Agenda - 03-25-1986 - Laserfiche WebLink

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` T Considerable effort has been expended by the Department of Natural Resources and Community Development to model Orange County's water resources and to determine a desirable instream flow for the Eno River. The Office of Water Resources has stated that a minimum release of about 5.4 cubic feet per second, or 3.5 MGD, is required from Lake Ben Johnson to meet instream flow needs. This is much in excess of the statistical 7-day, 10-year low flow for the Eno River, which the has been variously reported to be between 0.169.cfs to 1.7 cfs. Apparently, the 7Q10 has not been established. For comparison, HISARS flow data for the Eno River near Hillsborough show that in 1941 there were 8 consecutive days when flow was 0.3 cfs or lower and 9 consecutive days in 1954 when flow was 0.10 cfs or less. Differences between the reported low flows must be resolved and the impact on water supply capacity determined before a decision concerning instream flow requirements is reached. The limitations and other demands on stored water must be carefully examined prior to establishing the instream flow comnittment. Given the limited capacity of present water supplies and the fact that the primary purpose of these supplies at the time of construction did not include an instream allowance, OWASA supports the assurance of a 0.39 ads.-1440 instream flow above the Hillsborough Water Plant. The discharge from the 2 MGD treatment plant will increase the stream flow to over 2,0 cfs. which is above the 1.7 cfs flow suggested by the State. As future supplies are planned and developed, requirements for maintaining instream flows can be incorporated into project design, construction and financing. The State has completed an extensive computer evaluation of the Eno River flow. This model was used as a basis for recommendations presented by the Office of Water Resources. A better understanding of the model, including all of the assumptions utilized in running the model is needed to adequately evaluate the recommendations of the State. It is also important to evaluate the need to maintain flow in the Eno River at the Hillsborough wastewater treatment plant site. The State has based the issuance of the Town of Hillsborough's wastewater discharge permit on the assumption that the 7Q10 flow, reported to be 1.7 cfs, will exist. The permit as written indicates the Town's wastewater plant has a capacity of 3.0 MGD; however, the actual capacity is only 2.0 MGD and present plant flows are only about 0.9 MGD. The need to maintain a flow of 1.7 cfs to maintain water quality downstream of the Hillsborough plant needs to be reevaluated in light of the actual plant capacity and flows. For comparison, the OWASA WWTP with similar standards but 8 MGD capacity is based on only 0.25 cfs in the receiving stream. It will be beneficial to evaluate the water quality impact of instream flow release through the use of water quality and stream flow simulation models prior to any final decision on the amount of such release. The State uses water quality computer models to determine the NPDES Permit limits which are needed to protect water quality at the 7Q10 flow. With further knowledge and evaluation of these models, local governments will be better able to understand the basis for decisions which significantly affect the provision and cost of water and wastewater services and the need for capital improvements such as reservoirs and treatment plants.