Douglas A Dixon from EPRI gives an insight into the nature and severity of sedimentation problems at US hydro power facilities
Anecdotal information in the late 1980s and early 1990s indicated increasing reports of sediment-related problems at hydro-electric operations in the US. Such problems included:
•Loss of reservoir storage.
•Degradation of reservoirs’ functional purposes (power generation, flood control, public and private water supply).
•Damage to operational equipment (intake structures and turbines).
•Problems associated with the potential presence of contaminated sediments.
Many of these reports arose during investigations that support project relicensing by the US Federal Energy Regulatory Commission (FERC). Similar problems were also noted at non-licensed federal and state projects.
To further understand the nature and severity of these problems, a joint study was undertaken by FERC and the electric-power-research-institute (EPRI) to obtain a quantitative assessment of the sedimentation problems at US facilities, the remedial measures being taken, and whether sediment management plans were being used. Data were obtained through a survey questionnaire sent primarily to owners and operators of FERC-licensed hydroelectric projects. EPRI was responsible for preparation of the questionnaire, its distribution and solicitation of responses and FERC performed a preliminary analysis of the results. The survey results described represent subsequent analyses and data gathering performed by EPRI as part of its ongoing effort to develop sediment management guidelines for the US hydroelectric industry.
Questionnaire development began in late 1995. Questionnaires were mailed in mid 1996 and responses were tabulated for analysis later that year. The questionnaire was developed in two parts:
•Part one requested general information on the occurrence of sediment problems.
•Part two was aimed at gathering detailed information on the nature of sediment problems and the actions implemented to address them where reservoir sedimentation problems existed.
A multiple choice response format was selected because it made it easy for owners and operators to complete and encouraged a better response, and made data analysis easier according to the issues covered in the questionnaire.
Part one of the survey, in addition to gathering facility location and operational information, requested information on the occurrence of sediment problems at or related to the reservoir. Owners and operators were asked to check-off the type of problem (ie storage loss, operational, environmental, contaminated sediments, etc) and rank its severity as negligible, moderate, or severe. Owners and operators were also asked whether there were reasons to believe sediment problems would increase in the future; whether sediment deposits such as deltas and bars were observed during reservoir drawdowns or on aerial photos; and whether low level dam outlets (where applicable) produced sediment-laden flow into the tailrace over prolonged duration.
Part two of the survey requested detailed information on the nature of the sediment problems; remedial actions undertaken or planned; management plans implemented; and whether technical guidelines for sediment management would be useful. Specific responses on studies and sediment-caused problems were requested in the following areas:
•Performance of reservoir bathymetric surveys and quantification of storage loss, including the trend in storage loss.
•Operational problems with spillway gates, outlet works, power intakes and other civil structures.
•Degradation of reservoir functions such as power generation, flood control, navigation, flow diversion and recreation.
•Upstream and downstream channel degradation and aggradation, bank erosion and flooding.
•Ongoing sedimentation studies (other than bathymetric).
•Source of sediment inflow including those from natural erosion, agricultural runoff and development.
•Remedial actions (dredging, facility modifications, operational changes) and their planned duration.
•Sediment management plans in use (co-operative watershed erosion control, reservoir operation modification, sediment flushing and pass through) and the need for technical guidelines on reservoir sediment management.
•Environmental problems (fisheries, wildlife) and presence of contaminated sediments.
•Annual cost of remediation efforts.
The survey was mailed in April 1996 to approximately 660 owners and operators of hydroelectric projects, 614 of which were FERC-licensed projects and the remainder with federal and state operators. An initial limited response meant that a second request for completing and returning the questionnaire was issued in July 1996.
Responses were received from a total of 368 owners and operators (56% of those surveyed). Many of these have multiple developments, each with its own reservoir, dam, power house and other structures. As the survey required the completion of separate questionnaires for each reservoir, the total number of associated hydroelectric reservoirs covered in the responses was 547. The Tennessee Valley Authority (TVA) was the only US Government agency with hydroelectric projects that responded to the survey. Additionally, the frequency of response was higher for projects located in the US northeast and upper midwest. Large blocks of projects in the southwest did not respond. This uneven distribution qualifies the following results and conclusions because of differences in hydrologic and land use characteristics of the areas.
