Triple Bottom Line Risk Management_10 potx

The two most common mechanisms were pump stoppage,leading to sewage overflow from the emergency storage tank, and failure of therising main, most likely rupture, leading to sewage releas

This case study examines:

• Risk analysis and management strategy development for a large number ofassets

• Interpretation and simplification of large volumes of detailed information into

a format most usable for achieving the project objectives

• Use of release modes as a simplifying measure of consequence

• Advantages of preparation prior to workshops

• Quantification of environmental, community, and political risks

• Development of risk acceptability criteria

• Systematic development of cost-effective risk reduction actions to meet risk ceptability criteria

ac-B ACKGROUND

Metrowater owns and operates 102 wastewater pumping stations in AucklandCity, of which 90 are sewage pumping stations The remaining 12 stations pumpstormwater or combined sewage and stormwater Many of the stations are locatedclose to environmentally sensitive areas, such as coastal waterways and aquifers.Metrowater had identified the pumping stations as being critical assets and com-missioned the risk management study to identify which sewage pumping stationsposed the greatest risk in terms of operational and environmental consequencesshould any failure event occur

The corporation’s objectives for the study were to:

• Identify and document the existing management, technical systems, and cedures currently used by Metrowater to control risks

pro-• Identify and quantify the main risk contributors at each of the pumping stations

267

• Rank the pumping stations in order of the risk each poses.

• Develop risk treatment options and risk acceptance criteria

Metrowater’s overall objective in commissioning this study and pursuing therisk treatment actions resulting from the risk analysis was to reduce the frequencyand impact of sewage overflows from the pumping stations

A quantitative risk analysis was required to ensure that the pumping stationswere fully evaluated in a consistent manner and that a comparative ranking of therisk posed by each station was provided To enable effective risk management, foreach pumping station the study outcomes needed to clearly identify the risk quo-tients of each of the factors contributing to the overall pumping station risk.The input information was collected during a series of workshops The expertpanel was drawn from key Metrowater staff, all with direct operational and man-agement experience with Auckland’s sewerage reticulation and most with manyyears of service The panel members were selected to ensure that all the relevantoperating and management knowledge could be collected and included in thestudy inputs

The key stakeholders were identified as:

Groundwater users

P ROJECT S ETTING Sewerage System Characteristics

Metrowater’s files held a considerable volume of detail about each pumping tion’s characteristics, equipment, and operation, although many of these recordswere out of date to varying degrees In addition, a database contained the overflowrecords of the sewerage system While a good understanding of the systemcharacteristics and its operation was required to complete the risk analysis,this significant volume of information needed to be distilled into a manageableformat

sta-The general system characteristics derived from Metrowater’s records are marized in the following sections

sum-Pumping Station Operation Sewage pumping stations are required in the

Auck-land city region to pump sewage from houses and buildings into the city’s age network and ultimately into a regional sewerage collection and treatment

sewer-scheme The number of houses serviced by each pumping station ranges from five

to 150, with most pumping stations servicing around 20 houses Most of thepumping stations are located at relatively low elevations near the coast

Sewage flow from the catchment varies substantially Average dry weatherflows (ADWF) vary from less than 95 gallons per hour (gal/hr) (0.1 liters per sec-ond [L/s]) to around 11,400 gal/hr (12 L/s) For approximately 60 percent of thepumping stations, ADWF is less than 475 gal/hr (0.5 L/s) A small proportion(around 10%) have ADWF in excess of 2,850 gal/hr (3 L/s)

Maximum flows also vary substantially from station to station, from less than

950 gal/hr (1 L/s) to almost 57,000 gal/hr (60 L/s) Approximately 50 percent ofthe pumping stations have maximum flows of less than 1,900 gal/hr (2 L/s), andonly 10 percent of stations have maximum flows of greater than around 14,300gal/hr (15 L/s)

