CIVE 445 - ENGINEERING HYDROLOGY

SPRING 2003 - MIDTERM 1 - SOLUTION

PROBLEM 1

A= 265 km2

P= 750 mm/yr

Q= 1.9 m3/s

Total volume of precipitation VP:

VP = 265 km2 × 750 mm/y × (1000 m/km)2 / (1000 mm/m) = 198,750,000 m3/y

Total volume of runoff VR:

VR = 1.9 m3/s × 86400 s/d × 365 d/y = 59,918,400 m/y

Asbtracted percentage:

[(VP - VR) / VP] × 100 = 69.8%   ANSWER.

 

 

 

 

PROBLEM 2

fc = 1.5

f = 2.75 for t = 1

f = 1.9 for t = 2

Horton equation:

f = fc + (fo - fc) e-kt

f - fc = (fo - fc) e-kt

2.75 - 1.5 = (fo - 1.5) e-k

1.9 - 1.5 = (fo - 1.5) e-2k

1.25 = (fo - 1.5) e-k

0.4 = (fo - 1.5) e-2k

Dividing these two equations:

3.125 = ek

k = ln (3.125) = 1.1394    ANSWER.

Solving for fo:

fo = 1.25 ek + 1.5

fo = (1.25 × 3.125) + 1.5 = 5.406    ANSWER.

fc = 1.5    ANSWER.

Verification:

f = 1.5 + (5.406 - 1.5) e-1.1394 = 2.75   OK

f = 1.5 + (5.406 - 1.5) e-2 × 1.1394 = 1.9   OK

 

 

 

 

PROBLEM 3

T = 20oC

Qn = 620 ca/cm2/d

v2 = 150 km/d

RH = 75%

The saturation vapor pressure at the overlying air temperature of 20oC is: 

eo = 23.37 mb

The mass-transfer evaporation rate is:

Ea = (0.013 + 0.00016 × 150) × 23.37 × [(100 - 75)/100] = 0.216 cm/d

The evaporation rate due to net radiation is:

En = (620 cal/cm2/d) / (0.998 gr/cm3 × 586 cal/gr) = 1.06 cm/d

Penman's ratio (Table 2-4):

α = 2.19

The evaporation rate by the Penman formula is:

E = (2.19 × 1.06 + 0.216) / (2.19 + 1) = 0.796 cm/d.   ANSWER.

 

 

 

 

PROBLEM 4

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

  • Hydraulics solves for 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 when moisture is limited.

d. What four factors affect time of concentration?

  1. Length (hydraulic length) L

  2. Slope (channel slope) S

  3. Friction coefficient (Manning) n

  4. Rainfall intensity (effective) i
e. What is baseflow? Where does it come from?

  • Baseflow is the dry-weather flow or streams and rivers.

  • It comes from the groundwater.

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

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) or 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 mediterrean ecosystem is a product of latitude (30o-35o) and 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 every thirty years in all 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 due to ash particles produced by the fire.

 
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