Rivers of Sand

Nov. 1, 2002
Around the world, port districts are deepening shipping channels to accommodate global trade and its new international unit of measure: the container. The prospect of dredging brings up many questions related to erosion. Will turbidity increase? What will be the effects on aquatic life, shorelines, and river bottoms? Will contaminated sediments be disturbed? How can they be stabilized? Where and how will the sediments be disposed? In today’s regulatory environment, river dredging projects require much forethought and planning, and they rely on a unique set of best management practices (BMPs) to meet goals for protection of water quality, fish, and wildlife.The deep-draft ships that transport modular cargoes are fuel-efficient and can be operated by small crews. Some are as long as five city blocks and draw 40 ft. or more. They haul grains, logs, and raw minerals; cars, clothing, and myriad other manufactured items. They can be loaded and unloaded quickly. One large container ship can do the work of 100 barges, 1,500 railcars, or 6,000 semi-trucks, and its cargo can be transferred to these other modes of transport without being unpacked. In short, these superships are here to stay, and most port districts are deepening channels that already are artificially deep to take advantage of the several nested economies the container ships provide. A Historical Perspective
Animation of a clamshell bucket dredgeWater transport has long been the cheapest means of moving raw materials to processing plants and finished goods to market. It is no wonder that the world’s great cities have flourished on the banks of large, working rivers. In a thriving port city, as much as half the region’s economic activity might stem from the receipt, break in bulk, and reshipping of materials and goods through the port. Railroads, freeways, highways, waterways, and air routes radiate from water ports like the spokes of a bicycle wheel. These ancillary transportation systems allow raw materials to flow to industrial centers and allow manufactured goods to flow to markets near and far.Anyone who has lived for long in a city with a working harbor has seen the port undergoing continual growth and change in response to dynamic market forces. Turning basins are enlarged, terminals are added, docks are retrofitted, and waterways are reconfigured to accommodate ever-larger vessels. Such improvements have been going on in North America since the days when the kilt-clad boatmen of The Hudson’s Bay Company paddled pirogues up and down the streams of the New World. Snagging was among the earliest river channel improvements. This practice involved yanking or dynamiting logjams-sometimes miles long and scores of feet deep-from river channels to make way for vessels. The bedrock of riffle sections was blasted to improve the gradients of commercially used rivers. Shoals and shallow channels were dredged, then dredged again. Today, working rivers worldwide are being retrofitted so that ships with four-story drafts can navigate them and maneuver their 1,000-ft. hulls to berthing areas for servicing-not by swarms of stevedores packing grain sacks on their backs but by lone crane operators hoisting modular boxes from deck to wharf. Yet the prospect of deepening channels by just a few feet raises a host of questions about the environmental impacts of dredging and what can be done to avoid or lessen them.To answer these questions, project planners first need to get a handle on the nature of sediments that will be moved by dredging, particularly their size and quality. These variables will influence the equipment and materials they will select for the job, the design of the project, and its schedule and phasing. Sediment SizeDifferent sediment sizes are likely to be present along the shipping route between the ocean and the berthing area. The transition zone between the open ocean and a bay or river mouth is commonly a dynamic zone subject to complex patterns of longshore drift and bedload deposition. Here, sediments are likely to be sand-size. Closer to shore, the sands of barrier bars might be somewhat coarser, reflecting the higher-energy current and wave environments in which they form. The sediments of inland waterways tend to be finer, reflecting a lower-energy sediment transport environment. Harbors and berthing areas, being protected from direct ocean energies and river currents, tend to be sediment deposition environments. Here, fine materials settle out of the water column. The coarser materials being transported as river bedload also might come to rest in the quiescent water of the harbor.A large dredging project that will deepen a shipping channel from ocean to berthing area is likely to require a range of different dredging equipment to handle the different sediments. Various hydraulic dredges can operate with their cutting heads submerged in bottom sediments, thus minimizing turbidity generated by the dredging disturbance. Dipper and clamshell dredges might be capable of handling coarse sediments with little loss as material is excavated from the channel bottom and brought up to barges. For each setting, equipment needs to be carefully selected for the size of sediments to be handled and the distance and rate at which the sediments need to be moved.Sediment Quality
Dredges, such as the Yaquina on the Columbia River, work year-round to keep shipping channels open for deep-draft trade vessels from around the world.Sediment quality is a critical factor affecting the selection of dredging equipment as well as the means of its disposal. Legacy contaminants from past agricultural, industrial, stormwater, and sewerage practices have become topics of concern regarding the dredging of working rivers. In fact, they have put a bright spotlight on many of today’s river dredging proposals. Preproject sediment studies are increasingly required, and permit applications receive wide review.A number of concerns surface here: (1) the potential for resuspension of contaminated sediments, (2) the exposure risks to both people and wildlife in dredged areas, and (3) the exposure risks to people and wildlife at dredge spoil disposal sites. Contaminants of concern include metals, nutrients, and various organic compounds. Among the metals of concern are nickel, lead, zinc, mercury, and iron. Nutrients, whose uptake by aquatic plants can result in algal blooms and subsequent eutrophication, include nitrogen, phosphorous, and ammonia. The organic compounds, such as DDT, PCBs, and PAHs, are of concern because they bioaccumulate and, as a result, pose hazards to organisms that are highest in their food chain, including people. Thus, proposed dredging can open up a Pandora’s box of issues; for example potential conflict with local total maximum daily load standards and with the Clean Water and Endangered Species Acts.There do not appear to be any standard approaches for addressing contaminated sediments in proposed dredging projects. This is because each setting is unique with respect to the individual contaminants, their concentration and extent, the nature of the sediments involved, the river biota and water chemistry, and the dynamics of flow and sediment transport. In some cases, years of tests must be undertaken to arrive at scientifically sound conclusions about risk.
