Skip to main content.
Earth and Environmental Sciences Home > Rappahannock Watershed Information > Research Projects > Gen'l Environmental & Geomorphic Effects of Dams, & the Significance of Embrey Dam on Rappahannock

General Environmental and Geomorphic Effects of Dams, and the Significance of Embrey Dam on the Rappahannock River

Michelle Meyer, Freshman

References

The purpose of this study was to examine the removal of the local Embrey Dam and compare it to current trend of removals in the country and specifically to the removal of the two dams on the Elwha River.

Dams are a very common occurrence on American rivers, and the number of dams seems astounding. There are more then 68,000 dams two stories or higher in the US and about 2 million smaller ones. This makes 600,000 miles of rivers into lakes (Haberman, 1995). These dams are used for many reasons including flood control, water supply, navigation, water quality, sediment trapping, electricity, and recreation (Collier, Webb, & Schmidt, 1996). Views of dams, the effects, and their necessity are changing, and with these changes new era of dam removal has emerged.

Dams alter the rivers in many ways. They effect the water, sediment, channel, wildlife, and surrounding land. One of the main reasons for today’s interest in dam removal, is the very visible effect on fish. The fish most harmed by dams are migratory. Migratory includes the anadromous that live in the salt water and return to the fresh to spawn, and the catadromous fish like the American eel that returns to the salt water. These migratory fish can be blocked by obstacles only one foot in height (Chesapeake Bay Foundation [CBF], 1994).

Because of the harmful effects of dams, a new trend has emerged to try to remove them. Many agencies are removing dams, including the National Park Service which has removed upwards from 100 dams from parks, and the Wisconsin Department of Natural Resources has removed 15 dams from that state. The removal trend is gaining backing because many dams may be obsolete. Today there are better power sources, better irrigation techniques, and non-structural flood control methods (Haberman, 1995).

Two Examples of dam removal projects are on the Elwha and the Rappahannock Rivers. Currently the City of Fredericksburg is looking into the removal of the Embrey Dam on the Rappahannock River in the southeastern coastal state of Virginia. The Embrey Dam is situated 2.4 miles upstream of the City of Fredericksburg (Figure 1). The city owns the dam located in Stafford County. The dam is 770 feet long or 1070 feet long with the abutments. The dam is 22 feet tall except for the north end which is 43 feet tall because of scouring of the bed when the crib dam failed (Timmons, 1997).

Picture of Embry Dam

Figure 1 : Embrey Dam on the Rappahannock River

On the Elwha River in Olympic National Park, in Washington State on the West Coast of the United States, there are two dams that are being studied for removal. The 110 foot high 1913 Elwha Dam forms Aldwell Lake. The second dam is Glines Canyon Dam, which stands at 185 feet high and creates Lake Mills (Collier et al., 1996).

There are economic benefits to removing dams. In the Chesapeake Bay, the migratory fish are at an all time low. Part of this decline is due to dams and other obstructions. The fish are part of the economy of the surrounding states. For example, in Virginia, the shad declined from 11 million pounds 100 years ago to 385,000 pounds from 1978-1992. If there were 17 million pounds of shad harvested today like in 1900, it would be worth $8.5 million. Today the catch is only worth $42,169 (CBF, 1994).

The Elwha removals are expected to bring in $164 million over 100 years from recreation, tourism, and sport fishing. In addition, the nonmarket value, the worth of something that can not be bought like enjoyment, is worth a minimum of $3.5 billion to the American people ("Commonly", 1997).

These benefits help balance the high prices of dam removal. The cost for dam removal varies with the size of the dam and the presence of any heavy metals (Haberman, 1995). The cost options for the removal of Embrey dam would be $10,200,000 for a fish passage and a gravity dam, $7,450,000 for complete removal, or $7,400,000 for partial removal of the dam (Timmons, 1997).

The cost of the Elwha dams also is based on the sediment. The plan for Elwha's sediment is to allow the river to erode it. The removal cost is then $111 million. An alternative is to dredge and slurry the material for a removal cost of $127.5 million. For fish passage and restoration instead of removal, the cost is $38 million at the onset. The fish passage would increase the number of fish by much less than the expected levels from removal ("Commonly", 1997).

Another issue for dam removals is sediment management. Part of the reason this is very important is because sediment can harm habitats or even raise the channel and the water levels (Haberman, 1995). Behind Embrey Dam is 329 acre-feet of sediment in the reservoir. Options for its sediment removal are as follows: (1) hydraulically dredge all of the sediment and haul it away, (2) mechanically dredge all of the sediments and haul them away, (3) hydraulically dredge half of the sediment onto the bank, (4) mechanically dredge half of the sediment onto the banks, or (5) allow the river to wash it downstream. The sediment levels behind the dam seem to have been stable since 1965 at least, and the dam is not trapping any more sediment (Timmons, 1997).

It would be cheaper to allow the river to erode the sediment, but the environmental hazards would be higher. With a typical year's flow, the river will erode 15.6% of the sediment and leave 0-. 583 inch deep sediment through Fredericksburg. With a flood like the one in 1942, the river will erode 16.6% of the sediment and leave .51 inches of sediment just below the dam and 2.96 inches from 7475 feet below the dam through Fredericksburg. This is expected to require dredging which would be very difficult because of the thin layer of sediment (Timmons, 1997). The flood of 1942, the highest recorded on the river, had a flood crest of 42 feet at Fredericksburg where the flood stage is 18 feet (Evans, 1986).