Pumping station catchment area also varies substantially, from less than 2.5acres (1 ha) to 740 acres (300 ha) Around 80 percent of pumping stations havecatchment areas of less than 25 acres (10 ha), and all stations (bar three) havecatchment areas less than around 74 acres (30 ha)

As expected, there is a strong and significant relationship (p <<0.001 that therelationship is due to chance) between catchment area and ADWF Maximumflows coincide principally with large rainfall events

Emergency storage capacities vary considerably, so storage time before stationoverflows occur can be less than one hour up to approximately 110 hours Aroundseven pumping stations have close to, or less than, the minimum storage require-ment of four hours at ADWF required by the regulatory authority, the AucklandRegional Council Approximately 40 percent of the pumping stations have storagetimes between five and 10 hours, and around 20 percent of stations have storagetimes greater than 20 hours

Pumping station overflow records between July 1997 and October 1999 showthat there were 69 recorded overflows Of these, 13 were fault induced and therest (56) were rain induced Pumping stations E10, W07, and H04 had the mostfrequent overflows, with nine, eight, and seven events, respectively Eight otherstations (E01, E03, W26, H03, H05, T11, W01, and W20) recorded more thantwo overflows One or two overflows were recorded at another 11 pumpingstations

The duration of rain-induced overflows ranged from 0.1 to 15 hours, with amean duration of 2.8 hours In 50 percent of cases, the overflow duration was lessthan two hours; in 80 percent of cases, it was less than four hours; and in 90 per-cent of cases, the duration was less than six hours

Rain-induced overflows were 1.5 times more common where the emergencystorage time was less than eight hours (34 overflow events), which occurs in one-third of the pumping stations Rain-induced overflows were approximately 10times more common (51 events) in catchments of over 12 acres (5 ha) than insmaller catchments Approximately one-third of all catchments are over 12 acres(5 ha)

Tables 17.1 and 17.2 summarize key overflow statistics

Project Setting / 269

Failure Mechanisms Initial appraisal of the nature, location, and operation of the

pumping stations indicated that four potential mechanisms could lead to tial sewage overflows The two most common mechanisms were pump stoppage,leading to sewage overflow from the emergency storage tank, and failure of therising main, most likely rupture, leading to sewage release somewhere along thealignment of the rising main Other, less likely mechanisms were undercapacitypumps and sewage backflow to the water supply system (e.g., via the water supplyhose in the pump house)

substan-Preliminary identification was carried out of potential events that could triggerthe failure mechanisms Table 17.3 was prepared as an initial guide to the expertpanel and shows a summary of trigger events for each release mechanism

Sewage Overflow Impacts The preliminary evaluation carried out prior to the

first workshop indicated that although the impacts of sewage release to the ronment would be complex, the assessment of impacts for each pumping stationcould be made relatively simple due to commonality of settings for many of thestations For example, effluent discharge from many of the pumping stationswould enter the foreshore and the near-shore marine environment For other, moreinland pumping stations, the principal impact of effluent discharge would be onthe quality of the underlying groundwater system For the remainder, it was con-sidered that the discharge would be collected in the stormwater system some dis-

envi-Table 17.1 Overflow Frequency

Rain Induced Overflow Type Fault Induced Catchment <12 acres Catchment >12 acres

Table 17.2 Overflow Duration

Overflow Duration (hrs) Fault Induced Rain Induced

tance from the coast and although ultimate discharge would be to the marine vironment, the effluent would be substantially diluted by stormwater prior to dis-charge, thus reducing the impact on the receiving environment.

en-It was anticipated, and later confirmed at the workshops, that the engineeringand clean-up consequences of sewage overflows resulted in only minor cost im-pacts The major consequences would result from environmental impacts andfrom community and political reactions Table 17.4 summarizes the key impactsassessed for the marine discharges Many of the same events and impacts alsoapplied to the groundwater system

Pre-Workshop Preparation The preliminary understanding of the operation and

potential impacts of the pumping stations led to the following conclusions:

