The limits of applicability of equation (9.13) are h 6 q1/3 (m)
(equation (5.6) should be referred to, i.e. the overfalling jet should not
disintegrate) and the downstream pool depth dshould be bigger than
0.0041ReJ0.39FrJ0.24(m) (or d0.909h0.18q0.27(m,m^2 s^1 )) (as the major part
of oxygen uptake occurs in the downstream pool).
For the outflow under a gate with a hydraulic jump the deficit ratio is
r 15 1 Fr 1 2.1Re0.75 (9.14)(Fr 1 is the jump supercritical Froude number and Req/v).
The temperature correction from rTtor 15 is given by
(rT 1)/(r 15 1)(10.046T)/1.69. (9.15)Gulliveret al.(1998) reviewed various prediction equations for oxygen
transfer at hydraulic structures and concluded that for flow over sharp
crested weirs equation (9.13) in the form (Fr 8 gh^3 /q^2 )0.25,Req/v)
E 20 1 (1/(10.24 10 4 Fr1.78Re0.53))1.115 (9.16)gives the best results when tested against field measurements (E 20 is the
transfer efficiency indexed at 20 °C E 1
(
1
).
For ogee spillway crests they recommendE 20 1 exp( 0.263h/(10.215q) 0.203d) (9.17)and for gated sills
E 20 1 exp[ 0.0086(hq/s) 0.118] (9.18)wheresis the submergence of the gate lip.
For oxygen transfer at cascades see Chanson (1994) and for further
treatment of the whole subject see Novak (1994).
Aeration at hydraulic structures is usually, but not always, beneficial.
If the upstream water is fully or nearly saturated with oxygen then further
oxygen enrichment can lead to oxygen supersaturation that may have
detrimental effects as it can cause gas bubble disease in fish. This situation
is more likely to occur at high head structures with high velocities of flow
than at barrages. The problem can be alleviated by some structural meas-
ures and in any case is mostly very localized and does not propagate far
downstream of the dam.