7. urban pathways and pressures - john stack
TRANSCRIPT
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Urban Pathways and Pressures
John Stack, Executive Engineer Dublin City Council
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‘Urbanisation’
“The process by which towns and cities are formed and become larger as more and more people begin living and working in central areas”
• ∼ 40% of Irish population lives in Greater Dublin Area
• ∼ 527,000 people living in city • ∼ 80% of surface area is impervious
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Urbanisation
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Impact of Urbanisation on Water Cycle
• Urbanisation has fundamentally altered the way that water moves through the landscape.
• Increasing Impervious Surface Area: – Prevents groundwater recharge from rainfall, which – Reduces baseflow in rivers as they are no longer
adequately fed by groundwater – Increases frequency and severity of floods due to
greater surface runoff – Causes pollution of surface waters
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Urban Pathways
• Vast majority of urban pollutants enter waters via a pipe – Foul/combined drainage network – Surface/Storm water drainage network – Private drains
• Groundwater
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Drainage Infrastructure as a pathway
• ∼2,000 km of sewer – ∼ 660 km foul – ∼ 541 km combined – ∼ 788 km surface/storm
• 39 pumping stations • 212 combined sewer overflows • Largest wastewater treatment plant in Ireland • ∼ 54,500 road gullies (± 500) • 39 underground rivers
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Dublin City Drainage
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Pressures
• Classification: – Via foul/combined drainage infrastructure – Via surface/storm water drainage infrastructure – Direct to water
• Further classification – Acute: short term, infrequent – Chronic: continuous
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Pressure Description Impacts on Acute/ Chronic Upstream pressures Pressures in upstream catchments impact on water quality
entering into DCC functional area All water bodies Both
Foul Drainage Network • Network capacity issues • Effects of combined sewer overflow spil lages – spil lages
from foul/combined sewer network to waters as a result of rainfall
• Sewer chokes (impacts all water bodies) • WWTP (Liffey Estuary and Dublin Bay) • Exfiltration due to infrastructural deficiencies
All water bodies (WWTP relates only to Liffey Estuary Lower and Dublin Bay)
Both
Misconnections Discharges from household appliances and toilets to the surface water network
All water bodies outside combined area
Chronic
Urban Runoff • Road runoff • Runoff from other hardstanding areas
o Car Parks o Cobblelocked driveways o Roofs
• Il legal disposal of polluting matter to gull ies (e.g. old engine oil, etc.)
• Cross connections from foul to surface water network (both public and private)
• Animal/bird faeces • Private drain blockages • Car washing on kerbside/in driveways • Discharges not subject to l icence (e.g. business/people
washing shop/house fronts and floors and throwing the washwater into the gull ies
• Unlicensed discharges
All water bodies outside combined area
Chronic
Historical changes to morphology • Lots of reclaimed land in the region • River channels altered, culverted, canalised, diverted Leads to increase flowthrough, loss of riparian zones and natural river features (impacts all water bodies)
All water bodies Chronic
Historical landfills A number of historical landfills may be impacting on some rivers Liffey, Tolka, Santry Chronic
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Foul Drainage Network
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CSO Spill Date E coli
(MPN/100 ml)
Ammonia (mg/l N)
Phosphorus (mg/l P)
21/7/2011 129,970 11 1
Pipe φ 0.6 m
Pipe flow 33 l/s Manning’s equation
Load 43 million MPN/s
363 ml/s 33 mg/s
For 10 hour spill
1.5 Trillion MPN
13 kg 1.2 kg
Rainfall for July 2011
Monthly rainfall, July 2011 % of 1961 – 1990 average No. days rain during month (> 1 mm)
38.1 mm 88% 10
Perception that CSO discharges are heavily diluted – they’re not
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Illegal Dumping??
