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 converted to this format. The alternative format is binary, which stores data more efficiently and securely, but is not accessible by text editors. ASCII files can be displayed and changed in text editors and word-processing programs. DRAINS stores data in binary form in its DRN files, but details of components and results such as hydrographs can be exported to spreadsheet and GIS programs in text formats.
AHD (Australian Height Datum) is the standard elevation reference system used for surveying and mapping in Australia, based on sea level heights. Since DRAINS files provide records and models of a system for future use, levels should be related to a standard datum such as this, where possible.
A chainage is a surveying term, originating at the time when distances were measured with chains, for the length of some geographic feature. It is used to define distances along a channel, from some zero point. Chainages can run in both upstream or downstream directions. In HEC-RAS (see below), chainages are called 'river stations'.
The term crown can apply to (a) the high point on the centreline of a road cross-section, or (b) the top of the exterior of a pipe.
Event models, such as those 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, particularly the wetness of the catchment prior to the event. During the storm, the model must describe the catchment becoming wetter. In the other type of rainfall-runoff model, a continuous model, it is necessary to provide soil moisture and groundwater storages to model changes in soil moisture levels and interflow, the water flowing out of the partly-saturated soil layers located above the water table, forming seepages or springs. These models can describe both the wetting and the drying out of soils. DRAINS only models events.
A floodway is the area around a stream that can be occupied by floodwaters, and which is needed to convey large floods without overflows and consequent damage. As a general guide, the zone inundated by a 1% AEP or 100 year average recurrence interval flood can be considered to define a floodway. The term is also applied to open channels that are intended to convey floods, and sometimes to artificial channels constructed to pass floods around a protected area. Another use is to describe sections of road that are lowered to pass stream flows over them as an alternative to constructing culverts. These may also be called dips.
The HEC-RAS program was originally produced in 1996 by the Hydrologic Engineering Center (HEC) of the U.S. Army Corps of Engineers. RAS stands for 'river analysis system'. HEC-RAS superseded the Corps’ HEC-2 program, first introduced about 1970 as the most widely-used program for calculating water surface profiles. HEC-RAS is available from the HEC website: https://www.hec.usace.army.mil/software/hec-ras/download.aspx. Version 5, which performs 2-dimensional flow calculations, was released in early 2016.
A non-return valve (one-way valve, reflux valve) is a device such as a flap gate that only allows flow to occur in one direction in a pipe. It may be used in situations where tailwater can be high, causing water to flow backwards through a pipe. DRAINS models these valves in the Pipe property sheet, using the switch named Include Non Return Valve.
Open channels carry water that has an interface with the atmosphere. The water surface can take many shapes due to gravity and inertial forces, and the hydraulic grade line follows the water surface. Open channel hydraulics requires consideration of energy effects using friction equations, specific energy concepts and water surface profile calculations. Pipe systems operate as open channels when they are not flowing full. DRAINS models open channels as links or reaches between nodes. It is not possible to have overflows from a node, except in the Full unsteady hydraulic model, where it can be used to model a situation where flows may split into two paths.
A reach is a name given to a section of river, open channel or pipe. Sections are often identified by chainages (see above) measured along the length of the waterway or conduit. In discussing pipe or channels networks, the words 'link' and 'reach' are often used interchangeably.
Runoff is any flow that runs or flows off a surface. Thus we can speak of catchment runoff, stormwater runoff or surface water runoff. A related term is run-on, where flow runs onto a surface from adjoining areas.
Steady flows are flows that are constant with time. They are rare in urban stormwater drainage systems, since rainfall inputs and flows change rather quickly. However, it is common to perform hydraulic grade line or water surface profile analyses assuming that peak flows in various pipes or channels occur simultaneously. This gives a picture of the system that is usually conservative. Spatially-varied flows, where flows increase or decrease along a channel or pipe, are more characteristic of stormwater drainage systems. Contributing catchment areas and flowrates commonly increase along drainage paths such as street gutters. Unsteady flows constantly change with time, as hydrographs pass through conduits and storages. The unsteady flow calculations in DRAINS allow for these.
A storm event is an occurrence of a storm that produces rainfall and runoff through a drainage system. Event models such as DRAINS calculate the behaviour of the drainage system at a number of time steps, beginning at the start of the storm. The loss model used simulates the wetting of the catchment soils, but it usually cannot simulate their drying out. For continuous modelling, soil moisture storages must be used, which are capable of being filled and emptied.