County Studies Environmental Impact of Sediment Removal In Pasadena

A massive three-year cleanout of sediment in Pasadana's Hahamongna Watershed Park that had been planned to get underway this summer and fall has been put off for at least a year. In the meantime, a short-term project by LA. County Public Works will remove a comparatively small amouut of sediment close to the face of Devil's Gate Dam at the south end of the park near the 210 Freeway to keep the dam's openings from being blocked by debris during storms over the next winter or two while the bigger cleanout takes shape.

This shift is much more than just a change in a wort timetable, it represents a break from the county practice that wiped out scores of mature oaks and sycamores in Arcadia in January to clear space to dump sediment from Santa Anita Dam. Public outrage over that debacle embarrassed Los Angeles County officials and helped bring the changed approach to the northwest Pasadena project.

Environmentalist hope that the new pattern will continue during needed sediment removal projects at other county dams giving the public a voice in how the projects are planned and how they affect the environment.

There was almost no provision for meaningful public input when the original Hahamongna project was presented last fall. As described by County Department of Public Works officials on November 30, it would remove 1.67 million cubic yards (four Rose Bowls full) of sediment and debris from a 50-acre area north of Devil's Gate Dam and truck it to sites in lrwindale and Azuza. The excavation would destroy acres øf black willow and create a deep basin for water conservation and for future sediment cleanouts.

The $35 million project would move 300 to 400 truckloads a day, five days a week during the dry season from May to December for three years.

Environmentalists were deeply concerned. Hahamongna Is not just any debris basin. Covering 300 acres where the Arroyo Seco emerges from the San Gabnel Mountains. It is a paid city park. It is a favorite of hikers, bicyctists, equestrans, bird watchers and nature lovers and is home to softball, soccer and disc golf. The reservoir and its percolation ponds supply much of Pasadena's water.

Then the County Board of Supervisors changed the picture. Responding to public protests that such a huge project was, after all, more than just an engineering and earthmoving exercise, the Board on March 1 directed that the Department of Public Works have an Environmental Impact Report prepared for Hahamongna.

An EIR will consider alternative ways of handling the sediment, and the environmental Impacts of the onginal plan-and possible alternatives. It will allow the public to participate in devising alternatives during scoping meetings, to comment on potential effects of the original project and any alternatives and to suggest adequate mitigition for environmental damage.

The result could be a better way to handle the sediment or at least ways to reduce environmental effects on habitat, wildlife and air quality. In any case, interested citizens will feel that they have had an opportunity to help shape the project.

To ensure that Devil's Gate Dam can handle debris flows during the year or more the EIR is being prepared, the County will remove 25,000 cubic yards of sediment from an area extending out about 100 feet from the dam this summer, change the dam's operating plan to reduce future sediment deposits near the face of the dam, and modify the trash rack that keeps debris out of the sluice gate, an outlet through the dam, to improve its performance

The pressure to clean out Devil's Gate Reservoir is partly a result of the Station Fire that burned 160,000 acres in the San Gabriel Mountains in 2009. The fire denuded steep hillsides in the Arroyo Seco water shed above the dam so that winter rains washed huge amounts of debris into the reservoir.

Similar big debris flows rapidly filled many of the county's 13 other major dam reservoirs and 155 debris basins along the foothills. With Its available sites for storing the debris filling much sooner than anticipated, the county now is developing a long range plan for handling sediment. Through public meetings and a working group, county officials are gathering suggestions not only on viable disposal sites but on other methods of handling sediment such as somehow transporting it to ocean beaches

Managing the sediment and debris constantly being shed from the region's mountains is a continuing and costly struggle involving flood control, public safety, water supply, habitat and cost, among other factors. The aftermath of the Arcadia oak woodland destruction, the changed approach to Hahamongna, and the search for long-range disposal methods seem to signal a heartening new emphasis on environmental values in balancing those goals.