Fundamentals of Engineering Design
Substantial in-channel vegetation along the berm indicates lateral stability in the channel. In-channel
vegetation may impact the hydraulic efficiency of the channel.
Depending on the degree of channel incision, top bank vegetation may or may not contribute to
bank stability. Highly incised channel banks may not benefit from the erosion resistance offered by root
systems, and may even be overburdened by the weight of the trees. Streams which have numerous toppled
trees and other woody vegetation in the channel may have recently had an episode of degradational
instability.
i. Hydrologic Features. During the field investigation, estimates of Manning's n values are made
for the various reaches of the channel. These data are important for computing water surface profiles in
subsequent phases of the investigation. Roughness (n values) is determined for the active channel, the
berms, and the floodplain.
Vegetation frequently preserves evidence of water surface elevations during floods. Debris
transported during floods is often trapped in the vegetation. These high water marks are recorded at the
surveyed cross sections, even if the method of measurement is crude. High water marks are also used to
calibrate n values.
Any evidence of frequent overbank flows such as sand splays, overbank erosion, and crop
damage, etc., are noted during the field investigation. These areas may need consideration for flood control
measures during formulation of the watershed plan.
j. Existing Structures. The presence of man-made features, the extent of the features, and their
location along the channel is recorded on the aerial photos. Man-made features include bridges, bank
protection sites, drop inlet structures, culverts, and grade control structures. An assessment of the
effectiveness of the various features is made during the field investigation. Evidence of scour on bridge
pilings and culverts provides information on the amount of degradation that has occurred since the
construction of the structure. The overall effect of channel stability on the basin infrastructure is assessed.
5.2.3
CHANNEL, STREAMBED, AND STREAMBANK DESCRIPTIONS
The channel description characterizes the stream channel and the adjoining area. The study area
as well as the reaches just upstream and downstream of the site should be the main focus of the field
investigations. As much information as possible should be collected (within the financial constraints of the
project) to accurately analyze the fluvial processes occurring.
The following terminology is used to describe channel, streambed, and streambank characteristics:
Terraces are fluvial landforms produced by past vertical instabilities in the fluvial system. Leopold
et al. (1964) define a terrace as an abandoned floodplain not related to the present stream. The sequence
of events leading to the observed features in the field may include several periods of alluvial deposition.
If incision and aggradation occur repeatedly it is possible to develop several
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