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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