Dredging BMPsDespite these uncertainties, the erosion and sediment control industry has been developing BMPs for dredging. Administrative BMPs focus on informed and interdisciplinary planning, scheduling, and phasing. These practices seek to carry out the work at least-sensitive times of year for biota of concern or to take advantage of diurnal or seasonal conditions during which dredging can be accomplished with least disturbance to habitats and water quality. Selected administrative BMPs are summarized in Table 1.In-water practices focus on creating a separation between the dredged material and surrounding subaqueous environments. Selected in-water BMPs are shown in Table 2.Sediment Disposal
Almost universally, the earliest efforts to improve large rivers for commerce involved removal of shoals and wood jams.Beach nourishment, or the placement of dredged sand in the wave zone at river margins, counteracts shoreline erosion accelerated by the rolling breakers that rush to shore in the wakes of ships. The buffer of sand cushions the shoreline and back-beach vegetation from the erosive energy of the higher and more frequent waves generated by the passing vessels. It also provides a recreational resource-a wide sandy beach-that might not have been present before. Thus, disposal sites often are valued recreation resources.Such artificial beaches generally need to be replaced annually or biannually. Specific points along the shore are strategically selected as disposal sites for sediments generated through regular channel maintenance. When the beach can be augmented with coarse dredged materials, such as fine gravel, wave energy might not be capable of moving the sediments as quickly as sand, and less frequent nourishment might be needed. In any event, the sediments of the beach remain in the sediment budget of the river, a condition increasingly favored by scientists who study the relations between riverine habitats and bedload transport.Sediment is frequently transferred to the shore as a slurry in a pipeline. Care must be taken to filter the runoff before it reenters the river. This is usually done by confining the dewatering load behind a temporary dike or by means of sediment fencing or curtains. Upland placement is becoming more rare because available river-margin sites are becoming scarce along dredged working rivers.
Container ships have created a need for deeper shipping channels to enable deep-draft vessels to call at inland ports.In-water disposal of sediments is very common. This entails the release of dredged materials into bottom currents. This disposal technique is sometimes referred to as flow lane disposal or in-water disposal. In sand-bedded rivers, the released sediments are expected fall to the channel bottom fairly close to the point at which they were released. From here, they will continue to be transported by flows of varying stages. Dredged sediments also are frequently released into the open ocean. In both instances, releases are preceded by biological evaluations to ascertain that species or habitats of concern will not be affected and that shoaling will not create safety problems.In autumn, when flows in the lower Columbia River are at their lowest, a person can walk for miles along sandy beaches composed of dredge spoils. Here, where eagles wheel over the mile-wide river and the air is heavy with the scent of cottonwood, it is easy to forget that this river looks nothing like it did before dredging began almost a century and a half ago. A stranger here would not guess that an international port city lies just upriver. But in a little while, the thrum of powerful engines overtakes the small, chirping, lapping sounds of the river and the hull of a container ship glides into view. People on the beach stop what they are doing to look up in awe at the behemoth vessel. It takes long minutes to pass, finally sending huge breakers crashing up the beach.As the sun dips westward, people begin to straggle up the beach to their cars. They will drive along the peaceful levee road until it joins the blacktop and then the highway. On their way into the bustling city they will pass rail yards, refineries, shipping terminals, grain elevators, log decks, warehouses, and truckloads of imported cars. They make their way home as the lights begin to glitter in the office towers and on the bridges, not thinking that this settled and prosperous place has anything to do with maintaining a deep notch in a shallow channel that wends among islands of drifting sand.