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Evaluation of alternative design flow criteria for use in effluent discharge permitting




Paulson, Cynthia L., author
Sanders, Thomas G., advisor
Ward, Robert C., committee member
Evans, Norman A., committee member

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A design flow Is the value used to represent upstream or dilution flow in the calculation of effluent permit limits under the National Pollutant Discharge Elimination System (NPDES). The7Q10 low-flow statistic, or the 7-day moving average low flow that occurs once every ten years on the average, has traditionally been used for design flows. Recently, alternative approaches to conventional NPDES permitting techniques have been Investigated. The objective of this study was to research alternative design flows that would maximize use of the assimilative capacity of receiving waters while also maintaining water uses. This study addressed two major aspects of design flows. The first was the definition of a set of recommended methods to use in the calculation of design flows. These methods were gathered from the literature or developed in the process of the study. The second aspect was a comparison of alternative design flows and recommendation of guidelines to use in selecting appropriate values. Data at eight sites on streams along the Front Range of Colorado were analyzed. Three types of analysis were applied to define design flows for acute and chronic conditions. Traditional frequency/duration statistics were calculated on an annual, monthly and seasonal basis. An empirical, distribution-free approach developed by the U.S. EPA, called the biologically-based method, was also applied. A simplified version of this method, termed excursion analysis, was developed to augment the Information supplied by the biologically-based approach. Design flows calculated with these methods were related to acute or chronic durations and allowable frequency criteria recommended by the U.S. EPA for the protection of aquatic life. The results of the research highlighted the need to establish a standard set of methods to use in the calculation of design flows. Some methods were recommended while other areas requiring more research were pointed out. The lack of long flow data records above effluent discharge points, where NPDES permit limits are calculated, is a major problem. The results of the flow analysis showed that distribution-based frequency statistic flows do not relate as directly to aquatic life criteria as biologically-based design flows. The level of protection provided by frequency statistic flows varies widely from site to site, while biologically-based flows provide relatively consistent levels of protection. Seasonal or monthly design flows can be used to Increase the use of stream assimilative capacity. However, it was shown that the number of excursions, or flows below a given design flow, was substantially higher for monthly frequency statistic flews than annual values. The implication of this result Is that more stringent design flows may be required on a monthly basis, depending on the seasonal needs of aquatic life populations, and other water uses. The critical importance of design flow criteria, based on aquatic life protection, was emphasized in this study. Once criteria have been chosen, the selection of an appropriate annual design flow is relatively straightforward. The biologically-based method, or a similar approach that relates directly to use criteria, is recommended as a better alternative than the 7Q10. Seasonal or monthly flows are also recommended, but will require further research.


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Stream measurements
Sewage disposal in rivers, lakes, etc. -- Mathematical models


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