(a) Introduction

Introduced in 2010, the Standard and Premium hydraulic model procedures superseded the Basic hydraulic model that had been applied in DRAINS since its first release.

In 2020 the Standard hydraulic model was renamed to Lite hydraulic model and the Premium hydraulic model was renamed to Full unsteady hydraulic model.

Both the Lite hydraulic model and Full Unsteady hydraulic model solve the full St Venant unsteady flow equations in pipes and channels. Only the Full Unsteady hydraulic model solves these equations in overflow routes while the Lite Hydraulic model analyses over flow routes using simple normal depth calculations. This means that storage effects in overflow routes are ignored in the Lite hydraulic model and therefore peak flows, depths and velocities in overflow routes may be overestimated. Experience has shown that the peak flows in overflow routes with the Full Unsteady hydraulic model can be lower than with the Lite hydraulic model, especially when a model includes several sag pits. This can lead to more economical designs with the Full Unsteady hydraulic model.

When there is limited overland flow such as a well designed drainage network, the Lite hydraulic model storm results are similar to those for the Full Unsteady hydraulic model, as there are limited storage and tailwater affects. Differences are more pronounced in the 1% AEP runs, or in situations where flooding is an issue.

(b) Calculation method

The unsteady flow engine used with the Lite and Full Unsteady hydraulic models solves the full St. Venant equations of momentum and continuity using an implicit finite difference scheme with a staggered H-Q grid. This solution scheme is widely used in pipe system software (for example, in the SWMM and MOUSE programs). Links are divided into an odd number of reaches (1, 3, 5, etc.) with DRAINS automatically determining a suitable number. 

The Saint Venant Equations for conservation of mass and momentum in unsteady flow are:

where  Q is flowrate,
             A is cross-sectional area,
             H is water surface level, 
             t is time,
             x is distance along a channel,
             g is gravitational acceleration,  and
             Sf is friction slope .

The calculation procedure solves these equations (in finite difference form), together with boundary condition equations (at pits, headwalls, detention basins, fixed outlet levels etc), to determine H and Q at all points in a system at each time step of the simulation. 

Part-full pit pressure changes for pits are handled by assuming that the pressure change factor k_u is the same as the full pipe flow factor.

Full unsteady hydraulic model (formerly Premium)

(a) Introduction

In addition to the unsteady flow modelling of pipes and open channels that is provided in the Lite hydraulic model, the Full Unsteady hydraulic model applies the full St. Venant equations of unsteady flow to overflow routes. This allows water levels along these routes to be determined accurately, allowing for varied water surface flow profiles, including subcritical and supercritical flows. It also accounts for storage effects in overflow routes. In many cases this storage provides very significant reductions in peak flows and water levels. The Lite hydraulic model, using simple translation, can overestimate peak flows and water levels. The Full unsteady hydraulic model can often provide significant cost savings in the design of pipe systems.

At sag pits there are two HGLs, one describing the water level in the ponded water and the other describing the pipe HGL. Calculations for outlet weirs from sag pits, detention basins, headwalls, and culverts use tables of elevation vs discharge. To cover the situation where tailwater levels below these controls are high enough to submerge the weir, DRAINS uses the Villemonte equation to modify the table if the downstream water level rises above the weir crest.

Tests indicate that surface flows are likely to be smaller than with the standard calculations, mainly due to the specific allowance for the storage of stormwater above sag pits, and within water moving along overflow routes.

The Full unsteady hydraulic model can also model many situations that cannot be handled by the Lite hydraulic model, such as:

• two or more overflow routes from an on-grade pit or a sag pit,

• backwater calculations along overflow routes, defining depths influenced by high downstream water levels, including ones that may submerge the pit,
  

• complex looped systems of surface flows,

• modelling of both subcritical and supercritical flows on in overflow routes.

(b) Additional Inputs and Outputs

Most inputs to a Full unsteady hydraulic model are the same as for the Lite hydraulic model.  The extra inputs are:

• overflow routes must be defined by lengths rather than by a lag time,

• invert levels must be defined at the upstream and downstream ends of overflow routes,

• a weir control must be defined for an overflow route coming out of a sag pit.

DRAINS  provides a comprehensive system of checks to ensure that overflow routes are correctly set out.

The main differences in the outputs are that it is possible to view long-sections for overflow routes that show how water levels vary along the route.