06-09-22 Stormwater Management Plan_Issue F · MCU21/0175

 6 September 2022


5.4.3.8.2      Sediment
The TSS outflow is identified in Table 4 above. As can be seen from the above table the pollutant
load reduction achieves the Water Quality Objectives. The TSS level is therefore considered
acceptable.

5.4.3.8.3       Nitrogen and Phosphorous
The TN and TP loads are identified in the above Table 4. As can be seen the TN and TP loads leaving
the site achieve the Water Quality Objectives. The Nitrogen and Phosphorous residual loads are
therefore considered acceptable.

5.4.3.8.4    Hydrocarbons
Hydrocarbons will become trapped within the bioretention filter media.

When there is a build-up of hydrocarbons within the bioretention basin, the top layer of the filter
media can be removed and replaced ensuring that no hydrocarbons are discharged from the site.

Studies have shown that micro-organisms in the organic mulch layer degrade and break down the
petroleum-based hydrocarbons and hence they will not be discharged from site.

5.4.3.9        Surfactants
Washing of cars is not expected on site for the proposed development, as a result surfactants
should not be an issue for the drainage system.

5.4.3.9.1      Heavy Metals
Heavy metals in stormwater runoff generally become attached to fine sediment. The bioretention
basin will remove the majority of this fine sediment. The removal of the fine sediment should
effectively remove most of the heavy metals in the runoff.

5.4.3.9.2       Pathogens/Faecal Coliforms
Domestic animals within the development will be under the control of their owners at all times and
the owners will be expected to clean up after them.

5.5       Proposed Stormwater Quality Treatment Devices

5.5.1 Locations

The location of the proposed stormwater quality treatment device is shown on MPN Plan 8293-
CP.1.01 attached in Appendix 3.

5.5.2 Device and Size

The device type and size are depicted on MPN Plan 8293-CP.1.01 attached in Appendix 3.

5.5.3 Operation

The bioretention basin operates by filtering runoff through densely planted surface vegetation,
then percolating the water through a sandy loam filter media with details as per MPN Plan 8293-
CP.2.01 attached in Appendix 3. As the water is percolated through the soil, pollutants are
captured by fine filtration, adsorption and biological uptake.




 Excellence with Economy in Engineering                                                               13
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