The successful performance of thousands of watershed flood control dams across the US has led to the urbanisation of rural areas — at the cost of dam owners. Suzanne Moxon explains why the bill for increasing spillway capacity continues to escalate

The 50th anniversary of Cloud Creek dam in Oklahoma was a significant event for the US hydro industry. It was the first watershed flood control dam to be built in the US: its ageing, and that of others like it, raises new issues for US flood control.

Under the auspices of the US Department of Agriculture, more than 10,000 of these dams have been built in the US since 1948. With construction hitting a peak from 1960-73, the projects have provided flood control in 46 of the country’s states. But as the 20th century draws to a close such schemes are faced with problems. A large proportion of this US$14B infrastructure is nearing the end of its operational design life, and the success of dams built in Cloud Creek’s footsteps is creating a huge upgrading bill.

‘Part of the problem,’ says Ed Fiegle, president of the Association of State Dam Safety Officials (ASDSO), ‘is that these structures have done their job so well. Through them we have seen rural America transformed into urban America. Downstream development has occurred because the dams prevented flooding in areas which used to be inundated on a regular basis.’ The significance of downstream development is that it changes the hazard classification of many dams. As Darrel Temple from the US Department of Agriculture explains, these dams were originally designed to protect crops but are now surrounded by housing developments and people. To provide better protection in the event of flooding, the dam’s spillway capacity needs to be increased to prevent breaching. Upstream development has also played a role. ‘Having asphalt roofs where there used to be agricultural land increases the run-off and flow into the reservoir,’ Temple adds. So the stakes are raised as the potential for loss of life and property damage due to dam failure escalates.

A growing problem

The need to increase spillway capacity is a growing problem across the US. Jeff Bohrer, a project engineer from US company Bowser Morner, says that like many states Ohio is becoming more developed. His company is working on four spillway rehabilitation projects which have been recently classified as high hazard. Ohio regulations say that Class 1 high hazard dams must pass 100% PMF safely through the dam. One of the projects Bohrer is working on only passes 34% PMF — typical of many watershed flood control dams which were originally designed to protect agricultural land. ‘Existing dams just can’t handle 100% PMF on the spillway,’ says Bohrer.

The Federal Energy Regulatory Commission (FERC), which regulates over 2600 dams throughout the US, is not unfamiliar with spillway enlargements either. Twenty-two regulated dams are undergoing spillway modifications. For the period October 1981 to October 1999, 84 FERC dams had some form of spillway modifications.

In Georgia estimates suggest that 75-80% of rehabilitation carried out on dams involves spillway modifications. And as the state is finding out, this can only increase as the number of high hazard dams grows. Fiegle says that out of Georgia’s 357 watershed dams 141 are classed as high hazard, although this number will be creeping closer to the 200 mark as soon as the hazard classification of these dams is reassessed.

The problem, according to Joseph S Haugh, director of dam safety at the Virginia Department of Conservation and Recreation, is that the hazard classification system is not based on the condition of a dam but on the downstream potential for loss of life and property damage. ‘A dam’s classification can change through no action of its owners but rather due to land use changes that the owner had no control over, or perhaps even no knowledge of,’ he says.

The drive towards an increased spillway capacity is not only accelerated by up- and downstream development, ageing plays a part as well. Temple says that the dams were constructed with a 50-year service life and the sediment pool was also designed on this basis. As many dams approach the end of their design life sediment pools are filling up. Additional sediments accumulate in the flood control pool, reducing the amount of water which can be stored for flood control purposes. One way to rectify this is through increasing spillway capacity.

Temple is keen to point out that the large amount of rehabilitation facing watershed dams should not be seen as a negative function of the structures — it is merely a passing of time. ‘They were designed with a 50-year operational life and we knew we would have to revisit these dams one day,’ he says. ‘The rehabilitation of dams is necessary to maintain quality of life. Just because we are having to go back and reassess them, it does not mean that anything has gone wrong.’ The major issue in the US is that reassessment of dams, and reclassification, has far-reaching implications for dam owners. The US Department of Agriculture’s watershed flood control dams are sponsored by local communities who are responsible for their upkeep, and will foot the bills for spillway modifications.

