Rapid Resource Inventory for Sediment & Stability Consequence (RRISSC)

Budget, time and scale considerations are realistic constraints in sediment and stability assessments. Thus, screening-level watershed assessments coupled with localized, process-based analyses are practical temporary substitutions for more extensive analysis throughout much of the watershed, guiding the greatest effort toward the most critical areas. Co-occurring sensitive landscapes, potentially unstable stream systems, and sediment-generating land use activities need to be identified, prioritized, and assessed for potential impacts at a level of detail beyond their initial flagging in RLA. The Rapid Resource Inventory for Stability/Sediment Consequence (RRISSC) is designed to provide this finer level of analysis. The time required to assess a 3rd order watershed using the RRISSC is measured in a few weeks (rather than days or months), depending on availability of land cover maps, aerial photographs, soils information, and access to land use history.

In contrast to the key areas mentioned, there are many land use activities that occur on stable landscapes associated with sound land management practices. Screening out such areas based on their limited risk of significant sediment yield reduces the level of effort needed during Prediction Level Assessment (PLA). It is possible for a risk rating system to be applied in a consistent manner over large areas in a short time frame. The risk rating system within the RRISSC phase of WARSSS requires analysis of the type and extent of land uses, the erosion potential of the landscape and channel, and the relationship of potential sediment sources to hillslope, hydrologic and channel processes. The RRISSC phase builds on the methods and results of the RLA phase in a number of ways. RRISSC begins with the subset of key areas (2nd - 3rd order Strahler sub-watersheds, reaches, specific landscapes) identified during RLA as potentially significant sediment sources. The majority of process relationships used in WARSSS are empirically derived from smaller watersheds and stream reaches. Using generalized process relations, RRISSC again reduces the number of these key areas that will be moved onward in WARSSS to the next, most intensive PLA phase. Like the RLA, RRISSC can be applied in a consistent manner over large areas in a relatively short time frame, although in finer detail. The RRISSC rating system's more detailed screening requires an inventory of the type, location, nature and extent of land uses as well as the sensitivity or erosion potential of the landscape and streams and potential sediment sources. Again like RLA, RRISSC highlights hillslope, hydrologic and channel processes.

Hillslope Processes

RRISSC analysis of hillslope processes is oriented toward land uses that influence sediment supply. The extent of certain land use activities is analyzed and linked to erosional risk categories. These land uses are overlain on soil/geology hazard maps to identify potential erodibility risks, revealing specific locations with potentially accelerated erosion and increased sediment levels. Sediment delivery potential will only be quantified in the upcoming PLA phase for those sites that have a high to very high risk of erosion. The moderate risk areas may need mitigation or stabilization to reduce erosion/sediment levels, when associated with a relatively low consequence of impact. The moderate risk and low consequence areas probably do not warrant the level of detail required in PLA. If serious questions arise, or if extremely high resource value is involved and/or serious potential consequences may result, then the analysis should advance to the highly detailed PLA phase. For those areas that merit advancing to the PLA level, one advantage is the ability to design process-specific mitigation measures backed by quantitative field data, rather than to apply generic measures that may or may not be effective.

Hydrologic Processes

The risk rating of hydrologic processes in RRISSC evaluates the potential for increased water yield and associated flow-related sediment increases. Rural watersheds are assessed in terms of percentage of the watershed in a modified vegetative condition (i.e. clearcuts or non-forested land cover types), whereas urban watersheds are evaluated mainly for percentage of impervious surface. The higher the percentage that is altered, the higher the potential for flow-related increased sediment supply due to channel enlargement and incision. Magnitude, duration and timing of flow are also important, including both natural and human-altered patterns such as operational hydrology of reservoirs and diversions. In addition, the risk ratings of increased flow are affected by the dominant stream types, which are grouped into risk categories by degree of susceptibility for increased sediment due to flow increases.

Channel Processes

Stream type classification plays an increasingly important role in RRISSC. For example, if a stream was described as a gravel-bed stream, and a question arose as to the response of this stream to increased flow, there would be a big difference in flow response of a B4 versus an F4 or G4 stream type (all being gravel-bed streams). The interpretations and influence of flow-related impacts, vegetation controlling influence, recovery potential and sediment supply all vary with channel morphology and stability. The risk rating system utilized for this part of the assessment relies on an understanding of differential response of stream types to imposed change (recall Table 2, PDF, 168 kb, 1 p.).

An individual could spend a great amount of time and effort running very complex bedload transport equations and doing factor of safety analysis on streambanks when the potential for instability and/or disproportionate sediment supply problems may be minimal. The use of rapid assessment methods that eliminate low-risk areas systematically is not designed to overlook problems, but rather to isolate those stream types with unstable conditions and other variables that may be observed in a consistent, objective, and reproducible manner. When in doubt, one may advance to the prediction level, gather quantitative field data and decrease the level of uncertainty.

The RRISSC flow chart shows the sequence of analysis used for the RRISSC assessment phase. Clicking on each of the steps indicated in the flow chart will open a web page with instructions for that step, many of which contain other links to tables, checklists, diagrams, photos, and other useful information. Closing these links will return the user to the RRISSC step where the link was posted.

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You may wish to download and print out the RRISSC flowchart (PDF, 392 kb, 2 p.) to view as you follow the online steps.

Begin RRISSC