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4 Usually the preliminary surveys are conducted before the design phase and there is a considerable lapse of time before construction gets underway. Thus, it is sometimes advisable to remove the buoys until the construction stage begins. When this is the case, easily visible and relocatable bottom markers, such as colored numbered concrete blocks, may be used to facilitate re-establishing the route with marker buoys. Most small diameter HDPE outfalls of less than 1,000 M length take no more than a few days to install. During this construction period it is necessary to prevent tampering with the marker buoys through whatever measures are necessary or are recommended by local authorities. Among them are warning notices printed in the buoys themselves, notices posted in small boat harbors, notification of boaters and fishermen to not only avoid fouling of the buoys, but also to stay clear of the pipeline construction when it is in progress. At this same time, it is a good idea to arrange for the outfall location to be added to nautical charts and coastal maps as soon as possible with a warning not to anchor in the vicinity of the outfall. 5. PIPE DIAMETER SELECTION Selection of pipe diameter for HDPE outfalls is made through the same series of determinations as for other pipe material. This is usually done through a balancing of friction loss reduction against the flow velocities necessary to maintain sufficient scour to prevent deposition of suspended solids, or grease build up on the pipe wall. New HDPE pipe has excellent flow characteristics. Because of its exceptionally smooth its Hazen and Williams Formula coefficient of C = 155 but for outfalls that have been in use the C is usually estimated to be 140 due to build up of grease on the pipe wall. For sewage outfalls utilizing HDPE, the flow velocity ranges that have proven satisfactory from both a friction and a cleansing standpoint usually fall within the ranges presented in Table 1. The amount of and characteristics of the suspended solids and grease in the effluent influences the necessary velocity for self-cleansing. Table 1 Flow velocity ranges HDPE submarine outfalls Nominal Pipe Size Satisfactory Velocity Ranges (cm) meter/sec It is important that cleansing velocities be achieved at least one time every day for a sufficient period of time to obtain complete flushing of the line. If this does not occur, depositions of solids and bacterial growth on the walls will occur and it will be necessary to send a cleaning plug (a pig) through the outfall at regular intervals to prevent pipe constriction or closure. When designing an outfall for a 25-year projected flow it is important to check the velocities at the present maximum flows to see if sufficient scour velocities are obtained during the first few years of operation. If not, then a maintenance schedule utilizing a cleaning plug should be implemented until such time that flows reach a level to obtain cleansing velocities. Facilities for the removal of grit and grease from the effluent prior to its discharge into the outfall will help minimize problems due to deposition and is recommended. This removal of grease along with floatables serves a second purpose of maintaining acceptable aesthetic conditions. 4. PRESERVING THE ROUTING BUOYS 10-30 25-50 40-75 0.7-2 1.0-2.5 2-3PDF Image | SMALL DIAMETER (HDPE) SUBMARINE OUTFALLS
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