LOGAN RIVER RESTORATION (DOWNSTREAM)

Team

Moody Engineers
• Drew Basset
• Amanda Blair
• Brooklyn Morgan
• Dokota Sparks

Abstract

Historical projects by local landowners attempted to improve the passage of flood flows through the Logan River by channelizing the river. The removal of foliage in this process resulted in unforeseen bank erosion and channel scouring. This project analyzes existing river conditions and proposes changes to a section of the Logan River from the Crockett Diversion to Center Street to improve the floodplain, reduce river temperatures, and add recreation.

Existing Conditions

Though the portion of the Logan River from Third Dam to Cutler Reservoir is considered impaired by the EPA for exceeding the total phosphorous limit, the portion of the river from First Dam to Highway 89/91 does not appear to exceed any nutrient regulations. Rather, shallow flows in this section of the river result in high temperatures (nearly 16°C in some months) and decreased oxygen in the water, increasing metabolic rates and displacing cold-water fish species. In addition to high temperatures, flooding is also a concern in this portion of the river, especially near the Crockett Diversion. The 100-yr flood rate used in this design is 2380 cfs, while the low-flow flood rate was assumed to be 30 cfs.

Alternatives

Five main alternatives were considered in this project: no action, Riverside Drive, increasing channelization, recreation, and soft engineering options. These alternatives are not mutually exclusive. The no action alternative involves making no more alterations to the river. The Riverside Drive alternative entails purchasing a portion of Riverside Drive, straightening the road, and expanding the river into the purchased segment of land. Increasing channelization refers to reinforcing the riverbanks with concrete, deepening the river bottom, and straightening the river. Modifying the river to improve wading, kayaking, and overall river recreation is another option considered to improve the Logan River. Soft engineering encourages the natural flow of the river, removes concrete, and reinforces the banks with native vegetation. These alternatives are considered in the next section.

Methods and Analysis

The downstream section of the river was surveyed in August 2018 to measure depths and flowrates. Cross sectional depths of the river were taken every 100 ft. A Philadelphia rod was used to take point depths at 5 ft intervals across the river. Flow rates were calculated by multiplying the cross sectional area by the velocity. The velocity was calculated by measuring a specified distance and timing how long a buoyant object took to travel down between the two points. This information was using to create a HEC-RAS model.

A decision matrix was created to provide an unbiased analysis of the five alternatives. Each alternative was evaluated based on environmental impact, cost, public opinion, recreation possibilities, displacement of people, and implementation time and a total score was assigned to each. In the table, a score of 0 represents high impact, 1 represents medium impact, and 2 represents low impact. Weights, indicated in parenthesis next to design factors, indicate the importance of each decision factor. A weight of 3 indicates top priority design factors, while a weight of 1 indicates a low priority. These weights were assigned based on the client’s priorities, meaning that a high weight indicates one of the client’s main goals for this project.

The downstream Logan River section was modelled in two dimensional unsteady flow using HEC-RAS, an engineering application created by the US army corps of engineers to model the hydraulics of natural river systems. This model was run using high and low flow conditions (2380 cfs and 30 cfs).

To add recreation to the river, a wading pond was designed to be added near the Crockett Diversion. This wading pond slows the river near the riverbank and creates a small scour pond. The wading pond was designed based on a modified jhook vane. A physical 1:30 scale model was used to test the design. This predicted prototype velocities below 5 ft/s under typical, non-flood conditions.

Preferred Alternative

The HEC-RAS results show that the 100-year flooding outside of the river for this section consist of low velocities and depths. The maximum depth of the 100-year flood is 8.5 ft, and the maximum velocity of the 100- year flood is 14.2 ft/s. The maximum depth for 30 cfs is 1 ft, and the maximum velocity for the 30 cfs is 3 ft/s.

The preferred alternative selected for this project is a combination of soft engineering and recreation. Adding a small wading pond, access points, and vegetation surrounding the river would increase recreational options in and around the river while providing bank stability. Removing concrete and seeding the banks of the river to encourage plant life and root growth would provide shade (to address temperature fluctuations) and decrease bank erosion.

The banks of the river would be seeded with a variety of species to create a diverse and resilient ecosystem. River banks with abundant plant life are also more stable in the case of high flow. The addition of large trees would provide shade and control temperature fluctuations. Recreation along the river is one of the main goals of the public and the Logan River Task Force. A small wading pond would be a viable option, and it would require minimum cost. The approach for created a wading pool is to build a “reverse” J-hook to reduce velocities.

Conclusion

The selected design of the “soft engineering” for the Logan River provides a recreational area from the flow alteration due to the modified J-hook, removal of concrete along the river banks, removal of non-native vegetation, and the planting of native vegetation. The estimated cost of construction for this design is approximately $118,000.00 not accounting for unforeseeable maintenance. The additional native vegetation will assist in maintaining adequate water temperatures in addition to minimizing erosion.