Description & Background
The initial filling of a reservoir is the first test that the dam will perform the function for which it was designed. A carefully managed first filling is crucial to the future success of a dam. According to a study completed by the Bureau of Reclamation on internal erosion failure modes, “approximately two-thirds of all failures and one-half of all dam incidents occur on first filling or in the first 5 years of reservoir operation.” ¹
Dams are constructed primarily to impound and store a large body of water. During the construction of a new dam, diversion channels and cofferdams are used to prevent water from entering the construction area. However, when dam construction is complete, flow to the dam site resumes and the reservoir begins to fill with water. The first filling of a reservoir can be defined as the increase in water level behind the dam from the time construction is complete until it reaches the desired operating level. Depending on the location, type, size, and intended purpose of a dam, the duration and rate of its first filling can vary. Regardless of whether it takes several months, several years, occurs naturally, or with the aid of pumping units, the first filling of a reservoir should be planned, controlled, and closely monitored in order to reduce the risk of failure.
Because the first filling of a reservoir is a critical phase in the life for the dam, it is vital for dam operators and engineers to have as much control over the first filling as possible allowing as much time as needed for appropriate surveillance, including the observation and analysis of instrumentation data. The first filling should be scheduled to occur after construction of the dam and all necessary appurtenances (i.e. spillways and outlet works) have been completed as well as the installation of appropriate instrumentation. Specifications regarding the rate of reservoir rise should be developed to allow the dam to adjust to the forces it will experience as the water level behind it increases. These plans should be documented in a design memorandum that may also include “reservoir regulations during project construction, a water control plan, project surveillance, cultural site surveillance, flood emergency plan, public affairs, safety plan, and transportation and communications.” ²
In addition to dam failure, it is common for design, construction, and/or material deficiencies of a new dam to become apparent during the first filling. For example, evidence of seepage, cracking, and erosion are often noted when the reservoir is raised to new levels for the first time. Inspection and assessment of these potentially hazardous conditions prior to the completion of filling is important and it may be necessary to halt filling or in some cases lower the reservoir before the desired operating water level is achieved to investigate signs of seepage, cracking and erosion. Repairs to any project features that did not function as designed can be re-evaluated and modified to ensure the dam operates according to its original design.
Anita Dam (Montana, 1997)
Anita Dam is located about 22 miles north of Chinook, Montana, about 5 miles south of the Canadian border. The drainage, which normally flows only in response to snowmelt or heavy rain, is an unnamed tributary...
Austin (Bayless) Dam (Pennsylvania, 1911)
Austin Dam (also known as Bayless Dam) was constructed between May and November of 1909 just outside Austin, Pennsylvania, a town of approximately 2,500 people in Potter County.
Fontenelle Dam (Wyoming, 1965)
Fontenelle Dam is located on the Green River near La Barge, Wyoming and was originally built in 1964 to provide irrigation water and hydropower for a region of southwest Wyoming. The earthfill embankment structure...
Marshall Lake Dam (Colorado)
Routine inspections of Marshall Lake Dam in the early 21st century were relatively uneventful. The 75-foot-tall, 2,400-foot-long high hazard earth embankment dam had relatively flat slopes...
Penn Forest Dam (Pennsylvania, 1994)
Penn Forest Dam was constructed between 1956 and 1958 to be one of the main water supply reservoirs for the city of Bethlehem, Pennsylvania. The dam was originally an earthfill embankment structure, stood 145 feet high, 1,930 feet long, and stored 6.4 billion gallons of water...
St. Francis Dam (California, 1928)
Located approximately forty miles northwest of Los Angeles, California, St. Francis Dam was a curved concrete gravity dam constructed between 1924 and 1926 in order to provide a storage reservoir for the Los Angeles Aqueduct system...
Teton Dam (Idaho, 1976)
Teton Dam was located in southeastern Idaho about 15 miles from Rexburg in the valley of the Teton River. The dam and its reservoir were the principal elements of the Teton Basin Project designed by the Bureau of Reclamation to control flooding as well as provide a source of...
Additional Case Studies (Not Yet Developed)
- Vajont Dam (Italy, 1963)
- Wister Dam (Oklahoma, 1949)
Author: Interagency Committee on Dam Safety
Date Published: 2015