Responses were received for a wide range of reservoir volumes and installed generating capacities. They range from projects with practically no storage (projects with a small forebay or diversion dam) to reservoirs with storage capacity of more than 2M acre-feet. Because of this diversity and the effect reservoir volume may have on the nature and severity of problems, response analyses were segregated for reservoirs with a storage capacity greater than 5000 acre-feet and 50,000 acre-feet, respectively.
The survey’s results indicate that 25% of hydroelectric reservoirs in the US have some form of sedimentation problems. The frequency of sediment-related problems was slightly higher for reservoirs larger than 5000 acre-feet (27%) than for reservoirs greater than 50,000 acre-feet (22%). Approximately 15% of the respondents consider their sedimentation problem as severe. A comparison of the severity of the problem for various reservoir sizes indicates that the severity decreases with an increase in reservoir size. This is probably related to the increased sediment storage capacity within large reservoirs.
Sixty-four percent of reservoirs with sedimentation problems report storage loss as the major issue. This was followed by operational problems (47%), con-taminated sediments (36%), functional problems (33%), environmental problems (29%) and problems with upstream or downstream bank erosion and channel degradation/aggradation (26%). Except for environmental problems, reservoir size did not appear to affect the frequency of occurrence of each problem. Environ-mental problems, however, were reported more frequently for reservoirs with more than 50,000 acre-feet storage capacity (41%) than for reservoirs with storage capacity exceeding 5000 acre-feet (29%). Given that sediment problems decrease with increasing reservoir size, this finding is difficult to explain. It may, however, be a bias related to the fact that large projects (reservoirs) tend to attract greater resource agency and other stakeholder attention during FERC relicensing than small projects, thereby identifying potential environmental concerns.
Results of part two of the questionnaire on the details of sedimentation problems at hydroelectric reservoirs are presented opposite. There are inconsistencies in the results of part one and those of part two. For example, in part one, 47% of reservoirs with sedimentation problems reported them as operational in nature, while in part two only 26% reported specific operational problems. Similar discrepancies can be seen relative to environmental impacts and contaminated sediments. These discrepancies developed despite the use of virtually identical questions in parts one and two of the survey. The reasons for the inconsistencies are unknown but are likely to result from some combination of random reporting errors, misunderstanding, or different interpretations of the questions when completing parts one and two.
Key results of part two of the survey for reservoirs with sedimentation problems include the following:
•20% had impact on recreation.
•12% had some degradation of their power generation services.
•23% have conducted studies to address the sedimentation problem.
•28% have performed remedial action, with dredging and sluicing as the primary method of sediment removal.
•33% have implemented a sediment management plan, with co-operative watershed erosion control as the most widely used approach.
•23% report impacts on fish and wildlife and, of those reporting these impacts, 66% of them were on fisheries.
•42% report that technical guidelines would be useful for addressing their sedimentation issue.
•Annual direct costs for addressing sedimentation issues vary considerably. The maximum annual direct cost reported was US$300,000 and the average annual cost was approximately US$47,000.
A vital problem
Although approximately 55% of those surveyed responded to the questionnaire, the results do not represent one-half the hydro power projects in the US. The survey population did not include FERC-exempt or federal and state managed hydro power projects. It is also possible that the survey results include a response bias; ie those projects surveyed with sediment problems may have been more likely to return the completed question-naires, thereby inflating the results. Despite the less than optimal sample size, geographic coverage and inconsistencies in responses, the results of the FERC/EPRI survey clearly indicate that sediment-related problems are an important issue at FERC-licensed projects. It is quite likely that these issues extend to FERC-exempt projects and hydroelectric projects operated by federal and state agencies, as well as at non-hydroelectric reservoirs.
