River-Lab

Actively incising Lower Tassajara Creek in Dublin, California, was restored as a compound channel in 1999-2000 to mitigate incision and provide flood conveyance capacity to reduce flood risk to an adjacent greenfield residential development. The project benefitted from a wide floodplain corridor set aside during development, allowing a rough vegetated overbank channel with a design capacity to carry the 100-year flood. Several years of cross-sectional channel surveys conducted under an informal monitoring program from 2000 to 2006 indicated that active incision was largely attenuated following the restoration, with some minor incision still evident and low-flow channel complexity just beginning to develop. This research evaluates Lower Tassajara Creek 20 years after its restoration, investigating ongoing channel evolution, incision and aggradation, and floodplain reconnection, and discussing channel conditions in the context of the catchment land use history. I repeated and overlayed a series of cross-section surveys to detect channel incision and overbank deposition, documented field observations of channel condition, and cross-referenced against LiDAR terrain data to detect depositional landforms on the floodplain. I also developed a one-dimensional hydraulic model to better understand current flood conveyance capacity, using channel roughness back-calculated using measured high water marks from a recent storm. Preliminary results suggest overbank deposition has occurred in some locations, particularly in the upper reaches, while shallow slopes and fine sediment in the low flow channel throughout the project indicate a depositional environment. While active incision appears to have halted, entrenched conditions persist in the reach with a deeper low-flow channel sized to carry the 5-year flow. There may be opportunities to improve floodplain connection by adding woody debris to push more frequent flows out of the low flow channel, but additional hydraulic model validation and testing will be required to determine where flood conveyance capacity is sufficient to support such an intervention.

I provide this feasibility study for the future restoration of Tangxi Canal (Hangzhou, Zhejiang, China) to explore whether the canal district can become a social-interactive urban space with healthy environmental conditions and sustained connectivity with the canal waters. Overall, the Tangxi Canal District faces two challenges: first, pollution from fuel leaks from motorboats and stormwater runoff; and second, a lack of sociocultural interaction between residents and the canal. Therefore, the restoration of the Tangxi Canal area requires the following: (a) providing more green spaces and public spaces; (b) addressing environmental problems; and (c) providing more urban amenities. 

We conducted a post-project appraisal in November 2022 to evaluate the restoration’s performance using the goals outlined in the Sausal Creek Restoration at Dimond Park Monitoring Plan (Restoration Design Group, Inc. (RDG) 2017): restore native riparian habitat and improve channel stability. We performed vegetation surveys along the three transects (T1, T2, T3) initially defined by RDG and conducted a geomorphic survey along T3. We determined that the restoration project performed well in establishing native riparian habitat as we measured overall >90% canopy cover, which surpasses the percent cover criteria of 75% after five years of project implementation. Furthermore, we observed relatively high shrub survival rates of 138%at T1 and 173% at T2 and a limited shrub survival rate of 35% at T3. Despite meeting overall percent cover and survival rate criteria, we observed a decrease in native flora diversity, potentially due to excessive overstory shading from willows and invasive species outcompeting native plant species. Therefore, we recommend strategic thinning of willows, continued removal of invasive species, and additional replanting of native species. We compared the November 2022 channel geometry to previous years (pre-project, design, 2017, 2019, and 2021) and determined that the channel banks are stable (i.e., have experienced less than one foot of erosion). We also modeled the new channel geometry’s ability to convey a 10-year and 50-year storm event. Our model (HEC-RAS and the National Weather Service Normal Depth Calculator) results indicated that the channel would contain a 10-year event but not a 50-year event.

In 2019 grade control modifications were made to a tributary to Redwood Creek in Muir Woods National Monument as a way to evaluate the possible use of check dams to improve groundwater storage. This study seeks to evaluate the effectiveness of the grade control in 1) controlling or reversing incision due to prior land use changes and channel modification and 2) storing groundwater in areas adjacent to the tributary. Our results from initial monitoring efforts suggest that check dams could be effective in inducing aggradation of the streambed and reducing incision and in storing groundwater near the floodplains. However, we recommend continued monitoring to confirm their effectiveness as the tributary continues to evolve.

We conducted an appraisal of an urban creek daylighting project in San Pablo, California, to evaluate the ecological integrity of the restoration, understand if the project achieved its goals, and recommend planning guidelines for future projects. This 2013 project in John Herbert Davis Park daylit a 550 foot culverted section of Wildcat Creek, with objectives to improve flood control, create riparian and fish habitat, and expand trail access. We analyzed current project conditions by surveying vegetation at 8 transects, surveying vegetation by planting section, conducting 2 bird counts, mapping the creek bed and vegetation cover, and recapturing 15 photopoints. We compared results with initial plans and monitoring, which continued for 5 years.We found that the project created a riparian vegetation community with habitat supporting bird abundance, density, species richness, and diversity. Willow trees make up 80% of the overstory plants, as a result of an initial 560 willows planted. Overstory vegetation developed an almost fully closed canopy over the creek; we found 95% canopy cover. Little of the planted understory has survived, which we attribute to shade, human use, and competition from invasives. Invasive species dominated the understory composition, including Algerian ivy, veldt grass, and kikuyu grass. We found the surface cover consists primarily of live woody, non-woody vegetation, and vegetation litter. We did not observe streambed conditions for fish habitat given the seasonal lack of water.The project succeeded in increasing longitudinal connectivity through robust initial monitoring, extension of Wildcat Creek Trail, providing riparian habitat to support bird life, and constructing a channel and floodplain with an 100 year flood capacity. However, the project failed in overgrowth of vegetation, lack of continued monitoring funds, and death of the planted understory, in conjunction with invasive encroachment. Canopy overgrowth has additionally facilitated lack of bank-to-bank visibility, thereby encouraging encampments and dumping, resulting in decreased safety.At the site, we recommend thinning willow density and management for invasives. We also recommend appending guidelines for project planning and monitoring of urban creek restorations into the next version of the Wildcat Creek Restoration Action Plan for the City of San Pablo. We propose the following potential guidelines: centering principles of longitudinal and lateral connectivity, considering site-specific opportunities and constraints; selecting planting palettes appropriate for long term public and ecological use; planning for long-term physical, ecological, and social monitoring; and creating quantitative objectives prior to project implementation.