Normal Outlets With Optimization

Often for the design of retarding basins, it is necessary to gain full advantage of either storage limitations or discharge limits at various locations within the drainage systems. This can depend on the maximum storage available at a particular basin site, or maximum discharge the downstream reach can contain.

To enable the design of the retarding basin to incorporate one or the other of these features, optimization of the normal outlet pipe size is available.

It is usual to assume an initial pipe size and the program will adjust this size until is within a specified tolerance from the desired storage volume or basin discharge. The initial assumption should be as near as practicable to the final pipe size in order to minimize computing time. Lengthy optimization will generate lengthy computer runs.

Weir Type Spillways

At present RAFTS allows a broad crested weir spillway to be included separately and additional to a stage/discharge curve. A default coefficient of 1.7 has been adopted. Should a different coefficient be required then this may be assigned in the basin input data.

To allow for spillways of unusual geometry or hydraulic properties, RAFTS allows the input or a weir stage/discharge curve additionally to the normal outlet curve. This option then replaces the standard weir equation within the model.

Weir submergence is taken into account with assessing overflows with the interconnected basin option. RAFTS recognized different submergence coefficients for broad crested, ogee, and sharp crested weirs respectively. Differentiation is selected via the weir coefficient being 0 to 1.75, 1.75 to 2.00 and above 2.00 respectively. The adopted coefficients are listed below:

Fuseplug-Erodible Spillways

To enable the outflow characteristics of a fuseplug spillway in RAFTS, a routine has been included that models the time sequence of the flow through the fuseplug spillway over a range of storm events up to maximum probable flood.

Spillway erosion rates are included in the routine, and data is input in the form of; the base stage, the sill stage and effective width of the spillway.

Further details on this type of spillway, including required input data, are given in the figure below:

Parameters A1 and B1 used in the fuseplug spillway definition are also used to define hydraulically interconnected basins and therefore a fuseplug spillway cannot presently be modelled with an upstream interconnected basin.

Low Flow Pipes Under Basins

Often in the design of retarding basin, a low flow pipe is incorporated to carry flows up to the pipe capacity through the basin unrouted. XPRafts will model this situation.

The pipe capacity is input as the unrouted base flow and the program will separate this flow from the hydrograph obtained at the particular location. The remainder of the hydrograph is routed through the basin.

Multiple Orifice Type Outlets With or Without Optimization

A variation in outlet design for retarding basins can include the use of an orifice type outlet. This usually involves the construction of a tower with a number of openings at different levels. The lower openings act as the normal outlet and the upper openings act as spillway or secondary routing structure.

Additional weir type spillway can, of course, be incorporate into the basin embankment. Multiple orifice outlets are often used to maximize routing effects for both low and high flows within the limits of overall basin storage.

XPRafts presently allows for two levels of orifice type outlets with the option to optimize both outlet sizes to predetermined storage requirements.

Discharge through the orifice outlet of unit width is defined as follows:



Qo=discharge through the orifice (m3/s)

Cd=coefficient of discharge

h= height of orifice (m)

w=width of orifice opening=1.0

H=head over orifice invert (m)

g= acceleration due to gravity

Variations to the orifice width are achieved by a multiplication factor. Details of input variables related to single or multiple level orifice outlets are shown in the figure below:

Orifice stage/discharge data must be defined by a stage/discharge curve. PDIA is defined, in this instance, to be a width factor, i.e. PDIA should normally be set to 1 (1.0xS/D coordinates). A maximum of two orifices are allowed.

Other multiple level outlets other than an orifice can be similarly modeled as long as they can be represented by a single S/D rating curve or ratio thereof.