Table 7-6.—Minimum Slope for Sewer Pipe on the same grade. Therefore, an increase in pipe size to obtain the desired flow velocity is limited by   the   rate   of   flow.   Typical   minimum   flow velocities are 2 feet per second when the design flow fills the pipe and 1.6 feet per second at the average  rate  of  flow.  Maximum  velocities  must also be considered; too high a velocity will erode the  pipe.  A  typical  maximum  velocity  is  15  feet per   second   for   concrete   pipe.   Because   of   the differences in available slopes, smaller pipe may be  used  in  some  sections  than  is  required  in  an upper  section  of  the  same  sewer.  The  pipe  size should  be  reduced  whenever  better  flow  condi- tions  would  result. Manholes provide access to sewers for inspec- tion  and  cleaning.  They  are  placed  where  there is a change in grade, a change in pipe size, a junc- tion  of  two  or  more  sewerlines,  or  a  change  in direction. Otherwise, they are placed at intervals of  300  or  500  feet  of  sewerline.  The  manholes should be built so there is no decrease in velocity and a minimum of water disturbance. The channel should  be  deep  enough  to  prevent  sewage  from spreading  over  the  manhole  bottom.  The  covers should  be  of  a  weight  strong  enough  to  support the expected traffic. Perforated covers should not be  used  for  sanitary  sewer  manholes,  because openings in the sewer manhole would permit the entrance  of  sand,  grit,  and  surface  water.  The sewers  are  ventilated  by  the  stacks  of  the  building plumbing  systems. STORM  DRAINAGE  SYSTEMS Storm  drainage  systems  should  be  separate from sanitary sewage systems wherever possible. Some Navy bases may have combination systems still  in  use.  However,  storm  water  should  never be   drained   into   sewers   intended   for   sanitary sewage  only. EOs  and  BUs  generally  are  responsible  for building  ditches,  culverts,  and  other  structures that  are  a  part  of  storm  sewers.  Therefore,  con- struction of these facilities is not covered in this chapter. The UT is generally concerned with only the pipework itself. This involves laying storm drain lines both inside and outside buildings and other structures.  This  pipe  material  may  be  the  same as that used for the sanitary system. Storm sewer systems,   however,   may   include   pipe   of   much larger sizes than are needed for sanitary sewers. Plain  or  reinforced  concrete  pipe  (rather  than  clay, cast  iron,  or  asbestos  cement)  is  generally  used for  the  larger  lines.  Also,  it  is  not  so  important that   the   joints   be   watertight   in   storm   sewer systems.  In  fact,  the  mortar  is  sometimes omitted  from  a  portion  of  the  joint  and  washed gravel  is  placed  next  to  the  opening;  the  storm drain  thus  serves  also  as  an  underdrain  to  pick up  subsurface  water. Installation Considerations Storm and sanitary systems may differ in the installation of the piping. Building storm drains should generally be graded at least 1/4 inch per foot whenever feasible. This amount of drop per foot  provides  an  unobstructed  and  self-scouring flow.  However,  a  greater  drop  per  foot  may  be given as no fixture traps which might lose their seals are associated with it. When a change of direction is necessary, long radius fittings are used and a cleanout need not be installed. This is especially true in and under buildings.  But  a  manhole  is  used  outside  of buildings  when  a  change  of  direction  is  necessary, or  when  two  or  more  lines  are  connected  together. Storm drainage systems are designed to drain all surface and sometimes subsurface water that Sizing Building Storm Drains may  cause  damage  to  Navy  facilities,  property, or  adjoining  land.  They  consist  of  pipe,  inlets, To determine the size of building storm drains, catch  basins,  and  other  drainage  structures  to a number of factors must be considered, such as carry the surface runoff and subsurface water to rainfall  intensity,  roof  size,  and  pitch  of  roof. a  point  of  disposal. Tables have been made for use in estimating the 7-9