Lecture 6a - Runoff Prediction - Rational Method

Question 1

Name 3 streamflow generation mechanisms.

Answer 1

Saturation Overland Flow (Direct Runoff), Return Flow (subsurface flow intersects with land surface), Baseflow (groundwater)

Question 2

Name the stormflow hydrograph components.

Answer 2

baseflow recession, rising limb, peak, falling limb, inflection point (end of direct runoff), baseflow, direct runoff

Question 3

What are we typically interested in when we design and size stormwater management systems?

Answer 3

surface runoff

Question 4

What is runoff equal to?

Answer 4

runoff = effective precipitation = precipitation - abstractions

Question 5

What types of runoff predictions could stormwater management system design require?

Answer 5

peak runoff rates, total runoff volume, complete hydrograph

Question 6

What is time of concentration?

Answer 6

time for wave to travel from most hydraulically distant point to catchment outlet

Question 7

What is lag time?

Answer 7

time between centroid of rainfall and peak of the response hydrograph

Question 8

What is the rational equation mostly used for?

Answer 8

estimating peak flow rates from relatively small, urban watersheds

Question 9

What are the 3 assumptions you need to apply the rational method?

Answer 9

1. entire catchment area is contributing to flow at the watershed outlet, means that the duration of the storm must be equal to or greater than the time of concentration of the watershed, 2. rainfall is distributed uniformly over the entire watershed, 3. all catchment losses can be incorporated into one empirical coefficient

Question 10

What types of flow regimes could exist along the hydraulic length?

Answer 10

sheet flow, shallow concentrated flow, channel flow

Question 11

What is sheet flow characterized by?

Answer 11

runoff that occurs as a continuous sheet of water flowing over the land surface

Question 12

What does shallow concentrated flow represent?

Answer 12

the channelization of overalnd flows into small rills and channels

Question 13

Where does channel flow exist?

Answer 13

in larger watersheds where overland flows are eventually routed through larger defined stream channels

Question 14

What are the highest and lowest water velocities?

Answer 14

highest=channel flow, lowest-sheet flow

Question 15

When is shallow concentrated flow assumed to dominate?

Answer 15

after 100 m until the flow enters a channel