For the Elwha the sediment also is an important issue. The sediment in the reservoirs is 8.5 million cubic yards of course sediments and 9.2 million cubic yards of fine sediments. To dredge and slurry the sediment, 75% of the fine sediments would be added to water and put into the Strait of Juan de Fuca. The release of sediment is expected to temporarily cause excess turbidity. The Elwha dams are not flood control structures, but flooding might still be a danger. This is because the sediments will accumulate and raise the bed and the water level. This raised level is expected to be on average two feet ("Commonly", 1997).

The movement for removal of dams is a controversial one, and there are many people that argue against it. One group often against removal is homeowners. They are fearful that their property values will go down with the loss of their reservoir-front properties. This fear isn’t always viable as the riverfront property may be worth more (Haberman, 1995).

Land issues will occur once the Embrey is removed because the river is expected to decrease 500-600 feet in width up to the I-95 Bridge. One issue is the change in the views of the river. Another land problem centers on who will own the new land. A third issue is that Spotsylvania County will lose river frontage, and Stafford would own land on the south side of the river (Timmons, 1997).

Another fear of the homeowners is about the mud flats created by the low water levels. Evidence shows these mud flats usually vegetate quickly (Haberman, 1995). This re-vegetation is expected on the Elwha as the reservoirs will disappear and the river reforms. The reservoirs cover 684 acres, and 562 acres are expected to be reforested. The vegetation would look natural in three years, but it will not be completely restored for about one hundred years ("Commonly", 1997).

The main reason for removing Embrey Dam is the restoration of fish passage. Today, the Embrey Dam is the only dam on the 184-mile Rappahannock (Evans, 1997a). The Embrey cuts off 70 miles of the Rappahannock and hundreds of miles of the tributaries for migratory fish habitat. The migratory fish on the Rappahannock are American shad, hickory shad, alewife, blueback herring, striped bass, and yellow perch (Timmons, 1997).

There is a fish ladder on the Embrey’s southern abutment, but it does not work well. The passage for Embrey Dam would have to accommodate 150,000-300,000 shad and 1,500,000-3,000,000 herring. This necessitates a vertical slot fish passage (Timmons, 1997). The migratory bass would probably not use the fish ladder because in the Rappahannock today, the striped bass do not reach the dam. A fish passage on the Rappahannock would only be used at the times of migrations (Dewberry and Davis, 1990).

The importance of fish passage is a major issue for the Elwha removals. Without the dam there would be 400,000 pink, coho, chinook, and sockeye salmon a year (Evans, 1997b). The dam removal will restore 70 miles of habitat for fish on the Elwha and its tributaries (ÒCommonlyÓ, 1997).

Dam safety is a large issue: "In the National Dams Inventory, 32% of the dams have a ‘high’ or ‘significant’ downstream hazard potential . . ." (Haberman, 1995). This is true of the Embrey Dam where the dam is in ‘relatively poor condition’ and has seepage. The Embrey Dam is a also hazard with teens and drinking, and people have drowned. It also is a dangerous river portage for boaters (Timmons, 1997).

Not just individuals but local citizens of towns and cities are also at times against dam removal. The cities and towns look to the dam as a local landmark and as source of history (Haberman, 1995). Part of the interest in the Embrey dam removal is because of its history. In 1855 a crib dam was built, and it is still preserved behind the Embrey Dam. The Embrey Dam was built in 1909 (Trout, 1992). The Embrey Dam represents Fredericksburg’s history as a trading city. The Embrey Dam is a concrete Ambursen dam with slabs and buttresses with archways, and few of these Ambursen dams remain. A partial breach of the dam would leave the side chambers and abutments of the dam. This would allow for historic pieces to be left and slightly lowers the immediate cost. The later costs would have to include stabilization and maintenance of the abutments. Furthermore, if the historic crib dam was uncovered, it would be destroyed by exposure (Timmons, 1997).

With the high costs, complicated impacts, historic importance, and personal gain (Haberman 1995), many people think dam removal is not an option (Collier et al., 1996). Still, ideas are changing because of re-licensing, safety, fisheries, recreation on free rivers, less money for public works, and new technology (Haberman, 1995). The movement for dam removal is tearing down set ideas as fast as some dams.

 

References:

(Back to Top)

Chesapeake Bay Foundation. 1994. Chesapeake Bay passageways for migratory fish. 7 p.

Collier, M., R. Webb, and J. Schmidt. 1996. Dams and rivers: A primer on the downstream effects of dams. U.S. Geological Survey Circular 1126, 94 p.

Dewberry and Davis. 1990. Preliminary investigation and design for fish passage at Embrey Dam: City of Fredericksburg. Unpublished, Dewberry and Davis, 1990, Fairfax, 21 p.

Evans, L. 1986. "Floods are constant threat along the river" in The Free Lance Star. February 6. p. 28A.

Evans, L. 1997a. "Government dam-busting in Maine" in The Free Lance Star. December 5. p. A17.

Evans, L. 1997b. "Salmon will thrive without Elwha" in The Free Lance Star. December 5. p. A17.

Haberman, R. 1995. "Dam fights of the 1990s: Removals" in River Voices, v.5, # 4, p. 1+.

Timmons. 1997. Technical alternatives analysis to provide fish passage at Embrey Dam. Timmons, 50 p.

Trout, W. E. 1992. The Rappahannock scenic river atlas: A Virginia Canals and Navigation Society river atlas. 31 p.

Web Site:

Commonly asked questions about Elwha River restoration. 1997. Olympic National Park. http://www.halcyon.com/rdpayne/elwha.html

(Back to Top)