Drowned outlets are common in riverine areas and sometimes unavoidable. Due to site restrictions, drainage discharge outlets are often submerged as the water level fluctuates during high tides or during the monsoon. As the runoff cannot be discharged through the outlet the drainage system fills up faster, leading to flash floods caused by overspill from the drains. This study is focused on the application of an on-site detention system with submerged orifice to improve the runoff delay from a drowned outlet. The application was investigated through a reduced-scale laboratory set up and then visualized with computational fluid dynamics simulations. The model was tested under different perpendicular flow velocities to analyze the workability and flow characteristics of the submerged orifice. The study showed that, with different headwater and tailwater levels, the energy level can be restored upstream of the orifice and ensure full flow of water from the submerged orifice even when hindered by perpendicular tailwater flow. Besides, the orifice jet's pattern changes with high velocity tailwater flow, although it does not slow down the discharge rate.

INTRODUCTION

When a drainage outlet is submerged (drowned), it reduces or can even overwhelm the energy of the run-off discharge. This results in reduced rate or zero discharge from the outlet. As the effective headwater/hydraulic grade line is reduced, the surface drainage system fills quickly (UPRCT 1999). If an overflow occurs, flash flooding will follow and could cause inconvenience or damage to property. Drowned outlets have thus been tagged as unfavorable and their avoidance in design is strongly advised. Unfortunately, drowned outlets are at times unavoidable, especially on restricted sites, e.g., beside a river, so outlet drowning requires a solution.

An orifice is an opening that enables a fluid to flow out. Its sizing is designed to limit flow discharge. According to Buchanan et al. (2013), an orifice is also a specific type of outlet that can transform potential (elevation head) to kinetic energy (velocity) by accelerating flow. Hence, orifice flow has low volume but produces high-velocity discharge. A submerged orifice discharges under water (Figure 1). Bos (1989) showed that the discharge flow from a submerged orifice can be expressed using Bernoulli's theorem – Equations (1) and (2). Assuming there is no energy loss over the submerged orifice, so the streamlines are straight and the velocity of flow in eddies above the jet is relatively low. 
formula
1
Figure 1

Flow Pattern through a Submerged Orifice (Bos 1989).

Figure 1

Flow Pattern through a Submerged Orifice (Bos 1989).

Since , Equation (1) can be rearranged to: 
formula
2
Equation (2) is governed by H1-h2. If this condition can be established, then a flow of Vc is predicted theoretically. However, as shown in Figure 2(a) and 2(b), conventional drainage outlets are built with fixed invert levels. These invert levels are intended to create a drainage slope to enable flow during dry seasons. However, they pose a limitation during wet seasons when the outlets are submerged. To overcome the rigidity of fixed invert levels, a tank structure with an orifice as the outlet could add flexibility to H1 to deal with the dynamics of h2. A tank receiving water from the upstream drainage is equipped with a storage volume and the associated height of water, the H1 to its advantage. The tank surface area (width × length) has little impact on orifice discharge; but the height could be increased while maintaining the required storage volume (i.e. small width or length of the tank). Figure 2(c) and 2(d) show the potential design models compared to conventional drainage outlets. Note that the tanks could be lower than the invert level of conventional drainage depending on site conditions. By having a prolonged tank height, H1 could be maintained at higher level to establish the required condition of H1-h2. The authors admit that such a design has no impact on the h2 level. High rainfall or high flow events would render a condition of h2 = H1, but it is thought that would usually only occur during an extreme flood that no manmade structure could withstand.
Figure 2

Drainage Outlets, (a) Real Life Example, (b) Conventional Design, (c) and (d) Proposed Designs.

Figure 2

Drainage Outlets, (a) Real Life Example, (b) Conventional Design, (c) and (d) Proposed Designs.

