In this sheet, you must specify an elevation-surface area relationship (or an elevation-volume relationship), select the type of low-level outlet and if necessary, enter data for the low-level outlet of the basin. The characteristics of basin outlets are defined separately in property sheets for the outgoing pipe, and for one or more high-level outlets defined as overflow routes.

Note that DRAINS does not assume direct rainfall on a Detention Basin, unless you specify an attached catchment. This allows the modeller the choice of either option.

To make data specification easier and calculations more accurate, an elevation-surface area relationship is standard. This removes the need to calculate volumes from surface areas before entering data into DRAINS. However, if you wish to specify volumes directly, you can choose this option in the Options choice in the Project menu. DRAINS models set up previously with elevation-volume relationships for detention basins can still be applied.

(a) Name

First you must specify a basin name of up to 10 characters.

(b) Elevation-surface area relationship

The elevation or height of water in the basin is defined in terms of the datum used throughout the system for pipe inverts and surface levels. The relationship between elevation and the surface area of the ponded water is defined by a pairs of values entered in the table at the top right-hand corner of the sheet. You can also paste in a table from a spreadsheet, by setting out two columns of cells with elevations and corresponding areas, selecting and copying these, and then clicking the Paste Table button.

Surface areas corresponding to particular elevations are commonly estimated from contours defined in CAD drawings. Additional "quantity" calculations convert these to volumes. For accuracy, it is important to specify a suitable number of surface area points for some shapes.

DRAINS may not accept a zero surface area for the first point in the elevation-surface area table, and may recommend a small area instead.

Be sure to specify areas to a sufficiently high level. If the water level goes above the highest level in the table, DRAINS will extrapolate volumes from the last two sets of values, which may be inappropriate.

This is set out in the same way as the elevation surface-area information, but the associated calculations assume straight-line segments between the specified pairs of elevation-volume values, as shown in the graph produced by the Storage vs Elevation Graph option in the pop-up menu for a detention basin.

(d) Low-level outlets

You have options for modelling the lowest outlet from a detention basin – an orifice, a pit or sump, a circular conduit, a rectangular conduit and 'None'. The data to be entered for the various options differs. 

If you choose the Orifice outlet option, the sheet will appear as shown above. You must specify the orifice diameter and the level of its centre. The orifice is assumed to be a circular, sharp-edged hole in a plate, with a hydraulic discharge coefficient of 0.61. This is commonly used as a flow control in on-site stormwater detention systems.

Stormwater will pass through the orifice into an outlet pipe outlet pipe or channel that is defined separately. An orifice wizard calculator is now available in DRAINS that allows you to calculate orifice sizes, allowing for maximum depths of ponding, allowable outflows and orifice coefficients other than 0.61. A similar wizard is available for special orifices that run with the Full unsteady hydraulic model method.

High early discharge pits in OSD systems can be modeled by clicking the box in the lower left corner of the sheet and adding the height  and crest length of the weir in the HED pit.

In the View Water Level Tables output for a detention storage with a HED pit, the water levels in the pit and the main storage are presented separately.

If the Pit/Sump outlet option is chosen, the property sheet takes the form shown below. You must select a pit family and type from the pit data base.   Special pit types can be developed using elevation-discharge relationships calculated from the sag pit wizard in the Pit Data Base. The flow threshold (lowest point in the table) will be the elevation of the pit/sump. Alternatively, these can be developed in a spreadsheet and pasted into DRAINS.  Allowance for blockages should be included in these relationships, as the DRAINS detention basin property sheet does not provide for this.

This option can be used to model basins that have a sag pit as an outlet, or unintended storages occurring when overland flow paths are blocked by roads, fences and other developments.  Since pipes cannot enter a basin below its lowest level, you must include the pit and its lowest point in the basin.

Basins with pit outlets can be placed on-line if the incoming pipes are at levels above the surface of the storage, but this is a rare situation.  With such outlets, it is also possible to place the pit entrance above the base level of the detention basin, as in bioretention basins.

If you have chosen a circular pipe or rectangular culvert, the sheet shown below appears for a new basin.

It is necessary to enter the entry and bend head loss factors representing the losses at the pipe inlet and at any bends or changes of section in the pipe system. These factors are available from fluid mechanics texts.  Losses due to grates at the entrance or exit to a pipe may be included in the K_entry + K_bends factor. DRAINS assumes that the water at the upstream end of the pipe is stationary. At the outlet, it assumes that the energy loss will be 1.0.(V_p²/2g - V_c²/2g), where V_p is the pipe velocity, and V_c is the velocity in a receiving channel.

In some older DRAINS models, an older format for detention basins may be used, in which pipe information is provided in the detention basin object. This format is not available in new files, but still runs in older files.

If the 'None' outlet option is selected, no additional information is required about outflows. You will have to specify the outflow details as an elevation-discharge (H-Q) relationship in an overflow route link leading from the basin. This might be the case where the basin outlet is a vertical slot in the basin wall. Outflows will occur under weir control and the threshold for flows will probably be the lowest level in the elevation-storage relationship.

If the 'None' option is applied, hydraulic grade lines cannot be projected backwards through the detention basin, and so no backwater effects apply.  This is acceptable in steep locations, where the discharge pipe or channel can flow freely, but may be questionable in flat areas subject high tailwater levels and to backwater influences.

(e) Initial Water Level

(f) Infiltration

The second panel on the detention basin property sheet displays data that can be used to model stormwater infiltration out of a storage that has a permeable base and/or permeable sides.

The calculations involved are simple; the exposed surface of the storage at any time is multiplied by the hydraulic conductivity to define an outflow. The greater the depth in the storage, the larger the infiltration rate. Allowance is made for storages having permeable or impermeable floors and walls.

Infiltration procedures are discussed in detail in Argue, J.R. (editor) (2004) WSUD: Basic Procedures for 'Source Control' of Stormwater, University of South Australia Water Resources Centre, Adelaide. Indicative values of hydraulic conductivity (p. 44) are given as:

(g)  High-level outlets

A number of high-level overflow routes can be defined, leading out of the basin to various destinations. The information on the outflow characteristics is entered in the Overflow Route property sheet.