Fluvial Geomorphology

Understanding relationships between streams and the land around them is the key to successful stream restoration.


PWD aquatic biologists performing sediment particle size (left) and bank pin analysis (right)

Overview

Fluvial geomorphology (FGM) is the science of studying the shape of streams to understand how they interact with the land around them. Streams naturally take on different shapes and flow patterns depending upon whether the land around them is steep vs. flat, rocky vs. sandy, arid vs. rainy, etc. In addition to these natural differences, land development and urbanization also strongly influence streams. PWD conducts FGM survey work throughout Philadelphia's watersheds, complementing hydrologic monitoring conducted by USGS, with the goal of providing a solid foundation of information to guide stream restoration work.

FGM Principles and Definitions

Fluvial geomorphology is a complex subject. In this section we attempt to provide a brief explanation of several underlying principles of FGM. More information is available from the sources listed under Additional Resources, below.

Stream Stability and "Dynamic Equilibrium"

Streams naturally move and change their shape due to erosion and redistribution of bed and bank materials. Dynamic equilibrium is a term that is used to explain how a stream can be described as "stable" despite the fact that it actually changes shape. Though stable streams may move and change shape, overall dimensions tend to remain steady and they do not become noticeably wider, straighter, more entrenched, or accumulate lots of sediment. If the stream channel does become noticeably bigger due to erosion or starts accumulating lots of sediment, then the stream is no longer considered stable and no longer in a state of "dynamic equilibrium" with its watershed.

Aggradation and Degradation

As described above, erosion and deposition of sediment are natural process that occur in stable streams. Aggradation and degradation are used to describe contrasting processes that can occur when a stream becomes unstable. Stream gradients become steeper (aggradation) or less steep (degradation) due to excess deposition or erosion of sediment, respectively. Aggradation and degradation are often the symptoms of a problem within the watershed, such as changes in land use within the watershed that occur over a broad area and affect the stream's hydrology. However, a stream may also aggrade or degrade very quickly if the problem is caused by a very large storm event or a localized disturbance, such as earthwork without proper sediment controls.

Bankfull Discharge

The bankfull, or effective, discharge is the discharge that is most efficient at doing work within the stream channel. Small storms occur very frequently, but the resulting flows are incapable of moving much sediment or causing much erosion. On the other hand, very large storms cause floods that can move a lot of sediment and cause a large amount of erosion, but some of this work takes place outside the active channel and these events tend to happen very infrequently. In the middle of these two extremes is the bankfull, or effective discharge, the flow which has the optimum combination of power and frequency. This bankfull discharge is usually the focus of stream restoration designs.

The definition of bankfull flow is not universally agreed upon by FGM practitioners, but may be defined as a flow with a specific recurrence interval, such as the 1.3 or 1.5 year flow. It is also widely recognized that stable streams may exhibit various bankfull indicator features that can be identified in the field and correlated with hydrologic data.

Stream Classification

As described above, streams take on distinctive shapes depending on geologic and climatic factors within their watersheds. Scientists from a variety of disciplines find it useful to employ a classification system to describe streams. The most popular system is the Rosgen Stream Classification System. More information about the Rosgen Stream Classification System is available from the sources below, under Additional Resources.

FGM Data Collection


PWD engineers use a survey total station to measure stream dimensions

Stream Cross-sectional Profiles

PWD engineers and aquatic biologists survey stream cross sections at riffles (shallow points), recording elevations across the stream channel and into its floodplain. Water surface elevation is measured at each bank, along with the deepest point in the stream (thalweg), and the elevation of any field-observed bankfull indicators such as breaks in slope or vegetation/debris lines. A series of photographs are taken and permanent monuments are installed at each cross section location, allowing scientists to return to cross sections for re-survey in the future.

Longitudinal Profile

In addition to surveying across the stream in a cross section, a number of points are surveyed along the stream longitudinally in order to generate a thalweg profile, map out the locations of riffles, runs, and pools, calculate stream and water surface slope, and identify additional bankfull indicators when present.

Pebble Count and Bar Samples

During FGM surveys, scientists walk transects across the stream in a zig-zag pattern and measure stream particles within the surveyed riffle cross section. For most surveys, 100 particles are measured, using a gravelometer and sand gauge. When more data are needed for restoration project design, one or more gravel bar and/or sub-pavement samples may be collected. This technique is much more time consuming, requiring excavation of sediment from the streambed and subjecting it to sieving and weighing for a much more quantitative measure of the size distribution of sediment actively being moved by the stream.

Bank Measurements

PWD has been utilizing the Bank Assessment for Nonpoint Consequences of Sediment (BANCS) Model to estimate annual streambank erosion rates. Field observations of streambank dimensions and conditions are recorded, such as bank height, bank angle, root depth and density, etc. These data are used with the Bank Erosion Hazard Index (BEHI) and Near Bank Stress (NBS) predictive tools to estimate bank erosion potential. Estimates from these calculations are combined with Geographic Information System (GIS) data to estimate sediment loading due to stream erosion.

Bank Pin and Scour Chain Monitoring

Steel pins or lengths of iron rebar are installed horizontally into stream banks and measured over time in order to observe changes in the streambank profile. Bank pin measurements can then be used to estimate rates of erosion and sediment loading. This type of monitoring has been conducted primarily in the Wissahickon Creek Watershed, where PWD is researching contributions of streambank erosion from tributaries to siltation problems in Wissahickon Creek mainstem. Bank pins, along with scour chains (a similar technique for measuring changes in the stream bottom), are installed in PWD stream restoration sites to track changes over time.

FGM Data Analysis

Stream Dimensions

Field survey data are entered and processed with computer software to yield a number of different dimensions and measurements of the stream's shape. Some of these dimensions (or the ratio between two different dimensions) are used to describe the stream in a stream classification system. Stream dimensions are also used to calculate flows, estimate sediment transport capabilities, and compare a stream to reference reaches that are known to be healthy and stable.

Shear Stress

Shear stress is the force applied by flowing water parallel to the stream bed (or bank). Shear stress forces have the ability to move particles of a given size within a stream. This analysis may be performed for specific flow conditions, such as bankfull flow, in order to determine what size particles can be moved by a given flow.

Particle Size Distribution

Pebble counts are used to estimate the substrate particle size distribution, which in most streams approximates an s-shaped curve. Various points along the curve, such as the median particle size (d50) may be used in sediment transport calculations. PADEP may use the proportion of fine sediment in pebble count samples as a rapid assessment of whether a stream is impacted by stormwater runoff.

Stream Competency Analysis

Stream Competency is a calculation that takes into account shear stress and the particle sizes available in a stream. This analysis is performed in order to determine whether a stream is capable of transporting its sediment load from the watershed or whether the stream will have a tendency to aggrade or degrade.


Additional Resources


Fundamentals of the Rosgen Stream Classification System USEPA Watershed Academy Training Module
USEPA Watershed Assessment of River Stability & Sediment Supply (WARSSS)
Stream Morphology Modules, a series of spreadsheet-based tools for FGM analysis distributed by Ohio Department of Natural Resources
Natural Stream Channel Design Guidelines for Pennsylvania Waterways developed by the Keystone Stream Team
Wildland Hydrology Provides training materials, software, and courses in stream assessment and restoration.
Fluvial Geomorphology Training Modules from State University of New York College of Environmental Science and Forestry (SUNY ESF)

NOTE: These links are provided by PWD for the convenience of users of this website and are not an endorsement of any product or service.