Node Properties
Property | Description | Units | Default Value | Recommended Range | Constraints | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Cross-sectional area | Defines the cross-sectional or area of the tank base. | m2 | 2.5 | NA | >= 0 | ||||||
Height | Defines the total height of the tank. Includes detention and dead zone height and dead zone height. | m | 2 | NA | >= 0 | ||||||
Height of off-take invert | Defines the height of the offtake pipe on the tank; the offtake pipe supplies demands from the tank. The invert level of the offtake is the height of the lowest point of the pipe above the tank base. Once the tank is filled above the offtake invert level, the water depth in the tank will never fall below this level. Demand off-take occurs from the base of the tank just above the anaerobic or "dead" zone; a tank should therefore have a minimum "dead zone" depth of 0.1 m to allow for accumulation of sediment and other material. Water will never be drawn from the "dead zone". | m | 0.1 | [0, 2] | >= 0 | ||||||
Initial water depth | Defines the initial depth of water in the tank at the start of the model run. Default: 0.1 m Minimum: 0.0 m Maximum: 1000.0 m | m | 0.1 | [0, 1000] | >= 0 | ||||||
Overflow Configuration | |||||||||||
Detention volume depth | Defines the detention (storage) volume depth of the tank. The detention volume depth is the depth from above the overflow outlet invert to below the tank spill/top of the tank; hdet in the diagram. Detention volumes are used as buffers to decrease output flow rates in situations of high input flow. If the tank has no detention storage, set the detention volume depth equal to the overflow outlet diameter. | m | 0.1 | NA | [0, 2] | ||||||
Overflow outlet diameter | Defines the diameter of the overflow orifice, located at the base of the detention storage. | mm | 100 | [10, 320] | [1, 1000] | ||||||
Overflow outlet discharge coefficient | Discharge coefficient used in the evaluation of the orifice flow equation. | 0.86 | NA | NA | NA | ||||||
External top up | |||||||||||
Enable external top-up | If enabled, allows for the tank to be topped-up with mains water so that the tank always maintains a user-specified water level. You do not need to connect a mains water supply node to a tank node to provide inflow for top-up; when you enable external top-up, Urban Developer handles the mains connection internally. | NA | Disabled | NA | NA | ||||||
Top-up rate | Defines the rate of mains water top-up in litres per second. | L/s | 250 | NA | >= 0 | ||||||
Top-up triggered on | Defines the depth above the tank base at which the tank will start to top up. This depth must be greater than the height of the off-take. | m | 0.2 | NA | >= 0 | ||||||
Top-up triggered off | Defines the depth above the tank base at which the tank will stop automatically topping up. This depth must be greater than the Top-up triggered-on depth. | m | 0.3 | NA | > Top-up triggered on | ||||||
First Flush System | |||||||||||
Enable first flush system | If enabled, changes the tank inflow behaviour so that a user-specified volume of water will bypass the tank before inflow to the tank occurs. | NA | Disabled | NA | NA | ||||||
First Flush Volume | Specifies the volume of water the first-flush device diverts. | m3 | 0.015 | [0, 100] | >= 0 | ||||||
First Flush Outlet flow rate | Specifies the outflow rate to the selected discharge outlet. | m3/s | 0.01 | NA | >= 0 | ||||||
Discharges Connection | Specifies the tank outlet that first-flush outflow is discharged through. Detention outflow specifies that the first flush volume discharges via a connection to the detention outflow pipe. Spill specifies that the first flush volume discharges via spilling from the tank. | NA | NA | NA | NA | ||||||
Other | |||||||||||
Tank bypass | Not currently implemented in the Urban Developer Plugin. If enabled, all inflows flow directly to the detention outflow. This allows you to temporarily disconnect a tank without removing it from the model. | NA | NA | NA | NA |
Tank Technical Details
First Flush Separation
First flush separation is a commonly applied practice in many rain and stormwater harvesting systems. First flush devices are typically used to separate the initial runoff from a surface from subsequent flows and operate on the principle that the initial runoff commonly contains a higher proportion of accumulated surface contaminants, such as dust, sediment, litter, animal droppings, leaves and debris.
There are many forms of first flush separation devices. The storage tank configuration utilised by Urban Developer incorporates a simple volumetric first flush model conceptually illustrated in Figure 1. The device, shown in Figure 1 operates by allowing the first flush to fill a storage chamber, drained by a small orifice. Once full, inflow bypasses the storage chamber and leaves the device via the outflow pipe. Once rainfall ceases the first flush storage chamber is drain slowly as water leaves via a small orifice located at its base. For simplicity, the discharge is assumed to be at a constant rate.
Figure 1: First flush system.
where
Qin is the inflow volume (m³);
Qout is the outflow volume following first flush separation (m³);
qff is the constant outflow rate from the chamber (m³/s); and
Vff is the diameter of the first flush outlet (m³);
The Storage Tank
The rainwater tank model used by Urban Developer is illustrated below. The storage volume comprises three distinct storage zones.
where:
VDetention
is the Detention Storage Volume (m3)
VRetention
is the Retention Storage Volume (m3)
VDead
is the Dead Storage Volume (m3)
hoff-take
is the height of the supply off-take obvert from the base of the tank (m)
hretention
is the height of the retention storage volume (m)
hdetention
is the height of the detention storage volume (m)
htank
is the height of the storage tank (m)
hTT On
is the height of the trickle top-up on trigger (m)
hTT Off
is the height of the trickle top-up off trigger (m)
Tank routing
The Urban Developer storage tank allows for the inflow of water, Qin, as well as providing for an optional trickle top-up volume, Qtopup, that is triggered on and off at user-specified tank heights. Supply to meet consumptive demand, Qsupply is drawn from the base of the tank just above the "dead" storage zone
Inflows in excess of the retention storage volume are routed through the detention outflow, which is controlled according to the capacity and configuration of the outlet. During periods of very large and rapid inflows the detention storage capacity of the tank may be exceeded resulting in spillages, Qspill, from the top of the tank. This spillage volume represents the volume of water that is unable to enter the storage tank.
The routing algorithm adopted by the tank applies the generic first order Ordinary Differential Equation (ODE) solution scheme, to solve the governing water balance present in Eqn 1:
thqhQQQQVVtttttttttΔ−−−++=−−)()(1detentionspillSupplytopupin1
(Eqn 1)
where:
tV
is the volume at the end of time t (m3)
1−tV
is the volume at the end of time t-1 (m3)
intQ
is the inflow volume for time interval t-(t-1) (m3)
topuptQ
is trickle top-up volume for time interval t-(t-1) (m3)
supplytQ
is the demand volume extracted for time interval t-(t-1) (m3)
th
is the depth of water at the end of time t (m)
1−th
is the depth of water at the end of time t-1 (m)
)(spilltthQ
is the overtopping volume as a function of depth at time t (m3)
Urban Developer User Guide 143
thqttΔ−)(1detention
is the discharge rate from the detention storage for time interval t-(t-1) (m3/s)
Outflow from the detention volume is calculated as a function of depth, above the detention outflow orifice obvert, Outflow using the minimum discharge of the broad crested weir and orifice flow (Eqn 2) to account for the transition that occurs as the outflow orifice is drowned.
where:
maxh
is the maximum height of the detention storage (m)
h
is the depth of water above the outlet obvert (m)
outflowdetention q
is the detention storage outflow rate (m3/s)
outflowdetention φ
is diameter of the outflow orifice (m)
(Eqn 2)
where:
Cd
is the coefficient of discharge
g
is gravity (m/s2)