Design Alternatives
Data collected from the preliminary site characterization was used to develop design constraints and requirements. A number of design options for the channel restoration and bank stabilization project were then developed using these constraints and requirements. Section 3 provides a summary of the chosen design specifications, alternatives, and final design recommendation.
Design Specifications
Six design specifications were considered before the determination of an exceptional channel design was made. Longitudinal channel slope is an important aspect related to the overall stability of a channel, and can change depending on the size of the channel reach being restored. The channel width has been chosen for all design alternatives based on the reference reach analysis. Side slopes have also been chosen based on the reference reach analysis. Additional factors addressed in the design process include optimizing earthwork so that the amount of soil excavated, or “cut”, is offset by the amount of soil needed for “fill”. Optimizing cut and fill volumes will help minimize impacts on local flora and fauna. Cut and fill volumes vary with each of the different designs presented. Post-construction monitoring is also a necessary component of all natural channel restoration projects.
Preliminary Channel Designs
A decision matrix was created to determine the most adequate design for a final recommendation. Six design parameters were developed to compare the five design alternatives, as seen later in Table 3. The design alternative with the highest total score was chosen as the final design recommended for implementation. Shown in Table 2 below are brief descriptions of the design alternatives developed.
Table 2: Design Recommendation Tables: Design Descriptions
Design # |
Description |
1 |
No action |
2 |
Long: Complete channel restoration. Introduce sinuosity from culvert entrance to upstream reference reach. |
3 |
Short: Realign channel and culvert with one short downstream channel curve. |
4 |
Long w/ Rock Structure: Incorporate a short, steep entrance (~60', 5%) to the culvert protected with locally recovered rocks, followed by a completely graded channel connecting with the reference reach |
5 |
Short w/ Rock structure: Incorporate a short, steep entrance (~60', 5%) to the culvert protected with locally recovered rocks, followed by one short curve aligning the culvert and channel |
As seen in Table 2 above, four designs were developed, as well as a plan to take no action and leave the existing channel alone. Detailed descriptions of each design alternative are found below.
Long Reach Design
The Long Reach Design is the first design that was developed. This design provides a complete natural restoration extending from the culvert to the reference reach. The primary objective of this design was to use a constant slope of about 1.35%, which was found using the reference reach analysis. The long design extends 500 feet upstream from the culvert before connecting with the reference reach. A long design would provide a larger activated floodplain and flatter banks than what currently exist. The high quantity of excess cut resulting from this design make it less desirable, and relatively impractical. The tree interference associated with the length of this design is also problematic. Over ten mature Ponderosa Pine Trees currently reside in the lower floodplain 300 feet upstream of the culvert. Because the Ponderosa Pine Trees are located on the floodplain of this design, lowering the floodplain would be extremely detrimental to the grouping of trees. The Long Reach Design plan set is found in Appendix H.2.
Short Reach Design
The Short Reach Design was created to eliminate excess cut from the long reach design and to avoid disturbing the group of mature Ponderosa Pine Trees. The primary goals of this design were to stabilize the channel with a natural slope, align the channel with the skewed culvert, and reactivate the floodplain in the area. The Short Design extends 250 feet up from the culvert and utilizes one bend to align the existing channel with the culvert. In order to connect with the existing channel, a constant slope of 2.33% would be created, where a slope of 1.13% is ideal. Compared to the Long Reach Design, the slope is less natural, but the excess cut makes this design more favorable. Banks in the Short Reach Design have a side slope of 1:5, similar to those of the reference reach. There are three young Ponderosa Pine Trees that are in the design cut zone. These trees would be carefully transplanted out of the cut zone and into the floodplain. Plans for this design can be found in Appendix H.3.
Long Reach Design with Rock Structure
The decision to create a design with a rock structure near the culvert was made in order to reduce excess cut, provide a more natural slope, and stabilize the channel bed near the culvert. The Long Design with a Rock Structure extends 500 feet from the culvert to connect with the reference reach. The design utilizes two bends to align with the culvert. Unlike the Long Reach design that utilizes a constant slope, the Long Design with a Rock Structure is fitted with two different slopes. The first slope consists of 60 feet at a 5% grade covered in large diameter rock. The design plan for the rock riprap can be found in Appendix H.7. The second portion of the design utilizes a slope of 0.98% for the remaining 440 feet, which is a near-ideal slope according to the reference reach analysis. With the addition of rock riprap, the initial slope of 5% is very stable due to the nature of the rock structure. This 5% slope then enables the rest of the design to have the near-ideal slope of 0.98%. The plan view for this design can be found in Appendix H.4.
