13_EnvironmentalFlows(1)

C&ENVENG 4109 / 7109 “Environmental Engineering Design IVB”

Dr Seth Westra

LECTURE 13: FLOODS AND ENVIRONMENTAL FLOWS

HOW MUCH FLOOD STORAGE IS REQUIRED?

Design specifications (e.g. hold the 10% AEP flood)

Release capacity

FLOOD FUNCTION OF A RESERVOIR

http://stream2.cma.gov.cn/pub/comet/HydrologyFlooding/StreamflowRoutingInternationalEdition/comet/hydro/basic_int/routing/print.htm

Annual exceedance probability (AEP): The likelihood of occurrence of a flood of given size or larger in any one year, expressed as a %. (e.g. the 5% AEP)

Average recurrence interval (ARI): The long-term average number of years between flood events as large or larger than the design flood event (e.g. 20 year ARI).

AEP = 1-exp(-1/ARI)

FLOOD PROBABILITY

How probable is the X% AEP flood to occur over a design life of ‘n’ years?

1- (1-AEP/100)n

For example, the probability of a 1% AEP event occurring over a 70 year design life:

1-(1-(1/100))70

= 1-0.9970

= 0.505

FLOOD PROBABILITY

In your brief, you should design the flood storage to hold the 10% AEP flood. This means that the spillway will only potentially be activated once every ~10 years. These factors will influence you setting the spillway level. • This should be estimated based on your daily flow model in Source.

In this year’s assignment, no dedicated flood model is required.

In a real design project, the spillway capacity should be sufficient to handle the probable maximum flood (PMF). Note that you are NOT required to estimate the PMP.

SPILLWAY LEVEL AND CAPACITY

HOW MUCH FLOOD STORAGE IS REQUIRED?

Note: Flood storage depends on how quickly flow is released during a flood. For this assignment you can assume the average annual flow is equivalent to pre-dam “bank full” flow, and thus defines the point at which flows will exceed the river banks.

FLOOD FUNCTION OF A RESERVOIR

10% AEP flow…

…needs to be attenuated to the flow that occurs on average once per year. This will define the

HOW MUCH FLOOD STORAGE IS REQUIRED?

Flood storage ‘airspace’ will depend on the sequence of inflows, the reservoir level (which generally should be kept at or below active storage capacity) and the outflow capacity. Can only work this out by continuous simulation (e.g. in Source), with stochastic generation being useful to better understand the probabilities.

See lecture slides by eWater

ENVIRONMENTAL FLOW - THEORY

LOWER REACHES OF THE ONKAPARINGA

LOWER REACHES OF THE ONKAPARINGA

FOUR PHASES OF ENVIRONMENTAL FLOWS

ENV. FLOW REGIME FOR THE ONKAPARINGA RIVER

Source is a powerful but complex modelling tool. You may wish to use Excel to: • Check whether the computations performed by Source are reasonable

• Furthermore, you may want to explore alternative reservoir designs and reservoir operating rules in Excel – you are not under any obligation to use Source for all aspects of the modelling work.

DO NOT UNDERESTIMATE THE POWER OF EXCEL!

Read: • the User Guide and

• the Scientific Reference Guide.

LEARNING MORE ABOUT SOURCE…