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Assessing Sediment Problems

Past and current land uses and future development plans of watersheds must contain commensurate assessments of cumulative watershed impacts. Applications of this watershed-based assessment methodology potentially include developing TMDLs (Total Maximum Daily Loads) for clean sediment, as well as the sediment management components of watershed plans. Numerical Water Quality Standards for non-toxic or clean sediment have not been set in many States, due to the great complexity and uncertainty of prediction under extreme natural variability over temporal and spatial scales. This complexity and variability has precluded the establishment of universal sediment standards, but nonetheless, widespread adverse sediment impacts on beneficial water uses require an organized, consistent, quantitative assessment to address the problems.

The WARSSS methodology is intended to provide the following:

  • A mechanism to put fundamental geomorphic principles into practice
  • A consistent, quantitative, comparative analysis which minimizes subjective bias
  • A watershed-based as well as specific river reach assessment
  • Linkages between various land uses and their associated sources of accelerated sediment supply
  • A procedure that will rapidly prioritize high risk sub-watersheds and river reaches at broad "screening" levels, yet ultimately provide for a more detailed assessment
  • Methods to assess the probability, risk and potential consequence of sediment problems in the presence of imperfect knowledge, large uncertainty and spatial and temporal variability
  • A basis for mitigation/restoration plans that isolates processes responsible for high risk/adverse sediment related consequence of watersheds/river systems
  • A companion assessment that can be related to aquatic habitat/sediment relations
  • A summary of output parameters that is useable for assessment of sediment consequences
  • A time-integrated procedure to assess sediment and stability problems of the past and present, and help set environmentally sound management direction for the future

WARSSS seeks to use hillslope, hydrologic and channel erosional/depositional process relations to identify specific sediment sources and their locations that may impact beneficial water uses, rather than to attempt to predict total annual sediment export from the watershed. When used to implement clean sediment TMDLs, WARSSS offers more emphasis on the potential, proportional contribution of sediment sources by various processes influenced by land uses than on model outputs that predict "absolute" total annual sediment yield. Although annual sediment yields are calculated at the PLA assessment level, the values are used to apportion relative contributions of various sources and to assess likely consequences as related to beneficial uses. This emphasis allows for an assessment that can directly lead to appropriate management, improved design specifications and mitigation prescription.

Applications of this methodology are appropriate for evaluating physical water resources and their condition for watershed planning. The WARSSS methodology, which addresses physical attributes, is nevertheless compatible with the integration of biological assessments such as fish habitat indices. Restoration designs also require a stability assessment that addresses the cause, consequence and correction of river impairment. The majority of the procedures integrated into the WARSSS methodology have been developed and applied for stability analysis for over 15 years in river restoration designs.

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Land use influences on river stability/sediment

Landscapes and stream systems like the lower reaches of Fall River are often set up for failure, depending on the nature of land uses that potentially impact their natural function. WARSSS assesses relative risks and consequences of sediment generation and transport influenced by past, current and potential future land use activities that contribute to erosional and depositional processes, including their effects on floods.

Surface disturbance activities, roads, streamflow and channel alteration, and major vegetation changes in the landscape and along watercourses can affect multiple processes leading to stream channel instability and corresponding change in sediment supply and transport. Stream channel adjustments due to channel instability (dis-equilibrium) often result in accelerated sediment yields and associated potential adverse consequences. The terms equilibrium and stream channel stability are often used interchangeably. Stability is defined as a river or stream's ability in the present climate to transport the streamflows and sediment of its watershed over time in such a manner that the channel maintains its dimension, pattern and profile without either aggrading nor degrading (Rosgen, 1996).

WARSSS includes a river stability component that addresses processes of flow and sediment changes below reservoirs and for streamflow modifications due to urban and non-urban change in the hydrologic response to various land uses. The products of accelerated erosion and deposition, as well as their associated impacts on river stability and on corresponding physical and biological function, are key issues. The natural, geologic, erosional and depositional processes need to be understood in order to determine acceleration of these processes or departure from natural conditions due to poor land use practices. WARSSS is designed to isolate the location of specific problem practices that contribute disproportionate amounts of sediment. The identification of such problems will help the user design process-specific mitigation measures. WARSSS is also designed to help land use planners avoid potential future problems and prioritize mitigation through changes in land use practices, restoration and/or rehabilitation needs.

