Modesto WWTPs: Flood and Water Quality Risk Reduction

Updated Status on 2/4/2022: Short Project Description and Project Cost

Project Lead: City of Modesto

Potential Project Partners:
No partners identified at this time

Short Project Description:
The City of Modesto’s wastewater treatment facilities span two locations and both are situated within 100-year floodplains, one on the Tuolumne River and one on the San Joaquin River. The City has completed the Sutter Plant Relocation Feasibility Study and the Wastewater Treatment Facilities Master Plan and has determined that the Sutter Primary Treatment Facilities will be relocated and consolidated with the Jennings Treatment Facility along the San Joaquin River, new primary treatment facilities shall be constructed outside the floodplain. This will greatly reduce the flood risks to future Primary Treatment Facilities.

Long Project Description:
The City of Modesto’s Wastewater Treatment Facilities are both are situated within designated 100-year floodplains. The Sutter Treatment Facility, located on the Tuolumne River, has flooded multiple times, most recently in 1997.  A Flood Protection Analysis for this facility, done in 2014, concluded that it would be infeasible to effectively flood protect Primary Treatment processes at this location.

The levee that protects the Jennings Treatment Plant along the San Joaquin River is uncertified, but has  protected the Jennings Plant from all flood events since it was built. The relocation of Primary Treatment to the Jennings site, along with levee improvements and changes to the Treatment Processes, will provide opportunities to capitalize on underutilized infrastructure and vacated land.

Multibenefits of this project would include flood risk reduction, the capture and reuse of storm water (flood flows), irrigation of farm land and passive recharge to groundwater.

Phase I: Sutter

The Primary Wastewater Treatment Facilities located at 1221 Sutter Avenue, Modesto (Sutter Facilities) are reaching or have exceeded their useful life.

The Sutter Facilities have flooded four times since 1950 (1950, 1955, 1969 and 1997), due to rapid snow melt and rain.  During flooding from the Tuolumne River, the Sutter Facility wastewater comingles with flood waters and flows downstream.

The Sutter Primary Treatment Facility Flood Protection Analysis, as part of the 2014 Sutter Treatment Facility Feasiblity Study, found that it was not feasible to protect the primary treatment facilities, excluding the sludge drying beds, and still satisfy the “no-rise” FEMA criterion.

The relocation of Primary Facilities to the Jennings site will consist of constructing new Sludge Drying Beds, clarifiers and digesters at Jennings Facilities. With Primary Facilities removed, Sutter Plant will be reduced to a pumping facility. The City is currently replacing one of the pump stations at the Sutter Facility,  and relocating it to higher ground. Flood proofing key remaining infrastructure such as  junction structures, manholes, pumps and electrical equipment will further reduce flood risk.

To fully protect from the commingling of wastewater with floodwaters, the trunks  and mains bringing wastewater to the Sutter Plant will need to be secured or relocated out of the 100-year floodplain. Modifications include lining the interior of these gravity mains and trunks and installing water tight manhole covers. The City of Modesto is currently pursuing a design for the relocation of the River Trunk, which transports approximately 40 percent of the City’s Domestic flow to the Sutter Plant.

Phase II: Jennings

Secondary and tertiary treatment facilities are currently located at 7001 Jennings Road, Modesto at the Jennings Wastewater Treatment Facility (Jennings Facility).

The City is proposing a full evaluation of the levee adjacent to the Jennings Facility site to determine the improvements and actions needed to achieve certification. Additionally, new primary treatment facilities planned for the Jennings Facility will be constructed on higher ground to protect the facilities from a flood caused by levee failure or levee breach.

The City of Modesto has storage ponds that hold up to 7,830 ac-ft of water. Currently, these ponds are used to hold and discharge secondary-treated wastewater into the San Joaquin River from October 1st through May 31st annually. As the City moves to tertiary treatment of the wastewater and selling the recycled wastewater as a water supply to Del Puerto Water District, these ponds will be available to capture storm water and store flood flows from the San Joaquin River. This water will be used to irrigate 2,530-acres of ranch owned by the City.

The majority of the needed infrastructure to get the water onto the ranch land (ponds for storage, land for irrigation and passive recharge, pipes, pumps, reservoir, valves for conveyance) already exists. Construction of a conduit to get the water from the River into the ponds is the remaining element needed and the proximity of the ponds to the River (less than 600 ft in some reaches) should keep conduit costs down.  An on-site evaluation of the existing outfall and old pumps used for pumping irrigation water from the river will be completed along with consideration of possible alternate methods.

Finding new use for the ponds will also maintain the recreational element of this space, as avid birders visit the Jennings Facility (open to the public on the second Saturday of each month) to view the variety of birds the ponds attract.

Phase III: TRRP Implementation

The Tuolumne River Regional Park (TRRP) Master Plan (Dec 2001) has identified areas within and adjacent to the northern, western, and southern boundaries of the Sutter Facility as the Carpenter Road Area.

With the relocation of Primary Facilities, vacated land in this Area could be regraded to reduce flow constriction that occurs along the Tuolumne River near the Sutter Plant and reduce river elevations during floods. Grading and habitat enhancement would be coordinated with the The Tuolumne River Regional Park Commission, comprised of the City of Modesto, City of Ceres, and Stanislaus County.

Portions of the sludge drying beds may be removed and regraded to expand the existing riparian corridor,with activities including but not limited to adding trees (increasing tree canopy, increasing green space, reducing heat island effect) and creating recreation opportunities (additional and improved trails, enhancing stormwater wetlands, and increasing green space) within a disadvantaged community (DAC).

Unique Project Characteristics: 
This project is adjacent to another RFMP project site, the Tuolumne River Regional Parkway Carpenter Road Project, and would provide synergistic benefits to that project.

Project Status Pre-planning
Project Cost $80,000,000
Project Timeframe Undetermined
Cost-sharing No opportunities identified to date.
Multi-benefit Project Yes
Types of benefits The project would improve flood risk management, enhance groundwater recharge, protect the integrity of the San Joaquin and Tuoulmne rivers, support the use of recycled water, promote recreation, enhance ecosystem and encourage recreational use of vacated land.
Source of Project City of Modesto

Background Information:

Latitude: 37.531508

Longitude: -121.092553