Changing the Channel

• Oct 05, 2007

Urban channel design has traditionally focused solely on erosion control and the transport of floodwaters (flood conveyance), ignoring geomorphologic habitat sustainability (the evolution and configurations of landforms as habitat). In many cases, avoidance of sediment deposition can be just as important as avoidance of erosion. Historically, stable channel design has included installation of armor with large rock, gabions, concrete, and more recently, bioengineering techniques. Too frequently, tactics are used to "control erosion" without a strategy that considers the fluvial processes of the entire watershed.

In urban areas, down cutting - the response to removal of sediment from the channel bed - results from increased runoff rates and transport capacity in the receiving stream. As the bed elevation is lowered through erosion degradation, a reduction in channel slope and increase in bank height are commonly experienced. When the banks reach a critical height, mass failure occurs, the channel widens and large amounts of sediment are delivered to the channel, which may result in sediment deliveries that temporarily exceed the transport capacity of the stream system. Sediment is typically stored or deposited in the reach until fluvial action incises the newly deposited material.

As a result of urbanization, the Fort Branch of Boggy Creek in Austin, Texas, experienced significant down cutting and bank erosion, exposing utilities and threatening structures along the channel banks. Prior to our design study comparisons of historical channel profiles and cross-sections in the design reach (expanse of water visible between bend in a river or channel) - approximately one mile long with a drainage area of roughly three square miles - indicated that the channel bed of Fort Branch widened and lowered in elevation by approximately seven feet.

Using equilibrium approaches, the stream restoration and rehabilitation design of Fort Branch, attempted to arrest further down cutting and bank instability, while providing habitat features, including riffles, pools and vegetation. Therefore, a natural design approach, allowing some mobility in the channel bed and utilizing sediment transport as a basis for channel stability, was preferred by the client.

Typically, natural channel design attempts to provide a system that is in dynamic equilibrium and includes some structure, form and function similar to a natural stream. When a channel is in dynamic equilibrium, the hydraulic geometry and profile are compatible with the natural watershed conditions. Also, a geomorphically stable channel has more opportunity to sustain instream and riparian habitat as compared to a channel designed solely for erosion and flood control. Natural channel design allows a somewhat mobile channel boundary that is neither excessively erosive nor aggradational (filling or raising the level of the bed of a stream by deposition of sediment). This approach, however, requires a more comprehensive understanding by the civil or environmental engineering team of the watershed hydrology and stream mechanics.

Equilibrium methods - including hydraulic geometry relationships, sediment continuity and threshold methods - can only be used to evaluate long-term conditions toward which the channel is adjusting. The benefit and danger of these methods is that they can be employed relatively easily. In contrast to sediment routing, and in comparison with sediment continuity, the equilibrium approaches assume that sediment transport relationships are relatively unchanged over time.

Long-term channel response results from the integral effect of all the hydrological, meteorological and geological variables that influence the stream, but in stable channel design it is convenient to apply discrete values of the physical conditions affecting the system. The discharge presumed to contribute most significantly to the channel characteristics - known as the channel forming discharge, bankfull or dominant discharge - is the flow that cumulatively performs the most sediment transport over time. In a disturbed stream, the bankfull condition may not correspond to the dominant discharge and therefore an understanding of the channel sediment transport characteristics and geomorphology is required to evaluate the channel forming discharge.

Because Fort Branch was an ungaged stream and had not been measured for characteristics, such as velocity of flow and water surface elevation, empirical hydrology and rainfall-runoff modeling were used to estimate flow frequency values. The United States Geological Survey (USGS) regression equations for Texas were compared to results from a HEC-1 model developed by the city of Austin, and the regression equations for rural and urban areas resulted in lower peak discharge values than the HEC-1 model.

The comparison of the rural regression equation results to peak flows computed with the HEC-1 model demonstrated the effects of urbanization on peak flow discharges. Results from the two- through 100-year storms indicated that urbanization in this watershed has increased the peak flows by nearly a factor of four. For our analysis, the discharge associated with the runoff resulting from the one-year storm was selected, and the one-year HEC-1 discharge appeared to correspond to the bankfull condition at most cross-sections in the stream reach.

