downstream river and riparian system after dams building, (3) poor initial design at the time of completion and (4) modified priorities of watershed management. Safe design of flood spillways is a major concern, and requires to cope with low frequency flood hazard. One must estimate flood figures with high return periods (R ⩾ 1000–10,000 years) but statistical methods involve large uncertainties because of the short length of the available records. This paper investigates the return period of the design flood of existing spillways R_S of large dams in Italy. We used re-normalized flood frequency approach and regionalization using the Generalized Extreme Value distribution. The estimation of the site specific index flood is carried out by simple scaling with basin area at the regional level. The result show that 55% (245) of the 448 examined dams are equipped by spillway with R_S > 10,000; and 71% (315) of the dams have R_S > 1000. Conversely, 29% (130) of the dams display R_S < 1000 years, lower than acceptable hazard. The spillway of 14% (62) of the dams has R_S < 100 years, indicating potential exceedance of spillways capacity. Reservoir routing may dampen the outflow hydrograph, but one should carefully account for the need of achieving accurate dam safety assessment of these dams based on site specific investigations, also accounting for global change forcing.

Introduction

Dams have segmented most rivers with associated environmental impacts, sometimes disruptions. The interspacing of dammed, inundated, preserved and restored reaches has fragmented large watersheds by disconnecting once integrated free-flowing systems (e.g. [1]), and river impounding is often blamed for jeopardizing biodiversity downstream [2], [3], [4]. However, dams provided support to economic and social development worldwide, and a large number of dams under construction and planned is a matter of significance in the world today. The safe operation of dams has significant social, economic, and environmental relevance, and appropriate management procedures are necessary (e.g. [5], [6]). Dams will continue to provide valuable services, but rehabilitation is needed because of (1) the new hydrological safety requirements posed by increasingly risk-averse societies, (2) the changes in the downstream river and riparian system after the dam were built, and (3) the modified priorities of watershed management.

A large number of dams was built in Italy during the XX century under different engineering, social, economic, and possibly climate conditions from those nowadays. As a consequence large uncertainties affect the policies for mitigation of flood hazard in regulated rivers. Uncertainties descend from the complexity of both physical and man-controlled processes, including scale problems in observation and modeling, management strategies and operational practice. Additional uncertainty is given by the lack of knowledge of the effects of dams on floods downstream, let alone the unexplored evolution of social perception of flood risk after dams were built and operated for a long time. Lack of knowledge of worldwide dam accidents is a further factor of uncertainty, in spite of the role of incident reporting and data collection in enhancing reservoir safety [7].

Berga [8] reported that dam failures had been significantly reduced in the XX century. The percentage of failures before 1950 was 2.3%, while for dams constructed from 1951 to 1982 it reduced to 0.2%, and since 1982 it was 0.09%. Major advances are associated with ameliorated structural safety, but hydrological safety of dams, and safe design of flood spillways are also major concerns, and hydrologists worldwide continuously investigate new methods to approach spillway design under uncertainty (e.g. [9], [10]). The Committee on Failures and Accidents to Large Dams [11] of USCOLD estimated that overtopping covered more than 40% of dam failures worldwide. Charles et al. [12] showed that most of the failures of embankment dams causing loss of life can be attributed to the embankment breaching due to either of two causes, namely: (1) overtopping during an extreme flood, and (2) internal erosion due to piping or hydraulic fracture. Study of overtopping is largely within the province of hydrology, and so is design flood assessment for the provision of appropriate spillway and freeboard, and guidelines emphasize the importance of design flood as a key factor to dam safety [13], [14]. For instance, in the United States, over 2000 dams (3% of the 75,000 US dams) were identified as potential hazards to people living in upstream or downstream areas, due to inadequate spillway capacity [15]. The International Commission on Large Dams [16] suggested that the return period R of the spillway design flood should range from 1000 to 10,000 years, depending upon exposition and vulnerability of downstream riparian areas (e.g. [17], for a comparative study of regulatory frameworks for dam safety worldwide). One notes that the length of the available hydrological series (typically R ⩽ 50–100 years), is too short for any site specific flood estimation based on local data, if available.

The American Society of Civil Engineers [18] reports that the average age of the 84000 dams in the country is 52 years. Many of these dams were built as low-hazard dams protecting undeveloped agricultural land. However, with an increasing population and greater development downstream, the overall number of high-hazard dams continues to increase, to nearly 14,000 in 2012. The number of deficient dams is estimated at more than 4000, which includes 2000 high-hazard dams. The Association of State Dam Safety Officials estimates that it will require an investment of $21 billion to repair these aging, yet critical, high-hazard dams.

In Italy there are 548 large dams, i.e. those displaying dam height higher than 15 m, or impounded storage larger than 10⁶ m³ (e.g. [19], and Fig. 1). Design and building records are available for 488 dams only, 83% of which (403 dams) were built before 1970, and roughly 41% (198 dams) before 1950. About 63% (332 dams) are concrete dams (arch, gravity, or both) and 31% (165 dams) are earth dams. Spillway design flood q_S was generally evaluated using empirical formulas based on stream flow records available at the time of dam completion, without any probabilistic argument to support hydrologic hazard and dam safety. Governmental surveillance of dams was enforced after 1925, after a major dam’s failure occurred at the Gleno reservoir, on December 1st 1923, causing at least 353 deaths. However, a national authority was established in 1989 only, called SND and further RID. It operated for about 20 years before being discontinued, and its mission included generic infrastructure management, with unclear liabilities by the National and Regional Administrations. RID recommended the rehabilitation of dam spillways in order to accommodate the 1000 years flood, regardless of dam’s type.

To assess the capability of the existing dam spillways to accommodate the 1000 years flood one has to face large uncertainties. The assessment of low frequency flood flows is cumbersome in Italy, because the average length of Annual Flood Series (i.e. the largest observed annual peak flow, henceforth referred to as AFS) is about 28 years [20]. After 1970, systematic stream flow records decreased countrywide, because of lacking responsiveness by both governmental and local agencies. Also, a few dam sites, if any, are equipped to measure hydrological flows continuously, so site specific data of (long term) dam operation are unavailable in practice. Notwithstanding so, one needs to achieve a first, approximate but comprehensive assessment of flood hazard in Italian large dams, hopefully before some catastrophic event will focus the social perception of risk. The estimation of low frequency flood figures plays an essential role in assessing current and future hazard. This paper provides an answer to this question using the regional approach to low frequency estimation of flood flows.