Flood Bypassing Design

* Re “Safety Study Ordered for Landfill Road After Accident Kills 2,” Oct. 6.

A parallel can be drawn between the access road and the landfill flood discharge channel system. Just as the Ventura County Regional Sanitation District underestimated the dangers of the 8% Toland Road grade, the district has underestimated the dangers of its landfill with respect to the velocity of the flood discharge passing over the face, when topped-off, in the future.

There are some things that you do at your peril in our seismically active countryside:

* Pile up obstructive debris and dam up a mountain canyon that was eroded to a milder gradient over eons.

* Underestimate the flood peak in the mountain canyon drainage area while denuding the watershed.

* Provide an inadequate flood peak bypassing channel system during operations and after topping off the completed landfill that will require constant major maintenance.

After the Northridge earthquake, engineers concluded that “caution is warranted in the design of modern, geosynthetic-lined and / or covered landfills subjected to seismic loading.”

The mountains north and south of the Santa Clara River are in the process of eroding and washing down to the sea. The drainage system of streams and the Santa Clara River are the conveyor belts that carry the sediment.

In the present design, the velocity of the flow over the downstream face of the topped-off landfill was limited by a series of channels following the access road down the 50% sloped face of the 400-foot-high landfill. The road has a grade of about 10%, limited by a series of switchbacks crisscrossing the landfill face. The discharge channel follows these switchbacks. This is not acceptable hydraulic design. The mountain torrent before introduction of the landfill produced a velocity of 15 to 20 feet per second. This velocity would erode the compacted fill if the channel flow “short-circuited” the switchbacks and flowed directly down the topped-off fill. Just as the failed brakes of the trash truck produced uncontrollable speed resulting in a tragedy, the hydraulic design of this landfill requires the equivalent of a safety ramp design to slow up the velocity of flow in the topped-off landfill, to meet the design criteria and ensure that the flow is contained in the channels.

A rigorous analysis of the flood bypassing design, which must last forever, should be included in the proposed study.

CLARENCE N. FREEMAN

Fillmore