Stage/discharge data must be compiled for the normal outlets and emergency spillways. Stage/discharge data may be entered directly in coordinate form, or an option is available to use standard hydraulic equations for preliminary runs only.

Default equations presently used are:

Normal pipe outlet with h < 1.0 x diameter


h =

height of water in basin over invert of outlet pipe

d =

pipe diameter (m)

Qp =

discharge through pipe at stage h (m³/s)

Ap =

area of flow in pipe at stage h (m²)

R =

hydraulic radius at stage h (m)

S =

pipe slope

n =

Manning's roughness (empirically set internally to 0.021) to take into account additional entrance and exit losses

Normal pipe outlet with h > 1.0d


Qp =

discharge through pipe at stage h (m³/s)

d =

pipe diameter (m)

n =

Manning's roughness (presently set to 0.011)

L =

length of pipe (m)

H =

total hydraulic head (m)

g =

acceleration due to gravity

ki =

entry loss coefficient


exit loss coefficient

N =

number of conduits

Equation (29) assumes that the pipe flows under head when the headwater ratio (h/d) exceeds 1.0. The conduit is then assumed to operate under outlet control. 


The spillway is treated as a normal weir with an equation of the form:


Qs =

discharge over spillway (m³/s)

c =

coefficient of discharge, default set at 1.7 for a broad crested weir

w =

spillway width (m)

hs =

height of water above spillway.

Under basin options the weir coefficient can be set to any specified value or a weir stage/discharge curve may be used to replace the standard Equation (30).

At present the program also has the ability to handle spillways, orifice type normal outlets, fuseplug spillways, unrouted low-flow pipes through the basins. It can optimise the size of normal outlets for given basin storage volumes or maximum desirable outflow. These aspects are further discussed below.

Equations (28), (29) and (30) are generally only used for preliminary investigations. For detailed design of retarding basins it is essential to carry out separate offline detailed hydraulic investigations to derive accurate outflow characteristics as this data is the mainstay in defining the basin's operation and safety factor.

In this regard a stage/discharge curve is derived taking into account tailwater effects, flow transitions in the normal outlet flow regime, the operation of the spillways plus realistic maintenance and blockage considerations. An adopted stage/discharge curve is then inputted as a series of coordinate points.

It is possible to put in separate series of coordinates for the normal outlet and spillway provisions.

Types of Basins

  • Hydraulically Discrete Retention & Retarding Basins. Discrete retention and retarding basins are considered those which operate independently. In this case the stage/outflow relationship is treated as a unique function.
  • Hydraulically Interconnected BasinsHydraulically interconnected basins can be analysed using XPRafts. They are defined as basins that are potentially operationally dependent on the time/stage relationships of any downstream basin or basins.

XPRafts uses an iterative approach to solve this type of situation and can analyse as many interconnected basins as there are links in a catchment.

Interconnected basins or basins outletting into a river have the option through variable "IFLAP" to have free two-way draining outlets or a flap gate control outlet that prevents flows back into the basin from downstream.

Reduced and possible reverse flow is presently limited to the normal piped outlet or the stage/discharge rating curve. Submergence effects, or reverse flow over spillways are presently not included. Additionally, submergence or reverse flow in the normal pipe routine is restricted to inflow heads above pipe obvert only. To date this facility has not been utilised in the workbench modelling.