Storm Water management plan · MCU12/0133.03

Noosa Council documents


                  Bligh Tanner Consulting Engineers                                                                     Page 11
                  Noosa Flexi Learning Centre Stormwater Management Plan
                  August 2012




                  4 STORMWATER QUALITY ASSESSMENT

                  Modelling has been undertaken in accordance with Water by Design (2010) MUSIC
                  Modelling Guidelines Version 1.0 with the exception of the following:
                    • Forested parameters have been used to reflect the landscape areas—apart from the
                         turfed areas—in the post-development case, as there is no significant change to pre-
                         developed conditions.

                  Whilst the design objectives for the site (as outlined in Section 2.1) allow for the infiltration of
                  stormwater and do not explicitly require stormwater quality to be managed, given the
                  sensitivity of the receiving environment it is prudent to consider these impacts.

                  It is difficult to accurately estimate the impacts of the development on groundwater quality for
                  several reasons:
                       • The capacity of the in-situ soils to process stormwater pollutants, particularly the
                            denitrifying capacity of the soils and root zone, is difficult to estimate, and MUSIC
                            does not account for the potential retention or transformation of nutrients in natural
                            soils.
                       • The site’s sandy soils are far more permeable than the soils used to develop the
                            recommended rainfall runoff parameters outlined in the SEQ MUSIC Modeling
                            Guidelines (Water by Design 2010), and local calibration data are not available.
                            Permeable area parameters have a significant bearing on model results when
                            impervious areas are low (Water by Design 2010). For example, using a
                            recommended rainfall runoff parameters for a forested site results in an annual
                            volumetric runoff coefficient of 32% which is well above what would be reasonably
                            expected for this site a value of . Adjustments to these parameters in the absence of
                            calibration data would be somewhat arbitrary as they are calibration parameters and
                            not directly related to physical soil characteristics.

                  Notwithstanding the above limitations, using MUSIC in accordance with the recommended
                  guidelines produces the results shown in Figure 4 and Table 6 below.

                  A schematic of the MUSIC model configuration is presented in Figure 3.
                  Note that the architectural design of the site plays a key role in mitigating stormwater
                  impacts. If the site were to be developed as a conventional commercial development, and
                  the stormwater runoff was treated to current best practice standards using bioretention
                  systems, the runoff volumes and pollutant loads would be higher than those from the Noosa
                  Flexi Learning Centre even if there were no rainwater tanks or infiltration systems. In other
                  words the site, by virtue of the low impact design of the site, it will likely perform better than a
                  conventional ‘best-practice’ development.

                  This result highlights the logic of exempting development with low levels of imperviousness
                  from the stormwater quality load-based objectives.




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