Alongside the debates about whether and how large dams should be decommissioned, small dams are removed on a regular basis. But even at the lowest end of the scale, removing a dam can generate plenty of debate, as Janet Wood discovered
The US state of South Carolina houses around fifty thousand dams. Only the largest — fewer than 100 dams — are regulated by the Federal Energy Regulatory Commission. Of the remainder, those that impound more than 50acre-feet of water, or those that are more than 25ft high, are regulated by the Department of Health and Environmental Control’s Dams and Reservoir Safety Department — a total of around 2250. The others are not regulated.
The Department’s Steven Bradley explains that there are frequently dam removal cases underway. For these small dams the reasons for removal are very varied, and technical issues may not be the overriding factor.
At present, for example, the Department has one dam removal on its books relating to a dam built back in the late 1800s. The dam was built to provide a millpond allowing local farmers to grind grain, and it was constructed by using wagons to pull soil along the ground. The dam is too small to be regulated, but is affected by plans for another dam in the same region.
Carolina Development Company, which owns the dam, has submitted plans to build a new dam upstream, and a condition of the development was the restoration of the wetland habitat downstream of the new dam and, hence, removal of the original dam.
The old dam will not be much mourned. It was poorly built and had in the past been poorly maintained, and the amenities it provided will be replaced by those at the new dam. The owner will pay for the removal, which will be a matter of using bulldozers and other mechanical equipment to break down the dam structure.
This type of process is continually underway as the water needs of a region change. But it is not always such a simple procedure, as the inhabitants of the city of Denver could attest.
Within what is now the metropolitan area of Denver, in the US state of Colorado, there is a dam called Leyden. Built in 1902, its original purpose was to supply water to the local farmers, an example of many dams built early in the century by groups of local people who combined to develop their area. Each bought shares in an incorporated company (now the Farmers Highline Canal and Reservoir Company) and were allocated use accordingly of the water that became available when the dam and reservoir were built. The dam is 44ft high, and impounds a lake of around 1100acre-feet.
With 90 years in operation, the dam and lake have been part Denver’s amenities for as long as anyone can remember. But in 1972 flooding nearly overtopped the dam, and state officials realised that the dam had severe structural safety problems. The spillway is entirely inadequate, explains Mark Haynes, from the Colorado Division of Water Resources’ Dam Safety Office, which has regulatory authority over the dam. The structure should be designed to 75% of the probable maximum flood (PMF), he explains; in fact, it could handle only 20-25% of the PMF in safety.
The dam also poses other problems for those trying to ensure public safety, Haynes says. The Division has calculated that the flood plain mapping used in the past to assess the extent of likely flooding, in the event of a failure, was inadequate. Areas thought to be safe from flooding, it is now known, would be at risk if the dam was to fail.
Since the structural weakness of the dam was discovered, it has been partially drained and it now impounds around 400acre-feet — which, given the immense changes in use that have taken place in the area since the dam was built, is sufficient to fulfill the dam’s traditional functions. But it means that around the new, much smaller, lake, a wide area of mud and dust has been created.
The Division has only two options available to offer to the dam owner: improve the structural safety of the dam, or breach it.
The current plan for breaching the dam is intended to allow it to be recovered, probably at a lower level, in the future. But repairing and improving the dam has been costed at US$2M, while the breaching option costs out at around US$3/4M — costs that will be borne entirely by the Farmers Highline Canal and Reservoir Co. It seems that, for the owner, the choice is clear, and some time ago the company board took the decision to go ahead with the breach. What is more, breaching the dam will allow the company to develop the land that was previously beneath the impoundment, and plans for housing lots have already been mooted.
The Leyden dam is structurally unsound and the dam owner has a plan to remove it. But there is one problem: no-one wants the dam to be removed, and the issue has quickly become one of public interest.
Public interest says no There are a large number of residents who use the amenities of the lake: the local waterski club is extremely popular, for example. Similarly opposed are residents with ‘lakefront’ properties: even with the lake only partially drained they are now facing what easily, in hot weather, turns into a dustbowl.
The protestations of residents are one thing: the cost to the municipality of removing the dam is quite another. The effect of removing the dam begins, ironically enough, with relocation: some residents now calculated to be on the flood plain are likely to be moved if the dam is breached.
In addition, according to Mark Haynes, there will be a large amount of work required on existing infrastructure downstream of the dam. The municipality of Denver will be required to strengthen bridges and roads, as will other municipalities not incorporated into the Denver region. Drainage and flood control will also need to be strengthened in the region, and this work will be the responsibility of a raft of agencies.
Federal agencies will be involved because one road that requires strengthening is an interstate highway. What is more, water quality issues will be raised because a canal runs along the toe of the the dam.
All in all, Haynes estimates, in addition to the US$3/4M incurred by the dam owner for the removal, bills totalling at least another US$8M will have to be paid by the various agencies who have jurisdiction in the area. And all this to remove a dam local people would be more than happy to retain. It is even possible that the owner would be happy repair the dam, if it were the cheaper option.
Surely the answer is simple: split the cost of repairing the dam among the many affected agencies and municipalities. But in practice, it is not. The practical difficulties begin with the simple problem of getting all the agencies who have an interest in the situation into a position where they can discuss the options. And that is before any attempt is made to find a solution acceptable to all the local and federal bodies protecting their turf. The difficulty is well illustrated by the offer made by one municipality to contribute US$3/4M — but only in return for complete control. In the event the dam is breached that municipality will be facing work likely costing over US$1M.
Another attempt As IWP&DC went to press the Department of Water Resources was planning another attempt to bring affected parties to the table. Meetings in mid-March will attempt to save various agencies several million dollars, and leave the people of Denver with a safe dam and a lake that can still be used as an amenity. But Mark Haynes is not optimistic. The issue has been in abeyance for months now, and the company board had already decided against keeping the dam. If the agencies cannot pull a plan together, they will collectively wave goodbye to $8M.