• The attributes (e.g., trigger events, failure mechanisms, and impacts of sewageoverflow) that must be considered in the risk assessment were very complexand would be difficult to manage if each pumping station were to be consideredseparately in relation to all attributes There was a practical need to group thepumping stations (and if possible) to simplify the risk assessment

Project Setting / 271 Table 17.3 Summary of Trigger Events for Release Mechanisms

Pump Stoppage Rising Main Failure Pump Inadequate Backflow

Blockage (e.g., silt, Blockage (e.g., silt,

Table 17.4 Key Overflow Impacts

Fauna damage (e.g., shellfish) Recreational amenity Opposition to consents

• The pumping stations could be grouped on the basis of nature and sensitivity ofthe receiving environment and the likely volume of an overflow.

• Using these groupings, it was possible to identify a few specific release modesand to assess the likelihoods and consequences associated with each In thisway, the large number of pumping stations and their consequences could bemade manageable

• Information was then prepared to help focus the expert panel on appropriateoutcomes, thereby maximizing value from the panel members’ time

Management Procedures in Place

Metrowater was aware of the failure pathways and the potential impacts on thecommunity and the environment As part of its normal risk management proce-dures, it had implemented a number of management actions to reduce exposure torisk events

The existing risk management actions included:

• Installation of a comprehensive telemetry system to monitor pumping stationoperations

• Establishment of a rapid response team that was on call on a 24-hour basis

• Installation of a standby pump at all pumping stations

• Establishment of an emergency management plan to ensure generator ability in the event of power failure

avail-• Regular field inspections of plant operation and condition

• Implementation of a comprehensive maintenance schedule

• Progressive upgrading of emergency storage capacity

• Implementation of a community consultation process

Risk Assessment Structure

The risk posed to Metrowater by any pumping station is equal to the likelihood(annual probability) of a sewage overflow occurring, multiplied by the cost of theentire range of consequences The key tasks of the panel were to:

• Identify the potential mechanisms that could trigger effluent release to thewider environment

• Estimate the likelihood of a release (considering the available overflow sponse time at each station)

• Identify and estimate cost ranges of the potential consequences of sewage lease from each pumping station

re-The risk assessment structure relied on the simplified release mode approach toquantify the input data, from which was calculated the risk posed by each of the

90 sewage pumping stations, allowing each station to be prioritized in order ofrisk In addition, the structure retained the detail of each trigger mechanism to as-sist development of a staged risk reduction strategy

Numerical Inputs The schematic diagram of Figure 17.1 indicates the input

in-formation that was required from the panel to develop the risk model It shows thatthe likelihood of Event 1, which was overflow due to pump stoppage, was equal

to the sum of the likelihoods of all trigger mechanisms (i.e., power failure, ment failure, etc.) that could lead to Event 1 Similarly, the likelihood of Event 2,which was the release of sewage due to rising main failure, was equal to the sum

equip-of the likelihoods equip-of all release mechanisms (i.e., pipe failure, vandalism, etc.) thatcould lead to Event 2

Figure 17.1 shows that regardless of release mechanism (pump or rising mainfailure), any effluent would be released to the same environment and thereforewould have effectively the same consequences

Figure 17.2 is a flow chart that indicates (for the two main release mechanismsonly) the process the panel needed to follow to perform its key tasks The flowchart shows that the panel had to determine how sensitive each of the receivingenvironments (swimming beach, harbor, groundwater, and stormwater) would be

to an influx of sewage The panel then needed to determine, for each sensitivityclass, what volume of overflow would constitute a significant sewage flow result-

Probability of release due to main failure = p2 p2 = (p of each release mode) p2 = (pr1 + pr2 ptn)

Probability of release due to

ing in substantial environmental damage and/or strong community reaction Forexample, a release of several hundred gallons of sewage to a moderately sensitiveenvironment, such as Manukau Harbor, may not be considered a significant spill.However, the same volume at an extremely sensitive receptor site, such as a swim-ming beach, could possibly be considered a significant discharge.

