7. Poor Flow Patterns

The poor flow distribution which exists upstream from a measuring device often cannot be attributed to any one of the causes discussed above. The best solution, then, is to assume that the poor distribution has several causes. Start with the easy factors, work through the list, and address each probable cause of poor flow patterns until obtaining the desired flow conditions.

Turnouts located close to and upstream from a measuring device may cause poor approach conditions, as may bridge piers, channel curves, or a skewed measuring section. Relocating the measuring device may be the only remedy in these cases.

Submerged weeds or debris can cause excessive turbulence or local high­velocity currents. Eddies adjacent to the shoreline can cause the flow approaching the weir to contract into a narrow band. Sediment bars deposited from inflow or from sloughing banks can also produce undesirable flow conditions. More drastic remedial measures include deepening the approach area, widening the approach channel to make it symmetrical, or introducing baffles or other devices to spread the incoming flow over the entire width of the approach. However, 10 channel widths of straight, unobstructed approach should lie between baffles or other devices placed before the measuring device. Surface waves, which are usually difficult to reduce or eliminate by ordinary procedures, may require special treatment, as discussed under "Rough Water Surface" in section 5.