Thursday, April 28, 2011

More on Devil's Gate Interim Sediment Removal Plan

At its April 26 meeting the Hahamongna Watershed Park Advisory Committee received an update from Los Angeles County Flood Control concerning the interim measures to be taken to clean out sediment in Hahamongna behind Devil's Gate Dam. The most important update was a clarification of the haul routes to be used to remove the sediment.

According to the presentation, 25,000 cubic yards of sediment will be removed from the area immediately behind the dam. The work will proceed on an emergency basis under a CEQA exemption which will require that no further environmental work will have to be done.

Preparatory work began March 31, 2011. Actual sediment removal is anticipated to take 16 to 20 days with the removal of 1600 cubic yards of dirt per day. Clean out is scheduled to begin June 17 and to continue until August 15, 2011. Other measures to improve dam safety are scheduled to be completed between August 15 and October 15, 2011. These include heightening the sluice gate, replacing the damaged ladder system, installing a new catwalk and log booms, and modifying the Altadena West Storm Drain.

The County anticipates having to do a similar clean out effort each year until the environmental impact report for the final sediment removal project is completed.

The haul route as illustrated in the presentation will go north along an access road on the east side of the park to just below the east JPL parking lot where the trucks will turn south and go down Windsor Avenue to the 210 Freeway. The bulk of the sediment, over 90%, will then be trucked east to pits in Irwindale. The organic material comprising the rest of the sediment will be taken via the 210 Freeway to Scholl Canyon landfill in Glendale.

There were a number of questions concerning why the County had chosen the haul route which would cause the most neighborhood disruption as well as the most environmental impact within the park. The Committee and the public both asked about other alternatives. One mentioned was the road at the southwest corner of the park which has been used for prior debris clean outs. Another was the possibility of exiting the park from the southeast in a way which would not impact the neighborhood. The County assured the Committee that this alternatives had been considered.

The Advisory Committee passed a motion to request the Pasadena City Council to examine the County's transportation plan to identify areas to be impacted, to ask the County to consider alternative routes which would reduce environmental impacts, and to notify areas to be impacted.

If you would like to read County Flood Control's presentation, it can be found at Interim Measures 200110426.pdf

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