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Department of Civil, Structural & Environmental Engineering Trinity College Dublin

Characterisation of surface water runoff from an urban residential sewer separation scheme Dave Morgan, Paul Johnston and Laurence Gill Trinity College Dublin Phil Collins HRD Technologies Ltd Kwabena Osei Hydro International morgandt@tcd.ie.

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Department of Civil, Structural & Environmental Engineering Trinity College Dublin

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  1. Characterisation of surface water runoff from an urban residential sewer separation schemeDave Morgan, Paul Johnston and Laurence GillTrinity College DublinPhil CollinsHRD Technologies LtdKwabena Osei Hydro Internationalmorgandt@tcd.ie Department of Civil, Structural & Environmental Engineering Trinity College Dublin

  2. Introduction Motivation Catchment Separate Pipe and Gully System Mwo RG Mb Mg HDVS Msew Considerable research conducted on source-area or end of pipe solids characterisation and modelling..... but SS prediction still a challenging proposition Mout Mwo = Mg + Msew + Mhs + Mout Mhs • Can the consideration of a system mass balance: • Improve prediction of suspended solids and associated pollutants in the runoff • Inform SuDS retrofit Methodology -Field monitoring of small urban residential catchment (18 months) -Characterisation of sediment (loadings / PSD / TP / Cu / Pb / Zn) -Modelling of the suspended solids

  3. RiverPoddle Introduction Project Outline Catchment Stormwater sampling 450∅ 225∅ 225∅ 225∅

  4. Sediment yield Washoff Gully Runoff Comparison Build-up rates 0.5 – 10.5 g/m2/day (bitmac road) ~ 0.05 g/m2/day (concrete footpath) Particle size D50 = 1200µm 10% < 200µm

  5. Sediment yield Washoff Gully Runoff Comparison 1 2 9 10 4 8 3 Gully sediment characteristics Annual accumulation of 36kg (10 gullies, 30% of imp. area) D50 =1000µm 13% < 200µm 7 6 5

  6. Sediment yield Washoff Gully Runoff Comparison SSC -19 events/231 discrete samples - 30% of annual runoff volume -Site median EMC = 29mg/l -Total mass: 40kg PSD -D50 = 34 microns -90% < 200µm

  7. Sediment yield Washoff Gully Runoff Comparison

  8. Conclusions Gullies ineffective in trapping < 200 µm particles 2 – 4 times higher annual pollutant load in SS compared to gully sediments High uncertainty in source loading rates – modelling of SS washoff required to determine mass balance Current work Modelling of suspended solids at the outlet Partitioning of easily eroded and stable solids fractions Thanks for listening....

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