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MANAGING URBAN DRAINAGE:A theoretical case study of SUDS design
Malcolm Sutherland (0204783)
A coursework report submitted in fulfilment of the requirements for the MSc module in Sustainable
Urban Drainage Systems (WW545) at the University of Abertay Dundee, April 2003
REVISED May 2013
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CONTENTS
SECTION 1: the changing hydrology of a river catchment
Introduction
1.1: 19501970
1.2: 19701990
1.3: 1990 - 2010
SECTION 2: installation of a combined sewer overflow
2.1: Purposes and problems of a combined sewer over-flows
2.2: Determination of Formula A
2.3: CSO arrangements for discharge into two contrasting rivers
REFERENCES (websites may no longer be available)
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SECTION 1: THE CHANGING HYDROLOGY OF A RIVER CATCHMENT
This chapter concerns the hydrological and environmental changes caused by developments
along a section of a theoretical river. As shown in Figure 1, this section includes a small
tributary, which joins upstream from where a New Town has-been established. Two damshave been raised to create boating and Fishery lakes, and Hood barriers have been raised
downstream of the town, in order to protect agricultural land.
Figure 1: layout of the hydrological catchment
At the gauge point, the following changes in river depth and flow were noted for are being
predicted) (Table 1):
Table 1: records at the gauge point and historical information
Year Flow (m3/s) Water depth (m) Developments
1950 12.5 2.1 New Town established in 1952
New flood banks raised in 1965
Boating lake constructed in 1995
Fishery opened in 2000
1970 13.0 2.3
1990 13.7 2.2
2010 13.0 2.6
The following sub-sections address the possible causes of these changes in flow and depth,
how they correlate with the developments described above.
1.1: 1950-1970
The river flow increases from 12.5 m3/s to 13.0 m
3/s, and the water depth rises from 2.1m
to 2.3m.
HYDROLOG1CAL CHANGES
Both the river flow rate and the depth increased during this period. Two main events lake
place alongside the river, which are likely to have caused this to happen: (i) urban
development; and, (ii) the construction of flood banks where the gauge point stands.
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Urban development increases the area of impermeable surface cover, and it requires
drainage systems. This reduces the amount of water passing into the soil, and being
absorbed into vegetation, or evaporating. As a consequence, urbanisation can increase the
flow rate of receiving rivers by between 200% and 600% (Kiely, 1997).
This may drastically change the hydrologic cycle section (from rain hitting The ground until
water enters the river), and drain pipe networks with result in the direct and shortened
discharge of drainage into the river (CIRIA, 2000; Herricks, 1995). The rise in water depth
may owe to the construction of buildings and bridges, which reduce channel capacity
(Figure 2) (Bennett et al, 1997):
Figure 2:the encroachment of development upon a natural river, resulting in rising levels
MORPHOLOGICAL CHANGES
The increase in flow will lead to more erosion of the riverbanks and the riverbed. This
pattern was researched by Derricks (1995), who describes how increased runoff from
urbanised catchments contributes to faster and deeper riverbank erosion and widened
unstable river channels downstream. Straightened river channels (usually in urban settings)
are out of phase with the water and sediment discharge regime, resulting in increased bank
erosion (Bennett, 1997).
A SEPA report on the Weinflu Case Study (Kennedy et al (see References: websites),
described the effects of urbanisation and flood control along this river passing through
Vienna. These include changes to the river pathway, along with disruptions to sediment
transport and flow regime. Regular bank re-enforcement is a necessary precaution due to
increased erosion.
WATER POLLUTION CHANGES
Increased water pollution is likely to be the result of changes occurring at this period-
Typically, the flora and fauna within urbanised river channels deteriorate due to decreasing
water quality and damage lo habitats (Herricks, 1995). Turbidity will inevitably increase with
effluent/storm water discharges and increased erosion. There are several reports of changes
to water pH, BOD and DO levels, and increased levels of heavy metals, oil, pesticides,
nitrates, coliform bacteria, and dissolved ammonia. These are often directly attributed to
wastewater, storm drainage and soil erosion (Herricks, 1995).
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Construction areas and parking lots are an important source of suspended solids and heavy
metals. River sediment may be characterised by high BOD levels, along with contamination
by pesticides and hydrocarbons. Possible causes tor this include eroded (organic-rich) soil,
garden/field pesticides (Manahan, 2001), spilled substances such as fuel oil, or
trade/sewage discharges (Herricks, 1995; Radojevic et al, 1998).
Furthermore, the excavation of soil and sediment required for the construction of flood
banks will necessitate the use of heavy machinery, which contributes to soil erosion, and
increased turbidity (Dept. of Natural Resources & Environment (Australia), 2001).
1.2: 1970-1990
The flow rate increases from 13.0m3/s to 13.7m
3/s, and the water depth decreases
marginally, from 2.3m to 2.2m.
HYDROLOG1CAL CHANGES
The water depth falls slightly during this period, in spite of increasing flow-rate. The initial
impacts of raising flood embankments include increased water depth and riverbed erosion
(ESCAP Publications). Flood banks only re-locate floods to either end of the levied stream.
