1909-13367 SRA referral response approved docs

Table 1 – Existing site catchment summary

                                                           Details
     Subcatchment
                               Catchment 1             Catchment 2             Catchment 3

          Area                     54,640m2             15,980m2                 35,790m2

      % impervious                   0%                    0%                       0%


          Slope                     6.5%                  6.5%                     6.5%



3.1.1 E xi st i ng H yd rol og y

The XPStorm model has utilised “Laurenson’s” method to calculate the runoff hydrograph for each of
the design events. Below is a brief summary of the parameters used in the model.


Mannings Roughness

The developable site area is vegetated. The developable area is generally well grassed with some small
bushes. A high value for Mannings roughness has therefore been applied. The majority of the
impervious surfaces will be concrete and roof areas and therefore a single impervious roughness has
been applied. The values that have been adopted for Pervious and impervious areas are as follows:
                         Pervious Area n = 0.045

                         Impervious Area n = 0.014



Initial and Continuing Losses

The initial loss (IL) and continuing loss (CL) for pervious/undeveloped areas available from
ARRDatahub are shown below.
                         ARRDataHub IL = 32mm

                         ARRDataHub CL = 2.6mm/hr



The above values were sourced from ARRDatahub (for coordinates (-26.445, 152.897) on the 20 June,
2019).

The above IL and CL have been deemed too high for the site. The IL and CL that have been adopted
are listed below.

                         Adopted IL (Pervious) = 15mm

                         Adopted CL (Pervious) = 2.6mm/hr



                         Adopted IL (Impervious) = 1.0mm

                         Adopted CL (Impervious) = 0.0mm/hr




Preburst Depths
                                      Stormwater Management Report                                  7
S19-007                                 122 & 144 Myall St, Cooroy