Larry Caldwell from the Natural Resources Conservation Service in Oklahoma, says an April 1999 survey in 22 states assessed the extent of rehabilitation required. More than 10,000 of the watershed flood control dams are located in these states and it was found that 2200 require rehabilitation estimated to cost more than US$540M. Downstream development means that over 650 of these dams do not meet required design standards and need to be rebuilt and upgraded at a cost of US$400M.

The problem which dam owners now face is one of limited financial resources. ‘The sponsors of these dams have always had to carry out maintenance on a shoestring budget,’ says Fiegle. ‘To be faced with the cost of spillway modifications is somewhat daunting.’ Darrel Temple explains how the cost of increasing spillway capacity is site-specific. ‘If you have to change from an earthen spillway to a structural one you could be talking millions of dollars,’ he says. ‘Acquiring land rights to enlarge the spillway can also be expensive. But in other cases, if you have an earthen spillway with geological material resistant to erosion, it may be a minor job to enlarge the spillway.’ FERC’s division of dam safety and inspections recognises that spillway modifications can be an expensive venture but the benefit of protecting human life and property cannot be ignored. The average cost of spillway modifications at FERC-regulated dams is US$5.2M. This ranges from a low of US$3000 to a high of US$15M. Although FERC dams are perhaps in a different league to the watershed flood control projects, from his experience Jeff Bohrer knows that spillway modifications can be costly.

‘Most spillway projects I have worked on have all been in the US$1M price range,’ he says. He gave the example of a dam that required a much larger spillway — the cost of which was estimated at more than double the amount the private dam owners had budgeted for. The owners said they would go broke if they had to pay this money but the simple reality was that if the dam wasn’t fixed it would have to be breached. This again would be costly, and would also mean the homeowners would no longer have the lakeside properties which they desired. Bowser Morner and Ohio Department of Natural Resources went back to the drawing board and came up with a less expensive solution to satisfy the regulations. The dam face is to be covered with RCC and overtopping of the dam will be allowed.

Ed Fiegle believes that there is a real need to fund rehabilitation of private dams. ‘The owners do not have the financial resources, let alone all in one lump sum,’ he said. ‘They have to borrow money and we are finding in Georgia that many banks are not used to loaning money for dam rehabilitation.’ Bohrer spoke about a second dam in Ohio where community sponsors are faced with a bill of US$0.75M for spillway modifications. To pay for this they are getting a low interest dam safety loan from the Ohio Water Development Authority. The cost will be spread among the homeowners over a 30-year period, possibly adding several hundred dollars a month to each household’s bills.

Ohio is one of few states that provides loans for both public and private dam owners to utilise for dam safety repairs. In recognition of the huge gap which needs to be bridged between the repairs required on watershed flood control dams and the financial resources available, two pieces of legislation are being pushed through the US legal system.

Bill HR728 was introduced into the House of Representatives by Representative Frank Lucas of Oklahoma on 11 February 1999. Called the Small Watershed Rehabilitation Amendments Act 1999, it covers dams built under the public/private partnership of the small watershed programme. The bill requests that the federal government gives 65% of the total cost associated with project rehabilitation. Local sponsors and/or the state government will provide the remaining 35%. The fund formula will request US$60M annually from the federal government over the next ten years.

Bi-partisan support has been shown for the proposed legislation and a companion bill, SB1762, was introduced to the US Senate on 21 October 1999 by Republican Senator Covedell of Georgia and Democratic Senator Lincoln of Arkansas. Both bills are progressing well and there are indications that HR728 will be passed soon. Although the bills cover all areas of dam rehabilitation, Fiegle says: ‘There is no question that this requested money will be eaten up by spillway capacity requirements.’ While bills HR728 and SB1762 address the needs of watershed rehabilitation at the present time, Joseph Haugh believes that attention should be paid to future needs. ‘We need to address those dams whose classification could change in the future,’ he told the Association of State Dam Safety Officials. Appropriate authorities have to be made aware of the relationship between existing and proposed dams and downstream areas. In addition, policies regarding land approval procedures within the flooding limits of a potential dam failure have to be established. Haugh believes that people need to become aware of the effects that downstream development can have on the required standards of a dam.

