SI-3C1 (2/18/05 version)
Examples, Stream Integrity Assessment To Determine Ultimate Channel Form
This is an example of a stream morphology assessment conducted in conjunction with proposed residential development plans that included altering the channel in the existing ditch. In this case, the existing ditch morphology is not providing all of the potential environmental benefits possible, such as flood attenuation, aquatic habitat, pollutant assimilation, and a self-sustaining channel. Therefore, in this location, the plans for urbanization are an opportunity for stream restoration, because there is already a commitment to invest resources in altering the existing channel form.
The site is located upstream of Scioto Darby Creek Road, and is representative of hydromodification in a headwater stream in the Darby watershed in western Franklin county. At the request of the local Soil and Water Conservation District office, two cross sectional measurements of the existing stream channel were taken, as illustrated in the blue lines in Figures 1 and 2. Stream channel dimensions are approximate for illustration purposes only.
Figure 1. Worthington-Sherwood Ditch, cross section 1, midway between the upstream end and Scioto Darby Creek Road.
Figure 2. Worthington-Sherwood Ditch, cross section 2, near the lower end at Scioto Darby Creek Road
The red lines in Figures 1 and 2 denote the "desired" channel form that will provide the environmental benefits noted above. The "new" channel form recommended includes ample floodplain and does not include lowering of the channel bed. However, if the channel bed is lowered, the surrounding floodplain should be lowered a similar amount, such that the floodplain is the same height above the channel bed as illustrated above.
This is an example of a stream morphology assessment conducted on May 26, 2004 in conjunction with plans to construct the Licking Heights Local Elementary School in Jefferson Township, eastern Franklin county. Measurements of three cross sections typical of Ramey's Run, a tributary to Blacklick Creek, in the area of the school were taken as illustrated in Figures 1-3. A rapid survey was also done downstream of the site and in the adjacent watershed to north.
Figure 1. Ramey's Run cross section 1, midway through an open field.
At this location, the channel is quite straight with little meander development. The fence on a ridge was used as the benchmark for surveying the site, with the peak plotted at -10. The low area, plotted from 10 to 30, varies from wet grasses to scoured gravel. The bankfull elevation is shown by the blue line and the flood-prone area is shown by the red line. The channel is undersized for its watershed size relative to other cross sections in the area (see Figures 2 and 3). A significant portion of flow from typical rain events presently flows through the field.
Figure 2. Ramey's Run cross section 2, just downstream of the midpoint
At this location, the riparian area has more mature trees and channel form is sinuous. Again, the bankfull elevation is shown by the blue line and the flood-prone area is shown by the red line. Grade control is dominated by tree roots crossing the channel bed. The low area to the left of the plot appears to be scour associated with the reduced resistance of the open field and perhaps the "dead furrow" at the edge of the field.
Figure 3. Ramey's Run cross section 3, near the end of the woods.
Channel form at this location is sinuous before the stream enters a section of newly constructed ditch, which flows into a box culvert. Again, the bankfull elevation is shown by the blue line and the flood-prone area is shown by the red line.
Figure 4 shows these same three cross-sections again, overlain with target or "desired state" cross sections (based on the eastern U.S. regional curve) with proportions that are associated with high quality streams. For example, the target cross sections have a floodplain that is 10 times wider. The eastern U.S. Regional Curve provides typical channel cross sectional dimensions based on watershed size. In this example, the eastern U.S. regional curve was selected from several regional curves used in Ohio, based on agreement with dimensions measured in three similar channels in the area.
Figure 4. Target cross sectional dimensions (shown in red) for Ramey's Run
Typically, narrower floodplains are the most prevalent limiting factor to stream integrity in the region, because when the ratio of floodplain to channel width is less than three, streams are characteristically unstable and of poor quality. Therefore, creating additional low floodplain is typically the most urgent action that can be taken in conjunction with land development for long-term stream integrity. However, in the locations where Ramey's Run was surveyed, the existing cross sectional dimensions match very well with the high quality target. The floodplain on this site is surprisingly low and wide.
For this reason, the following options are practical to enhance and protect the existing channel:
- Control the scour occurring parallel to the channel at the field edge with regrading or with channel plugs constructed with fill at intervals; and plant shrubby/woody vegetation to increase flow resistance.
- Abandon the channel in the upstream half of the site, where the existing channel form is straight and build a new channel with meander geometry in the floodplain as shown in Figures 6 and 7. The cross section shows the lowest part of the valley is generally north of the existing fence row out in the open field.
- Refine the floodplain grade in the upstream half of the site to create and improve the riparian wetlands, as shown in the convex profile plotted below. Generally, the upstream portion of streams with this type of profile have a low bed-load sediment supply, high sinuosity, and wetland floodplains. The pond upstream from this reach insures low bed-load supply and further indicates the appropriateness of working to create a stream system with this character.
Figure 5. Profile, Recommended Floodplain Grade, Ramey's Run.
Figures 6 and 7 show the dimensions of two meander patterns based on an 8 foot wide channel and a sinuosity of 1.3 and 2.0 respectively.
Figure 6. Ramey's Run restoration with sinuosity of 1.3.
Figure 7. Ramey's Run with a sinuosity of 2.0.
Figure1 compares the dimensions of a conventional ditch to those of a two-stage ditch, characteristic of a 2 square mile agricultural watershed in northwestern to central Ohio. The pink dashed line denotes conventional ditch morphology, whereas the blue solid line shows the dimensions of a two-stage ditch.
The two-stage ditch design is based on the width of the channel anticipated to form within the ditch. Typical designs have used that width, multiplied by 3, for the width of the ditch at the bench or bankfull stage. The following are the differences (from conventional ditch dimensions) in top widths for a range of drainage areas:
|Increased Width Each Side|
The two-stage configuration provides more capacity at higher stages of flow. For example, the capacity of the conventional ditch design in the 2 square mile drainage area example is 440 ft3/s compared to 660 ft3/s with the two-stage design, or 150% of the conventional design. Using the 2 mi2 northwest Ohio drainage area as an example, Figure 2 compares the flow stages in a conventional versus a two-stage ditch during storms with a variety of recurrence intervals, from a very frequent 0.2-year recurrence interval, up to the 100-year storm.
Note that while the flow elevations during larger rain events (lower recurrence interval) are lower in the two-stage design, elevations of more frequent rain events (higher recurrence intervals) are higher. Flows contained within the smaller, narrower portion of the channel are deeper, with a greater ability to scour and reduce the accumulation of fine sediment building up on the stream bed.
Figure 3 illustrates that the two-stage configuration moderates the bed shear stress. The shear stress is higher for frequent flows when accumulation of fine sediment is a concern, but is less at high flows when erosion is the greater concern.
The same information is presented again in Figure 4, except the amount of work done by the shear stress is multiplied by the number of times each particular flow occurs. The results indicate that two-stage channels should be significantly less prone to filling in with sediment, thereby requiring less long-term cost for maintenance.
Often within conventional ditches, small benches begin to form. However, these benches are often removed during periodic ditch maintenance. The cross sections shown in Figures 5 and 6 illustrate this phenomenon and are from a ditch in Sandusky County. The black dashed line delineates the original channel cross section for the conventional ditch and the red line delineates the new cross section as it would occur if the ditch were modified to a two-stage channel.
Figure 5. Cross section 1, Sandusky County ditch.
Figure 6. Cross section 2, Sandusky County Ditch.