By considering the coefficient of discharge, velocity head and orifice area, Equation (2) can be further developed, as shown in Equations (3)–(5), to determine the total discharge through a single submerged orifice (Malloy 2000; Chanson et al. 2002; Iowa State University 2008). 
formula
3
Or 
formula
4
Or 
formula
5
where: Q is the orifice flow rate, m3/s (ft3/s);
  • Cd is discharge coefficient (0.40 to 1.0);

  • Ao is the orifice area, m2 (ft2);

  • The cross-sectional area of a rectangular orifice is b × d;

  • b is orifice width, m;

  • d is orifice depth, m;

  • The cross-sectional area of a circular orifice is ;

  • r is orifice radius, m;

  • Ho is effective head, the difference in elevation between the upstream and downstream water surfaces;

  • (h1–h2), m;

  • h1 is effective headwater elevation, m;

  • h2 is tailwater elevation, m; and

  • g is gravitational acceleration, 9.81 m/s2 (32.2 ft/s2).

METHOD

It is noted that h2 in Equations (2) and (3) represents linear flow from upstream to downstream after the orifice. Yet, the flow from a submerged orifice collides perpendicularly with the flow in the stream or river. Because of this, laboratory measurement is the best way to understand flow behavior. However, a real tank could be costly to build without a detailed study. To investigate the workability of a submerged orifice in improving retarded runoff from drowned outlets, a reduced-scale experiment was carried out, imitating orifice flow under various submerged conditions. Observations of the flow characteristics of a submerged orifice in an experiment can be enhanced using computational fluid dynamics (CFD) simulations.

Experimental works

The laboratory set up involved a scale-model based on a prototype proposed in a real life example, so that the model's behavior could be used to describe a similar system in real-world conditions. Equations (3)–(5) confirm that on-site detention (OSD) tank size has no influence on orifice discharge rate. A 5-liter water sampling bottle was used to represent the OSD. Fitting the 155 × 270 × 340 mm bottle to a 300 × 400 × 8,000 mm flow channel, yielded a scale model for design testing before implementation in the field. A scale factor of was adopted and a reduced-scale experiment set up using the specifications in Table 1. Full water sampling bottles were arranged in series along one side of the channel to represent OSD adjacent to the river, to ensure a geometrically similar boundary, and a flow ratio of 0.049 was used when upscaling the discharge rate to achieve dynamic similarity, where:

Table 1

Specifications of reduced-scale model

Item Parameters Prototype Model Remarks 
OSD Tank Shape Rectangular Rectangular 
  • Similar

 
Height 1,000 mm 300 mm 
  • Vertical axis

  • Ratio of prototype to model is 1:

 
Vertical Orifice Shape Circular Circular 
  • Similar

 
Orifice Size 40 mm 12 mm 
  • Ratio of prototype to model is 1:

 
Orifice Area 0.00125600 m2 0.00011309 m2 
  • Ratio of prototype to model is 12:

 
Effective Head, Ho
(H1-h2/ Different of head- and tail- water level) 
Height 50 mm 15 mm 
  • Ratio of prototype to model is 1:

 
150 mm 45 mm 
350 mm 105 mm 
550 mm 165 mm 
Item Parameters Prototype Model Remarks 
OSD Tank Shape Rectangular Rectangular 
  • Similar

 
Height 1,000 mm 300 mm 
  • Vertical axis

  • Ratio of prototype to model is 1:

 
Vertical Orifice Shape Circular Circular 
  • Similar

 
Orifice Size 40 mm 12 mm 
  • Ratio of prototype to model is 1:

 
Orifice Area 0.00125600 m2 0.00011309 m2 
  • Ratio of prototype to model is 12:

 
Effective Head, Ho
(H1-h2/ Different of head- and tail- water level) 
Height 50 mm 15 mm 
  • Ratio of prototype to model is 1:

 
150 mm 45 mm 
350 mm 105 mm 
550 mm 165 mm 
Model ratio Dynamic similarity 
Length Ratio,  Flow Ratio =  
Model ratio Dynamic similarity 
Length Ratio,  Flow Ratio =  

By making the model similar to the full-scale unit, geometrically, dynamically, and kinematically, the scale model was a good representation of the real application – see Figure 3.
Figure 3

Experimental Set Ups.

Figure 3

Experimental Set Ups.

During the experiment, potential variables like orifice size (0.012 m), shape, and position, and the channel setting (slope, roughness) were kept constant. Variables like effective head (differential between head- and tail- water levels), and tailwater velocities were varied.