It should not come as a surprise that one in four hydroelectric reservoirs in the US may be experiencing sedimentation problems. Over a decade ago, Scheuerlein (1986) wrote that the management of reservoir sediment will become a vital problem for the future and a challenge to dredging technology. Sediment pollution is also the most commonly reported pollutant in US waters. Under Section 303(d) of the US Clean Water Act, states are annually required to provide the Environmental Protection Agency with lists of impaired waters and pollutants of concern. As of 1998, the latest compiled results clearly list sediment as the primary pollutant of concern among the states. Furthermore, the US hydroelectric industry is ageing. The US Army Corps of Engineers’ (USACE) National Inventory of Dams currently lists nearly 77,000 dams in the US. Of this total, approx-imately 3% (2278) of the dams support hydroelectric power production. Eighty-seven percent of these hydroelectric dams are over 30 years old and 73% are more than 50 years old. Taking into account the age structure of hydroelectric dams and their associated reservoirs, that sediments are the most frequently reported pollutant in the US, and the persistent accumulation of sediments in reservoirs, sedimentation problems will undoubtedly increase in the future.
Storage loss is expected to be the most prevalent sedimentation issue reported. This affects numerous reservoir functions including reducing generation capacity and flood control, and impairing recreational activities. As storage loss increases and reaches capacity, more sediment is passed through project works, causing problems with the operation of intakes, outlet works and spillways, including wear of turbine blades and associated machinery.
Addressing sediment pollution and sedimentation problems is a complex technical challenge to public agencies at the federal, state and local level, as well as to the hydroelectric industry. The challenge has two parts: remediation and prevention. Remediation of existing problems is an engineering and environ-mental protection challenge; however, it is a challenge within the grasp and control of project operators. Prevention is a much greater challenge because sediment control is beyond the control of the operator or any single entity, and is an ecosystem or watershed management issue. Identifying and managing sediment erosion and transport is very complex and involves all watershed entities (including private homeowners). Though a major water quality challenge for the future, significant progress has been made in developing and implementing best management practices through watershed stakeholder collaboration.
Almost half of the hydroelectric owners and operators reporting reservoir sedimentation problems noted that tech-nical guidelines on sediment management would be useful. Currently such guide-lines do not exist but relevant technical information is embodied in guidelines for navigation dredging and guidelines for remediation of hazardous waste sites. The American Society of Civil Engineers also recently published Guidelines for the Retirement of Dams and Hydroelectric Facilities (ASCE 1997). While targeted, in part, at approaches for remediating impounded sediments before dam removal, this manual has information on sediment management which will prove valuable to operating facilities.
EPRI has initiated a programme to consolidate sediment management infor-mation of relevance to hydroelectric sediment problems. The long term goal (~2003) is to specifically develop sediment management guidelines for operating facilities. These will include methods for characterising the physical and chemical quality of sediment, assessing environ-mental impact, managing remediation and sedimentation prevention.
EPRI is about to release to its members a Scoping Study Report on Sediment Issues at Hydroelectric Projects (EPRI 2000). This will identify important areas for research and development and estab-lish a roadmap for the future develop-ment of sediment management guidelines.
|Siltation versus sedimentation?|
|These are essentially the same but generally the US prefers the term sedimentation. Sedimentation is a process of depositing sediments in a reservoir. Sediments include all that is ‘solid’ including clay, silt, sand, gravel and cobble etc. Siltation technically is more specific referring to the deposition of silt but it can also be used more generically to include sediments.|
|About the Electric Power Research Institute|
|In 1973 the US electric utilities established the Electric Power Research Institute as a non profit research consortium for the benefit of utility members, their customers and society. Now known simply as EPRI, the institute provides a wide range of innovative products and services to more than 1000 energy-related organisations in 40 countries. EPRI’s multidisciplinary team of scientists and engineers draws on a worldwide network of technical and business expertise to help solve today’s toughest energy and environmental problems.|
| ASCE, 1997. Guidelines for Retirement of Dams and Hydroelectric Facilities. New York, NY.
EPRI 2000. Scoping Study on Sediment Issues at Hydroelectric Projects. TR-114008. Palo Alto, California, US. April 2000.
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