Fig. 9.9 Inverted filter and flexible (launching) apron
(c) Concrete aprons and inverted filter
The aprons are of plain concrete blocks of about 1 m1.5 m0.75 m deep,
castin situ. The downstream apron is laid with 70–100 mm open joints filled
with spawls (broken stones), so that the uplift pressure is relieved. An
inverted filter of well-graded gravel and sand is placed under the concrete
apron (Fig. 9.9) in order to prevent the loss of soil through the joints. The
upstream apron is laid watertight so that the uplift pressure and downward
flow is reduced (due to the increase in creep length). Aprons of boulder or
stone are laid downstream and upstream of the concrete aprons (Fig. 9.10).
9.1.7 Effect of barrages on river water qualityThe flow of water over and/or under barrage gates as well as over spill-
ways and weirs results in aeration with a beneficial effect on oxygen con-
centration levels in the downstream river reach.
The most frequent case is aeration at overfalls. On the basis of labo-
ratory experiments (Avery and Novak, 1978) with results corroborated by
extensive field measurements (Novak and Gabriel, 1997) the oxygen
deficit ratio r(ratio of upstream to downstream oxygen deficit) at 15 °C is
given by
r 15 1 kFrJ1.78ReJ0.53 (9.13)wherek0.627 10 4 (for water without salinity), FrJ(gh^3 /2q^2 J)0.25and
ReJqJ/v,his the difference between the upstream and downstream water
level and qJis the specific discharge (m^2 s^1 ) at impact into the downstream
pool; this equals the specific discharge at a solid weir crest (qqJ) but with
air access below the nappe (e.g. at flow over gates) q 2 qJ. Equation (9.13)
(which is dimensionless) can be transformed for the given value of (k) (h
(m),q(m^2 s^1 )) into:
r 1 0.18h1.34q^ 0.36 (9.13a)