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 (m2) | 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 (m2) | 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 (m3/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 On-Line 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: qin = flow at upstream node qout = flow at downstream node DV = change in volume |
In addition, one or more of the following equations apply
Weir equation
where: Cdw = weir discharge coefficient g = gravitational constant L = length of weir crest (across flow) (m) h1 = water level in pond p = crest level of weir (mAD) |
Sluice equation
Surcharged flow (h1³d+p)
|
where:
|
Free surface flow (h1<d+p)
|
where: Cds = sluice discharge coefficient A = area of opening (m2) P = sluice invert or crest level (mAD) d = depth of opening (m) |
Rating curve equation
|
where: f = user-defined 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 un-steady 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 low-level throttle pipe through an embankment capable of passing the dry weather flow without surcharge, but restricting flood flows to a specified limit. A high-level 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 Cdw 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 | Cdw |
|---|---|
Rectangular thin-plate weir (full width and ventilated) | 0.59 |
Rectangular thin-plate weir (side contractions) | 0.56 |
Rectangular profile, broad-crested weir (sharp u/s edge) | 0.46 |
Rectangular profile, broad-crested 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 Cds for submerged flow can be selected from the following table.
Structure | Cds | |
|---|---|---|
Circular, sharp-edged orifice | 0.60 | |
Circular, sharp-edged short tube, flush with headwall (L=2d to 3d) | 0.82 | |
Concrete pipe culverts, square-edged entrance, flush with headwall (running full) (L = culvert length, d = diameter) | L/d < 10 | 0.70-0.80 |
10 < L/d < 50 | 0.62-0.75 | |
50 < L/d < 100 | 0.50-0.60 | |
The equation for submerged flow (h1 ≥ d + p) is reasonably accurate for moderate to high heads, provided an appropriate value for Cds 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 part-full circular orifices. Another possible source of error in both equations is the use of a constant value of Cd. It is usually satisfactory to select Cd 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 - n1
Line 5 to line 5+n1 - hi, Ai
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 - Cdw, Bw, zcw
If OUTFLOW SLUICE is specified, then
Line 10+n 1 - Cds, As, zcs, ds
If OUTFLOW RATING is specified, then
Line 10+n1 - n2
Line 11+n1 to Line 11+n1+n2 - Qoi, hoi
End of repeated block
