CHAPTER 7 - WEIRS
18. Care of Weirs
The weir and weir pool should be kept free of weeds and trash. The weir pool should be cleaned of sediment as it accumulates. Frequent trimming of the channel and cleaning of the weir box structure or approach channel with shovel or scraper are necessary to maintain accurate flow measurement. This cleaning should extend from the bulkhead to upstream past the head measuring station. The invert of the pool must be kept low enough to maintain at least the minimum distance below the crest of the weir. This minimum distance is twice the maximum head on contracted weirs and three times the maximum head on suppressed weirs.
Frequent inspections should be made to determine whether leakage is occurring around the weir structures and through any sluicing clean out gate or cover. If leakage does occur, remedial action should be immediately followed with careful rechecking to see that the weir is level and that its elevation corresponds to the zero elevation of the measuring gage. In any case, the crest of the weir should be checked periodically to verify that it is level and to verify correspondence to gage zero.
Care must be taken to avoid damaging the weir notch itself. Even small nicks and dents can reduce the accuracy of an otherwise good weir installation. Any nicks or dents that do occur should be carefully dressed with a fine-cut file or stone, stroking only in the plane of the upstream face of the weir plates or the plane of the beveled surface of the weir plates. Under no circumstances should any attempt be made to completely remove an imperfection, which will result in a change to the shape of the weir opening. Instead, only those portions of the metal that protrude above the normal surfaces should be removed.