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H. Wayne Harper, PE
State water quality agencies typically impose turbidity standards on pipeline construction across water bodies primarily because it is a widely used water quality measurement, is easy to determine in the field, and provides instantaneous feedback to regulatory personnel. However, most state water quality regulations pertaining to turbidity were originally developed for use with chronic long-term point-source discharge situations. The use of these criteria without some adjustment for the short-term nature of construction projects may be a mis-application of the basic concepts behind their original intent. Additionally, regulatory personnel will often use turbidity data to infer fishery impacts. Turbidity, however, has a lesser biological effect on fish than does its often-related measurement, suspended sediment. Portland Natural Gas Transmission System (PNGTS)/Northern Ecological Associates, Inc. (NEA) used established engineering models and grain size analysis to conduct a detailed study of turbidity and suspended sediment dynamics caused by pipeline construction across streams.
To predict total suspended sediment (TSS) distribution and transport, PNGTS/NEA developed scenarios for typical waterbody crossings by assuming representative stream characteristics including: width, cross-sectional area, bed composition, mean velocity, estimated transport distances, material lost during excavation, and the increase in suspended solids expected downstream of the crossing. PNGTS/NEA used sediment grain size analyses that were collected from representative stream crossings as input parameters in the model. PNGTS/NEAs estimates were then input into Trows 1996 model to estimate sediment dispersion for three stream types: low, medium, and high energy. Predicted suspended sediment values were then used to determine lethal and sublethal fishery impacts using Newcombe and Jensens mathematical model which assigns a Severity of Ill Effect (SEV) value for fish species guilds based on dose (TSS/ml) and duration (hours) of exposure. The results of this analysis were used in negotiations with state regulatory personnel to help describe potential realistic fishery impacts, rather than hypothetical effects that may be caused by elevated turbidity values.
This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.
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