Having defined the volume of a significant release into a given receiving ronment, the minimum allowable period of overflow could be calculated for a

envi-Figure 17.2 Flow chart describing the process used to tify the likelihoods and consequences of sewage overflows.

quan-Decide RECEIVER sensitivity

Determine SIGNIFICANT FLOW volume

Look Up ADW OVERFLOW RATE

Identify TRIGGER MECHANISMS

Calculate MINIMUM OVERFLOW period

Identify TRIGGER MECHANISMS

Determine LIKELIHOOD of prolonged overflow

Determine LIKELIHOOD of prolonged overflow

Identify IMPACTS (e.g., env, social, political)

Identify IMPACTS (e.g., env, social, political)

Determine CONSEQUENCES, Likelihoods, & Costs

Determine CONSEQUENCES, Likelihoods, & Costs

specified rate of sewage flow (e.g., the ADWF) The panel would then be in aposition to evaluate the likelihood of an overflow lasting longer than the minimumperiod for each trigger mechanism.

Similarly, for each receiving environment (classified according to sensitivity),the panel could consider the nature of significant effluent discharges and identifyspecific impacts (e.g., damage to fish, community outcry, and political backlash)and their potential consequences (e.g., compensation, remediation, public rela-tions, and business losses)

The information that the panel provided would then be used as input to the riskmodeling process

R ISK I DENTIFICATION AND Q UANTIFICATION Expert Panel

The fields of expertise represented by the panel members were:

analy-Receiver Sensitivities and Significant Flow Volumes

The water supply aquifer was considered extremely sensitive because sewageoverflowing from a pumping station would move directly to the groundwater

Risk Identification and Quantification / 275

supply The panel considered that even if only a very small volume of sewage wasreleased, it would be noticed by the public and would cause substantial reactiondue to the potential health hazard to the potable water supply A significant release

to the water supply aquifer was considered to be 50 gal (200 L)

Swimming beaches were considered to be highly sensitive to sewage releasedue to obvious visual presence, recreational contact, and loss of beach amenity.Other aquifers occur within the area Although they are mostly of potable stan-dard, they have not yet been developed as water supplies These other aquiferswere also considered to be highly sensitive due to their future water supply po-tential A significant release to the swimming beaches and other aquifers was con-sidered to be 530 gal (2,000 L)

The harbor areas, used predominantly for fishing and recreation, were ered to be moderately sensitive due mainly to visual impacts rather than, for ex-ample, the potential for recreational contact A significant release to harbor areaswas considered to be 5,300 gal (20,000 L)

consid-Stormwater, or the stormwater reticulation, was considered to be sensitive (thelowest sensitivity category) because considerable dilution by stormwater wouldoccur prior to ultimate discharge of the effluent to the coastal environment A sig-nificant release to stormwater was considered to be 35,000 gal (130,000 L).Table 17.5 summarizes the receiver sensitivities and significant overflow vol-umes determined by the panel

Identification of Trigger Mechanisms

The panel recognized that, notwithstanding the design, operations, and nance efforts that have been applied to the pumping station system to reduce theamount of risk, there remained some residual risk that failures leading to substan-tial sewage release would occur in future Accordingly, the panel identified arange of mechanisms that could trigger release of effluent, either as overflow or as

mainte-a result of rising mmainte-ain fmainte-ailure

Overflow Triggers The following trigger mechanisms were considered to have

the potential to lead to uncontrolled overflow from pumping stations:

Table 17.5 Receiver Sensitivities and Significant Flow Volumes

Receiving Environment Sensitivity Significant Sewage Release (gal)

• Equipment fault: Pump stoppage due to mechanical breakdown, failure of

sen-sor probes, telemetry system failure The potential for equipment failure wasreduced by installation of backup pumps at each station, round-the-clock mon-itoring of telemetry information, and regular maintenance schedules and peri-odic operational inspections

• Power failure: Unscheduled power outages are relatively common due to the

combined effects of events such as provider failure, vehicle accidents, and clement weather The likelihood of extended pump stoppage due to power out-age was reduced by installation of generator power plugs and development of

in-a comprehensive emergency generin-ator plin-an

• Impact: A number of pumping stations were constructed below ground surface.

