HYDRAULIC DESIGN OF POOLED STEP CASCADES by 4mD4ar

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									         HYDRAULIC DESIGN OF POOLED STEP
                    CASCADES


                                       Detlef Aigner

                    Technische Universitaet Dresden, 01309 Germany

                     Tel +4935146334397 E-mail detlef@d-aigner.de



  Abstract: Cascades and stepped chutes are used for the discharge of water over dams or
steep slopes. These are sequentially placed drops which either enable the step by step
dissipation of the hydraulic energy or overflow by skimming flow. The stepped
transitions ensure a good aeration of the water as a result of which an air-water mixture
type of flow takes place. If the steps are designed as small stilling basins to dissipate the
energy, they are called pooled step cascades. The hydraulic design of these structures will
be dealt with in this paper.

1 ADVANTAGES OF CASCADES AND STEPPED CHUTES
The advantages of pooled steps and stepped chutes are the stepwise reduction of kinetic
energy of the water through transformation into heat, so that the size of the stilling basin
can be reduced. They are preferably applied for dam spillways or combined sewer
systems for the safe discharge of water over large drops.

Stepped chutes are often combined with the spillway of the dam. They are also used in
parks and are suitable to the type of river course design improving natural surroundings.

Each step in a pooled step cascade corresponds to on isolated stilling basin (Fig. 1) for
hydraulic jump. On stepped chutes the water falls from level to level and the optimal
dissipation is expected if a hydraulic jump builds. For steeper stepped chutes and large
discharges, so called skimming flow occurs in which the step acts as roughness element
and causes a strong mixing of the water with air.

Most research work has been conducted with flat horizontal steps, and there is little work
on pooled steps. Press (1954) wrote in his book “stauanlagen und Wasserkraft”
(reservoirs and water energy) that the use of cascades as energy dissipaters was neglected
because the structures are highly susceptible to the effects of ice and weathering. Recent
publications and the international workshop in Zurich (March 2000 in Switzerland)
indicate that interest in hydraulics of stepped spillways has significantly increased in the
past years.
2 HYDRAULIC DESIGN OF POOLED STEP CASCADES
Fig. 2 enables the calculation of the minimum dimension of pooled steps (full energy
dissipation) considering a safety addition of 5% for the conjugate depth and the
minimum length of one step. The design underlies the following principal equations for
the two-dimensional calculations.

Definition of parameters:

s —— height of step                    hc —— critical depth

h —— water level of overflow           h1,2 ——        sequent depths of hydraulic jump

L —— length of step                    Lp —— jet impact position

w —— height of weir step               Lj —— length of hydraulic jump

q —— specific discharge                Cd —— discharge coefficient

v —— velocity                          g —— gravitational constant

The slope of pooled steps is calculated as the ratio of the step height and the step length,
including the thickness of the weir wall

                                                                (1)


The critical depth hc is used as an auxiliary variable to represent the specific discharge as


                                                                  (2)


The specific discharge is calculated with a standard overflow formula.

The discharge coefficient Cd (for the case of a sharp-crested weir, Bollrich 2000) should
include the effect of approach velocity.


                                                                      (3)



                                                                                (4)
The velocity (v1) at the downstream end of the jet is calculated from the energy equations
as (without loss of energy)


                                                                             (5)



                                                               (6)


The hydraulic jump is assumed to develop fully and calculated with the conjugate depth
equation (Bollrich 2000)


                                                                     (7)


The height of the weir w plus a 5% safety margin for h2 amounts to

                                                                       (8)

The length L of a step required is determined based on the so called throw length of the
jet Lt according to Oficerow (Bollrich, 2000) and the necessary length of a hydraulic
jump Lw according to U.S.B.R.. There is further possibility of optimising the length, for
instance, through the minimize the length of the jet.

                                                               (9)


                                                                (10)


                                                              (11)

The above described equations allow to design pooled step cascades with full hydraulic
jump. Figs. 2 and 3 illustrate the results.



