Basic Hydrology & Hydraulics: DES 601

Module 2General Hydrology Concepts

Hydrologic Compartments• Compartments (where water occurs):

• Atmosphere• Ocean• Ice (Polar and Glaciers)• Surface (Lakes, Rivers)• Subsurface (Soil, Aquifers)

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Hydrologic Processes

• Fundamental Processes:• Precipitation• Evaporation• Infiltration• Storage• Runoff

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Process-Compartment Relationships• The processes and compartments are inter-

related and used to help explain various hydrologic outcomes:

• Channel storage• Floods and droughts• Direct runoff• Base flow• Shape, size, and number of river channels

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• Concept central to hydrology• It is conceptualized as a continuous process,

without start or finish• Typically presented in a diagram that illustrates how

water in different compartments move from one compartment to another

Water Cycle

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The Water Cycle

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Clouds

Clouds

Surface Water Body

Lake or Stream

Ocean

Groundwater Flow

Surface Runoff

Precipitation

Transpiration

Evaporation

Sun

Water Cycle

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Probability, Risk & Risk-Based design

• Hazard is a situation (driving through deep water) that is likely to cause harm (loss) in the absence of its control. Identification of hazards is the first step in performing a risk assessment

• Loss is the dollar value caused by a hazard that manifests itself

• Risk is the probability that a particular criterion is exceeded

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Probability, Risk & Risk-Based design

• In the HDM risk is the consequence associated with the probability of flooding attributable to a project

• Assessing hazards, risk, and loss are done in both a qualitative and quantitative manner

• Hazards are mitigated (losses reduced) by:• Insurance (financial loss)• Education/barricades (DOS during the hazard)• Structural measures to lower the probability

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Probability, Risk & Risk-Based design

• Risk-Based design (as used herein) assumes that the goal is to convey the particular discharge without rendering the infrastructure temporarily out of service

• If this requirement can be relaxed the hydraulic structure can be sized much differently

A hydraulic structure can convey huge discharges (rare events) if one is willing to accept the surrounding infrastructure to be out of service for awhile – a culvert can be really small if we allow the roadway to be under water for awhile

It might make sense to allow a portion of infrastructure to flood to protect a far more valuable (hard to replace) portion of the infrastructure nearby.

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Annual Exceedance Probabiltiy (AEP)• In TxDOT HDM, the terminology is

Annual Exceedence Probability (AEP)

• Expressed at “percent chance”

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Risk Based Design – Suggested Values• The HDM, Table 4.2, pg 4-13 has recommended risk

(probabilities) to consider for infrastructure design in Texas• “Structures and roadways should be

serviceable (not inundated) up to the design standard”

• “All facilities must be evaluated to the 1% AEP flood event. … The intent of the check flood is not to force the 1% AEP through the storm drain, but to examine where the overflow would travel when this major storm does occur”

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Table 4.2 Exercise• Table 4.2 and narrative (pp 4-13 to 4-15) in

the participant package.

• Consider the watersheds delineated in Exercise 1. What would be the appropriate AEP for the two crossings on Exercise 1?

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Probability and Magnitude

• The magnitude of a discharge is associated with a probability

• Expressed as a flood frequency curve

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Risk of EXCEEDANCE

Mag

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de (d

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Flood Frequency Curve

1

99

10

90

5

95

50

50

2

98

20

80

30

70

40

60

10000

20000

30000

40000

50000

600007000080000

100000

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Summary• Water Cycle Concept

• Hazard, Risk, Magnitude

• Table 4.2 in HDM

• Flood Frequency Curve concept

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