Utah Lake, part 3: Dredging, islands, and crossings

 

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Part 3 of a series on Utah LakeThe following observations and insights concerning Utah Lake have been developed during my nearly 50 years in Utah Valley as a professor, researcher, environmental engineer, consultant, and local citizen.


At times, one hears speculation about dredging the lake with the goal of converting it into a nice, clear lake. In fact, many years ago the Utah County Commission purchased a used dredge to use in the lake. Fortunately, the plan was abandoned when insurmountable transportation problems arose. Also, projects to develop direct transportation links across the lake have surfaced from time to time. The feasibility of such projects depends on many, many factors, but the nature of the lake itself must always be carefully considered if such projects are to have any possibility of success.

A Primer on Lake Stratification

In this climatic region, when clear, ponded water is deeper than about 20 feet, summer thermal stratification is common and often persistent. The top 10 to 15 feet of surface water is considerably warmer and doesn’t mix with bottom waters for weeks or sometimes even months. With stratification, ongoing natural decay of accumulated organic debris at the bottom often depletes oxygen—first at the bottom and then upward in the overlying water. Under these conditions the anoxic water at the bottom is stagnant and becomes septic. These anoxic conditions kill normal aquatic organisms and sometimes even kill fish if they can’t escape to refuge areas still containing oxygen, e.g., near the surface or near inflowing streams or springs. Usually, aesthetics, water quality, and aquatic habitat deteriorate significantly when persistent stratification occurs, particularly in eutrophic lakes. In turbid lakes more sunlight energy is absorbed near the surface and thermal stratification can occur in shallower water, perhaps only 12 to 15 feet deep.

Dredging to “Clear Up” the Lake

In Utah Lake frequent wave action mixes water to about 10 to 12 feet deep. If the lake were deeper than about 12 feet, settled precipitated-minerals would tend to stay on the bottom thus reducing turbidity. As a result, the lake would probably be clearer in early spring and late fall, but during the summer would suffer large scum and biological turbidity increases from heavier algae growth driven by the increased light availability. In fact, the lake likely would often become “pea soup” from algal growth and suffer a major deterioration in quality. The most damaging effect would likely be episodes of oxygen loss during decay of algal blooms. Stresses on other aquatic life, high biological turbidity—rather than mineral turbidity—and noxious odors are common problems accompanying the growth and decay of excessive algae. Incidentally, in Utah Lake beneath winter ice, clear water occurs since with no wave action flocculent sediments settle to the bottom while algae growth is limited by low temperatures and reduced light due to the ice-snow cover.

Maximum Dredged Depths

When full, the lake now averages about 9 feet deep with its deepest parts 13 to 14 feet deep. To avoid a persistent thermal stratification problem in the summer, the maximum dredged depth would need to be limited to about 16 feet. Then as drawdown occurred through into the summer, the normal drawdown of about 3 feet would result in maximum depths of about 13 feet thus preventing persistent stratification. If dredged to 16 feet deep, the lake’s average depth (and water volume) could be increased about 75%—but the volume of dredged sediments would be gigantic. If only a relatively small area were dredged, the dredged area would still be turbid since water circulating from other areas would carry re-suspended sediments. Small dredged areas would tend to rapidly fill back in since the relatively quiescent bottom would foster accumulation of settling sediments.

In addition to its water quality and habitat problems, large scale dredging is not likely for a variety of ecological, engineering, and economic reasons. For example, dredged bottom sediments are clayey—when exposed to the air to dry, they shrink, crack, and become very hard, but when wet they swell and become mucky.

For large-scale dredging, the massive amount of dredged muck would be a colossal disposal challenge. If the entire lake were dredged an average of just two feet deeper, the dredged material could cover an area 5 miles wide and 5 miles long to a depth of about 10 feet. But proactively, this amount of dredged material could be used to construct islands with total area of perhaps 7 square miles–requiring some 25 to 35 feet of initial fill that, after a few years, would settle and result in islands standing about 10 feet above a full lake.

In summary on dredging, if the lake were deeper than about 16 feet, lake water would be clearer in the early spring followed by increased algal growth during the late spring and summer. Likely lake stratification coupled with heavier algae growth would likely cause oxygen depletion problems and bad odor events. The dredged areas would hover back and forth as latent biologically-dead zones.