Example

Given a dam with a height of H = 10 m and an inclination of tan = 0.15, develop a
pooled step cascade with a selected height of s = 1 m and number of levels N = 10 in
order to convey a discharge of 5 m³/s.
Figure 3:      tan = 0.15   q = 0.6 m2/s    b = Q/q = 5/0.6 = 8.33 m

Figure 2:      tan = 0.15   w/s = 0.41      w = 0.41 m

               tan = 0.15   L = s/tan = 1/0.15= 6.7 m

The values obtained are nearly the same, if the discharge is determined with the critical
depth.

             control: hc = (q2/g)1/3 = 0.332 m      hc/s = 0.332   w/s = 0.41

The height, length and width of the pooled step cascade are easily and quickly
determined to have values of 0.41 m, 6.7 m, and 8.33 m respectively. With rounded
overfall (close-lying jet) the length of basin according to shortened by approximately
25%.

3 DIFFERENTIATION OF CASCADES
Basically two types of overflow are differentiated, the nappe flow and the skimming
flow. The transition between the two types of regimes can be determined by Chanson
(1994b) simplified equation

                                                                      (12)


Nappe flow is differentiated between stepped chutes without and pooled step with
hydraulic jump. The sizing of pooled step cascades is an approximation to the result in
Figs. 2 and 3. The limits of application for a cascade with fully developed hydraulic jump
can be estimated with equation (13). The use of cascades above the limits of this equation
results is without hydraulic jump and the kinetic energy hand over to the next basin.


                                                                   (13)


4 COMPARISON OF ENERGY CONVERSION FOR POOLED
  STEP CASCADES AND STEPPED CHUTES
For stepped chutes (w = 0) a large number of levels and a higher roughness produce a
better energy conversion (Chanson, 1994a). The strongly aerated skimming flow achieves
a better energy dissipation than the nappe flow.

The maximum possible energy dissipation can theoretically be determined with the
assumption that the total dam H is converted to energy head through the overcoming of
cascades. The energy dissipation is referred to as H. The degree of dissipation (the
energy dissipation related to the maximum possible energy height Hmax) is expressed as a
function of the ratio of the height H and the critical depth hc. For cascades, for example,
the maximum energy dissipation is

                                                                       (14)


The energy conversion with stepped chutes always lies under this limiting curve. The
analysis of Chanson (1994a), Mates and Quintela (1995) shows results of extensive
experiments of different authors. The friction coefficient has, for example, a large
influence on the calculation of the energy dissipation with skimming flow. With nappe
type flow, the number of levels have an important role and the friction influence is not
represented here.

5 CONCLUSION
Investigation in the past years facilitated the possibility of designing cascades for
discharging flood over dams and steep gradients. From literature, it appears that the
empirical investigations were particularly done for stepped chutes. To supplement these
works, the author examined the limits of application of pooled step cascades and
compared these with the results for stepped chutes. The outcome was the development of
a simple and quick technique which enables the design of pooled steps and the
comparison of these with stepped chutes.

                                        References

[1]   Bollrich, G. (2000). Technische Hydromechanik, Band 1, 4. Auflage, Verlag fur
      Bauwesen Berlin (in German).

[2] Chanson, H. (1994a). Comparison of energy dissipation between nappe and
    skimming flow regimes on stepped chutes. Journal of Hydraulic Research 32, S.
    213-218.

[3] Chanson, H. (1994b). Hydraulics of skimming flow over stepped channels and
    spillways. Journal of Hydraulic Research 32, S. 445-460.

[4]   Matos, J.; Quintela, A. (1994). Discussion for paper of Chanson (1994a and b) in
      Journal of Hydraulic Research 32, S. 135-139.

[5] Press, H. (1954). Stauanlagen und Wasserkraft. II Teil: Wehre. W. Ernst & Sohn,
    Berlin (in German).
     Fig. 1   Pooled step cascade




Fig. 2   Design of pooled step cascades
                   Fig. 3   Inclination of pooled step cascade




Fig. 4   Boundary between nappe flow and skimming flow over a stepped chutes and
         between overflow of pooled step cascades with and without hydraulic jump

								
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