Fundamentals of Engineering Design
terraces. Figures 5.20 and 5.21 illustrate stages of terrace development. The presence of terraces as well
as the number should be noted.
Overbank deposits describe the presence and amount of material deposited directly onto the valley
floor by out-of-bank flows. The magnitude of overbank deposits belies the sediment transport capabilities
of out-of-bank flows. Deep, fast overbank flows are usually indicative of active floodplain processes which
are usually associated with aggrading streams.
Trash lines are remains of floating trash and vegetation left after a flood flow recedes and often
provides a good indication of the high water mark. Most often the debris is found in the floodplain and can
usually be found attached to trees and bushes. Trash lines degrade quickly once the flood flows recede.
The frequent appearance of trash lines resulting from flow rates with short recurrence intervals suggests
that the stream may be aggrading.
Adjacent land use describes the type of activity or land modifications taking place in the areas
adjacent to the site. Generally, cultivated areas have higher runoff potential and sediment yield than natural
settings. Urban and suburban catchments produce flashy runoff hydrographs and extremely varied sediment
Riparian buffer zone and strip width describe the presence of natural vegetation buffer zones
along a stream. The riparian zone provides several important ecological functions such as providing wildlife
habitat, intercepting surface runoff, reducing sediment yield, providing a sink for pollutants in surface and
subsurface flows, reducing near bank velocities, reinforcing bank materials, and limiting access to the bank
by grazing animals.
Flow type defines the regime of flow in the stream at the time of observation. Flow type is a
function of bed forms and bank material and is highly dependent on the stream gradient. Grant et al.
(1990) developed a relationship between bed forms and gradient (Figure 5.22). Uniform/tranquil flow is
characterized by uniform flow velocities and channel characteristics. Uniform/rapid flow involves
significant changes in velocity along the channel. Pools and riffles generally are seen at low flows and
represent a flow regime that alternates between shallow and deepened features which produce non-uniform
flows. Pools are areas of deep, slower moving flow with a gentle water surface slope that generally result
from localized scour. Keller and Melhorn (1973) distinguish two types of pools in meandering channels:
primary and secondary. Primary pools, which exhibit deep scour, are usually found at bends and are
typically associated with point bars. Secondary pools, which are scoured to a lesser depth than primary
pools, are not necessarily associated with point bars. Riffles are shallow areas characterized by fast moving
flow which results from bed material deposition. Steep and tumbling flows occur in high gradient streams
with coarse bed materials. These flows produce localized supercritical flow between and over boulders.
Steep and step/pool flow is found in very steep channels with boulders or woody debris arranged in
periodic steps across the channel and plunge pools in between.
Bed controls describe the presence of local geology, materials, or human structures that resists
being eroded by river processes and thereby controls vertical instability.