QUDM pipe system design procedures
The Queensland Urban Drainage Manual (QUDM) sets out detailed pipe design procedures that include the estimation of pit pressure changes using a set of charts that includes the Missouri Charts (Sangster et al., 1958) and graphs developed by Hare (1983). The charts are set out in Appendix B of the 2013 version of QUDM. In DRAINS the procedures can be applied with all hydrological and hydraulic models for design and analysis.
In a design situation the method is applied as follows:
(a) Develop a model layout and set up a DRAINS model, assuming trial Ku values (typically 1.5), providing pit alignment and width information for each pit using the above sheet.
(b) Carry out a design run, analyse the system and produce minor flows.
(c) From the Run menu, select Revise Pit Loss Coefficients and then Using QUDM Charts.
The following messages appear as the Ku values are altered.
You can copy out a summary of changes to an Excel sheet, producing an output like the one below:
Separate Ku, Kl and Kw values are calculated for pit HGLs, lateral branch HGLs and pit water levels in appropriate cases. (Mostly these K values have the same value.) The QUDM Chart number and the ratios used in deriving the K values are noted.
For some complex situations, equations presented by Hare and O'Loughlin (1991) are employed.
(a) Next, repeat Steps (b) and (c) until the Ku factors, calculated flowrates and HGL levels become constant.
(b) Review Ku factors (either in the table placed on the Clipboard or by examining pit property sheets) and change them if necessary.
(c) Perform a design run followed by a minor storm analysis, check the results and adjust the model until conditions are satisfactory for minor flowrates.
(d) Repeat Steps (b) to (f) for major flowrates.
The model should stabilise after two or three iterations. The Ku values for the minor and major cases will be different, with minor flow values being not necessarily smaller than the major flow Ku values. If this procedure is applied with the rational method, part-full pressure changes will be calculated using the QUDM method when producing Check HGL sheets, even though another option (constant depth or full-flow coefficient multiplier) may be specified for these and used in DRAINS calculations.
Currently, DRAINS gives chart numbers from the 2008 edition of QUDM. The equivalent chart numbers in the 2013 draft edition are shown below:
Adjustments to pit pressure changes for analysis calculations can be performed in the same way as design calculations, leaving out the design step and simply analysing the system.
Although pit pressure changes are difficult to determine, the QUDM procedure organises the process into a logical structure that can be easily applied in DRAINS to obtain reasonable estimates of Ku and related factors. Results like those presented above can be checked against the QUDM charts. With practice, the procedures will be easy to apply.
Some Queensland procedures require estimation of the characteristics of flows approaching pits at the top of a drainage line. This can be done by including additional nodes and overflow routes, as shown below, and in an example.
The property sheets for these approach flow routes must specify suitable values for % of the downstream catchment flow carried by this channel. For flow from a single direction this will be 100%. Where flows approaches a pit from two directions, two approach flow links can be provided with their % of the downstream catchment flow carried by this channel percentages adding to 100%.
References
- Hare, C.M. (1983) Magnitude of Hydraulic Losses at Junctions in Piped Drainage Systems, Civil Engineering Transactions, Institution of Engineers, Australia, Vol. CE25
- Hare, C. and O'Loughlin, G.G. (1991) An Algorithm for Pressure Head Changes at Pits and Junctions, Urban Drainage and New Technologies (UDT '91), Dubrovnik, 1991
- Queensland Department of Energy and Water Supply (2013) Queensland Urban Drainage Manual,3rd Edition, Brisbane
- Sangster, W.M., Wood, H.W., Smerdon, E.T. and Bossy, H.G. (1958) Pressure Changes at Storm Drain Junctions, Engineering Series Bulletin No.41, Engineering Experiment Station, University of Missouri
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