Guidelines and Technical Information
ARR 2019 Initial loss - Continuing loss (IL-CL) hydrological model
The IL-CL hydrology procedure in DRAINS is an alternative to Horton (ILSAX). Both methods are accepted in the ARR 2019 guidelines and discussed in Book 5 Chapter 3. The IL-CL model and its parameters are set out in Section 3.5.3 of ARR 2019. ARR 2019 ...
Urban stormwater management
This is the complex process of dealing with the problems associated with stormwater drainage in the urban environment. It has three main concerns – (a) the reduction of flooding and waterlogging effects, and (b) the control of pollution carried ...
Urban stormwater drainage systems
These are systems constructed by humans to convey stormwater through urban areas without undue damage to property or danger to persons from floods, and to minimise environmental damage to drained areas and receiving waters. Older systems are combined ...
Time-area method
This is a form of catchment routing model in which a hyetograph of rainfall is combined with a time-area diagram to produce a flow hydrograph. All of these must use the same time step, Dt. The procedure effectively divides a catchment into a number ...
Time of travel (time of entry, time of concentration)
Generally these names refer to the time required for water to flow from the farthest point on a catchment to its outlet during a storm, following flow paths dictated by natural topography and human changes. Times of flow are complex, since they ...
Tailwater level
This is the water level occurring at the outfall or lower end of a drainage system, which can influence water levels and flows through a system due to backwater effects. Setting an appropriate tailwater level can be difficult. The following comments ...
Storage routing calculations
These are carried out over a series of time steps, with the information obtained from solving equations at one time step being used as an input to the next step. In reservoirs, the information needed is: a relationship between storage and outflow, ...
Soffit (culvert or bridge)
This is the level of the top of the interior of a culvert, or the underside of the deck of a bridge.
Simulation
Simulation is the most common way of performing analysis and design checks. This process, usually in mathematical form, imitates or mimics the behaviour of some real thing or system. In models such as DRAINS, the behaviour modelled is the passage of ...
Routing (attenuation)
Hydrological routing is the process of converting a hydrograph that passes through some part of a flow system to allow for the changes that occur during its passage. There are three main types of routing: catchment routing, which converts a rainfall ...
Queensland Urban Drainage Manual (QUDM)
This 1992 publication extended the methods and information provided in Australian Rainfall and Runoff, 1987. The methods set out in QUDM are more rigorous than those applied in most other parts of Australia. Accurate pit inlet capacity and pressure ...
Part-full HGL changes
Throughout the development of DRAINS, various procedures have been applied to handle pressure changes at pits when flows are part-full. This has been difficult because there is little literature or guidance on this subject. In the Lite and Full ...
Overflows
These are stormwater flows that escape from a pipe system or a detention storage for one of these reasons: flows reaching a pit cannot be accepted by the pit inlet, and flow away as bypass flow, flows are forced to upwell out of a pit because the ...
On-site stormwater detention (OSD)
This is the practice of providing storage devices within properties, with the aim of counteracting the hydrological effects of urbanisation from the increase in impervious surfaces. Storages are usually small and can consist of underground tanks or ...
Orifice control
In hydraulics the term 'orifice' refers to a hole or inlet through which water must pass. Orifices restrict flows and hence they control the flows going into downstream pipes or channels with larger flow capacities. Orifice control occurs when the ...
Obvert (or soffit)
This is the highest point on the inner surface of a pipe or closed conduit.
Normal depth
If a steady flow occurs in a long, straight channel, the velocity and water level will become constant, as the loss of energy through friction will equal the energy lost through the fall along the channel. The water surface slope will be the same as ...
Map projections
Spatial data (with x and y coordinates) is set out in a number of possible map projections. When DRAINS imports a background and other data from CAD or GIS files, these will be to one of the common projection systems used. Generally, you will not ...
Manning’s equation
This equation or formula calculates the velocity and flowrate of the flow that will occur in a long channel of uniform cross-section (a 'friction' equation). It has the form: Where the depth of flow is known, the equation allows the velocity and ...
Loss model
In rainfall-runoff modelling, loss models are methods or procedures that determine the 'losses' from rainfall that do not contribute to runoff. They are abstractions from the rainfall, producing rainfall excess, equal to rainfall minus losses. The ...