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Misconnections Appliance BOD (mg/l) pH Suspended Solids
(mg/l) Phosphorus (mg/l) Population Equivalent
Washing Machine Flow (l itres) Required Dilutions
1,534 58.5
17,948
10 366 0.53 1.50
Dishwasher Flow (l itres) Required Dilutions
313 25.0
1,565
9.2 19 0.32 0.13
Car Wash* Flow (l itres) Required Dilutions
3,160 7.0
4,424
- - - 0.36
Washing Machine BOD P
Concentration 1,534 mg/l 0.53 mg/l
@ 60 litres 92,040 mg 31.8 mg
4 washes/week 208 washes/year 208 washes/year
Annual Load/miscon
19.1 kg/year 0.0066 kg/year
3,168 miscons* 60, 649 kg/year 21 kg/year
Annual PE > 1 million
* Assume 60% of misconnections are washing machines
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Car Washing
Specification Karcher K2
Hose length 4 metres
Motor power 1.4 kw
Actual Bar Rated Pressure
110 Bar Max Water Pressure
Flow Rate Max 360 litres/hr
Estimated Annual PE from car washing
BOD 3,160 mg/l
Assume 10% of householders wash cars fortnightly
6,600 washes/fn
Wash takes 20 minutes 120 litres
BOD kg/fortnight 2,502 kg/fn
BOD kg/year 65,070 kg/year
Annual PE > 1 million
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Car Wash – backstreet garage
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Historical landfills
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Morphological Changes
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Heavily Modified Water Body
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Urban Runoff
• Storm/Rainfall runoff created by urbanisation • Major source of flooding • Major source of water pollution • Urban runoff is typically untreated
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Urban v Rural
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Research: Nationwide Urban Runoff Programme (USEPA, 1979 – 1983)
• Study objectives: – Assess the water quality impact of urban runoff
across a number of cities – Assess the impact of urban runoff on overall water
quality – Implement storm water management best
practices.
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NURP Findings • Heavy metals (especially copper, zinc and lead) are the most prevalent priority
pollutant substances found in urban runoff • Coliform bacteria are present at high levels in urban runoff • Nutrients are generally present in urban runoff but, in general, concentrations are
not high compared to other sources • Oxygen demanding substances are present in urban runoff at concentrations
approximating those in secondary treatment plant discharges • The physical aspects of urban runoff (i.e. erosion and scour) can be a significant
cause of habitat disruption and can affect the type of fish present • Detention basins and recharge devices are capable of providing very effective
removal of pollutants in urban runoff • Wet basins (a design which maintains a permanent water pool) have the greatest
performance capabilities • Wetlands are considered to be a promising technique for control of urban runoff • Interestingly, the NURP found that street sweeping was ‘ineffective as a technique
for improving the quality of urban runoff’.
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Our findings – monitoring during rainfall event
Date: 21st July 2011 Ammonia B.O.D. Dissolved
Oxygen Dissolved Oxygen
E. coli Nitrate Nitrite Phosphorus (React)
Suspended Solids
TON
Number of Locations = 25
mg/l as N
mg/l
% Sat.
mg/l
MPN/ 100ml
mg/l as N
mg/l as N
mg/l as P
mg/l
mg/l as N
Average
0.96
17.89
92.80
9.30
13286.25
0.65
0.04
0.24
125.05
0.69
Max
11.08
154.00
99.00
10.00
129970.00
1.54
0.09
1.01
415.00
1.60
Median
0.22
8.00
94.00
9.40
5803.00
0.51
0.04
0.10
119.50
0.52
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Examples of Urban Runoff
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Traffic
3 km slick
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Birds
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LOTS of birds
NPWS = approx 13,000 birds in South Dublin Bay
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Horses and other animals
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This??
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Construction
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Cobblelock
Ave. 26m2
132,000 houses in Dublin If 20% of these convert front garden to cobblelock, that’s 686,400 m2 new impervious area x 2.54 cm (1”) of rainfall = approx. 17,500 m3 additional runoff
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Private Drain Blockage
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Blocked drains = big business
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It’s not just Dublin and it’s not static
Urban Creep,…or crawl perhaps
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Urban centres Population density >10,000 >20,000 >50,000
Leinster 25 14 2
Munster 13 4 2
Connaught 4 2 1
Ulster* 2 0 0
*ROI counties only
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Surface Area of Major Towns/Cities Surface Area (km2) Potential Impervious Area
(km2)
Cork 37 29
Dublin 114 91
Limerick 51 41
Galway 53 42
Waterford 41 35
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Housing Agency’s National Statement of Housing Supply and Demand
New Housing requirements to 2020 City/Town Demand
Dublin Min 33,000
Cork 8,434
Limerick 3,436
Galway 2,316
Waterford 713
10 other towns >800
> 30 town > 300
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Ireland to 2040
“…unmanageable sprawl of housing areas, scattered employment and car based commuting, presenting…adverse impacts on peoples’ lives and the environment.”
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Summary • Urban environment extremely complex • Very broad range of pressures
– Difficult to classify – Difficult to quantify
• Urban runoff has a serious negative impact on water quality
• No quick fixes or easy solutions • Solutions will require creativity, investment,
community engagement, boots on the ground and time