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For example, in some situations, the large scale rainstorms associated with hurricanes may greatly impact sediment loading to the water body through erosion of watershed soils, but have little effect on stability of the in-water sediment bed itself. When considering the potential impacts of disruptive forces on sediment movement, it is important to assess these forces as they relate to the overall watershed and in terms of current and future site characteristics.
Many site characteristics affect sediment movement, but primary among them are the flowinduced shear stress at the bottom of the water body during various conditions, and the cohesiveness of the upper sediment layers. In most environments, bottom shear stress is controlled by currents, waves, and bottom roughness (e.g., sand ripples, biologically formed mounds in fines). A preliminary evaluation of the significance of sediment movement should include at least site-specific measurements of surface water flow velocities and discharges, water body bathymetry, and surface sediment types (e.g., by use of surface grab samples).
In some cases, empirically measured erosion rates are lower than anticipated from simple models, due to natural armoring. Winnowing (suspension and transport) of fines from the surface layers of sediment is one common form of armoring. Others are listed in Highlight 2-9, including the effect known as “dynamic armoring,” which describes the effect caused by suspended sediment or a fluff, floc, or low density mud layer (present in some estuaries and lakes) that decreases the expected erosion rate of underlying sediment.
• Compaction of fine-grained sediment
• Chemical reactions and weathering of surface sediment
• Suspended sediment dampening turbulence during high flow events
• Physical protection and sequestration by rooted aquatic vegetation • Mucous excretions of polychaetes • Erosion-resistant fecal pellets or digested sediment 2-26 Chapter 2: Remedial Investigation Considerations Sediment properties that affect cohesion and erosion in many sediment environments include bulk density, particle size (average and distribution), clay mineralogy, the presence of methane gas, and the organic content. It is not unusual for erosion rates to vary by 2 to 3 orders of magnitude spatially at a site, depending on currents, bathymetry, bioturbation, and other factors (e.g., pore water salinity). In a fairly uniform cohesive sediment core, erosion rates may drop several orders of magnitude with depth into the sediment bed, but in more variable cores this may not be the case.
Biological processes by macro- and microorganisms also affect sediment in multiple ways, both to increase erosion (e.g., gas generation and bioturbation by lowering bulk density) and to decrease erosion (e.g., aquatic vegetation, biochemical reactions which increase shear strength of sediment). The process of sediment mixing caused by bioturbation is discussed further in Section 2.8.3.
A wide variety of empirical methods is available to assess the extent of past sediment and contaminant movement. Highlight 2-10 lists some key examples. Each of these methods has advantages and limitations, and generally none should be used in isolation. The help of technical experts is likely to be needed to determine which methods are most likely to be useful at a particular site.
2.8.2 Routine and Extreme Events
Naturally occurring hydrodynamic forces such as those generated by wind, waves, currents, and tides, occur with great predictability and significantly influence sediment characteristics and movement (Hall 1994). While these routine forces seldom cause changes that are dramatically visible, they may be the events causing highest shear stress and, therefore, the most important factors in controlling the physical structure of a given water body. In northern climates, formation of ice dams and ice scour are also routine events that may have significant effects on sediment. It is important to note that seasonal changes in water flow may also affect where erosion and deposition occur. Depending on the location of the site, (e.g., riverine areas, coastal/marine area, inland water bodies), different water body factors will play important roles in determining sediment movement. To determine the frequency of particular routine forces acting upon sediment, project managers should obtain historical records on flows and stages from nearby gauging stations and on other hydrodynamic forces. However, project managers should keep in mind that residential or commercial development in a watershed may significantly increase the impervious area and subsequently increase the frequency and intensity of routine flood events. While the intensity of most routine forces may be low, their high frequency may cause them to be an important influence on sediment movement within some water bodies.