Lag time (Lag factor)
In hydrology, catchment lag or delay has a general meaning, as the time taken for runoff in general to flow off a catchment to its outlet. It relates to the time difference between the centroids of the rainfall hyetograph and the runoff hydrograph. ...
Kinematic wave
This refers to a method of calculating the characteristics of flows across planes and in channels, based on a simplified or cut-down part of the St. Venant Equations for conservation of mass and momentum in open channel flow. Kinematic wave ...
Invert
For a pipe or closed conduit this is the lowest point on its interior surface. For a channel it is the lowest point on the channel bed. Levels of inverts are used to define the vertical alignment of pipelines and channels.
Initial loss
This is a hydrological loss that occurs at the start of a storm event. It is expressed in mm and is subtracted from the beginning of a hyetograph by adjusting the rainfall values. Initial losses are an important feature of IL-CL (initial loss - ...
Infiltration
This has two meanings. Applied to the rainfall-runoff process, it is the passing of rainwater and other forms of precipitation into the soil. Under the action of capillary and gravity forces, rainwater is absorbed into the soil to be held as soil ...
ILSAX
ILSAX is a public domain MS-DOS program developed from the South African ILLUDAS-SA program, which was itself a development of the American ILLUDAS program and the British Transport and Road Research Method (known as the TRRL or RRL model). It was ...
I-F-D (intensity-frequency-duration) relationships
This is a three-way relationship that defines the statistical distribution of rainfalls expected to occur at a particular location. It is derived from statistical analysis of rainfall pluviometers or automatic raingauges, which record rainfalls ...
Hyetograph
This defines a rainfall pattern, usually shown as a block diagram. Rainfall is presented in blocks because depths are accumulated over a time period, rather than being measured at a particular time, like streamflows or water levels, which are usually ...
Hydrograph
This is a time series of some water charachteristic, usually flowrates, expressed graphically as a series of ordinates, representing the values at different times. Drainage engineers are mainly interested in the peak flowrate and in the volume.
Horton equation
Horton’s equation is the most common relationship for describing infiltration capacity in a soil. It describes the decrease in capacity as more water is absorbed by the soil, and has the form: f = fc + (f0 - fc)・e-kt where f is the infiltration ...
Hydraulic grade line (HGL)
The HGL is a line that can be drawn along a pipe or channel, indicating the pressure. In a pipe flowing full it is above the pipe obvert so that the pipe is pressurised. In an open channel it follows the water surface, except in some special ...
Hydraulic grade line analysis
This is a form of hydraulic analysis of piped drainage systems that traces the positions of hydraulic grade lines throughout the system. It is usually applied as a steady state model, calculating HGLs for the peak flowrates at each pipe in a system. ...
Head loss
This is the loss of energy or head (height of water expressing energy per unit weight) occurring in a pipe or channel carrying flows of water. The loss is caused by friction between the water and the conduit’s walls and within the flow, and by ...
Gutter (Channel) and roadway hydraulics
Since DRAINS models catchment flows that flow in street gutters or channels and overflows that travel along streets, it has some facilities to deal with various gutter and roadway sections, as shown below: Road cross-section characteristics can vary. ...
Glossary
ASCII stands for American Standard Code for Information Interchange, and describes the plain text format used in many computer data files. Many common date formats used by spreadsheet, CAD, database and GIS programs are in ASCII, or can readily be ...
GIS Programs
Geographic Information System (GIS) programs combine a mapping facility with a data base of information on the spatial position of components, such as drainage pits and pipes, and on their other attributes, such as pipe diameter. Objects can be ...
Friend's Formula
This equation appeared in early versions of Australian Rainfall and Runoff and is still presented in the 2013 Queensland Urban Drainage Manual (QUDM) and other guidelines. It has the form: The graphical version of this equation is presented in QUDM ...
Freeboard
In hydraulic design, this is a height that is added to an allowable water level for design purposes. For example, in determining the height of a flood wall, or a channel that is required to contain flows of a given frequency (ARI or AEP), a level ...
Flood modelling
To cope with flood problems, it is necessary to use a number of technical procedures and models to define the physical nature of flooding in terms of flood frequency, flowrates, depths and widths. This information is combined with social and economic ...
Event model
The rainfall-runoff models in DRAINS simulate particular storm events, rather than extended and continuous time series of rainfalls and runoffs that might contain many events. With an event model it is necessary to specify antecedent conditions, ...
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