Property sheet for overflow routes from pits

Property sheet for overflow routes from pits

The appearance of this property sheet will vary depending on whether:

  • the storage network routing module is enabled, or
  • you have started to specify extra data for the Full Unsteady hydraulic model.

The figure below shows the property sheet for an overflow route from an on-grade pit assuming (a) that you are using the demonstration version of DRAINS, or (b) that your license allows you to use the Full Unsteady hydraulic model. This is the most detailed form of this property sheet.  



If your license only allows you to use the Lite hydraulic model, the property sheet will appear in the simpler form shown below.

 

Both of the sheets shown above are for a sag pit, since an overflow weir is required.  The Overflow Weir Properties tab will not appear for an overflow from an on-grade pit.

Travel time is used in the Lite hydraulic model. It can be calculated by DRAINS from information specified for the overflow route, but it can also be specified directly. If you have purchased the Storage Network Routing Module, you can use Kinematic Wave routing instead of Simple Translation in the Lite hydraulic model. In that case you must specify the reach length.

If you wish to use the Full Unsteady hydraulic model you must specify Reach length, Upstream invert level (IL) and Downstream IL. Reach length can be scaled from the DRAINS model if this is drawn to scale. You can choose a length based on the straight line distance between the nodes at the ends of the overflow route, or you can specify the full length of the poly-line representing the overflow route. If neither of these is valid, any length can be entered directly.

The travel time of the flow path can be calculated by DRAINS, or you can specify a value. DRAINS calculates the time based on the safe major system flowrate occurring on the route. This will be conservatively low for minor storm events.

If the overflow route comes from a sag pit, and you have the Full Unsteady hydraulic model, you will see an additional page labelled Overflow Weir Properties: 

 

In the Full Unsteady hydraulic model, it is necessary to define a hydraulic control that is used to model the outflow of water from the sag pit. This can take four forms:

      (a)   a rectangular broad-crested weir, with a weir coefficient of 2.0;

      (b)   a parabolic weir representing a low point on a street, with the road crown acting as a weir crest (The longitudinal sections of roads are designed as parabolas.);

      (c)   a general depth-discharge relationship opened using 'You specify', that can be imported from a spreadsheet.

     (d)   a 'No weir' option that assumes that water will spill onto an open surface, which should have its upstream end at the same elevation as the pit overflow level.

The parabolic weir input in the property sheet shown above requires the two representative lengths shown below:



The crest elevation for all options is assumed to be the spill level of the sag pit, equal to the pit elevation plus the ponding depth specified in the Pit property sheet.

For the common case of flow across a road crown, the weir control operates as shown below. The overflow route should not include the ponded area on the higher side of the street.


Situations where a weir may not be required are shown below:


The last page of the property sheet is labelled Cross Section Data:

 

The drop-down menu labelled Shape allows a cross-section to be selected from the Overflow Route data base. It is necessary to select a shape for all overflow routes. Since some of these will be unimportant, such as flow between pits a short distance apart, a dummy overflow route shape can be set up as a default that can be selected for these cases. As with all dummy values used in DRAINS, the cross-section should be physically realistic for the flows that will occur. It will be used for unsteady flow calculations in the Full Unsteady hydraulic model. Unrealistically large shapes would include non-existent storage in the model. Unrealistically small shapes can lead to numerical instabilities.

The Full Unsteady hydraulic model model includes a flood mapping capability for overflow routes. An overflow route cross section is often not symmetrical. There are extra radio buttons where you can specify whether the cross section represents the view looking upstream or downstream. Refer to Flood mapping for more information.

The box labelled Safe Depths and Flow Rates allows you to set values to be used in design runs, and for reporting of unsafe values. 

The % of downstream catchment flow carried by this channel can be used to simulate lateral inflows along the overflow route:

  • If the percentage is zero, there is zero lateral inflow and all the downstream catchment outflow arrives directly at the downstream pit.
  • If the percentage is 100%, then all the downstream catchment outflow is applied as a lateral inflow distributed along the length of the overflow route.
  • If the percentage is say x%, then x% of the downstream catchment outflow is applied as lateral inflow distributed along the length of the overflow route, and 100-x% is applied directly at the downstream pit or other overflow routes leading into the downstream pit.

In more complex situations, such as two overflow routes arriving at the same downstream pit, the downstream catchment outflow can split up and applied as lateral inflow along both overflow routes (e.g. x%, y% and (100-x-y)%).

The channel slope is used for depth calculations in the Lite hydraulic model. You can use the Calc Slope button to calculate the slope if the drawing is to scale. You will have the choice of using the actual length of the overflow route polyline or the straight line distance between the upstream and downstream nodes. The fall will be the difference between the upstream and downstream node surface levels.

When a run is performed and there is an overflow, the results on the cross-section window of the Overflow Route property sheet will be:



In this example, some of the overflow routes display one red number while others show two numbers. One of the latter type, OF K4, had the % of downstream catchment set as 85%, in the property sheet shown above.



The first flowrate shown in red for OF K4 (0.012 m3/s) is the peak of the bypass overflow leaving the pit.  The second flowrate is the combined peak flow rate (bypass plus lateral inflow). This hydrograph considered in design is the overflow hydrograph plus 85% of the hydrograph draining to the downstream catchment (which has a peak of 0.050 m3/s).  Rather than being 0.012 + 0.050 x 0.85  = 0.054 m3/s, the peak of the approaching flowrate is 0.045 m3/s when the different times of hydrograph peaks and the time delay for the overflow are taken into account. The differences in maximum flows are due to variations in the way that hydrographs combine.

With the Lite hydraulic model the flow characteristics such as depth and velocity shown for the overflow route are calculated from a normal depth corresponding to the maximum flowrate, determined using Manning's Equation. DRAINS calculates this after the main computations, when this window is opened.

The Full Unsteady hydraulic model flow characteristics such as depth and velocity shown for the overflow route are calculated from St. Venant equations of unsteady flow to overflow routes, performed during analysis.

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