AWWA WQTC65708

The City of Oregon City's Mountain View Reservoir #2 is a 10.5 MG finished water reservoir. The facility was originally constructed in 1915 as an open 4.0 MG facility. The original facility design was a rectangular, concrete, in-ground reservoir with a single inlet/outlet (I/O) configuration at one corner of the reservoir. Operationally, the facility was originally designed in conjunction with a pumping station housing two pumps to fill and draw on the reservoir through the common 18-inch vertical, floor elevation I/O. In 1952, the reservoir was expanded, by adding reinforced concrete walls above-grade, to its present capacity of 10.5 MG. At that time an emergency auxiliary outlet and pump station with separate power generation was added to the reservoir for redundancy and reliability. The auxiliary outlet was designed at elevation that only allowed access to the newest 6.5 MG of storage. The original 4 MG was unavailable through the auxiliary outlet. Typical operation of the reservoir maintained water levels within three ft. range of the overflow. Due to the tight operational range freshwater brought into the tank during fill cycles was, at a maximum, only 14% of the total capacity. Field monitoring of the reservoir had not historically been conducted. However, CFD modeling of the original configuration of the reservoir showed that minimal mixing was occurring and hydraulically the reservoir was short-circuiting in a "last in, first out" manner. Structurally the facility did not meet current codes for seismic stability and in 2005 seismic improvements were planned. During design of seismic upgrades to the reservoir it was decided to rectify water quality problems resulting from the common I/O design, primarily loss of chlorine residual within the reservoir due to minimal mixing and hydraulic short-circuiting. To address the problem the City requested solutions for separating the common I/O to improve chlorine residual maintenance, mixing and reduction of short-circuiting and "dead zones." The I/O improvements separated the inlet and outlet by extending the existing auxiliary outlet to a 24-inch diameter inlet 240 ft. into the reservoir. A 90º degree elbow was installed and the inlet pipe extended down the slope and to the floor of the original 1915 reservoir. The new manifold was now located approximately 240 ft. from the outlet and 80 ft. from the back wall. The I/O improvements were combined with the replacement of the primary and auxiliary pumping stations with a single pump station. This provided the capability to maintain the existing common I/O intact and hydraulically convert it to function solely as the reservoir outlet. The reconfigured fixed directional inlet manifold was designed to distribute fill water across the reservoir horizontally while at the same time the lower elevation mixed the water vertically. To maximize the hydraulic momentum of the inlets, reducers were added to create discharge jets. The separation of the inlet and outlet provides the operational flexibility to operate the reservoir in a fill then draw mode, or in a continuous fill-draw mode. CFD modeling results of the relocated inlet and manifold designed to promote mixing show a more mixed condition in the reservoir. Actual field testing has not yet been conducted. Includes figures.