Tuesday, April 12, 2011

Hahamongna sediment removal report to City Council

At their April 11 meeting the Pasadena City Council heard a presentation by Steve Sheridan of the Los Angeles County Department of Public Works concerning the County's plans for sediment removal behind Devil's Gate Dam in Hahamongna Watershed Park.

County Public Works is working with a consultant to prepare an Environmental Impact Report as directed by the Board of Supervisors to study the effects of removing the massive amounts of sediment which have washed into the basin as the result of the Station Fire. As yet no schedule has been set up for outreach meetings. When the schedule is set up, the City and the public will be notified.

The bulk of Mr. Sheridan's presentation, however, focused upon the measures which the County will take while the EIR is being prepared to protect the integrity and continued operation of the dam and to insure flood protection to downstream communities. Chief among these measures is the removal of 25,000 cubic yards of sediment from along the face of the dam. This excavation will be within 100 feet of the dam and no vegetation will be removed. The work is expected to begin in June or July of this year, depending upon how quickly the basin dries out, and to last about one month. Mr. Sheridan in his presentation stated that there would be 250 truck trips per day. The report to the Board of Supervisors (link given below) estimates 150 to 200 truck trips per day, however.

To remove the sediment from the basin, the access road along the east side of the dam will be re-established. The haul route will be along this road to just below the JPL east parking lot, then on to Windsor Avenue to the freeway and ultimately to Scholl landfill. According to Mr. Sheridan, at present no alternate site has been identified so that the sediment would not have to be deposited in the Scholl landfill. Other interim operational measures include extending the height of the existing sluice gate trash rack, replacing part of the damaged ladder system on the face of the dam, replacing the existing wooden boom logs, installing a new catwalk around the spillway ports, and modifying the Altadena West Storm Drain to allow discharge in the event the outlet becomes blocked by sediment.

The County is also working on two emergency plans to be in place until the reservoir cleanout is complete. The first will notify agencies and residents in response to potential flooding due to high debris flows. The second will be used to notify agencies in response to a dam safety emergency.

To read a copy of the Board report, go to