Plan View, Rock Structure Detail
Short Reach Design with Rock Structure
The Short Reach Design with a Rock Structure was created to reduce excess cut and eliminate the need to remove mature trees from the Long reach Design with a Rock Structure. The Short Reach Design with a rock structure extends 250 feet from the culvert, utilizing one bend to align the channel with the culvert. The rock structure design extends 48 feet from the culvert with a slope of 5.0%. Rock embedded in the channel will act as armor and prevent sediment transport. Like the short reach design without rock riprap, this design will result in the least impact on the local vegetation, as well as the least amount of excess cut. Although this design does not achieve a complete natural channel restoration, it is practical due to its low cost, low excess cut, culvert alignment, slope stabilization, and bank stabilization. The plan view for this design can be found in Appendix H.5.
Plan View, Longitudinal Profile, Rock Structure Detail
Feasibility Assessment
In order to determine the most feasible project design, two decision matrices were created. The decision matrices created contained six parameters dictating which design alternative was the best. The six parameters were: cut/fill volume, final slope, vegetation restoration, natural aesthetics, required labor and stability. The cut/fill volume was based off the amount of excess cut/fill determined in AutoCAD Civil 3D. The final slope was determined by how steep the slope was, with the steepest being the worst, which was calculated in AutoCAD by dividing the change in elevation of the stream by the length of the reach. The vegetation restoration parameter was determined by how much vegetation would be removed and replaced along the newly aligned channel. The natural aesthetics was based on the final visual of the channel and the natural features associated. The required labor was determined by the amount of cut/fill needed and the effort to relocate and plant surrounding vegetation along the channel. The stability of the channel was based on the final slope of the channel, sinuosity, and bank stabilization with regards to having vegetation or not. A cost parameter was not used in the design matrix because ACE will be providing the equipment and labor. Table 3 below illustrates the original decision matrix with each design rated by parameter.
Table 3: Design Recommendation Tables: Criteria Ratings
Design # |
Cut/Fill Volume |
Final Slope |
Vegetation Restoration |
Natural Aesthetics |
Required Labor |
Stability |
Total |
1 |
10 |
4 |
4 |
5 |
10 |
1 |
34 |
2 |
2 |
7 |
3 |
10 |
4 |
5 |
31 |
3 |
7 |
5 |
8 |
9 |
8 |
4 |
41 |
4 |
3 |
9 |
3 |
7 |
3 |
8 |
33 |
5 |
8 |
9 |
8 |
8 |
7 |
7 |
47 |
* 1 = Poor; 10 = Desirable |
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As illustrated in Table 3 above, designs were assigned ratings between one and ten for each parameter, with one being a poor rating and ten being a desirable rating. After the first decision matrix was completed, it was decided that certain parameters were much more important to the functionality of the project designs than others. A second decision matrix was then created using weighted parameters to provide a more accurate rating for the design alternatives. Table 4 below illustrates the weighted decision matrix used.
Table 4: Design Recommendation Tables: Weighted Criteria Ratings
Design # |
Cut/Fill Volume (25%) | Final Slope (22.5%) | Vegetation Restoration (7.5%) | Natural Aesthetics (10%) | Required Labor (10%) | Stability (25%) | Total (100%) |
| 1 | 2.5 | 0.9 | 0.3 | 0.5 | 1 | 0.25 | 5.45 |
| 2 | 0.5 | 1.575 | 0.225 | 1 | 0.4 | 1.25 | 4.95 |
| 3 | 1.75 | 1.125 | 0.6 | 0.9 | 0.8 | 1 | 6.175 |
| 4 | 0.75 | 2.025 | 0.225 | 0.7 | 0.3 | 2 | 6 |
| 5 | 2 | 2.025 | 0.6 | 0.8 | 0.7 | 1.75 | 7.875 |
As illustrated in Table 4 above, each design parameter was assigned different weighted percentages based on the importance and necessity of the parameter. The excess cut/fill volume and stability parameters were each weighted 25% because they were determined to be the most important of the parameters. The cut/fill volume is indicative of the amount of labor required. The stability parameter correlates the slope and the amount of vegetation restoration needed. The final slope was weighted to be 22.5% because the slope is an important factor for providing channel stability. Vegetation restoration was assigned the lowest weight of 7.5% due to the fact that the primary project objectives were to align the culvert and stabilize the channel, with vegetation restoration being a secondary project objective. Natural aesthetics and required labor were both assigned weights of 10% because they are especially important to MNA and ACE. Natural aesthetics is important to MNA and ACE because the channel lies on both of their properties. The required labor is mostly important to ACE because the equipment and workers will be provided by ACE. After the weighted scores from Table 4 were analyzed, it was determined that the Short Design with a Rock Structure achieved the highest score, making this the recommended design.