The extent to which land use practices might generate sediment levels to the point of impairment needs to be assessed in a rigorous, consistent and quantitative manner. The stability of the lands and streams affected and the type of land use practice influence the risk associated with potential adverse sediment problems within a watershed. The major categories of erosional/depositional processes that are potentially influenced by a wide range of land uses are as follows:

  • Surface erosion
  • Mass wasting (slump-earthflow, debris torrents)
  • Stream channel instability/channel adjustments
  • Streambank erosion
  • Channel degradation/ incision
  • Channel aggradation/excess deposition
  • Lateral channel containment/confinement
  • Channel enlargement

Current sediment problems are not only a product of the various geologic erosional rates, but also are a legacy of historical land use activities. Many past land uses, such as placer mining at the turn of the last century, are responsible for channel adjustments that continue to add sediment to stream systems (Pickup et al, 1983, Pickup, 1988, James, 1991, and Riefenberger and Baird, 1991). Accelerated sediment deposition following poor farming practices in the Piedmont province of the Eastern United States aggraded many rivers. Today, with improved farming practices resulting in reduced hillslope erosion, these rivers are now routing excess sediments out of the system. To correct past sediment and stability problems, many projects to restore aquatic ecosystems have been implemented in recent years (National Research Council, 1992). For restoration projects to succeed, the geomorphic challenge is to differentiate adjustments in the fluvial system from natural versus anthropogenic influences. The sedimentological state is based on past events as well as current influences. An assessor must understand natural recovery following disturbance, evolution of morphological states, response of systems to imposed change, and quantifying the natural stable state.

Channel changes can be adversely affected not only from excess sediment but also by results of reduced sediment loads leading to degradation, loss of aquatic habitat and long-term stream channel instability. This has been well demonstrated in the literature due to "hungry water" degradation below reservoirs and clear water discharge from storm water drains in urban rivers. An equilibrium state or balance of energy and sediment supply is a central theme in the evolution of stable morphological forms and associated beneficial water uses.

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Assessment methodology discussion

It is necessary to isolate and locate the various processes associated with accelerated sediment supply in order to identify their relative contributions and to appropriately prescribe effective and practical mitigation. Not all land uses that present a potentially high risk for sediment problems occur on unstable lands or are necessarily associated with poor practices. As a result, it is necessary to identify the location, nature and extent of land uses that require an initial assessment of potential sediment source problems. The assessment analyst must know the following:

  • How do various land use practices generally affect sediment production?
  • How is increased sediment generated?
  • Can stream systems accommodate sediment increases/decreases? and,
  • What are the consequences of sediment changes and sediment related processes?
WARSSS is designed to provide the necessary tools to help answer these questions.

An initial screening approach is generally recommended in order to eliminate areas that most likely are not contributing or do not require special study and/or mitigation. Screening levels are designed to initially detect the "hot spots" or potential problem areas. Depending on the nature, severity, and/or potential consequence of the hot spots, generalized mitigation may correct or minimize the problem. A more detailed assessment, however, may be needed or required to identify specific measures for reducing site-specific potential sediment problems.

The reality of sediment-caused problems is widely accepted. Solutions, however, are often constrained due to political and economic issues as well as scientific disputes of cause/effect relations. Guidance to assist agencies, states, municipalities, and property owners on conducting sediment assessments is essential for consistency in the recognition and prevention of continuing sediment problems. Watershed/river assessments can provide managers with timely understanding to make informed decisions for management, mitigation and/or restoration Although some rightfully question such assessments when data are sparse, monitoring data should ease their anxiety of uncertainty and continually improve our understanding and prediction of sedimentological relations. Another great benefit of monitoring is to demonstrate the effectiveness of sediment reduction and improving river stability due to management/mitigation…the central purpose for watershed and sediment assessments.

There is currently no one model available that will accomplish all of the assessment protocols mentioned. There are, however, surface erosion components such as RUSLE or WEPP that have been modeled and other computer assisted iterations that speed up the mapping and analytical process. WARSSS is a "framework" for assessment that gives the user an option to select a model separate from those presented. The assumptions of various models preclude their universal applications for all conditions or sites. Thus, it is important to be certain that whatever model is selected for prediction, the output is relevant to the appropriate processes and site conditions. Many professionals have had extensive, positive experience in using and validating certain models within a given region, and these can be substituted in place of a particular process relation model contained in WARSSS. The data outputs, however, must be comparable for consistent, quantitative results. The procedures offered in WARSSS are not intended to preclude application of other models, but rather provide an example of how the relations presented can and have been utilized for sediment and river stability prediction at the watershed management level. The validation monitoring portion of the method will help users gain confidence in certain selected prediction models.

