Launch Slideshow

Unexpected Flood Debris

Unexpected Flood Debris

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    Once storm sewers filled with sediment and debris, flooding on the streets got worse.

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    After the flood waters subsided, plows were brought in to clear the streets. Mounds of sandy soil and rocks lined the streets throughout Boulder and in many areas, the sediment was piled more than a foot deep on the right-of-way between the street and sidewalk.

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    Pine logs washed down from the mountains above Boulder were stripped of bark and deeply scarred by the time they wound up on the city’s streets.

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    Lines of wooden flotsam mark what had been the edge of the floodwaters.

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    Debris that floated was left piled wherever it reached a barrier that kept it from flowing any farther.

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    Sidewalks had to be shoveled to make them passable.

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    Formerly lush green lawns were buried with sandy soil and strewn with rocks washed out of nearby creek beds.

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    Part of this yard was filled with sand a full foot deep which gave the appearance that deciduous trees were growing on a beach.

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    This apartment house was right in the active floodway and was pummeled with rocks tumbled along by strong water currents.

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    This urban alleyway was deeply filled with sediment, even after a plow went through.

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    A section of this mountainside gave way in a mudslide after the soil was oversaturated with record amounts of rainfall. It’s not clear whether soil from the slide inundated the nearest residential property a few hundred yards below or if an adjacent irrigation ditch was to blame.

(Boulder County Colo.) Some waste after a rain-induced flood is expected--like damaged building material and ruined belongings from inside homes and businesses--but a surprising amount of waste created in our recent flooding were natural materials that flowed in from the surrounding environment. Mud, silt, and sand sifted into structures and filled every crevice in the landscape while rocks and driftwood piled up in yards and on the streets. It was a surreal scene to find the scoured and battered trunks of pine trees that tumbled down from the mountains onto the city streets miles below.

During the City of Boulder's curbside pickup in the weeks after the flood, seven times more sediment (by weight) was removed than building materials and other trash.

"Dirty" Dirt

Surprisingly, the earthen mix of mud, silt, and sand picked up throughout the city was not usable as fill. The unknown chemical and sewage content of the combined sediment made it hazardous enough that it all had to be landfilled. However, sediment deposited on bike paths, parks, and other public lands away from houses will be used as fill according to the City of Boulder. The city itself has an estimated 20,000 tons of this sediment to deal with, independent of the tonnage in Boulder County's other municipalities and unincorporated areas.

Ruined Crops

Another type of unexpected waste created by the flooding was the loss of crops that came into contact with flood waters. The possible contamination from pathogens, chemicals, and even heavy metals from terrain upstream was enough to make most producers destroy their yields during what would usually be a profitable harvest time.

Farms that decided to sell affected crops had to follow stringent procedures that involved soaking their produce in a chlorine bleach solution.

Vegetation Debris

Discarded crops, trees and weeds washed out by flooding, and other related vegetative waste represent another category of debris that must be dealt with after a flood. Here in Boulder County, much of it may be fodder for local commercial composting yards, but matter contaminated with chemicals or heavy metals should be excluded because its toxins will survive the natural decomposition process and be present in the finished compost used in yards and vegetable gardens around the region.

Due to its proximity to the mountains and its greater population density, the City of Boulder took a conservative stance and landfilled the vegetative waste it collected along with the flooded building materials, trash, and sediment. However, organic materials being picked up by the county in mountain communities and unincorporated areas will be commercially composted. In addition, Boulder County residents can take in organic flood-related matter to the yard waste collection center for commercial processing. The fees for this drop-off service will be waived for the foreseeable future as residents gradually clean up their property.

New Waterways

Creeks and rivers brought down so much mud and rock during the storm that they reshaped the landscape in many parts of the county. Some creeks and drainages ended up hundreds of feet away from their original beds in places after the storm and had to be routed back with heavy equipment during the weeks after the flood. These new waterways blocked access to homes, schools, and other buildings that were suddenly on the wrong side of a creek.

After evaluating and shoring up all the bridges over waterways, the county's next step will be to scan creeks and rivers with LIDAR to find areas that they have to dredge to get back to normal for the flow of water and for the sake of wildlife. Fish will then be restocked in some areas where the native population was wiped out during the devastation of the flooding.

Sediment Damage

Besides choking out wildlife and its habitat, flood sediment also causes a lot of specific property damage you may not think of until you've seen it firsthand. The infiltration of sand and silt into a building can cause a lot more damage than water alone. And since it contains suspended organic matter and unknown levels of bacteria, any sediment washed in should be removed--you can't count on it to dry into harmless dirt and sand.

Water can be drained and dried out of walls, basements, and crawlspaces, but pervasive sediment will fill up every available nook and cranny while clogging drains and ruining electrical devices and gas appliances. For example, the tubular gas burner in a domestic water heater may be salvageable if it just filled up with water briefly, but if the orifices are clogged with silt, it's probably toast. The same goes for electric motors and other furnace components.

The Value of Sand Bags

During a flood--especially in flowing flood event as compared to flooding from rain and rising groundwater--half of the protection provided by placing sand bags around a building is reducing the infiltration of destructive sediment. Water may crest a protective wall made of sand bags (or staked-down straw or hay bales), but much of the heavy sediment won't go over the top with the water. So it you have a chance to make even a short barrier wall around property you are trying to protect, doing so may reduce the damage you suffer significantly.

Keep checking back for more coverage. Links to guidelines and standards for clean up and restoration work are next followed by information on modified building standards during a declared state of disaster. Also coming is the account of one man's flooded workshop and what we learned about salvaging power tools and equipment.

This is the seventh in a series of reports intended to help inform both the victims of natural disasters as well as the contractors they look to in these times of crisis.