Ed Fiegle stressed that meeting such standards is not an easy task. He explained that even though Georgia is leading the way in rehabilitating watershed dams, progress is slow. ‘Look at it this way,’ he adds, ‘it won’t all happen in my lifetime. It’s a fact that we need more money to make this happen. Across the US there is a need for a realistic, ongoing funding mechanism which dam owners can tap into to rehabilitate their dams.’

Watershed flood control dams

The watershed flood control dams were built under the following programmes: The Flood Control Act of 1944 (PL534). More than 3400 flood control dams have been constructed in 12 states.
The Watershed Protection and Flood Prevention Act of 1954 (PL566). Referred to as the Small Watershed Programme more than 6300 flood control dams have been constructed across the US.
The Pilot Watershed Programme was a transition between the PL534 and PL566 acts. More than 400 flood control dams were constructed in 33 states.
The RC&D programme has also provided technical and financial assistance to local sponsors for planning, design and construction of more than 200 flood control dams since the 1960s.
Since 1948 more than 10,450 earthen embankments ranging from 6-24m in height have been built. The projects provide more than US$800M in benefits annually and were built with federal funds. Local sponsors had to pay for land rights and all operation and maintenance costs, and after construction the dams became their responsibility. The majority of watershed projects have been constructed for flood prevention and watershed protection but others have included water management; municipal and industrial water supply; recreation; fish and wildlife habitat improvement; water quality improvement; and water conservation.
[Source: Rehabilitating Our Nation’s Ageing Small Watershed Projects, Larry W Caldwell, ASDSO Annual Conference 1999]

Spillway modifications

Darrel Temple from the US Department of Agriculture says that generally to increase spillway capacity the spillway will be widened, but problems can occur if urban developments surround the project, and there may be difficulties in securing land rights. Other alternatives include deepening the spillway and raising the dam. With earthen embankments this may lead to too much erosion and will require a structural spillway. Computer simulation using programs such as SITES (see article on pp24-25) can help determine if this is feasible. The US Department of Agriculture’s Agricultural Research Service also has research under way to improve prediction of vegetated earth embankment breach due to overtopping in extreme flood events. This research addresses timing of the breach, the rate of breach and measures that may be taken to improve resistance to breach.
Jeff Bohrer from Bowser Morner explains how the use of RCC is increasing in popularity in spillway modifications in Ohio. There is a growing trend to let the dam overtop and use RCC or similar erosion protection on the dam face. ‘Generally you do not want a dam to overtop,’ he says. ‘But these dams are in place and RCC facings have worked well and are economical in such circumstances.’

Critical flood guidelines

Critical flood information may help to stem the flow of spillway capacity enlargements across the US. The government is recognising that in certain situations passing 100% of the PMF over the spillway may not be necessary, and that overtopping dam failures may be insignificant in the amount of damage they cause to downstream areas in major flood events. The critical flood rule is based on the assumption that if all flood damage occurs due to a base flow flood that is less than the regulatory design flood for the dam, then requiring additional spillway capacity above this flood to satisfy design flood requirements would not be necessary. The critical flood must not result in additional loss of life, health or property along a critical routing reach downstream of the overtopping dam, when compared to damages caused by the flood in the absence of an overtopping dam failure.
The critical flood rule is not common practice in many states but Jeff Bohrer from Bowser Morner believes it may grow in importance. The Ohio Department of Natural Resources is currently reviewing it. Critical flood studies do require a lot of engineering effort and cannot guarantee that study results will be favourable to the dam owner.