Fundamentals of Engineering Design
5.2
FIELD INVESTIGATION
One of the most important aspects of any study of watershed geomorphology or channel stability
are field investigations. Data critical to understanding the physical state of the project area can be obtained
from thorough field studies. Field work can be divided into quantitative and qualitative studies. The
quantitative aspects of field work involves site surveys, hydraulic measurements, estimate of channel
roughness, soil sampling, and sediment sampling. Channel surveys are designed to provide information for
determining average channel geometry, channel slope, and bed and bank profiles. Cross-section surveys
are often taken at intervals based on channel width. The interval length depends upon economics, position
of controls, dimension and character of channel related failures, sediment characteristics, and channel
configuration. Within the economic constraints of the project, surveys must be extensive enough to
accurately represent the channel and the pertinent features. The survey sections should also sufficiently
extend beyond the top of the banks to record the general level of the immediate floodplain. In addition to
the cross-sectional surveys, the water surface and bank profiles are surveyed to determine the slope of the
study area.
Hydraulic measurements such as average water surface width, stage, and water temperature
supplement the channel survey data. In addition, long-term measurement of flow velocity and subsequent
discharge calculations at selected channel cross sections provide valuable hydraulic historical data.
An estimate of the roughness coefficient should be determined for the left overbank, right
overbank, and channel. Roughness coefficients are a function of surface roughness, amount of vegetation,
channel irregularities, and to a lesser degree, stage, scour, deposition, and channel alignment. These
coefficients will be used in numerical models for computing water surface profiles, sedimentation, and
channel stability. It is important to recognize that roughness coefficients may vary dramatically from left
overbank to right overbank and even within the channel. These variations can significantly model
calculations and care should be taken to ensure that accurate estimations are made. If the stage, discharge,
and slope are known at a given cross section, the roughness coefficient can be calculated. Chow (1959)
outlines Cowan's approach to estimating Manning's n. Two excellent pictorial references are estimating
the roughness coefficients using a visual comparison method are: Roughness Characteristics of New
Zealand Rivers (Hicks and Mason, 1991) or USGS Roughness Characteristics of Natural Channels
(Barnes, 1989).
Soil samples should be collected from the bed and banks and analyzed to determine geotechnical
characteristics such as unit weight, angle of repose, angle of internal friction, cohesion, and soil particle size.
If the bank is stratified or layered, a sample should be taken from each layer. Edwards and Glysson (1988)
define bed material as the sediment mixture of which the bed is composed. Descriptions of bed material
sampling can be found in Julien (1995), Edwards and Glysson (1988), or Petersen (1986). For coarse bed
material, Hogan (1993) describes several procedures that can be performed to determine the bed material
composition. This information is used for the stable channel design computations and slope stability
analysis.
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