Basic Hydrology & Hydraulics: DES 601 Module 2 General 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 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

Summary Water Cycle Concept Hazard, Risk, Magnitude Table 4.2 in HDM Flood Frequency Curve concept Module 2