Highlight 2-10: Key Empirical Methods to Evaluate Sediment and Contaminant Movement Bathymetry (evaluates net change in sediment surface elevations) • Single point/local area devices • Transects/cross-sections (with known vertical and horizontal accuracy) • Longitudinal river profiles along the thalweg (i.e., location of deepest depth) • Acoustic surveys (with known vertical and horizontal accuracy) • Time-series observations (event scale and long-term seasonal, annual, decade-scale) • Comparison of core pattern or changing pattern in surface sediment, with pollutant loading history • Patterns of grain-size distribution (McLaren and Bowles 1985, McLaren et al. 1993, Pascoe et al. 2002) • Cs, lignin, stable Pb (longer-lived species to evaluate burial rate and age progression with depth) Pb, 7Be, 234Th (shorter-lived species to evaluate depth of mixing zone) • • Land and water body geometry and bathymetry; physical processes
• Upstream and tributary loadings (grain size distributions and rating curves) • Tidal cycle sampling (in marine estuaries and coastal seas) • Sampling during the rising limb of a rain-event generated runoff hydrograph (frequently greatest erosion)
Dissolved contaminant movement:
• Seepage meters at sediment surface
2-28 Chapter 2: Remedial Investigation Considerations In contrast, some water bodies are significantly affected by short-term extreme forces that are much less common. In many cases, these “extreme” forces originate by the same mechanisms as “routine” forces (e.g., wind) but are significantly stronger than routine conditions and capable of moving large amounts of sediment. Some extreme events, however, have no routine event counterparts (e.g., earthquakes). Meteorological events, such as hurricanes, may move large amounts of sediment in coastal areas due to storm surges and unusually high tides that cause flooding. Flooding may occur from snowmelt and other unusually heavy precipitation events resulting in the movement of large amounts of upland soil and erosion of sediment, which are then deposited in other areas of the water body or on floodplains when the flow slows during the falling limb of the runoff hydrograph. Scour of the sediment bed may also result from the movement of ice and/or natural or man-made debris during extreme flood events. To obtain a preliminary understanding of extreme event frequency at a site, it is important to examine both historical records (e.g., meteorological and flow records) and site characterization data (e.g., core data and bathymetry).
Floods are frequently classified by their probability of occurrence; for example 50-year, 100-year, 200-year, and probable maximum flood. Although the term “100-year flood” suggests a time frame, it is in fact a probability expression that a flood has a one percent probability of occurring (or being exceeded) in any year. Similarly, 200-year flood refers to a flood with a 0.5 percent probability of occurring in any year. Probable maximum flood refers to the most extreme flood that could theoretically occur based on maximum rainfall and maximum runoff in a watershed. It is not uncommon for multiple low probability events to happen more frequently than expected, especially when the hydrograph record used to determine these probabilities is not very long or where land use or climate is changing.
It is important to consider the intensity of extreme hydrodynamic forces as well as their frequency. Intensity is a measure of the strength, power or energy of a force. The intensity of a force will be a significant determinant of its possible impact on the proposed remedy. Tropical storms (including hurricanes) are often classified according to their intensity, that is, the effects at a particular place and time, which is a function of both the magnitude of and distance from the event. Tropical storms such as hurricanes are commonly classified by intensity using the Saffir-Simpson Scale of Category 1 to Category
5. Other physical forces and events, such as earthquakes, may be classified according to magnitude, that is, a measure of the strength of the force or the energy released by the event. Earthquakes are most commonly classified in this way (e.g., the Richter scale) although they may also be classified by intensity at a certain surface location (e.g., the Modified Mercalli scale).
For sites in areas that may be affected by extreme events, project managers should assess the record of occurrence near the site and determine the appropriate category or categories for analysis. The recurrence interval that is considered in a project generally relates to the magnitude of the resultant impacts. The choice of design event gives consideration to the impact of the event and the cost of designing against the event. For evaluation of contaminated sediment sites, project managers should evaluate the impacts on sediment and contaminant movement of a 100-year flood and other events or forces with a similar probability of occurrence (i.e., 0.01 in a year). A similar probability of occurrence may be appropriate for analysis of other extreme events such as hurricanes and earthquakes. At some sites, it may be appropriate to analyze the effects of events with lower and higher probabilities to understand the cost-effectiveness of various design decisions. Recorded characteristics of physical events, such as current velocities or wave heights, may provide project managers with parameters needed to calculate or model sediment movement. If information from historical records is insufficient or the historical record is too short to be useful, project managers should consider obtaining technical assistance
to model a range of potential events to estimate effects on sediment movement and transport. Section 2.9 of this chapter discusses modeling in more detail.