These pumping stations usually had a surface-mounted electrical control box,which was, in many cases, exposed to collision with passing traffic Some con-trol boxes were protected by steel or concrete bollards In some cases there waspotential for a vehicle to crash through the protective bollards (where present),thus causing equipment failure that could lead to sewage release This eventwould cause a major failure, resulting in the potential release of substantial vol-umes of sewage effluent

• Fire: There was some potential for fire to break out in a pumping station

(mainly due to some kind of electrical failure) Such a fire could cause a widerange of damage The likelihood of major fire was reduced by the absence offuel material within the pumping station

• Flooding: In cases where the pumping stations were located in low-lying areas

of the topography, there was some potential for flooding to cause equipmentfailure leading to substantial downtime

• Tide: In some cases the pumping stations were located in close proximity to the

shore zones and had some potential to be flooded by the tide Conditions thatpromote tidal flooding of stations included “king tides” and strong onshorewinds

• Blockage: Deposition of silt and debris in the pump wells was ongoing, and

there was potential for this material to be entrained within the pumps and causemechanical damage (e.g., to impellers), which would cause pump failure Thelikelihood of blockage was reduced by scheduled cleaning of pump wells

• Vandalism/sabotage: Many of the pumping stations were relatively isolated

from view or human activity, and there was potential for vandals to break intothe pump houses and damage equipment The likelihood of vandalism was re-duced by construction of steel security fencing, security doors, and enclosure ofequipment

• Seismic event: There was some potential for earthquake activity to cause

dam-age to equipment, emergency stordam-age, pipes, and pump house that could lead torelease of sewage

• Land subsidence: In some cases, pumping stations were located in areas with

some potential for geotechnical instability A geotechnical failure (subsidence

Risk Identification and Quantification / 277

or landslide) could lead to a sewage release In many cases the likelihood of lease was reduced by design and sound construction of the pumping stationfacilities.

re-• Volcanic activity: There was some minor potential for volcanic activity to

cause sewage release

• Coastal erosion: Some pumping stations were located close to shore, where

wave action under certain circumstances could undercut the pumping stationfoundations and cause sewage release

Rising Main Failure The following trigger mechanisms were considered to have

the potential to lead to uncontrolled overflow from rising mains:

• Pipe failure: Rising mains could rupture due to structural failure, corrosion

(where applicable), or joint failure

• Impact: There was some potential for pipework to be ruptured during unrelated

earthworks activities or, in the case of unburied pipes, by collision with a truck

or digger

• Vandalism/sabotage: In cases where pipes were on the surface, there was some

potential for vandalism and sewage release

• Blockage: Silt or other debris could potentially cause a blockage in the rising

main, leading to pressure buildup and pipe rupture

• Seismic event: Earthquake activity could rupture the rising main.

• Land subsidence: Geotechnical failure had potential to rupture the rising main

in some cases

Likelihood of Prolonged Overflow

In each case where a potential trigger mechanism was considered to be applicable,the panel estimated the likelihood that sewage release would be sustained forlonger than the relevant minimum period of uncontrolled release

Impacts and Consequences

The expected impacts of sewage release and their subsequent financial quences were determined by the panel according to the sensitivity of each recep-tor (extremely sensitive, very sensitive, moderately sensitive, and sensitive) Thepanel also evaluated, for each receptor, the severity and likelihood of causingeach of the potential impacts The key impacts and consequences were listed

conse-in Table 17.4, and a brief discussion on the outcomes of the workshops on eachfollows