Velocity scouring

If stream velocities, scour and erosive forces are high, abutments with wingwall construction maybe necessary. Drift will affect the horizontal clearance and the minimum vertical clearance line of the proposed structure. Field surveys should note the size and type of drift found in the canal. Design based on the 50-year flow requires drift clearance. On major streams and rivers, drift clearance of 2 to 5 m above the 50-year discharge is needed. On smaller streams, 0.3 to 1 m maybe adequate. A formula for calculating freeboard is... 

For sand, the transport capacity is first calculated using either the Colby ( ), or Toffaleti (10) method, or a user supplied power function of velocity. If the calculated transport capacity exceeds the load present scour is simulated and if the opposite is true deposition is simulated. 

Spiral wound modules can be difficult to clean. There are dead spaces within the module where high velocity cannot scour the surface of the membrane, and cleaning solution does not mix well to remove debris. 

Figure 5.6 shows an RO array with concentrate recycle. A concentrate recycle is generally used in smaller RO systems, where the cross-flow velocity is not high enough to maintain good scouring of the membrane surface. The return of part of the concentrate to... 

Recirculation rate the velocity of recirculation will impact the ability of the cleaning solution to remove debris from the membrane. Higher flow rates (see step 5 above) are usually necessary to scour debris off of the surface of a membrane. 

A particle possesses both downward terminal velocity v or Vp, and a horizontal velocity (also cdlltd flow-through velocity). Because of the downward movement, the particles will ultimately be deposited at the bottom sludge zone to form the sludge. For the particle to remain deposited at the sludge zone, v should be such as not to scour it. For light flocculent suspensions, should not be greater than 9.0 m/h and for heavier, discrete-particle suspensions, it should not be more than 36 m/h. If A is the vertical cross-sectional area, Q the flow, the depth, W the width, L the length, and the detention time ... 

While the formed-in-place or dynamic hydrous zirconium oxide membranes on porous stainless steel supports have been studied mostly for biotechnology applications, they have also demonstrated promises for processing the effluents of the textile industry [Neytzell-de-Wilde et al, 1989]. One such application is the treatment of wool scouring effluent. With a TMP of 47 bars and a crossflow velocity of 2 m/s at 60-70°C, the permeate quality was considered acceptable for re-use in the scouring operation. The resulting permeate flux was 30-40 L/hr-m. Another potential application is the removal of dyes. At 45 C, the dynamic membranes achieved a color removal rate of 95% or better and an average permeate flux of 33 L/hr-m under a TMP of 50 bars and a crossflow velocity of 1.5 m/s. 

Generally with wet or mist applications, as the deposited material is retained on a wetted surface, operational gas velocities can be appreciably higher than that of a dry precipitator, as the risk of particle scouring is significantly reduced. 

A method of scouring raw wool in which the detergent is forced through jets, under pressure, and impinges upon it with considerable velocity has... 

When looking at Colby s publications, it becomes obvious that he considered journal papers as unsuitable for his works. Instead, most of his research is published in Reports of the USGS or the US Department of Agriculture. This characteristics was reflected in his personality A person who did not want to generalize findings but applied these to detailed studies, both in time and in location. Colby also exclusively worked in the narrow field of sediment mechanics, yet his knowledge was so profound that he counted among the then top American experts. He further took interest into the relation of sediments and the surface water chemistry, the effect of sediment on scour, or the effects of sediment transport on the velocity patterns in sand-bed streams. 

In contrast to the case examined previously (see p. 349), in river flow the detachment of particles adhering to the bottom takes place not from a surface, but from a layer of solidly packed particles. In this case, the same as in the case of detachment of particles by air or water flow, we can distinguish critical velocities for detachment of the adherent particles. The minimum critical velocity at which the first particles are detached, in application to riverbed evolution, is termed the nonscouring velocity, and the velocity for first mass movement of the particles is termed the scouring velocity [351, p. 288]. 

In addition to the scouring velocity, which characterizes the detachment of adherent particles, we need to know for practical purposes the nonscouring velocity. With a stream velocity equal to or less than the nonscouring velocity, there is no detachment of adherent particles. Also, the nonscouring velocity, in application to the near-bottom and suspended drift material, characterizes the process of preventing the precipitation and adhesion of particles. 

From the foregoing discussion, it is clear that adhesive interaction determines the course of riverbed evolution. From the magnitude of the adhesive interaction, we can estimate the velocity required to move the bottom deposits (bed load) and also the velocity below which no scouring of the bottom will take place. 

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