Tassajara Creek Restoration

Challenge

Channel straightening and years of grazing had led to significant incision of Tassajara Creek near the City of Dublin, hydraulically disconnecting the creek from its historical floodplain. Subdivision of the Livermore Valley and proposed housing and commercial developments on either side of the creek necessitated a solution to stop the incision and effectively convey flood waters. Along the upstream portion of the site, 11 oak trees 18-36 inches in diameter and other younger trees were in danger of being undercut by the creek. These oaks were some of the few remaining trees in the area, and protecting them was an important goal. Furthermore, the presence of woody vegetation threatened to preclude easy access to the creek and limit the ability of residents to cross the creek. The development of new housing along the west bank of the creek and the Emerald Glen Regional Park on the east bank brought this issue into clear focus and created a question about how members of the new community could easily access park facilities and enjoy the amenity the creek provided.

Solution

A compound channel would stabilize the creek by incorporating a constructed floodplain terrace within the incised channel margin, allowing water to flow over a wider cross-sectional area and reducing flow velocities and bed-shear stresses in the channel during high flow events. Three meander bends were designed to reroute the creek to protect the mature oaks on the project site. In order to provide pedestrian access across the creek, steps were integrated into a grade control structure so that it doubles as a low-flow crossing with gates that can be locked to prevent use during the rainy season.

• The creek restoration was divided into two reaches. In the upstream reach (north of Dublin Boulevard), the low-flow channel conveys the 5-year flow before overtopping onto a floodplain terrace, which conveys the 100-year flow. The existing low-flow channel was left mostly intact and the flood terrace was created by excavating to the 5-year flow (34 m3/s) water-surface elevation in the low-flow channel. The terrace was then planted with native vegetation.
• In the downstream reach (between I-580 and Dublin Boulevard), the entire channel was reconstructed with a low-flow channel designed to convey the 2-year flow (14-18 m3/s) before overtopping onto the floodplain terrace designed to convey the 100-year flow. This design allows the creek to overtop its banks every other year without threatening local businesses.
• In the upstream reach, north of Central Parkway, three meander bends totalling 900 ft were created in order to reroute the creek to protect existing mature oak trees.
• Five grade control structures were installed to maintain a consistent slope of 0.8% in the upper reach and 0.38% in the lower reach. These include a drop structure, two velocity check dams, and two grade stabilizers constructed of buried riprap running horizontally under the channel.
• The drop structure between Central Parkway and Gleason Drive incorporates steps, which allow it to serve as a creek crossing for pedestrians during low flows. The popular crossing connects the neighborhood and trail on the west bank of the creek to Emerald Glen Park and a trail on the east bank. A metal gate at each entrance remains open during the dry season and is locked to prevent use during the rainy season.
• Within the riparian corridor, 18 native plant species were planted, 9 mature oaks were preserved, and 3 volunteer plant species (2 native, one invasive) appeared.
• Of the 18 species planted, all have been found thriving along both reaches in the past five years with survival rates of 76-100%. These plantings increased woody vegetative cover along the upper reach of the riparian corridor from 6.3% in 1993 to 31% in 2012, and along the lower reach from 3.5% in 1993 to 72% in 2012.
• Paved, multi-use trails were added on both sides of the creek, connecting the new residential neighborhoods to Emerald Glen Park and the Tassajara Creek Regional Park located 0.76 miles north of the project site. Wide sidewalks and bike lanes along Dublin Boulevard link this trail to the Iron Horse Regional Trail, which is part of the 300-mile East Bay Regional Trail network.
• Removal of an existing drop structure at I-580 and regrading of the channel eliminated the need to import earth fill to raise the rest of the site. Material from the drop structure and a former military bridge was repurposed as buried riprap along the channel. This represents 18% of the total volume of riprap buried along the channel for bank protection.

• According to staff at the Alameda County Flood Control District’s Zone 7 Water Agency, maintenance of the channel along the restored section of Tassajara Creek costs an estimated $18,000-$35,000 per year, while maintenance of a traditional trapezoidal channel of the same length costs an estimated $40,000-$60,000 per year. This results in an annual savings of $5,000-$42,000.
• Reusing 208 cubic yards of concrete from a bridge and a drop structure demolished on the site as buried riprap resulted in an estimated savings of $8,500 in material costs and $10,300 in disposal costs.

• In the upstream reach between Dublin Boulevard and the new low-flow crossing, project planners left the existing low-flow channel intact and excavated above the 5-year flow level to create the floodplain terrace. This decision was driven by a desire to stay out of the creek channel and thereby remain outside the jurisdiction of the US Army Corps of Engineers. In the downstream reach, the entire channel was reconstructed with a low-flow channel designed to convey the 2-year flow. Upon implementation, however, project planners realized that the additional effort to create a different terrace level was unnecessary and added a layer of complexity to an already complicated process. Future projects used the 2-year low-flow channel and floodplain terrace (for higher flows) throughout with success.
• Halting channel incision by regrading the creek and installing velocity check dams was a key goal of the creek restoration. Monitoring since the project’s completion has indicated that incision has largely halted at most cross sections, though in a few cases, minor down-cutting was noted. Continued monitoring of channel cross sections is needed to confirm whether this data indicates sporadic or chronic incision and can be used to inform future management efforts.
• Miscommunication between project planners and those tasked with maintaining the site after project completion initially led to excessive trimming of riparian vegetation along the channel. Developing maintenance strategies that promote riparian health while meeting flood control requirements – and then clearly communicating them – is key to the continued success of a restoration project.

Project Team

Client/Landscape Architect: Alameda County Surplus Property Authority
Engineering Consultant: BKF Engineers
Contractor: RGW Construction, Inc.
Planting Implementation: Circuit Riders
Environmental Planning: Davis Environmental
Initial Project Planning: Rio Design

Role of the Landscape Architect

Numerous organizations and agencies worked together to design the Tassajara Creek Restoration Project. Rio Design, a small design firm no longer in operation, provided the initial project concept, which was then reworked and carried to completion by Davis Environmental and the Alameda County Surplus Property Authority.