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
    of on-grade pits, and makes accurate modelling of on-grade pits
    difficult or impossible.
•   DRAINS checks for inlet control at culverts, headwalls or
    detention basins. This is not an issue in sewers.
•   shock losses are an important factor determining the capacity of the
    subsurface pipe system. They are easily handled in DRAINS.
•   A form of Inlet Control can occur at pits in steep terrain (shallow
    pit depth, steep outgoing pipe). If this is ignored the results can
    grossly overestimate the hydraulic capacity of the pipe out of the
    pit. DRAINS checks for this condition.

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