During storm events, the restrictive effect of the banks produces increased flow rates. The
same effect leads to more concentrated sedimentation within the protected channel, which
may explain the decrease in depth over this time (Bennett, 1997).
MORPHOLOGICAL CHANGES
The establishment of the flood banks will lead to an increase in the flow-rate, due to these
preventing the dissipation of floodwaters onto the surrounding land. However, the water
depth will be lower, as the banks were raised, using dredged material from the old river bed
banks, leading to higher flow velocity, and riverbed erosion in the new cutting (Figure 3):
Figure 3: morphological river bed changes. Sediment is dredged up from the old river bed (bold brown line), to
be used for building the flood barriers (dotted brown line). The blue bold line is the old water level; the dotted
blue line shows the new river water level (Bennett, 1997).
As mentioned, scouring of the embankments and the riverbed will occur within this
modified river course, and increased flow conditions can exacerbate riverbank erosion
(SUDS Working Party, 2000). The new town will still continue to expand during this time,
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leading to further culverts being constructed. River modifications within the town and
alongside the flood banks can lead to increased sedimentation and a changing river form
downstream (Natural Resources and Environment, 2001).
WATER POLLUTION
As the New Town will still be expanding, there will be further increases in surface drainage
and treated sewage/industrial discharges into the river. The effects of this have already
been discussed, and (if this was in Scotland) these may not yet have been addressed under
any environmental legislation. (As recently as 1996, sewage effluent discharges were the
main causes of river pollution in Scotland (SUDS Working Party).) Since the fishery has not
been considered yet, there may be no economic driver behind imposing water quality
improvements as yet.
The flood hanks will have reduced the amount of sediment and soil being washed into the
river, along with diffuse agricultural pollution such as pesticides, fertilisers or manure. (NSWGovernment). This would result in reduced nitrogen and phosphorus, pathogens, turbidity,
pesticides and hydrocarbons (e.g. from spilled oil) within the water column.
1.3: 1990-2010
It is predicted, that the flow rate will decrease from 13.7m3/s, to 13.0m
3/s, and that the
water depth will increase from 2.2m to 2.6m.
HYDROLOGICAL CHANGES
The creation of the fishery and boating lakes will require damming the river upstream and
downstream of the town and embankment. This may lead to reduced flows downstream
from these artificial lakes. These may also be weirs behind the lakes, and the water depth at
the gauge point will have risen, if this is a short distance upstream from the fishery lake.
The effects of a dam on river flows downstream are more significant during storm events, as
these are mitigated with more steady flows passing out An average decrease in flow rate
also requires a significant effort being made to control the diffuse inputs from the new town
and rural land upstream of the gauge point (Bennett, 1997.)
MORPHOLOGICAL CHANGES
The construction of a dam results in sediment being trapped behind this barrier under
tranquil reservoir conditions. The resulting reduction in the river's sediment load
downstream may result in abrasive, sediment-free water causing rapid channel incision. A
study of the Stony Creek river in California revealed that this impact, resulted in the river
changing from a braided pattern to a single-channelled meandering stream, which migrated
laterally (across lad), and restricting land-use in that area (Bennett et al, 1997). This is
illustrated in Figure 4over-page:
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Figure 4:fluvial changes downstream of a dam along the Stony Creak river (these are not accurate maps)
The construction of a fishery will raise the altitude of the water surface level, downstream of
the gauge point. This may serve to raise the water depth as this retained water may extend
upstream past this point, and increased sedimentation will occur within the reservoir.
WATER POLLUTION
The implementation of sustainable urban drainage systems under SEPA has been prolific;
since 1996, over 760 SUDS systems have been developed across Scotland (CIRIA, 2000).
Improvements would have to be made, if the river passing into the fishery can support the
aquatic population therein. The UWTD (Urban Wastewater Treatment Directive) requires all
sewage being discharged to rivers, sensitive waters, and those used for commercial
purposes, to be treated to a very high standard of effluent purity. The instalment of SUDS
within the new town would serve to reduce levels of turbidity, BOD, heavy metals,
pesticides, floating solids (e.g. litter), oils and other hydrocarbons within the water column,
and passing into the river sediment (CIRIA, 2000).
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SECTION 2: INSTALLATION OF A COMBINED SEWER OVER-FLOW
2.1: PURPOSES AND PROBLEMS OF COMBINED SEWER OVER-FLOWS
Combined sewer systems are single-pipe sewer networks, conveying bothdomestic/industrial effluent, and storm-water run-off, to an STW. During periods of intense
rainfall however, these can be overwhelmed, and excess wastewater may have to be
discharged straight into receiving waters, in order to prevent flooding at the STW.
This emergency procedure is called the combined sewer overflow (CSO), and it has been
permitted on the basis that the large volume of wastewater will be dilute enough, for some
of it to be discharged into a river/lake, without causing serious environmental damage.
Nevetheless, in practice this has not always been successfully achieved; the environmental
damage to rivers in the UK by CSOs has been a major cause of urban river pollution for many
years (Herricks, 1995).
