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Protecting dam is high priority

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By Arin McKenna

Infrastructure and property in Los Alamos County escaped the Las Conchas fire relatively unscathed.

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However, 2,100 acres of the Los Alamos Canyon watershed burned, and the aftermath has the potential to destroy a project 10 years in the making.

High to moderate intensity burns, which completely consume a forest, burned roughly 60 percent of the canyon watershed.

With no pine needles, leaves, brush or duff layer (layer of plant debris on the ground) to slow rainfall and allow it time to soak into the ground, runoff is intensified. High intensity burns–which occur in highly flammable pine forests–magnify the problem.

Pine needles in the duff layer burn very hot and leave a waxy layer over the ash, creating “hydrophobic tendencies”–a slick surface the water slides off.

Without vegetation to anchor it, the soil also is eroded. The mixture of water and soil pours into the steep canyon, which channels it downstream in a flash flood with the power to uproot trees, lift boulders and send anything in its path racing downstream.

And one or more of these could be heading straight toward the extremely vulnerable Los Alamos dam and reservoir.

The dam became county property shortly after the Cerro Grande Fire, when the Department of Energy (DOE) transferred the county’s water system to Los Alamos County.

That placed the dam under the jurisdiction of the State of New Mexico, which required the State Engineer Office to complete a detailed analysis of capacity and stability, including storm event modeling. The analysis called for a redesign of the dam, which was approved in 2009.

Reconstruction of the dam began in March. Shotcrete (laid down to stabilize the dam after the Cerro Grande fire) had been removed and rebuilding had barely begun when the Los Conchas fire brought the project to a halt.

At present, there is nothing more than a mound of dirt holding back the reservoir. Earthen steps with five-foot concrete barriers – as yet unanchored – had been constructed on the downstream side of the dam. The next stage calls for mounding dirt and Shotcrete over those barriers, with spillway channels down either side to handle overflow.

With the dam in its current state, there is an elevated risk that monsoon floods could breach it, adding the reservoir’s 30,000 acre feed to the floodwaters. Such a flood could send tons of earth from the dam itself, as well as the concrete barriers, careening down the canyon.

The Los Alamos County Department of Utilities has been at work to prevent that scenario. Crews spent the week excavating a channel to redirect flood waters around the dam, which involved cutting through the two-foot cement wall of the original dam. Work was to be completed by Friday. If the channel works, floodwaters will bypass the dam on their way down canyon.

“I think the channel around the dam is a great idea. We can only hope that it works,” said Craig Martin, Los Alamos County open space and trails specialist, who is currently serving as community liaison for the BAER team.

Los Alamos County Ice Rink – the only structure in the canyon – is at risk, especially if the dam collapses. County paving crews built a three-tiered concrete barrier wall to protect the rink as soon as Burned Area Emergency Response (BAER) teams identified the danger.

Los Alamos National Laboratories (LANL) has closed West Road, and entrances to Camp May and the Ski Hill, to prevent vehicular traffic in the canyon. Flash floods can start high in the mountains while the town itself is dry, and race down the canyon before vehicles would have time to evacuate.

There is no accurate warning system for flash floods. Kyle Zimmerman, acting director of public works, said that BAER installed rain sensors up canyon after the Cerro Grande fire, but they were unreliable, since rainfall anywhere along the watershed could trigger flooding.

Martin said the worst flooding danger is right now, before the moderate intensity burn areas start to recover. Those burns, which occur in areas populated by less flammable trees like aspens, burn at lower temperatures and do not destroy seeds in the soil. With some monsoon moisture, those seeds will germinate and begin to anchor the soil and help with water absorption.  Gentle rains, like those in the last week, will also mitigate the problem by softening the soil and making it more absorbent.

BAER maps show the majority of the water shed had moderate burns, with only a small area experiencing high intensity fires. High intensity areas will require major restoration efforts, since nothing remains alive in the soil.

Even after moderate burn areas start to revive, flash flood danger remains elevated throughout the monsoon season. West Road will remain closed until the danger is past.

Work on the dam, which was scheduled for completion in September, will not resume until the fall. Access to the dam is along a mile-long dirt road through the narrow canyon.

The risk to workers and equipment is too great to continue until after the monsoons.

How long it takes to restore the dam depends on what nature throws at it this summer. “The worst case scenario is that we have a huge monsoon rain up canyon that knocks everything out, in which case we’ll have to start all over,” said Department of Utilities Director John Arrowsmith.