c--------------------------------------------------------rise_fall_vel
	subroutine rise_fall_vel(vv)
c--------------------------------------------------------------------c
c vv > 0 : rise  and  vv < 0 : fall  vv = 0.0 : nuetral              c
cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc
c      This program is to  assign a velocity to one of the           c
c      sewage particles which have various sizes and densities and   c
c      paricle size and density are uniformly distributed among      c
c      the particles                                                 c
cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc
c
c       yo and y1 are coefficients of speed regression equation 
c       which has form 10.0**((m-y0)/y1)
c       particles which fall have possitive velocity
c       particles which rise have negantive velocity
c
cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc
c
        include 'rise_fall.cb'
     	REAL    rnumb, vv, ran3, fspe, rspe 
        common /randm/isd,isd1
	INTEGER isd,isd1
c       
        rnumb=100.0*ran3(isd)
        vv=0.0
c
        if(rnumb.le.rf) vv=fspe(rnumb)
        if(rnumb.ge.(100.0-rr)) vv=rspe(rnumb)
	return
	end

	function fspe(rnumb)
        include 'rise_fall.cb'
	real fspe, rnumb
        fspe=-10.0**((rnumb-y0f)/y1f)/3600.0
        return
	end

	function rspe(rnumb)
        include 'rise_fall.cb'
	real rspe, rnumb
        rspe=10.0**(((100.0-rnumb)-y0r)/y1r)/3600.0
	return
	end