CIVE 445 - ENGINEERING HYDROLOGY

SPRING 2008 - MIDTERM 1 - SOLUTION

PROBLEM 4

 

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

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

  • Hydrology solves for discharges from precipitation.

2. What types of mathematical models are used in hydrology? Define each type.

  1. Deterministic: based on general physical laws

  2. Probabilistic (Statistical/stochastic): based on laws of chance

  3. Conceptual: simplified representation, valid everywhere

  4. Parametric (empirical): valid only for specific site

3. What is the difference between potential and actual evapotranspiration? Which one can be calculated with existing formulas?

  • Potential evapotranspiration occurs under an ample supply of moisture.

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

  • Only potential evapotranspiration can be calculated with existing formulas.

4. 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.
5. What is baseflow? Where does it come from? How can baseflow be lost?
  • Baseflow is the dry-weather flow of streams and rivers.

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

  • Baseflow can be lost through pumping of groundwater near the exfiltration point.

6. What is the range of bottom (river, creek) slopes likely to be encountered in practical hydrologic applications?

  • Sometimes steeper than 0.1, and as mild as 0.00006.

7. Under what circumstances can recorded hydrologic data (precipitation, discharge) become obsolete?

  • Anthropogenic change in watershed properties will change precipitation and surface runoff.

  • Climate change will change precipitation (and runoff) patterns.
8. What is a rating curve? When is a rating unique? When is a rating nonunique or looped?

  • A rating is a relationship between discharge and stage.

  • A rating is unique under uniform or critical flow.

  • A rating is nonunique or looped under unsteady flow (floods).

9. 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.

10. 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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