Stop­ping the fan al­lu­vial

Advertiser (Grand Falls) - - Editorial - Rus­sell Wanger­sky Rus­sell Wanger­sky’s col­umn ap­pears in 30 SaltWire news­pa­pers and web­sites in At­lantic Canada. He can be reached at rwanger@thetele­gram.com — Twit­ter: @wanger­sky

Wa­ter’s an amaz­ing thing. Com­bine it with dirt and sus­pended silt and grav­ity, and it’s even more as­tound­ing. It etches and carves and re­shapes, and it can do all of that re­mark­ably quickly.

Take a walk af­ter a big rain, and you can see what wa­ter does quite clearly, even on paved roads with gut­ters: the speed of the wa­ter sus­pends soil, gravel and silt and car­ries it down­stream un­til the wa­ter slows, and then it spreads a broad fan of its loot in an or­ga­ni­za­tion set by pro­gres­sively de­scend­ing weight — some­thing called an al­lu­vial fan.

On a gravel path in a wooded park, it’s not sur­pris­ing af­ter a

heavy rain to find scores of such fans reach­ing their fin­gers out from the trail into the sur­round­ing woods.

Some­times, fast-mov­ing wa­ter car­ry­ing sed­i­ment and stone stops more abruptly, such as be­hind a dam or other im­ped­i­ment, and the sud­den drop in speed means the wa­ter’s abil­ity to carry ma­te­ri­als stops so sud­denly that the car­ried ma­te­ri­als drop out be­fore they can build deltas or sand bars or bar­risways.

In­stead, they fall to the bot­tom and start to fill the pond or pool where the wa­ter is caught.

Fish­ing on a stream or a river, you can find places where the bot­toms of pools fill in with tons of small rock ev­ery year, where there’s only oc­ca­sion­ally enough storm-fu­elled flow to gouge out the ac­cu­mu­lated stones, gravel, sand and fines and re­store the pool closer to its orig­i­nal shape.

For big dams, that can cre­ate a se­ries of prob­lems. Ev­ery year, the wa­ter in a reser­voir to gen­er­ate power can shrink, re­duc­ing the amount of stored en­ergy be­hind the dam and the amount of en­ergy its tur­bines can pro­duce. The Three Gorges dam on the Yangtze River in China — the world’s largest hy­dro­elec­tric de­vel­op­ment — is a poster-child for what hap­pens when huge vol­umes of silt be­gin drop­ping out of en­train­ment in the wa­ter col­umn: silt is pil­ing up be­hind the dam, not only af­fect­ing fu­ture power lev­els, but re­mov­ing the silt from be­ing able to sup­ply needed nu­tri­ents to farm­ing op­er­a­tions be­low the dam.

It’s fine to write about the scale of the prob­lem. But pic­tures are truly worth thou­sands of words.

Six years ago, the White Salmon River’s Con­dit Hy­dro­elec­tric dam in Washington state — which only pro­duced a max­i­mum of 14.7 megawatts — was in­ten­tion­ally breached, its base blown out. On­line, you can see video of that breach­ing in this col­umn, and it’s shot in a way that’s de­cep­tively beau­ti­ful.

What’s in­ter­est­ing to me about the video — be­sides the blast it­self — is some­thing that comes at about 1:29 into the time-lapse video, when the cam­era moves from the wa­ter gush­ing out of the dam to the reser­voir it­self. It’s the mas­sive amount of soft silt that has piled up be­hind the dam in 100 years or so.

The silt is a gi­ant, choco­late-pud­ding-like gelati­nous mass, one that slumps down with the shrink­ing reser­voir and then trav­els down­stream with it. All of it ma­te­rial that, with­out the dam, would have con­tin­ued on its river-ride to­wards the sea years ago.

And that’s the silt caught and car­ried in just one small trib­u­tary of the Columbia River, one that sup­plied a tiny frac­tion of the power of Muskrat Falls.

All of that ma­te­rial, drop­ping out of so­lu­tion so slowly that that wouldn’t be vis­i­ble to the naked eye, pil­ing up year af­ter year af­ter year. The al­lu­vial fan in­ter­rupted, de­layed per­haps, but not stopped.

We think we’re win­ning over na­ture.

It’s only be­cause the speed of the los­ing is so in­cred­i­bly, glacially slow.

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