In the first part of the experiment, the model was placed in tailwater flowing perpendicular to the orifice opening at 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 m/s, and discharge rates were measured under various submerged conditions (different effective heads). After that, the flow characteristics of the submerged orifice were observed at different perpendicular tailwater flow velocities.

CFD validation

CFD can provide both qualitative and quantitative results for fluid flow patterns and is used to validate laboratory measurement as well as improve visualization (Sayma 2009; Bouillot et al. 2016). The CFD model here was used to study flow patterns at reduced scale model.

RESULTS AND DISCUSSION

Discharge rate from the submerged orifice

In the experiment, the orifice acted as an aperture on the side of the sampling bottle, enabling water discharge under submerged conditions. Water was discharged continuously from the bottle until the headwater level (inside the bottle) was the same as that of the tailwater (perpendicular channel flow). For a fully submerged outlet, the headwater/energy change/momentum flux passing through orifice area Ao in unit time is critical. It is expressed as a function of the velocity head, and entrance and friction losses over the depth of the tailwater (Manly Hydraulics Laboratory 1993; UNSW Water Research Laboratory 2004). Generally, discharge from the outlet is hindered by tailwater flow. However, with adequate energy available from the effective head, the submerged orifice jet appears to work against the tailwater, enabling continuous discharge. The orifice discharge rate depends on the effective head derived from the headwater (rising trend in Figure 4).
Figure 4

Discharge rate under different velocities of perpendicular tailwater flow.

Figure 4

Discharge rate under different velocities of perpendicular tailwater flow.

Flow patterns of the submerged orifice

In the second part of the experiment, the submerged orifice's jet patterns were observed with respect to different perpendicular tailwater flow velocities. The differences in the patterns were minute between flow rate increments – 0.1 m/s steps – so only those for tailwater flows of 0.1 and 0.6 m/s are presented here.

Experimental work and CFD outcomes showed that the flow pattern from a submerged orifice is affected significantly by the velocity of tailwater flowing perpendicularly across it. At 0.1 m/s (Figure 5), the effective heads enable discharge from the orifice, which continues to disturb the low velocity channel flow. The velocity cut plot of the CFD simulation (Figure 5) also shows a clear orifice discharge cutting across the channel flow as the diffusing jet's velocity is higher than that of the channel flow surrounding it.
Figure 5

Observed and Simulated Flow Patterns in Channel at 0.1 m/s Tailwater Flow.

Figure 5

Observed and Simulated Flow Patterns in Channel at 0.1 m/s Tailwater Flow.

With the same effective head (same differential in head- and tail- water level) but a tailwater flow of 0.6 m/s, the orifice jet is dominated by the channel flow, as depicted in Figure 6. This is because the channel flow velocity exceeds that from the orifice jet, so that the channel flow pattern is dominant. Although the orifice jet's pattern changes and is dominated by relatively high velocity tailwater, discharge from the submerged orifice is continuous and there is a slight increase in its rate. This implies that high tailwater velocities do not hinder submerged orifice discharge but rather act to assist it. Since the outcomes are similar, the experimental work is therefore confirmed by the CFD modelling.
Figure 6

Observed and Simulated Flow Patterns in Channel at 0.6 m/s Tailwater Flow.

Figure 6

Observed and Simulated Flow Patterns in Channel at 0.6 m/s Tailwater Flow.

CONCLUSIONS

With the effective head provided by OSD, the head above the orifice enables the stored water to discharge continuously through a submerged orifice against tailwater flow that is perpendicular to it. The orifice's discharge rate increases slightly when the perpendicular tailwater's flow rate increases – i.e., the submerged orifice's jet pattern changes with high velocity tailwater flow and discharge remains continuous. An OSD system with submerged orifice could be used to improve retarded runoff from drowned outlets into tailwater flowing perpendicularly.

ACKNOWLEDGEMENTS

The authors wish to thank the financial support through Special Grant Scheme F02/SpGS/1405/16/6 rendered by the Universiti Malaysia Sarawak. The first author receives financial support through the Zamalah Penyelidikan Naib Canselor (ZPNC) of Universiti Malaysia Sarawak.

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