25F - AGMT - DELHI DIVERSION PROJECT

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Preliminary Design Report Addendum No.2 3.0 RECOMMENDED PRELIMINARY TREATMENT ALTERNATIVE This section provides discussion of the preliminary treatment system. The preliminary treatment system will consist of an in- channel flow diversion structure, intake structure, coarse screening, fine screening, sedimentation chamber, storage chamber, a conveyance pump station, access manhole /cleaning structure, outletibypass structure. Detailed discussion of the force main will be discussed separately in Section 4. 3.1 IN- CHANNEL DIVERSION STRUCTURE The in- channel diversion structure will capture up to 100 cfs of water to a maximum depth of 8 feet in the Delhi Channel. The in- channel flow diversion structure will be in the closed position during all dry- season periods and pre /past- winter storm events. During the winter season, the diversion will remain in the closed position to promote near continuous trash and debris screening of all urban discharge events. During the rainy season, from October 15th to April 15s the in- channel flow diversion structure will remain closed during the first flush event of any impending storm event (approximately 15 minutes after the start of the storm) before moving into the open position. Upon the passing of a storm event, the diversion can be placed back into service once the stream flow events have subsided and returned back to typical urban flows. Two types of diversion structures have been considered: radial steel gates and an air - inflated rubber dam. 3.1.1 Radial Steel Gates Radial gates are designed for a wide, clear waterway opening. Radial gates are usually applied for diversion of water where wide, clear waterway openings are necessary and where economical control of water is important. The main advantage of radial gates is that friction is concentrated at the pin and is usually much less than that for other channel flow control structures, such as sliding gates. Radial gates are lightweight, economical gates that can be opened and closed with minimum effort and with a comparatively small number of turns of the hand wheel on the hoist. 3.1.2 Air - inflated Rubber Dam Air - inflated rubber dams are made of synthetic materials such as rubber, and are inflated with air and/or water. They usually have an inner shell and outer casing, and can be used to close very long spans with heads up to 18 feet. The main advantages of an air - inflated rubber dam are low cost, low weight, absence of a heavy lifting mechanism, little need for maintenance, and ease of installation. The disadvantage is that the fabric /rubber material can be damaged and may have a limited life. Rubber dam service life can be improved by including a stainless steel wire mesh. It is recommended that a rubber dam be considered that includes the stainless steel wire mesh. Based on operational familiarity and experience of the existing OCFCD staff member, the rubber dam is the favored diversion configuration. 3.1.3 Hydraulics Both types of structures are capable of efficient performance under dry- weather flow conditions and 15- minute first flush wet - weather flow conditions. Two radial gates in parallel have been proposed in the original PDR and PDR addendum No. 1. As for the rubber dam, one single piece would have to be installed on a single clamp plate. Radial gates would provide a superior clean water way, compared to the inflatable rubber dam, which would partially block the water way after deflation. Operationally, the radial URS 6 25F -138