To compare the sediment transport capacity of the design and supply reaches, reach-averaged hydraulic parameters were used to characterize representative cross-sections for analysis. The reach-averaged channel hydraulics were calculated from the HEC-RAS model output for 85 cross-sections, including three riffles and three pools. Reach-averaged hydraulic parameters for the supply reach were then calculated from the city's HEC-RAS model for Fort Branch Creek, and a historic city HEC-RAS model used to analyze hydraulics for a period in 1976, before the watershed was considered to be "urbanized." Further, particle size distributions representing the bed material in transport during bankfull conditions were obtained from field measurements. The sediment gradations were used to determine stable channel width and slope such that sediment continuity is achieved between the supply and design reaches.

Three methods were used to develop a family of curves representing stable channel dimensions under a variety of sediment supply conditions: "Equation 3," which, over a hydrologic period, yields the net accumulation of sediment within a design reach; "Equation 4", which can be used to estimate amounts of downcutting or deposition expected for short- and long-term evaluations; and the "SAM method," which partitions channel roughness between the channel bed and banks, and uses resistance and sediment transport formulae to compute stable channel dimension curves.

Sediment transport functions, which were applied to reach-averaged hydraulic conditions for the entire channel upstream of the design reach yielded supply transport rates more than 200 percent of the design reach capacity. Sediment transport for a channelized reach immediately upstream was computed at 150 percent of the design reach transport. With respect to sediment continuity, such conditions would suggest aggradation and are inconsistent with the long-term trend in Fort Branch. Observations of severe degredation in Fort Branch indicated a sediment supply less than the design reach capacity, illustrating that blind application of sediment transport functions to quantify supply can yield erroneous results.

The objective of the sensitivity analysis was to assess an appropriate sediment supply assumption for developing a stable channel design. Because the channel is degrading and geomorphic indicators do not suggest any natural restabilization, a supply estimate was assumed between the existing design reach capacity and zero supply.

Observation of historical trends provided some additional insight into the stream's behavior. The historic channel condition in 1976 was compared to the existing condition in 2000 to illustrate the channel evolution over the last 25 years. It is reasonable to assume that the rate of change in channel geometry would decrease some time following a cessation in development in the watershed. Much of the watershed above the Fort Branch design reach was developed prior to 1980 and little expansion has occurred since then. Additionally, hydrologic controls are now required by the city in its land development code for future development in all Austin watersheds.

Consequently, on the basis of our design studies, a reasonable estimate for the stable channel dimension curves would be 25 to 50 percent of the design reach capacity. Such a supply deficit more accurately reflects the degrading conditions of the design reach. A design width of 60 to 65 feet and an energy slope of approximately 0.2 to 0.3 percent would satisfy sediment continuity. The trend in channel widening indicates an increase in channel width of five to 10 feet may be expected during the next 50 years. Using incipient motion to determine stable channel dimensions would result in a design slope less than 0.1 percent. Given that the upstream reach continues to supply some sediment from eroding banks and bed, an assumption of zero sediment supply is not accurate.

Unfortunately, the magnitude of sediment loadings may be difficult to quantify for most urban streams. However, a comparison of historical trends, in conjunction with field observations and a sensitivity analysis of channel adjustment to sediment loads, can provide the designer with a more intimate understanding of the stream's reach response characteristics. As illustrayed by the case study described in this article, the results of the Fort Branch project show that sensitivity to sediment transport can be evaluated readily using equilibrium methods, considering the upper and lower bounds of stable channel dimensions. The sensitivity analysis for Fort Branch indicated that an overall reduction of 0.1 to 0.3 percent of the energy slope should help stabilize the stream design reach. Given that incipient motion (stable slope equals 0.1 percent) represents the conservative, lower bound of stable slopes, the design for Fort Branch proceeded assuming a stable energy slope between 0.2 and 0.3 percent to account for upstream sediment supply.

On the basis of our studies, the conceptual design to achieve channel stability and increased habitat value included a series of grade control structures in the form of constructed rock riffles, preferred because of their flexibility, availability of suitable material and habitat value. General riffle geometry was based on findings from this analysis and subsequent analysis of flood flows and channel forming discharge of the frequent flow inset channel. Conceptual design incorporated terraced side slopes to achieve additional flood flow conveyance and reduce in-channel shear stress via floodplain reconnection.

This article originally appeared in the March/April 2002 issue of Water & Wastewater Products, Volume 2, Number 2, page 32.