Quantcast Theory

 
  
 
The labyrinth (Hauser 1995, 1996) assumes a "W" shape in plan view to fit within the channel,
as shown in Figure 4.8.2b. By increasing the crest length, the labyrinth weir reduces specific discharge,
thereby improving both aeration and safety of the labyrinth compared to a conventional linear weir
(Hauser 1991). The labyrinth weir's efficient head-discharge relationship allows it to pass the same
flow at a reduced head compared to a linear weir spanning the same river width. Labyrinths can
therefore be constructed closer to an upstream dam with less impact on turbine head.
The infuser weir (Figure 4.8.2c) was developed to achieve aeration equivalent to a labyrinth
weir with a compact design that requires less space in the channel. Like the labyrinth, the infuser
(Hauser and Brock 1994, Rizk and Hauser 1993) uses a long length of waterfall to achieve a high total
nappe perimeter to flow ratio. Unlike the labyrinth, however, the infuser splits the flow into a tight
series of transverse water curtains that impinge on a plunge pool beneath the infuser deck. Grating over
the deck openings creates turbulent irregular-shaped waterfalls that further increase the nappe perimeter
to flow ratio. Hydraulically, the infuser behaves like a broad-crest weir. Turbulent regions beneath the
infuser deck are rendered off limits to people using a barrier cage at the downstream face of the infuser
deck. Clogging of the infuser deck requires somewhat more routine maintenance than conventional or
labyrinth weirs. The infuser weir concept has been patented by TVA, and is protected under U.S.
Letters Patent Nos. 5,462,657 and 5,514,285. TVA can license this technology on a non-exclusive
basis to interested organizations for their particular applications.
4.8.2.1 Theory
The hydraulics of labyrinth weirs are quite complex, but considerable literature (Hay and Taylor
1970, Indlekofer and Rouve 1975, Lux 1993), Tullis, et al (1993) exists because these weirs are used
frequently for spillway applications.
The primary parameters influencing weir aeration efficiency are 1) drop height, defined as
headwater minus tailwater across the weir; 2) specific discharge, defined as flow per unit length of the
waterfall(s); 3) roughness of the waterfall perimeter as it impinges on the plunge pool; 4) plunge pool
geometry. Weir aeration occurs along the surface and underside of the falling nappe, across the plunge
pool surface, and across the surfaces of bubbles submerged in the plunge pool. Most of the total
aeration at a weir occurs in the submerged bubble region. Air is transported along the boundaries of
the free-falling nappe and introduced into the plunge pool. In the submerged bubble region, most of
oxygen transfer occurs in the descending phase, as air introduced by nappe impingement is sheared into
small bubbles and plunged downward toward the channel bed. Transfer also occurs as the bubbles
ascend, but this transfer is less than during descent due to bubble coalescence, reduced turbulence, and
buoyant separation of bubbles from the fluid jet containing lowest DO. A rougher surfaced nappe will
typically have more perimeter per flow at impingement, improving its aeration efficiency. Aeration
efficiency is lowest at high specific discharges (high flow to perimeter ratio) and at very low specific
discharges (nappe impinges as a spray or mist); thus, an optimal specific discharge exists for various
weir types
4.8-7

 


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