What Happened to Lake Delhi Dam?

Delhi Dam was breached on July 24, 2010, after two days of heavy rain in the drainage basin above the dam. What follows are details about the dam’s history, the Iowa Department of Natural Resources’ role in forming and assisting the Independent Panel of Engineers to investigate the breach, the dam breach investigation process, the likely cause of the dam breach and lessons learned.

The dam breach initiated about 1 p.m. on July 24, 2010, with an estimated peak breach outflow of 69,000 feet3/s. The flood and the dam breach resulted in extensive property damage in the reservoir above the dam and in the communities downstream of the dam. No loss of life occurred as a result of the dam breach.

There were a number of factors taken into consideration in the investigation of the breach. These included:

• The design of the embankment dam, which included a reinforced concrete core wall as the primary impervious element in the dam;
• The embankment materials, which appear to have consisted of a low plasticity sandy clay;
• The limited ability of the dam to pass a major flood (given the spillway capacity was initially designed to about 25,000 ft3/s, at reservoir water surface elevation 900 ft), and;
• The binding of one of the spillway gates preventing its full opening during the flood event.

Delhi Dam: History and background

Delhi Dam is on the Maquoketa River, about 1.4 miles south of the town of Delhi. The Maquoketa River in northeastern Iowa is a tributary of the Mississippi River. The dam was constructed between 1922 and 1929 by the Interstate Power Company for hydroelectric power generation.

Generation of power was terminated at the dam in 1968. The dam is currently owned and operated by the Lake Delhi Recreation Association. In 1991, the Lake Delhi Combined Recreation and Water Quality Tax District was formed pursuant to Iowa Code Chapter 357E to allow lake residents to tax their property an additional $4 per $1,000 of assessed value, to support the dam and lake. In 2005 the District issued tax-exempt bonds to finance dredging of the lake.

The flood gates and wicket gates of the dam were damaged in 2008 and dangerous scouring of the underwater rock armor of the dam was discovered. At the invitation of Federal Emergency Management Agency representatives, the Tax District applied for and was approved for repairs to the lake and dam. Several repair projects were completed and additional projects were underway at the time of the 2010 flood.

While the lake frontage was primarily privately owned, public access was allowed and the lake was patrolled by Iowa DNR enforcement officials. The dam was inspected every five years by the IDNR dam safety staff. The last full inspection was in 2009. At this inspection, the need for repairs to the spillway gates were noted, and IDNR required the repairs to be completed by the Lake Delhi Recreation Association by the end of 2009.

Follow-up contact made by the IDNR in January 2010 found that repairs were currently underway but were not completed. The repairs do not appear to have been completed prior to the July 2010 breach of the dam

Delhi Dam was designed as a concrete dam and earthen embankment. The 704-foot long structure consists of: A 60-foot long concrete reinforced earthfill section abutting the left limestone abutment; a 61-foot long conventional reinforced concrete powerhouse containing two S. Morgan Smith turbines with two Westinghouse generators (each rated at 750 kW); an 86-foot long gated concrete ogee spillway with three 25-foot by 17-foot vertical lift gates; and, a 495- foot long embankment section with a concrete core wall.

The embankment section was originally constructed with 1V:3H upstream slopes and 1V:2H downstream slopes, and extends to the south (right) abutment of the dam. The crest of the south embankment section of the dam is 25 foot wide and the dam crest is at elevation 904.8 feet NGVD29.

The maximum section of the concrete portion of the dam has a height of about 59 feet and the embankment section has an estimated maximum height of 43 feet. Lake Delhi, the reservoir behind Delhi Dam has an area of approximately 440 acres and a storage volume of 3,790 acre-feet at normal reservoir (elevation 896 ft) and a reservoir volume of about 9,920 acre-feet at the crest of the dam (elevation 904.8 feet). The spillway crest is at elevation 879.8 feet and the hollow inside of the spillway crest structure is filled with rock.

The concrete reinforced earthfill section of the dam at the left abutment was originally constructed with two parallel concrete retaining walls, founded on rock and spaced 20 ft apart. Rock fill was placed between the walls.

In 1967, a concrete crib wall and additional fill was placed upstream of the original walls.

The area downstream of this section serves as a parking and staging area for performing maintenance in the powerhouse.

Formation of the dam failure investigation team

In response to the dam failure, the Governor of the State of Iowa requested assistance from the National Dam Safety Review Board in providing an Independent Panel of Engineers to evaluate the cause of the overtopping and breach of Delhi Dam. This request was made to the Administrator of the Federal Emergency Management Agency (FEMA) dated August 6, 2010. The National Dam Safety Review Board includes representatives from federal and state agencies as well as a member from the private sector and operates under the direction of FEMA. The National Dam Safety Review Board is statutorily established under the Dam Safety Act of 2006 (Public Law 109-460) and provides the Director of FEMA with advice in setting national dam safety priorities and considers national policies affecting dam safety.

In an Aug. 6, 2010, letter, the State of Iowa identified the scope of the Independent Panel of Engineers review as follows:

• Review the operational characteristics of the project leading up to the breach of the upper reservoir.
• Perform an evaluation of the breach of the dam to determine the specific failure mode.
• Submit a final report documenting the results of their findings on the cause of the breach of the upper reservoir and the important lessons learned from the failure.