Saturday, August 1, 2009

The Pitfalls of Small Hydro Dams

By Douglas Watts

A recent article in the western U.S. "High Country News" has sparked discussion at the political website "Daily Kos" about the value of building or re-building small dams for hydropower in the U.S. Given the renewed interest in sources of non-fossil fuel power, the subject deserves a closer look. The thoughts below were spurred by a comment from a New Hampshire woman who wrote:

"I've been pushing to convert them to fish safe small hydropower sources for years but no one here wants to listen.  I've talked to the power companies and they say that NH law doesn't allow hydropower from these dams. I suggest that laws can be changed to match the changed technology but that just causes people to turn away. I used to think that the NH motto was "you can't get theah from heah" but lately I think it is "It will nevah happen heah."

It is important to note as you read on that I am not dismissing or denigrating the concept of "green" small hydro power production. It is conceptually possible for small hydro facilities to be constructed that have minimal environmental effects. However, as detailed below, the technological and economic hurdles to achieving such a goal are formidable -- and have only been partially achieved at perhaps a few dozen of the thousands of existing hydro dams in the United States. This does not mean the goal is unattainable and should be abandoned. It does mean that before considering building new dams or retro-fitting old ones for electricity generation, we must get to know the lay of the land.

In my 20 years of experience attempting to restore rivers in Maine and Massachusetts, I have found the key reason conservationists look at small hydro as a "green" or "greener" energy source is that they are unfamiliar with the enormous body of scientific literature on the subject. A few highlights:

1. Few, if any, hydro dams are equipped with fish passage that actually works, especially downstream passage, where the fish are forced to swim through turbine blades that literally chop them up.

2. The impoundments of these dams eliminate the type of habitat that native fish require to spawn and grow -- and at the same time create ideal habitat for invasive, non-native generalist fish species that prey upon and eat native fish.

3. These impoundments radically alter the habitat required by the riverine aquatic insect community that forms the base of the food chain in rivers, ie. stoneflies, mayflies and caddisflies. Extensive studies in Maine rivers shows that hydro impoundments are literally "too fast to be a lake and too slow to be a river." This creates a situation where lake and pond insect communities do not develop (they need true lakes and ponds) and riverine insect communities cannot survive (they cannot live in deep, slowmoving water). Maine studies show that, for these reasons, hydro impoundments contain far less insect diversity and abundance than lakes or rivers, which for native fish, means they are biological deserts.

4. Hydro dams increase water temperature levels, lower dissolved oxygen levels, and prevent diurnal (overnight) cooling of water temperatures that are critical to the survival of salmon and trout during hot weather.

5. Dams have been principal cause of the extinction of migratory fish populations in the United States. From the Columbia to the Susquehanna to the Penobscot and Kennebec in Maine, the construction of dams is the principal reason why these large rivers, which used to produce some of the largest sea-run fish populations on Earth, have lost most or all of their historic salmon, shad, eel, sturgeon and other migratory fish runs.

In response, the same woman from New Hampshire wrote back:

"I firmly believe in fish protection and restoration.  Newer turbine technologies allow power production without endangering fish.  Also, most of the dams in NH aren't being removed -- many are used for flood protection and control. It is these dams that I'd like to see producing power and if they are inhibiting fish populations, modified so that they don't."

This is a typical and understandable response of many people who are conservation-minded, ie. they want to do the right thing, but are not well-versed in the scientific literature regarding the impacts of hydroelectric dams on rivers and watersheds.

As explained in punch list above, dams on rivers and streams inhibit native fish populations due to the effects of their artificial impoundments on habitat character, water temperature and dissolved oxygen. This is especially true for rivers in northern New England, which are (or, more often, used to be) inhabited by native trout and salmon. These habitat and water quality effects cannot be mitigated with improved fishways, because fishways at a dam do not affect the size and character of the impoundment behind the dam, which is the cause of the habitat and water quality effects described above.

Second, turbine design, except to increase generation efficiency, has not progressed in a century. To my knowledge there are no new turbine technologies being designed specifically for the purpose of reducing the number of fish killed. And if there are any turbine designs on the drawing board that might do this, they are decades away from actual use. And unless ordered to do so by a court, a small dam owner is not going to remove a working turbine and replace it with a more "fish friendly" turbine (if and when such a thing becomes available) because the cost is prohibitive.

This illustrates the key problem with small hydro: the economics of scale. The fish passage, minimum flows and other environmental mitigation necessary for a 0.5 megawatt dam cost nearly the same as for a 5 megawatt dam. The mitigation costs, even amortized over 20 years, will still be higher than the value of the power the dam can generate during those 20 years. This makes the rate of return on the capital investment extremely poor.   The owner of an idle small dam  would get a better rate of return by taking the same amount of $$$ and putting into a money market fund. Which is what they do.

Isn't there some way to make a turbine that is rigid enough to spin but will bend if it strikes a fish? A hydro turbine operates in the same way as a boat propellor or a window fan or a food processor, but in reverse. In the latter three examples, supplied energy causes the blades to spin and do work: to move air, to move water, or to chop up a tomato. For this reason, in order to do work, the boat propellor, window fan blade or food processor blade must be more inflexible and rigid than the medium it works in. Otherwise the blades would just collapse once they stared spinning and do no useful work. Think of a boat propellor made of flexible rubber. Once it started spinning fast enough to move the boat, the rubber blades would bend flat.

A window fan with blades would quickly shred. A food processor blade less rigid than the skin of a tomato would leave you with a nice, whole tomato no matter how many times you pressed the "puree" button. Think of a windmill. In order for the blades of a windmill to spin, the blades have to be rigid enough to stand up to the force of the wind. If a windmill was made with flexible rubber blades, they would fold up and collapse every time the wind speed went above a gentle breeze. This would defeat the entire purpose of having a windmill in the first place.

The same goes for hydro dam turbines. Water is a far denser and heavier substance than air. In order for a turbine blade to not fold and bend and crumple under the force of thousands of pounds of water, it must be made of a extremely strong and rigid material: steel. Any material less rigid would cause the blades to bend or be ripped off by the force of the water. When a fish swims through a turbine and is struck by the spinning blade, something has to give, either the body of the fish or the blade, just like a tomato in a food processor. If you designed a turbine blade flexible enough to bend around the body of a fish, by definition it would be too flexible to withstand the force of thousands of pounds of flowing water and would be useless for generating power, just as a boat propellor made of soft rubber will not move your boat.

Lastly, most dams exacerbate floods -- they do not prevent or control them. This is for several reasons. First, dams destroy wetlands and the natural flood plain of rivers, which eliminates the natural flood controlling capabilities of these features. By this metric alone, dams create a net loss of flood prevention capabilities. Second, dams are nearly always kept at or near "full pond", usually at the request of people who have cottages and boat docks on the impoundments. This means that when we get a lot of rain, the "pond" is already at or near full capacity, and the rising water has no place to go except downstream. In order to operate a dam as a flood control device, you have to keep the normal water level well below the spillway of the dam, at least by 3-5 feet, and in New England this is almost never done, because people like to see the pond nice and full.

Unfortunately, keeping the pond nice and full during dry weather destroys any flood control or prevention the dam might possibly have and actually increases the severity of flooding during periods of heavy rain. Ironically, it is at the direct demand of shorefront camp and homeowners that dam impoundments are kept at or near full pond throughout the summer which negates any potential flood prevention benefits the dam could offer.