 23 Aug 2022
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Pond
 Updated on 23 Aug 2022
 4 Minutes to read

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The Pond is used primarily for unsteady flows and relates the rate of water level rise to the net discharge. It includes the action of one or two simple outlet structures and is intended mainly for use in flood routing models.
Data
Field in Data Entry Form  Description  Name in Datafile 

Inflow Label  label of upstream node  label1 
Outflow Label  label of downstream node  label2 
n/a  number of values in stage/area table  n1 
Elevation  elevation (mAD)  hi 
Plan Area  corresponding surface area (m^{2})  Ai 
n/a  number of structure groups included in POND unit  always 1 in the present version of Flood Modeller  nstruc 
Outflow Structure  Select outflow structure from: WEIR, SLUICE, WEIR & SLUICE RATING  noutflow 
Weir: Discharge Coefficient  weir discharge coefficient  Cdw 
Weir: Width  width across flow (m)  Bw 
Weir: Crest Elevation  weir crest elevation (mAD)  zcw 
Sluice: Discharge Coefficient  sluice discharge coefficient  Cds 
Sluice: Area of Opening  area of opening in sluice (m^{2})  As 
Sluice: Invert Level  sluice invert level (mAD)  zcs 
Sluice: Height of Opening  depth of sluice opening (m)  ds 
n/a  number of values in outflow stage/discharge relationship  n2 
Rating: Outflow Discharge  outflow discharge value (m^{3}/s)  Qoi 
Rating: Stage  corresponding stage (mAD)  Hoi 
Theory and Guidance
The Pond is used primarily for unsteady flows and relates the rate of water level rise to the net discharge. It includes the action of one or two simple outlet structures, and is based on the OnLine Pond in the software package RIBAMAN. It is intended mainly for use in flood routing models.
The Pond has two components. The first is the storage area, defined by a series of values of plan area at different water levels. The second is the outlet structure, which can be a weir, a sluice, a weir and sluice combination or a rating curve.
Only two nodes can be connected to a pond. Reverse flow is not allowed.
Equations
The conservation of mass equation

where: q_{in} = flow at upstream node q_{out} = flow at downstream node DV = change in volume 
In addition, one or more of the following equations apply
Weir equation
where: C_{dw} = weir discharge coefficient g = gravitational constant L = length of weir crest (across flow) (m) h_{1} = water level in pond p = crest level of weir (mAD) 
Sluice equation
Surcharged flow (h1³d+p)

where:

Free surface flow (h1<d+p)

where: C_{ds} = sluice discharge coefficient A = area of opening (m^{2}) P = sluice invert or crest level (mAD) d = depth of opening (m) 
Rating curve equation

where: f = userdefined relationship between discharge and water level. 
General
If the calculated water level exceeds the highest level in the stage/area table for a Pond , then the volume is assumed to increase at the same rate as between the highest two values in the table. Similarly, if the water level exceeds the highest level in the stage/discharge relationship for the outflow, the relationship is extrapolated upwards linearly.
Both weir and sluice are assumed to operate undrowned (modular) at all times. Thus no account is taken of possible backwater effects in the downstream channel. This should not cause problems with routing models, but may be important when Ponds are used in steady or unsteady flow hydraulic models. If drowning is thought likely to occur, then a Reservoir together with an appropriate outflow structure should be used instead of the Pond .
The following combinations of outflow structure are allowed:
 Weir
 Sluice
 Weir and Sluice
 Rating curve
For example, a common form of outfall control consists of a lowlevel throttle pipe through an embankment capable of passing the dry weather flow without surcharge, but restricting flood flows to a specified limit. A highlevel emergency overflow weir in the embankment limits pond level during extreme floods. You can model this combination by specifying parameters for the throttle and weir or, alternatively, by providing a data set for a rating curve, which combines both structures.
Discharge coefficients for weirs
If a weir is used, then approximate values for the coefficient of discharge C_{dw} can be selected from the table below. However, note that the actual value varies with the type of weir and the head over it.
Structure  C_{dw} 

Rectangular thinplate weir (full width and ventilated)  0.59 
Rectangular thinplate weir (side contractions)  0.56 
Rectangular profile, broadcrested weir (sharp u/s edge)  0.46 
Rectangular profile, broadcrested weir (rounded u/s edge)  0.54 
Rectangular section flume (L=throat width)  0.54 
Floodbank  0.63 
Triangular profile (Crump weir)  0.63 
Ogee weir  0.67 
Discharge coefficients for sluices
If a sluice is used to represent an orifice, sluice or throttle pipe at the outfall, then appropriate values for C_{ds} for submerged flow can be selected from the following table.
Structure  C_{ds}  

Circular, sharpedged orifice  0.60  
Circular, sharpedged short tube, flush with headwall (L=2d to 3d)  0.82  
Concrete pipe culverts, squareedged entrance, flush with headwall (running full) (L = culvert length, d = diameter)  L/d < 10  0.700.80 
10 < L/d < 50  0.620.75  
50 < L/d < 100  0.500.60 
The equation for submerged flow (h1 ≥ d + p) is reasonably accurate for moderate to high heads, provided an appropriate value for C_{ds }has been chosen, but is less accurate when the water level in the Pond is near the soffit. The equation for free surface flow through the outlet (h1 < d + p) has been derived for rectangular orifices and will tend to over estimate the discharge through partfull circular orifices. Another possible source of error in both equations is the use of a constant value of C_{d}. It is usually satisfactory to select C_{d} for the outfall when it is operating at high heads and to accept that errors will occur during the early filling or late emptying phase. If a more accurate representation of the outlet control is considered essential, then a rating curve should be specified. Alternatively, a Reservoir with a more appropriate structure attached could be used.
Datafile Format
Line 1  keyword POND [title]
Line 2  keyword ONLINE
Line 3  Label1, label2
Line 4  n_{1}
Line 5 to line 5+n_{1}  h_{i}, A_{i}
Line 6+n 1  nstruc
Line 7+n 1  keyword OUTFLOW
Line 8+n 1  noutflow
The following block of data is repeated noutflow times
Line 9+n 1  keyword OUTFLOW WEIR,
OUTFLOW SLUICE
or OUTFLOW RATING
If OUTFLOW WEIR is specified, then:
Line 10+n 1  C_{dw}, B_{w}, z_{cw}
If OUTFLOW SLUICE is specified, then
Line 10+n 1  C_{ds}, A_{s}, z_{cs}, d_{s}
If OUTFLOW RATING is specified, then
Line 10+n1  n2
Line 11+n1 to Line 11+n1+n2  Q_{oi}, h_{oi}
End of repeated block