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Basic assumptions for the use of WARSSS

The importance of field measurement

The WARSSS methodology is not exclusively a desktop modeling procedure. It relies heavily on field data collection and corresponding analysis. It is essential due to the nature of this assessment methodology that the predictions be conducted by individuals with training and experience in geomorphology, hydrology, engineering and other scientific disciplines. Individuals should be specifically trained and experienced in hillslope, hydrologic and channel processes.

Identification, prediction and measurement of sediment sources.

The most direct and definitive approach to the quantification of sediment contributions is to make direct measurements at the locations of known sources. Since sediment is associated with a great variety of temporal and spatial variability, it requires considerable effort to

  • Identify natural variability with flow/season/geology
  • Determine contributing erosional processes
  • document land use influence on magnitude/duration
  • Understand consequences of sediment changes
  • Assess departure from a stable reference condition
  • Compare sediment from past versus existing land use
  • Quantify the influence of sediment source/stability on upstream and downstream reaches
  • Conduct measurements of stream discharge, velocity, slope, and channel characteristics to accompany sediment data for analysis

Thus, measurements must be accompanied by an assessment of hillslope, hydrologic and channel processes that identifies the nature, extent and location of specific erosional processes linked to past and present land uses. The assessment of potential departure from a reference condition requires measurements of sediment from several source locations including the following:

  • Above versus below specific channel/sediment source type and locations
  • Before versus after contributing source activities are initiated
  • Control or paired watersheds with concurrent measurements
  • Reference stream that represents the stable form of the same valley type, stream type and vegetative type

It is obvious that measurements must also be obtained in more than one location to address the beneficial uses of the whole water body. The challenge for users in collecting direct sediment and streamflow measurements at one location at one point in time, lies in the interpretation of the data. Without the stratification described above and data obtained over a wide range of flow conditions including bankfull discharge or normal high flow, rising limb and recession limb of both stormflow and snowmelt hydrographs, it is difficult to correctly interpret the sediment data. The high costs, specialized equipment required, difficulty of high water measurements and training and experience in standardized measurement techniques often preclude proper sediment data collection and analysis. Sediment measurements, however, can and should be obtained to determine the reliability of prediction to calibrate specific models used over large areas, and to determine the effectiveness of management/mitigation and/or restoration efforts at specific locations. Sediment measurements are not required in every location for assessment or to establish a sediment TMDL, but are necessary in those locations and specific river reaches where controversy, risk and consequences may be high.

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Using existing spatial data: GIS and remote sensing

It becomes important at all levels of assessment to obtain and interpret inventory information that identifies various potential land use impacts on a diverse landscape. Much of this initial inventory can be accomplished by the use of aerial photography, which is widespread, or multi-spectral and hyper-spectral imagery when available. The most recent data from satellite have improved resolution, frequency of coverage, and product availability, and are often suitable for landscape analyses in large watersheds. Appropriate systems for specific applications are summarized by Lillesand and Kiefer (1999).

It is increasingly commonplace to find GIS data such as land use/land cover or detailed drainage mapping from already completed remote sensing analyses, but new analyses more suited to sediment are also possible. For example, suspended sediment concentrations sufficient to affect spectral reflectance can be identified on aerial photography or imagery and validated through ground truth. A sediment source study conducted on the West Fork Madison River in Southwestern Montana utilized color-infrared photography taken from helicopter supplemented with ground truth in order to quantify sediment source areas during snowmelt runoff (Rosgen, 1973, 1976). Photo density readings from the transparencies of the color-infrared photographs were correlated with actual suspended sediment measurements taken over a wide range of concentrations, size of channels and elevation. The relations produced a correlation coefficient R2 of 0.95, which was highly significant at the 99 per cent level (Rosgen, 1973, 1976). Once this correlation was established, photo density readings could be obtained for 100 per cent of the streams during snowmelt runoff to determine sediment contributions by specific reach and sub-watersheds. This approach proved to be a cost-effective and reasonable method to determine sediment source areas over a large area in a short period of time.

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