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 onsite detention system with submerged orifice to improve the runoff delay from a drowned outlet. The application was investigated through a reducedscale 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 runoff 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.
C_{d} is discharge coefficient (0.40 to 1.0);
A_{o} is the orifice area, m^{2} (ft^{2});
The crosssectional area of a rectangular orifice is b × d;
b is orifice width, m;
d is orifice depth, m;
The crosssectional area of a circular orifice is ;
r is orifice radius, m;
H_{o} is effective head, the difference in elevation between the upstream and downstream water surfaces;
(h_{1}–h_{2}), m;
h_{1} is effective headwater elevation, m;
h_{2} is tailwater elevation, m; and
g is gravitational acceleration, 9.81 m/s^{2} (32.2 ft/s^{2}).
METHOD
It is noted that h_{2} 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 reducedscale 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 scalemodel 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 realworld conditions. Equations (3)–(5) confirm that onsite detention (OSD) tank size has no influence on orifice discharge rate. A 5liter 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 reducedscale 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:
Item .  Parameters .  Prototype .  Model .  Remarks . 

OSD Tank  Shape  Rectangular  Rectangular 

Height  1,000 mm  300 mm 
 
Vertical Orifice  Shape  Circular  Circular 

Orifice Size  40 mm  12 mm 
 
Orifice Area  0.00125600 m^{2}  0.00011309 m^{2} 
 
Effective Head, H_{o} (H_{1}h_{2}/ Different of head and tail water level)  Height  50 mm  15 mm 

150 mm  45 mm  
350 mm  105 mm  
550 mm  165 mm 
Item .  Parameters .  Prototype .  Model .  Remarks . 

OSD Tank  Shape  Rectangular  Rectangular 

Height  1,000 mm  300 mm 
 
Vertical Orifice  Shape  Circular  Circular 

Orifice Size  40 mm  12 mm 
 
Orifice Area  0.00125600 m^{2}  0.00011309 m^{2} 
 
Effective Head, H_{o} (H_{1}h_{2}/ Different of head and tail water level)  Height  50 mm  15 mm 

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 = 
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
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.
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.