# Random Notes On Drains Unsteady Flow Engine by lindayy

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```									               Notes On Drains Unsteady Flow Engine
1. The unsteady flow engine in DRAINS solves the full St Venant equations of
momentum and continuity using an implicit finite difference scheme with a
staggered H,Q grid. Links are divided into an odd number of reaches (1 or 3 or
5 etc). When DRAINS reports the flow in a link it is referring to the flow
calculated at the centre of the link. Water levels are reported at the upstream
and downstream ends of the link. In long links, water levels are also obtained
at intermediate points along the link and these appear in long sections and
animations. Animations can be seen from the View/Animation menu item.
2. The unsteady flow model in Drains includes channel storage and inertia
effects in the underlying equations. These effects are not included in the basic
Drains calculation engine. Channel storage effects can attenuate the peak
flows (similar to a detention basin). This can be significant in long channels
and overflow routes. Inertia effects mean that to change the velocity of a body
of water (accelerate or decelerate it) a head difference (greater or less than that
required to overcome friction) must apply for a finite time. This can lead to
some unusual effects (eg water can be seen to flow uphill for some finite time.
This is OK provided the water is decelerating while it happens. Such effects
should not occur with the basic Drains engine).
3. A big advantage of the unsteady model over the basic model is the accurate
modelling of surface flows in overflow routes. The unsteady model provides
full 1D unsteady flow routing in overflow routes compared with simple
translation or, at best, kinematic routing in the basic model. These hydrologic
routing procedures did not provide accurate depths in overflow routes and they
allowed water to flow uphill. Although Drains warns of water going uphill,
these warnings were often ignored by users, sometimes with unfortunate
results. The new unsteady model prevents silly results like these. It can even
allow flow reversal in overflow routes where appropriate.
4. Another advantage of the unsteady flow model over the basic model is the
more accurate modelling of sag pits. The basic model assumed water did not
rise above the “maximum ponded depth”. The unsteady model uses a weir
equation or a table of depth vs flow over the weir. This can lead to greater
depths at a sag pit, with additional storage above the weir crest included in the
calcs. The extra storage can attenuate peak flows and the greater depth can
force more water down the subsurface pipe system.
5. The unsteady model also allows flow splitting at pits and nodes where 2 or
more overflow routes are possible from a pit or node. The correct flow split is
determined automatically because the overflow routes are now modelled
hydraulically as a network of channels. This surface network is connected to
the subsurface pipe network by pits, headwalls etc.
6. DRAINS was developed specifically for separate drainage systems. It has
significant advantages over models that were developed to model sewerage
systems (eg MOUSE and SWMM) which overlook some important
differences between drainage and sewerage systems:
• at on-grade pits the inlet capacity is a function of approach flow.
Most sewer oriented models force the user to treat it as a function
of depth. This means the user cannot use the large body of
published research data or the HEC22 procedure for inlet capacity