CSOs may contain raised levels of suspended solids, BOD, oils, floating debris, pathogens,
heavy metals, suspended particles and other potentially toxic compounds. These pollutants
pose a threat to aquatic species and human health, and may exceed water quality
standards. The ecological and chemical changes downstream from a CSO or a sewage
effluent discharge are well known. Increases in BOD, pathogens (including protozoa and
bacteria) and suspended solids, are accompanied by depletion in DO, and clean water fauna
(Figure 5) (Harrison, 2001):
Figure 5: ecological and chemical changes in a river downstream from a CSO
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DETERMINATION OF FORMULA A
The maximum combined sewer flow into an STW is conventionally predicted using this
equation. The calculated volume was believed to be dilute enough for excess amounts of
wastewater to be discharged to receiving waters, without any adverse effects. The equation
is reproduced (Read and Vickridge, 1997), and following calculations are provided below:
Formula A(l/s)= DWF + 130P + 2E = (PG + I + E) + 1360P + 2E
DWF: wastewater produced by the population (daily water flow)
P: Population (persons)
I: Infiltration
E: Industrial flow
For a new CSO being constructed at the lowest point of a local sewer system, the following quantitiesof wastewater need to be considered:
P = 4600 persons I = 0.005m3/s E = 0.012 m
3/s
DWF => (4600 0.225m3/day) + (0.005 86400 seconds/day) + (0.012 86400seconds/day)
1035 + 432 + 1036.8= 2503.8 m
3/day
Formula A => 2503.8 + (1.36 4600) + (2 1036.8)
2503.8 + 6256 + 2073.8 10833.4 m3/d or ~10.8Ml/day 10,800,000 litres per day 86400 seconds/day
= 125.4 l/s
SECTION 2.3:
CSO ARRANGEMENTS FOR DISCHARGE INTO TWO CONTRASTING RIVERS
The composition and environmental impacts of combined sewer overflow discharges have
been discussed. The nature of these aspects will vary considerably, depending on the
receiving water. The quantity of the receiving water is one important parameter for
determining the effects of CSOs (Table 2) (Moffa, 1997):
Table 2: general qualitative effects of CSOs on rivers
Significance of impact on
receiving water
D.O. Nutrients S.S. Toxics Pathogens Turbidity Sanitary
debris
Small river High Low Moderate Moderate High Low High
Large river Moderate Low Moderate Moderate High Moderate High
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Under the UWWTD, any discharge must not have any deleterious effects on water flora or
fauna, and must not have any aesthetic effects such as oily layers, discoloration, etc (SEPA,
2003). Incorporating a 6mm or 10mm mesh to remove screenings is now essential for all
CSOs, and any incompliant CSOs must be disabled under the UWWTD.
DISCHARGE INTO A LARGE RIVER PASSING THROUGH AN INDUSTRIAL AREA
It is unlikely that the river passing through an industrial area will have any amenity value
within the vicinity. For discharging into large low-amenity rivers, Read and Vickridgc (1997)
prescribed basic suspended solids removal techniques, including high-sided weirs, stilling
ponds and vortex separation devices. That was in 1997. SEPA now recommend (2003) that
where a discharge entering a river is diluted by a factor greater than 8 (>8:1 dilution), the
CSO may not require storm water storage facilities. In addition to a sewer of Formula A
capacity, a screening mesh of max. 10mm is essential for all CSOs.
Nevertheless, wastewater being released into this river may need to be screened to astandard, which does not compromise water abstraction by industrial firms downstream (if
appropriate). Industrial water uses include washing, processing and cooling processes.
Problems with using poor quality water include scaling (caused by calcium salts); metallic
corrosion (due to dissolved solids and metals); bacterial growth (which poses a health risk to
workers); or, the fouling of pipes (inorganic and microbial slime deposits) (MetCalf and
Eddy, 2003). Suspended sediment in abstracted water can lead to machinery damage and
oilier operational problems.
DISCHARGE INTO A SMALL RIVER PASSING THROUGH A RECREATION AREA
The CSO must attain a high performance standard, in terms of its aesthetic effects on a
receiving river, which is used for recreation. For a new CSO discharging into EU-designated
waters (i.e. bathing waters and sensitive waters), there should not be more than one
spillage per annum or season. This implies that a storm water tank volume of around 820m3
is required. This would be above the 652m3threshold volume, which would meet the SSD
standard (for a dilution of 1:1 or less).
The permitted number of discharges does not reflect the damaging effects, which may arise
from these intermittent events. Since the wastewater is being discharged to a smaller riveras well, it may be advisable to choose a larger storm tank volume than the 800m
3capacity,
or to increase the flow to the STW.
These would incur considerable capital costs, and so the use of SUDS is another option to be
considered, if the CSO discharges lead to any legitimate complaints (e.g. by anyone using
the waters for recreational purposes). Secondly, a finer screen (6mm or less) may be
necessary to prevent large particle-sized matter passing into the river and harming the
aquatic environment.
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