Islands

Construction of wildlife-reserve, residential, and recreational islands in the lake, with some linked together by causeways or bridges, is appealing in some ways. Sale of some constructed islands for residential and commercial use could provide hundreds of millions of dollars to fund the island-building projects and perhaps other lake development, recreational, and environmental-enhancement projects. One or more islands reserved as wildlife refuges would be great wildlife (and ecological) assets—rubble and rock shorelines could be used to enhance lake fisheries; the open land could host numerous plants and animals. Dredged sediments could be used for most of the needed fill material; top soils would need to be added for vigorous growth of grass, trees and other vegetation. Following initial construction, a few years of settlement would be needed before beginning facility construction. Initial fill would need to be some 25 to 35 feet above the lake bottom, to order to result in fairly stable ground levels at least 10 feet above full-lake elevation.

Ferries, Road Causeways, and Bridges

Long-anticipated developments and suburban growth is continuing on the west side of the lake. Some think that growth would accelerate considerably with more direct accesses to economic hubs on the east side.

Ferries that could carry passengers, cars, trucks, and other cargo have been suggested in the past, since their cost is relatively small compared to crossings. However, shallow depths, seasonal and year by year fluctuations in water depth, and rather large waves—relative to the limited size of ferries that could be considered—largely rule out ferries, although some types of hovercraft might be operationally feasible.

Lake crossings could be constructed but numerous environmental, engineering and financial problems exist. They represent major engineering and construction challenges and would be very expensive—likely costing over a half-billion dollars for a major crossing near the middle of the lake.

Technically, crossings could be built; practically and economically, it would be very difficult since the bed of Utah Lake is a very poor foundation. If a fill causeway were used for a highway, it would need a very wide base (several hundred feet wide) or would need to be placed on driven piles. The roadway surface would need to be some 10 feet above the full-lake level to protect it from wave action, and to also protect it from major ice-sheet movement. (Occasionally in early spring, as the ice breaks up, wind-driven ice sheets can stack up 10 feet high, or more, along the shoreline and even be shoved inland several hundred feet. However, this condition is rare and short-lived—perhaps a few hours every few years).

When loaded with the weight of a fill causeway, the lake bed underneath would settle several inches a year for many years—decreasing as underlying sediments compacted. Uneven settlement would occur in places and result in an undulating road surface; problems of pavement cracking and breakup might be persistent. Two or three short bridges and many very large culverts would be needed along a fill causeway to allow for good water and aquatic biota circulation through the causeway to minimize adverse impacts on the lake’s ecosystem and recreational use. Likely the best solution both structurally and environmentally would be a bridge roadway, supported on pilings driven into the bottom, perhaps used together with structural floats if underlying sediments are not sufficiently strong for piles alone to support the bridge. Engineering studies would determine how deep piles would have to be driven into the lake bottom—perhaps 50 to 100 feet. Also, at perhaps two locations, higher spans would be needed for up to medium-size sail boat passage—the keels of large sail boats are too deep to operate in this shallow lake.

A generally north–south causeway or bridge across Provo Bay would have less-challenging foundation problems than across the main lake, since sediment layers there contain more stable soils, particularly more sand and gravel layers, than under the main lake. However, if a dike causeway were used to control water levels in Provo Bay at levels much different than the main lake, the project would need a large hydraulic and pumping station(s) component—flood bypass channels would have to be built for Hobble Creek and other tributaries. The pumping stations large enough to pump flood-year water over the dike would be very costly.

Reclaiming Some of the Lake for Agriculture, Land Developments and/or Additional Wildlife Habitat

At this point in our national experience, most lake and wetland areas are considered too valuable to allow further significant encroachment or destruction. Current environmental laws and requirements make it very difficult to dike or fill shoreline and wetland areas. Though very unlikely, if at some point diking and dewatering were considered to “reclaim” land, only the Provo Bay area has bottom sediments (soils) somewhat suitable for farming. But Provo Bay is one of the lake’s most important wildlife areas and mitigation would be more difficult. The rest of the lake has clayey, marl sediments (mainly calcium and carbonate) that are not suitable for farming or most other “land” uses.

Major environmental issues would arise with dewatering projects of this type. Given current environmental requirements, stabilized water levels would be needed to support the large “replacement” bay and wetland areas. Therefore, the net outcome would likely be little, if any, additional lands for farming or development, as well as little, if any, water savings from reduced evaporation. Also, diked areas might suffer up to several feet of flooding for a few months about every 30 years or so during the peak of wet cycles since the cost of dikes high enough to isolate diked areas from high lake levels, along with the cost of standby pumping or bypass channels for tributaries to keep the diked area water levels low, would be prohibitive.

LaVere B. Merritt

LaVere B. Merritt is a professor emeritus of civil and environmental engineering at Brigham Young University. His research and public service have included many multidisciplinary studies on Utah Lake. He served as member and then chair of the Provo Metropolitan Water Board for many years and is a consultant to both public and private entities on Utah Lake matters.

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