Decision Matrix Scoring
In this section, the scores assigned for each design in the original decision matrix are justified, which validates the final design recommendation. For the cut/fill parameter, Design #1 (taking no action) was assigned the highest score of 10 because no cut/fill is necessary. Conversely, Design #2, which requires the 1134 cubic yards of excess cut to connect to the reference reach, was assigned a score of 2. Designs with rock structures have less excess cut than those without rock structures, giving those with rock structures higher cut/fill ratings. Design #4 had an excess cut of 983 cubic yards, earning it a cut/fill score of 3. Design #3 had an excess cut of 170 cubic yards while Design #5 had an excess cut of 138 cubic yards, earning them scores of 7 and 8, respectively.
For the final slope parameter, the ideal slope is 1.13%. Taking out of consideration the slope of the rock riprap sections, Designs #4 and #5 were the closest to the ideal slope with a slope of 0.95% each. Therefore, both Designs #4 and #5 earned a final slope rating of 9. Design #3 had the steepest final slope of 2.33%, resulting with a low score of 5. Design #2 had a slope of 1.35%, which was greater than Designs #4 and #5, but less than Design #3, resulting in a score of 7.
For the vegetation restoration parameter, higher vegetation disturbance resulted in lower scores and less disturbance resulted in higher scores. Current conditions of vegetation (i.e. Coyote Willows) was considered to be poor because they are located in the channel bed, which hinders normal flow and prevents the channel slope from normalizing. Therefore, taking no action earned a vegetation restoration rating of 4. The Long Designs both required the same amount of vegetation disturbance, transplanting the Coyote Willows from the bed to the banks and removing the mature grouping of Ponderosa Pine Trees. Therefore, Designs #2 and #4 earned a rating of 3. The short designs both require much less vegetative disturbance, transplanting only the Coyote Willows in the channel bed and the three young Ponderosa Pine Trees. Designs #3 and #5 both earned scores 8.
The natural aesthetics parameter was based on how aesthetically pleasing and how natural the design restoration would look after construction. Because taking no action leaves the channel in its unnatural and unrestored state, Design #1 earned a score of 5. The Long Design would be the most natural looking design with a constant slope and no structures installed, earning Design #2 a score of 10. The Short Design also does not utilize a rock structure, but it also does not restore the entire reach, earning Design #3 a score of 9. Because Designs #4 and #5 both utilize rock structures, they earned lower scores of 7 and 8, respectively. Design #4 earned a lower score than Design #5 because the rock structure implemented would be larger. All of the designs consist of a native plant restoration to emphasize a native riparian habitat, besides taking no action.
For the required labor parameter, more labor resulted in lower ratings and less labor resulted in higher ratings. Labor is based on the amount of cut and fill as well as the amount of vegetation transplanting. The construction of rock structures was also considered to be a part of the required labor parameter. Because taking no action requires no labor at all, Design #1 earned a perfect rating of 10. The Long Designs both earned low scores because of the high amount of cut and fill. However, taking into consideration the construction of the rock structure earned Design #4 a slightly lower labor rating than Design #2. Design #4 earned a labor rating of 3 and Design #2 earned a labor rating of 4. The short designs were then assigned much higher ratings because they have much lower cut and fill values. Again, the construction of the rock structure was taken into consideration, which resulted in Design #3 being assigned a labor rating of 8 and Design #5 being assigned a labor rating of 7.
The stability parameter was based on several factors. Channel stability is increased when the slope is natural, when the banks are supported by vegetative root growth, and, in the case of Schultz Creek, when the culvert is aligned with the channel. The addition of rock armoring also increases channel stability by preventing head cuts. After examining the current status of Schultz Creek, it was determined that the channel is very unstable. The slope is unnatural due to the Coyote Willows in the channel bed preventing it from normalizing. The banks of the channel are too steep and are eroding further away from the channel bed. The channel is also unaligned with the culvert, causing any flow to further erode the banks. Therefore, taking no action earned Design #1 a stability ranking of 1. Normalizing the slope for longer distances provides more stability, which is why the longer designs have higher rankings than the shorter designs. The addition of rock armoring also provides further stability which is why the rock structure designs earned higher ratings than the designs without rock structures. Therefore, Designs #2, #3, #4, and #5 earned ratings of 5, 4, 8, and 7, respectively.