Orange County Branch Newsletter

May 2017

Environmental & Water Resources Institute - EWRI

Santa Ana River Parkway Extension Branch Lunch

By Portia Gonzalez, P.E.

The ASCE Orange County Branch March luncheon meeting presented on the Santa Ana River Parkway Extension Project.  The key speakers include Mr. Jeff Dickman, Planner with the Orange County Public Works; Mr. Jerry Flores, Associate Principal with AECOM, and Mr. David Jaffe, Senior Project Manager with Psomas.  

Figure 1 - Project Location Map

The County of Orange is proposing to extend and realign the Santa Ana River Bikeway and Riding and Hiking Trail to better serve users and to complete its portion of the proposed 100-mile recreational parkway adjacent to the Santa Ana River. The project site is located along the Santa Ana River Parkway between Gypsum Canyon Road and the Orange County Boundary as shown in Figure 1. The Santa Ana River Parkway (Project) will extend the Santa Ana River Bikeway and the separate Santa Ana River Riding and Hiking Trail within this parkway. The Santa Ana River Bikeway is a regional Class I (off-road, paved) bikeway; it accommodates pedestrians and commuter and recreational bicyclists. The Santa Ana River Trail is a regional riding and hiking trail (unpaved); it accommodates walkers, hikers, runners, joggers, mountain bicyclists, and equestrians as shown in Figure 2.  The project also includes two new bridges across the Santa Ana River.

Figure 2- Project Elements

The hydraulic and sediment analysis of the Santa Ana River was developed to quantify the project’s impacts to habitat resulting from the proposed condition. Normally impacts analysis examines changes in river hydraulics; in some cases, impacts analysis looks at how the changes in hydraulics impact habitats. In the case of the present project the analysis examined how the impacts to hydraulics and the changes in sediment transport impacted the habitat. GIS-based tools were used to further quantify area of impacts for each habitat type by location within the channel.

According to the “Santa Ana River Draft IS/MDN”, RBF, 2011  the new structures would have a less than significant impact in the conveyance flows during the 100-year storm event.   HEC-RAS hydraulic analysis was performed for this project to compare the river’s hydraulic characteristics between existing and proposed conditions.  The Army Corps of Engineers design discharge used for the model is 30,000 cubic feet per second.  Results of the analysis show a maximum increase of 0.7 feet in water surface elevation and maximum decrease of 1.2 feet per second in velocity.  The changes in water surface and velocity occur within approximately 1,500 feet of the proposed structure.

Impacts to habitats were analyzed using a quasi-two-dimensional analysis and by mapping the hydraulic data over habitat maps in GIS.  Within the habitat area, the changes to the velocity is between -0.25 to -1.0 fps as shown in Figure 3.

Figure 3- Hydraulic Impacts

The most impacted habitat is the Cottonwood-Willow Riparian Forrest totaling 2.4 acres as shown in Figure 4.  Eight other habitats are slightly impacted.

Figure 4- Habitat Impacts

To study the impacts to habitat related to the potential aggradation or degradation of channel, HEC-6T was used to analyze sediment transport of the study reach.   Results indicate that the proposed condition will have less degradation compared to the existing condition which could be a result of the placement and operation of Prado Dam and not directly related to the project.   In summary, the proposed condition sediment transport analysis shows minor changes from the existing conditions and will have no significant impact to the habitat.