Characterisation of surface water runoff from an urban residential sewer separation scheme

Dave Morgan, Paul Johnston and Laurence GillTrinity College DublinPhil CollinsHRD Technologies LtdKwabena Osei Hydro International

morgandt@tcd.ie

Department of Civil, Structural & Environmental EngineeringTrinity College Dublin

Motivation CatchmentIntroduction

Considerable research conducted on source-area or end of pipe solids characterisation and modelling..... but SS prediction still a challenging proposition

Can the consideration of a system mass balance:-Improve prediction of suspended solids and associated pollutants in the runoff-Inform SuDS retrofit

Mb

Msew

Mg

Mhs

Mout

Mwo

Mwo = Mg + Msew + Mhs + Mout

Methodology-Field monitoring of small urban residential catchment (18 months)-Characterisation of sediment (loadings / PSD / TP / Cu / Pb / Zn)-Modelling of the suspended solids

RG

HDVS

Separate Pipe and Gully System

Project Outline CatchmentIntroduction

River Poddle22

5∅

225∅450∅

225∅

Stormwater sampling

ComparisonRunoffGullySediment yield

Zone 1

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

Washoff

ComparisonRunoffWashoffSediment yield

1

3

5

7

9

6

8

104

2

Gully sediment characteristics

Annual accumulation of 36kg

(10 gullies, 30% of imp. area)

D50 =1000µm

13% < 200µm

Gully

ComparisonRunoffWashoff GullySediment yield

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

ComparisonRunoffWashoff GullySediment yield

>80002000-8000

600-2000

212-600

63-212

<63

0

5

10

15

20

25

30

35

Gully Suspended solids

Particle size (microns)

Sedi

men

t m

ass (

kg)

TP Cu Pb Zn0

10

20

30

40

50

60

70

80

90

100GullySuspended solids

Pol

luta

nt m

ass (

g)

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

Thanks for listening....

Current work

Modelling of suspended solids at the outlet

Partitioning of easily eroded and stable solids fractions