Geology 111G/Lecture 17
Surface Flow/ Stream Flow
Types of Flow
Surface Flow Parameters
Hydrograph
Floods
I. Surface Flow. There are two types of surface flow.
A. Laminar flow: stream lines or flow pathways are sub-parallel and do not mix. Laminar flow is found sometimes in very slow flow, but is uncommon in natural streams.
B. Turbulent flow: Flow pathways or streamlines are complex, erratic, crossing. Mixing is common.
C. Surface flow parameters. These are variables that enter a mathematical equation.
1. Velocity. Distance water travels per unit time, usually per second.
2. Flow dimension: Consists of width, w, and water depth, d. The product of these variables is the cross-sectional area, A, of the flow.
3. Gradient: slope of the stream bed, y/x.
a. Influences stream velocity.
b. Typically decreases from the headwaters
of a stream to the point that it enters the ocean. Colorado River in the
4. Discharge: Q, the volume of water that passes a given point in a unit of time.
a. cfs or m3/sec
b. Discharge varies from one stream to
another and seasonally for a given river system. Western rivers have greatest discharge
in the spring during snowmelt.
Discharge usually increases downstream due to addition of water from
tributaries. Last class I discussed
rivers in which Q decreases.
Mimbres,
D. Hydrograph: chart or record of varying discharge with time.
1. Base flow: that part of the graph attributable to groundwater, the constant discharge occupying the stream channel that does not vary with time.
2. Lag time: The response time between a rainfall event upstream and an increase in discharge.
3. Peak flow or discharge:
2. Stream Parameter Adjustments. Changes in discharge must be accommodated by the other parameters.
A. Discharge=flow width x flow depth x velocity
Q (m3/sec) = wdv (m3/sec)
Thus, if discharge increases, at least one of the other parameters must increase. Because the discharge of rivers increases downstream, channel dimensions generally increase in that dimension.
B. Flood: discharge exceeds capacity of channel dimension, forcing water over banks.
1. Bankfull discharge. This is the flow that just fills the channel. Statistically, this flow takes place every 1.5-2 years.
2. Recurrence interval. Statistical time period between floods of a specified flow. Named according to that time interval: "100-year flood." Brings up the point that floods are natural events and that we must plan for them. How big a flood to we need to plan for? Can we inadvertantly modify the expected flood events?
C. Effects of urbanization. There is a tendency for development to change the characteristics of a hydrograph.
1. Increased volume and velocity of surface runoff. This decreases the lag time and increases the peak discharge.
2. Causes a fall in the water table due to decreased infiltration. This in turn decreases the base flow.
3. Bankfull discharge increases, about 2.7x with 50% urbanization.