Risk Perception, Assessment and

Management

Environmental Risk• Prior to 1980s assumed that pollutants had

a threshold level, below which they were harmless

• Increasing contradictory evidence, especially with carcinogens

• Initial approach was to demand zero levels

• Came to understand that zero not possible• Led to development of field of

environmental risk assessment

Definitions• Risk -- the probability of injury, disease or

death under specific circumstance (Environmental protection agency, EPA)

• Health – a state of complete physical, mental and social well-being, not merely the absence of disease or infirmity (WHO)

• Hazard – the agent or means by which an adverse effect can occur in a particular situation

Definitions• Risk perception – what people believe

poses a risk or hazard• Risk assessment – quantifying the risk

associated with a hazard• Risk management – evaluating whether

real or perceived risks are acceptable, and if not, addressing them

Risk Perspectives

Risk Perspectives

Risk Assessment Model

Hazard Identification• First indication that a hazard exists• Initiator of the Risk Assessment process• Conventionally thought of as toxicological

evidence• Can be more broadly viewed as any initiator

– contaminant levels -- releases– health concerns -- public outcry

Dose-Response Assessment

• Also termed toxicity assessment• Commonly presented as dose-response

curve• Generally produced in animal studies• Assumes response of the population

follows Gaussian statistics (normal distribution)

• Capable of detecting risks ~1%

Extending Dose-Response Data to Environmentally Relevant Doses

• Environmental contaminants normally found at much lower doses than used in animal studies

• Many methods available to extrapolate • Controversial because results can differ

widely• Generally, no data to prove or prove false

any approach

Carcinogens• A carcinogen is any substance or radiation

that is an agent directly involved in causing cancer.

• EPA uses linear model – risk decreases with dose but always some risk no matter how small the dose

• Calculates a slope factor (SF) – risk per unit dose,

• EPA maintains a data base of slope factors

Exposure Assessment

Key Steps in Exposure Assessment

1. Identify significant pathways2. Determine concentrations in

environmental media that are contacted3. Assign exposure factors4. Calculate chemical intake5. Adjust for administered vs. absorbed

dose

• Understand the physical, chemical and biological properties of the agent to assess fate and transport potential of the agent

• Understand the system and specific fate and transport processes that result in elevated concentrations reaching the organism

• Understand the behavior of the organism to assess contact with the media

• Eliminate pathways where concentration or contact is likely to be low

Calculate Chemical Intake

(day) exposure of period timeaverage theis (kg)t body weigh theis

(yr)duration exposure theis (day/yr)frequency exposure theis

(L/day) rateingestion theis (mg/L)in water chemical ofion concentrat theis day)-(mg/kg water drinking ofingestion theis

1

1

1

111

avg

B

t

E

avgB

tE

tWDfRCIwhere

tWDfRC

I

Example: intake of drinking water

Risk Characterization• Last step in the risk assessment process• Integrates first three steps

– hazard identification– toxicity assessment– exposure assessment

• determines probability of an adverse impact to individuals or to a defined population

• provides the basis for risk communication to stakeholders, determination of risk acceptability, and evaluation of risk management strategies

DESIGN OF STORM SEWERS

DESIGN OF STORM SEWERS

The first step in the design of STORM SEWERS is the estimation of flow which they will receive.

There are different methods to estimate storm flow from URBAN AREAS.

Primary source of storm flow is RAIN FALL and RATIONAL METHOD is minimally used for estimating the storm flows in urban areas and semi urban areas

RATIONAL METHOD

All techniques for estimating storm flow are based upon use of rainfall data – either directly or indirectly and rational method is not an exceptional to it.

Rational method relates the flow to:(a) rainfall intensity (b) the tributary area and(c) a coefficient

Q = CiA (Rational Formula)

WhereQ = amount of rainfall which appear as runoff , m3 / hr.i = intensity of rainfall , m / hrA = Area upon which the rainfall, m2 C= Runoff coefficient, i.e the fraction of incident rainfall which appear as surface flow. It depend upon the nature of area.

Typical values of ‘C’Typical values of ‘c’ as used by design engineers are given in table below:-

Type of area C value

Density built areaWell built areaDetached houseSub urban areas(with few building)

0.7 ~ 0.90.5 ~ 0.70.25 ~ 0.50.15 ~ 0.25

TIME OF CONCENTRATION

Definition: It is the time required for the max runoff to develop. When rainfall event occur upon an area served by a storm sewer the runoff will flow over roofs yards and pavements to the gutter and eventually to the sewer INLET. This travel require measurable time and the areas immediately adjacent to the inlet will contribute flow quickly, areas which are distant will not.

The max rate of runoff for a given rainfall intensity will occur when the rainfall has continued for a period sufficient to permit flow to reach the inlet from the most remote point of the drainage area. Consider the rectangular water shed shown below:-

TIME OF CONCENTRATIONC

B

A

InletSewer

5 min

5 min

5 min

Inlet Time

Time of Flow in sewer

I2

I1

So, only the rainfall events are of interest which are of sufficient duration to develop max runoff.

Inlet Time Mathematically:-Time of conc = inlet time + time of flow in sewer.(For Lahore time of conc= 2 hrs)

Where INLET TIME is the time required for rainwater to flow from the farthest point to the sewer inlet.

  Usually Inlet time = 3 – 20 min

For most urban areas 2 hrs is taken as critical duration to produce max runoff.

RAINFALL INTENSITY

In determining rainfall intensity for use in RATIONAL FORMULA it must be recognized that the shorter the duration the greater the expected avg. intensity will be and vice versa. The storm sewer designer thus require some relationship between DURATION AND INTENSITIES. The relation is of form

I = A/(t + B)

Where. I = intensity of rainfall, mm / hrt = duration of rainfall, minutesA,B = Constants, determined using rainfall data

Constants ‘A’ & ‘B’Usually 5 year storm frequency is used for residential areas.

For Lahore Camp and Dresser found the values of A and B based on 5 years storm frequency.

I = 7190 /(t + 103)

USA is divided in 7 zonesA = 810 to 9520B = 10 to 38

SUMMARY OF PROCEDURE FOR CALCULATING STORM FLOW

• Select a suitable critical rainfall duration / time of concentration on (thoroughly consult rainfall records and project area).

• Find out rainfall intensity using above duration form a relation of type.

I = A/(t + B) • Use above intensity in rational

formula to find flow.

ProblemFind the maximum storm flow for the sewers shown in figure below:- Area

Inlet time for each area = 8 minutesTime of flow between manholes = 5 minutesRainfall intensity I = 2670/(T + 15) mm/hr

A4 ha0.8

B3 ha0.7

C2 ha0.8

4 3 2 1

Solution Using rational formula • Q = Ci A

From MH

To MH

Area Serv

d

C =AC ∑AC Time of

Conc.

(min)

Im/hr

Qm3/hr

Qm3/Sec

432

321

40000

30000

20000

0.80.70.8

32000

21000

16000

32000

53000

69000

81318

0.1160.095

40.080

9

3714.8

5053.9

5582.7

1.031.401055