HYDROLOGIC PROCESSES

PURPOSE:
This lab examines 1) the data and methods used to analyze the potential for and causes of flooding by examining a case history and 2) the migration of contaminants in the groundwater.

KEY TERMS AND CONCEPTS: Be sure you understand these.
Physical:
• Stream
• Channel, bank, floodplain
• Meandering vs. braided streams
• Inside vs. outside of meander bends
• Cross sectional area = Width * Depth
• Watershed-drainage area
• Gauging station
• Storm intensity; runoff
• Watershed, headwaters
• Tributary creek
• Water velocity (V) e.g. ft/sec Hydrological:
• Discharge (Q = A x V)
e.g. cfs = cubic feet per second
• Hydrograph
• Bankfull discharge
• Flood
• Stage (water height above channel bottom)
• Lag time
• Groundwater contour line
• Groundwater flow line
• Thalweg
• Point Bar
• Cut Bank

INTRODUCTION:
About 97% of the Earth’s water is in the oceans. The remaining 3% is fresh H2O, ice, liquid, and water vapor. Of that 3%, the majority, about 69%, is locked up in glaciers and icecaps, mainly in Greenland and Antarctica. You might be surprised that of the remaining fresh water, almost all of it is below your feet, as ground water. Practically anywhere on Earth, at some depth underground the rock/sediment is saturated with water. Of all the fresh water on Earth, only about 0.3 percent is contained in rivers and lakes—yet rivers and lakes are what we are most familiar with.

San Joaquin Valley residents rely heavily on groundwater as our primary source of drinking water. Keeping the groundwater supply safe and free of contaminants is vital to our wellbeing. Understanding how groundwater flows and what contaminant risks exist in our valley can aid us in keeping up the quality of our drinking water.

Water is our most valuable commodity but also can pose various hazards, including flooding and groundwater contamination. If one lumps together all kinds of flooding, including that caused by hurricanes and other severe weather, it is the most lethal of all the natural disasters. Each year, inundations result in thousands of deaths and billions of dollars in damages. Most of these could be prevented by understanding the causes of flooding, wise land use planning, and developing adequate emergency procedures.

Groundwater contamination in the San Joaquin Valley can come from many sources including septic systems, storage tanks, fertilizers, pesticides, and landfills. The U.S. today, is thought to contain 20,000 known abandoned and uncontrolled hazardous waste sites, with the numbers increasing annually. Such sites can lead to groundwater contamination, when containers like hazardous materials storage tanks and barrels leak onto/into the ground and the contents eventually make their way down through the soil and into groundwater.

SECTION 1. SURFACE WATER: FLOODING

BACKGROUND:
In the early morning of September 18, 1998 floodwaters from Green and Blue Creeks surged down Rush Creek, hitting the town of Woodhaven with unprecedented force (Fig. 11.1). Woodhaven, situated on the inside of a meander bend, was severely damaged. Downstream, the town of Millbrook, also situated on the inside of a meander bend, fared only slightly better; Vista Bluffs on the outside of a meander bend was unaffected save for severe bank erosion that required stabilization. The floodwater levels on Rush Creek are indicated by the three cross sections A-A’, B-B’, and C-C’ on Figure 11.1.

The flood was initiated by a brief but intense thunderstorm that began about midnight and dropped nearly an inch of water onto the watershed. The ground was dry before the storm. Flooding of this extent had not been recorded since Woodhaven was founded in 1922.

The area’s economy is almost entirely supported by logging; the foothills north of town are heavily forested in fir trees (conifers). However, clear cutting removed most of the timber in the headwaters of Green Creek during the summer of 1998.

Basic Stream Hydrology:
How water flows in a stream depends on the shape of the stream’s channel. The channels of meandering streams are fairly deep and narrow, and they wind and curve, forming meander bends whose inside banks are lower elevation than on the outside of the bend. Consequently, floodwaters will first inundate the inside banks. The channel network of braided streams are shallow, broad, and fairly straight. Their banks are usually the same height, so flood waters flow out of both sides of the channel equally.

Water does not flow uniformly down a stream channel, but rather, it varies in velocity and volume. The volume of water that flows past a certain point in the channel in a given time is called the discharge (Q), and is the product of the water’s velocity (V) multiplied by the cross sectional area of the channel (A); therefore, Q=V*A. 

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