CIVE 445 - ENGINEERING HYDROLOGY

SPRING 2004 - MIDTERM 1 - SOLUTION

PROBLEM 4

 

a. What is the difference between hydraulics and hydrology? Be specific.

  • Hydraulics solves for flow depths, velocities, and pressures.

  • Hydrology solves for discharges from precipitation.

b. What three conditions are necessary for precipitation to occur?

  1. Moisture availability.

  2. Condensation through cooling.

  3. Coalescence to develop rain-size drops.

c. What is the difference between potential and actual evapotranspiration?

  • Potential evapotranspiration occurs under an ample supply of moisture.

  • Actual evapotranspiration occurs in reality, when moisture may be limited.

d. What four factors affect time of concentration?
  1. Catchment length (hydraulic length) L

  2. Channel slope (channel slope) S

  3. Friction coefficient (Manning) n

  4. (Effective) rainfall intensity i.
e. What is baseflow? Where does it come from?
  • Baseflow is the dry-weather flow of streams and rivers.

  • Baseflow comes from the groundwater as it exfiltrates at springs located at lower elevations.

f. What is a rating curve? How many types of ratings are there?

  • A rating is a relationship between discharge and stage.

  • Types of ratings:

    • Uniform flow (equilibrium or Manning's) rating

    • Critical flow rating

    • Nonuniform (gradually varied flow) rating

    • Unsteady flow (flood wave, looped) rating

    • Unstable rating due to erosion and deposition in the channel bottom

g. What do the Creager curves depict?

  • They depict the diffusion that is present in streamflow.

  • The Creager curves state that the greater the catchment area, the smaller the peak flow per unit of catchment area.

h. To what five factors is to be attributed the recurrence of debris flows in the San Gabriel Mountains of Northeast Los Angeles? State them in causal order.

  1. The uplift (through tectonism) of the mountain range (reported to be the highest in the U.S.).

  2. The type of vegetative ecosystem (mediterranean, chaparral), which has developed adaptations to survive through droughts, including waxed leaf surfaces to minimize evapotranspiration. The mediterranean ecosystem occurs in midlatitudes (30o-35o) that have exposure to westerlies (trade winds from the west).

  3. The wind storms (Santa Ana), which affects the region.

  4. The wildland fires, propelled by drought and wind, which recur approximately every thirty years in chaparral ecosystems. The fires vaporize the waxy substances (in the litter and standing biomass) at the surface, and condense 1-5 cm inside the soil, creating the hydrophobic soil layer.

  5. The intense rainfall events, exceeding 1 in/hr, which follow the fire because of enhanced coalescence in the lower atmosphere due to ash particles produced by the fire.
Problem 1

 
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