Basic Hydrology & Hydraulics: DES 601 Module 2 General Hydrology Concepts.
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Transcript of Basic Hydrology & Hydraulics: DES 601 Module 2 General Hydrology Concepts.
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)
Module 2
Hydrologic Processes
• Fundamental Processes:• Precipitation• Evaporation• Infiltration• Storage• Runoff
Module 2
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
Module 2
• 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
Module 2
The Water Cycle
Module 2
Clouds
Clouds
Surface Water Body
Lake or Stream
Ocean
Groundwater Flow
Surface Runoff
Precipitation
Transpiration
Evaporation
Sun
Water Cycle
Module 2
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
Module 2
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
Module 2
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.
Module 2
Annual Exceedance Probabiltiy (AEP)• In TxDOT HDM, the terminology is
Annual Exceedence Probability (AEP)
• Expressed at “percent chance”
Module 3
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”
Module 2
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?
Module 2
Probability and Magnitude
• The magnitude of a discharge is associated with a probability
• Expressed as a flood frequency curve
Module 2
Risk of EXCEEDANCE
Mag
nitu
de (d
isch
arge
, ele
vatio
n, e
tc)
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
Module 2
Summary• Water Cycle Concept
• Hazard, Risk, Magnitude
• Table 4.2 in HDM
• Flood Frequency Curve concept
Module 2