Prediction Level Assessment (PLA) Index
The prediction level assessment (PLA) is the most detailed level of investigation for slopes, sub-watersheds and river reaches previously identified as being high risk associated with sediment and/or river stability problems. Due their sensitive nature, the value of the resource, the severity of the adverse consequence of impairment, and other compelling reasons, it becomes necessary to conduct this level of analysis.
The PLA assessment utilizes "reference condition" that represents stable natural land and/or stream systems to compare direction, rate, nature and extent of departure from natural rates of sediment and/or natural stability using the same assessment methodology. These comparisons to reference sediment and stability condition are used to identify potential departure, and to document acceptable erosional/sedimentation rates.
A major benefit associated with PLA is the ability to link quantitative evaluation of sediment sources and/or river stability problems to an individual source at a specific location, affecting a particular process. The analysis results allow the user to identify proportional distribution of sediment yields, consequences of sediment on river stability, and the influence of river instability on sediment yields. This enables the design of very well-targeted, site-specific and process-specific management prescriptions. To address PLA's uncertainty of prediction, validation monitoring is a key objective for this level of assessment that compares predicted to observed values. The same monitoring approach can also determine the effectiveness of the mitigation.
The estimated time required to assess a 3rd order watershed using the PLA is measured in months (rather than days or weeks), depending on availability of basic data, maps, photographs, soils mapping, size of watershed, miles of stream length, experience of observer, and access to the site. This method relies heavily on field measurements relating to hillslope, hydrologic and channel processes.
Analytical Concepts of PLA
The PLA approach is to complete and compare the same field measurements and analyses for stable reference stream reaches as well as for potentially impaired reaches. The same analysis is applied first to stable reference reaches in order to calibrate the method and then to impaired reaches to quantify departure. The procedure may appear laborious, but with familiarity it can be accomplished within a reasonable schedule. This assessment is designed for trained and experienced individuals in geomorphology, hydrology, engineering, and related fields. All of the prediction methods included in this approach can be validated by process-specific monitoring procedures. If concern exists over the uncertainty of prediction, then validation monitoring will help provide better understanding and adaptively improve the prediction.
The methodology involves detecting sediment sources, measuring sediment sizes, and determining the ability of the stream to transport sediment without adverse channel adjustment. Sediment storage and routing dealing with seasonal and annual distribution and re-distribution of sediment would require a complex three-dimensional model. Rather than calculate storage, potential impairment is assessed by determining increases in sediment supply for the receiving stream channel. Calculation of competence (ability to move the largest particle made available) and capacity (ability to accommodate the sediment volume) is predicted. Potential aggradation/degradation based on changes in the sediment regime, channel characteristics and streamflow is a key portion of the assessment.The PLA methodology provides recommended models and analytical procedures to predict sediment and stability conditions, but allows the user to substitute a model with which the user is more familiar to complete a particular process prediction. It is important however, that the output variables of the substitute model are consistent with the methods presented for consistency of interpretation.
Many of the documents listed on this site are PDF files. Viewing a PDF file requires use of Adobe's free Acrobat Reader software. *EPA's PDF page provides information on downloading the software.
You may wish to download and print out the PLA flowcharts (PDF, 789 kb, 4 pp.) to view offline as you follow the online steps. In addition to a copy of the online PLA flowchart, this PDF file contains three other flowcharts that illustrate key components of PLA in expanded detail.