2.8.3 Bioturbation In some depositional environments, the most important natural process bringing contaminants to the sediment surface is bioturbation. Broadly speaking, bioturbation is the movement of sediment by the activities of aquatic organisms. Although this movement may be in many directions, it is the vertical mixing that is mainly of concern for project managers because it brings contaminants to the bed surface, where most exposures occur. While many discussions of bioturbation are focused on sediment dwelling animals, such as worms and clams, bioturbation may also include the activity of larger organisms such as fish and aquatic mammals. The effects of bioturbation can include the mixing of sediment layers, alteration of chemical forms of contaminants, bioaccumulation, and transport of contaminants from the sediment to interstitial/pore water or the water column. Many bottom-dwelling organisms physically move sediment particles during activities such as locomotion, feeding, and shelter building. These activities may alter sediment structure, biology, and chemistry, but the extent and magnitude of the alteration depends on site location, sediment type, and the types of organisms and contaminants present.
One factor of concern for understanding exposure is the depth to which significant physical mixing of sediment takes place, sometimes known as the “mixing zone.” The depth of the mixing zone can be determined by examination of sediment cores (especially radioisotope analysis of core sections), or other site characterization data that displays the cumulative results of bioturbation through time, but useful information may also be gained from a sediment profile camera and other results. It is also useful to be aware of the typical burrowing depths of aquatic organisms in uncontaminated environments similar to the site. Project managers should keep in mind, however, that population density has a tremendous effect on whether organisms present at the site may have a significant effect on the mixing zone. It is important to understand the depth of the mixing zone in the various environments at a site because, where sediment is not subject to significant erosion and contaminants are not significantly mobilized by ground water advection, contaminants below this zone are unlikely to contribute to current or future risk at a site.
Typically, the population of benthic organisms is greatest in the top few centimeters of sediment.
In fresh waters, the decline in population density with depth is such that the mixed layer is commonly five to 10 cm deep (NRC 2001), although it may be deeper, especially in marine waters with high populations of deep burrowing organisms. Highlight 2-11 provides examples of organisms that cause bioturbation, their activity type, and the general depth of the activity. However, project managers should also consider the activity type, the intensity of the activity, and organism population density, when determining the extent bioturbation should be considered in site evaluation. For example, the depth and effectiveness of bioturbation may be very different in a highly productive estuary and in a heavily used commercial boat slip.
A project manager should be aware of at least the following parameters when assessing the depth
of the mixing zone and the potential role bioturbation will play on a given sediment bed:
• Site location - Salinity, water temperatures, depths, seasonal variation);
• Sediment type - Size distribution, organic and carbonate content, bulk density); and 2-30 Chapter 2: Remedial Investigation Considerations
This analysis may be done for naturally deposited sediment as well as potential in-situ capping material or dredging backfill material. Where bioturbation is likely to be a significant process, it is important to evaluate the depth over which it causes significant mixing, using site-specific data and assistance by technical experts, to assess alternative approaches for the site.
2.8.4 Predicting the Consequences of Sediment and Contaminant Movement Depending on its extent, movement of sediment or contaminants may or may not have significant consequences for risk, cost, or other important factors at a specific site. A number of differing factors may be important in determining whether expected or predicted movements are acceptable. Historical records or monitoring data for contaminant concentrations in sediment and water during events such as floods may be valuable in analyzing the increase in exposure and risk. Where this information is not available or has significant uncertainty, models may also be very useful to help understand and predict changes. This analysis should include increased risk from not only contaminant releases to the immediate water body, but wherever those contaminants are likely to be deposited. Increased cost may include remedy costs such as cap repair or costs related to contaminant dispersal, such as increased disposal cost
of downstream navigational dredging. There may also be societal or cultural impacts of contaminant releases the project manager should consider, such as lost use of resources.