(a) Introduction

This model, now obsolete, was the original hydraulic calculation procedure applied to pipe and open channel systems in DRAINS. In December 2010, this method was superseded by the Lite & Full Unsteady hydraulic models. It has been retained in existing models, but is not available in new DRAINS models.

Originally, the hydraulic procedures in DRAINS were intended to be rather simple, based on projections of HGLs upwards through pipe systems from a defined tailwater level. This was done at each time step during a storm, with flowrates and water levels varying. The state of the system was defined by quasi-unsteady means, allowing for the transfer of flows through the system and keeping track of water levels. No attempt was to solve the equations of mass and momentum conservation, and a different method was required when pipes ran full under pressure. The 'engine' from Watercom’s PIPES program was used to model pressurised flows. A different set of calculations were applied for open channels, and yet another for kinematic wave routing along overflow routes.  

The basic hydraulic procedure permits two outlet pipes to be specified for a pit but the Lite and Full unsteady hydraulic models can handle this better.

The following section describes how the basic hydraulic calculations are carried out.

(b) Pipe system calculations

In the basic model, drainage systems are analysed by making downwards and upwards passes through the pipe or channel network at each time step, going from each pit or node to the next one downstream or upstream. The first pass moves downwards from the top of each line in the system, establishing the surface flows arriving at each node by adding flows from the local catchment, overflows from upstream and user-provided flows. Using the pit inlet capacity relationship, bypass flows are determined. The flow entering the pit is then added to any flows through upstream pipes and possible user-provided hydrographs to define provisional pipe flows.  

When the calculations reach the system outlet or outlets, DRAINS makes the upwards pass, starting from the tailwater level at the outlet. Allowing for pipe friction and pressure changes at pits, it defines the position of the HGLs at pits and nodes, and if necessary, modifies the flowrates in the pipes and the corresponding overflows. For part-full pipe flow, this process is carried out by projecting HGLs upwards and allowing for pressure changes at pits. If a pipe flows full, a pressurised flow calculation procedure is used to define HGLs at pits and flowrates in pipes. This is done at each time step, so that a run may involve hundreds of HGL traces.

Whenever it encounters a junction, DRAINS projects HGLs up both branches from the pit water level.  If the calculated water level in a drop pit is determined to be below the invert level of an incoming pipe, the tailwater is set at the critical depth in this pipe, and upwards HGL projections are continued. If a HGL rises above the obvert of a pipe, DRAINS switches to the pressurised pipe model, which may use shorter time steps, thus producing an apparent slowing down of calculations.

This model provides information on water levels at pits and nodes and flowrates through pipes. When part-full flow in pipes is sub-critical, the standard step method is used to compute backwater curves from the downstream pit or node water level. Where part-full flow is supercritical, the normal depth, lower than the critical depth, is typically assumed to be the water depth along the pipe.

The flowrates in the hydrographs presented for pipes are those calculated at their upper ends, so that the flows displayed in DRAINS outputs at a particular time will probably differ from the flowrates emerging from the pipe at this time. If a pipe is unpressurised, these outflows will equal the flows that entered a conduit a certain number of time steps previously (depending on the pipe length and flow velocity). If it is pressurised, there is no time delay. DRAINS manages the transfers between part-full and full-pipe flow so that there are only small continuity errors.

In a Design run, DRAINS determines pipe sizes and invert level positions by determining the peak flows of calculated hydrographs and designing for these in a downwards pass followed by an upwards one. You then need to run major or minor analysis runs separately.

(c) Open channels

Along flowing open channels the depth of water will vary due to the flowrates, boundary conditions at the beginning and end of the channel, changes to the channel geometry (longitudinal- and cross-sections) and the presence of structures such as weirs, gates, bridges and culverts.

DRAINS allows for abrupt changes, as at a submerged culvert or weir, and for gradually-varied flow controlled by a downstream feature such as a weir or the tailwater level. The basic hydraulic calculations perform backwater calculations at each time step, working upwards from a downstream tailwater level. For flows at depths below the critical depth for a channel, the water levels are set at the critical depth level (This is similar to running HEC-RAS in subcritical mode). This assumption, which differs from that applied to pipe flows (see above), will set water levels conservatively high if the flow is supercritical, but will be correct for sub-critical flow. It will however, underestimate velocities at some locations.

(d) Overflow routes