Two EC professionals look at how the profession has evolved in the last 20 years.
By
Andrew Johnston,
Gary Shaw
There will be no mathematical equations in this article. There are no earth-shattering revelations or "Eurekas." Some of the best presentations at recent IECA conferences have dealt with on-the-job-training retrospectives. That got us to thinking: Where have we been, what have we done, and how have we grown in erosion control? The goal of this simple article is to look back over the last 20 years into the "rear-view mirror" of our combined erosion control experiences to see where we have been and to make mental adjustments for the future. |
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Andy's first encounter with erosion control was back in 1980 working for the City of Shreveport, LA. Bill Walker of Erosion Control Systems in Houston, TX, introduced him to excelsior blankets and Enka mats. It is amazing how large an industry erosion control would become over the next couple of decades. Now many types of geosynthetic materials are available that can provide armoring assistance over a very wide range of shear and soil conditions. Back in 1980, the main objective simply was to keep the sediment from individual street and drainage projects out of the nearby bayous. Today we have access to sediment logs (sausages) made with excelsior, wattles made of straw or coir, and a wide variety of biodegradable blankets and permanent matting. As we look in the rear-view mirror, we even see ourselves using hay bales in some situations. Now there are many more products to choose from that actually do work.
Pavers Vs. Concrete
Our experience with the City of Longview, TX, began in 1984 with the timid use of an open-cell concrete paver in a small swale from the parking lot on Timpson Street across parkland from the nearby creek. This small drainage swale was constantly muddy, which was not a good situation for the park patrons. When maintenance equipment was used to mow the park, ruts would form across the swale that caused even more ponding and boggy conditions. Instead of paving this section with concrete, Director of Public Works L.K. Smith, P.E. (now deceased), encouraged us to expand the envelope of erosion control and "try something new." Looking back, the opportunity to work for L.K. was truly a gift. He allowed us to stretch the boundaries. Therefore, across this small swale, we used open concrete pavers to form a simple trapezoidal section by installing one paver on the bottom and one on each side. There were no interlocking ears on these rectangular pieces. We did not lay them on any type of geotextile fabric. But what we did do was provide for drainage such that the mowers could now cross without rutting the channel. One small step for maintenance, one giant leap for erosion control.
The success of that small project gave us the courage to try the same type of erosion control on a larger, bona fide drainage channel. The channel wound through a residential area, and the sideslopes were caving in. Again, instead of paving with concrete, we created a much more ambitious channel armoring section with six rows of pavers on each side of the trapezoidal channel. It was during this general time frame that city staff began to hear of the new environmental requirements coming from some of the state and federal agencies to allow for water exchange between the channel and the groundwater (e.g., do not block this interchange with concrete ditch paving). It was becoming evident that the environmental benefits of the open block systems compared to the flat concrete channels were going to have to be reckoned with. The photos show a view from a street crossing with the concrete section tapering to the open paver channel section running between the homes (see photo 1). Again, these pavers did not have any interlocking ears or cabling ability. Construction crews had to use j-hooks made of rebar to anchor the lower sections of the sidewalls to keep them from being pushed down and out of alignment. No geotextile was installed underneath the pavers.
With this method, we were able to provide a vegetated, nonerosive channel that was more environmentally friendly than concrete paving. Some sections of channel were undermined, and some pavers were lost (see photo 2). Looking forward, the pavers (or articulating concrete blocks) would be a potential solution again, but this time with the use of geotextile fabric underneath. Several of the tougher turf reinforcement matting products could also be considered in projects of this type, as well as some bioengineering techniques. The need for a low-flow channel as well as a "benched" high-flow channel could also be evaluated. Other design changes, such as using some geomorphologic features instead of our engineer's "straight line" alignment, were also on the horizon.
Progressive Headwalls
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Another area of exploration in drainage maintenance dealt with headwalls at pipe outlets–or the lack thereof. Many times the headwalls for medium- to large-diameter pipes would be damaged by a flood, leaving the next upstream pipe section to fend for itself. As the flow would continue to exit, it would undermine the next pipe section–allowing it to fall–and then the next section, and the next. Many times we would have three or more pipe sections lying in an eroded trench line with the active headwall now lying well up into someone's backyard. To deal with this "progressive headwall" situation, we began looping a metal cable wire through the lift holes of the last three joints of pipe, connecting them to a turnbuckle, and then tightening the cable. This provided a "tension member" with the concrete pipe being the "compression member." If the headwall were lost, the water would still wash out from under one or two pipe lengths, but then it would stop. The energy to wash farther back under the pipe was lost, and we were left with an exposed pipe but a dormant erosion problem.
From Field to Office
One of the most beneficial things we were able to accomplish in erosion control was to take the experiences of the field maintenance crews and plant them back into the office engineering department. By learning from the designs of the past, we could make changes that remedied several of them. The office-to-field-to-office loop was critical. There were times we found ourselves needing to provide erosion control for locations with limited accessibility. In one case there was a confluence of two fairly large channels that had been stable for many years. After one particular flood, an entire backyard was removed, leaving about a 15-ft. vertical face just a mere 20 ft. from a residence. Something needed to be done quickly. We used a product called R-Rap, consisting of a bag of specially blended cement and sand that can be impaled upon large-diameter rebar. Successive bags were placed like bricks one on top of the other, with rebar used as interlocking anchor pins (see photo 3). A geotextile fabric was placed behind the wall, with compaction effort applied to the fill material. The result was a battered back (not vertical) bag wall that reclaimed the entire backyard (see photo 4).
Within months after we installed the bags, another major flood occurred, yet the wall was undamaged. The fact that each bag articulated upon the rebar pins allowed for the wall to settle without cracking, and the terrace of bags along the channel wall provided egress from the channel during the storm event. Another advantage was that the entire wall could "breathe" and release groundwater across the entire face instead of using a few weep holes as in other designs that always seem to get plugged and quit working. With this single success, the method was expanded for use as a headwall and a wing wall along a flood-damaged section of channel (see photos 5 and 6).
A similar product, made of concrete and formed as individual blocks, was used in several situations to provide a terraced-wall channel section (see photo 7). The rear-view mirror indicates that we were on the right track. The success of these block wall projects meant the materials and procedures would be used again. The advent of geogrid materials and computer software to detail slope modeling also allowed these products to reach large wall heights.
Geocell Walls
Looking in the rear-view mirror, we see we were fortunate to have worked for and with many key people over the last 20 years, particularly L.K. Smith, who encouraged, not discouraged. We see cautious "baby steps" trying new products with public funds. We see that, for the most part, we had very good fortune in that the things we tried were successful. We had heard of situations in which a potentially good product failed because some strange event took place and the product was no longer used. Many times the good intentions of offering valuable engineering to an erosion control project by attempting to provide environmentally aware solutions were derailed by one or two "failures." At one point of construction on one of the innovative jobs, the city manager at the time asked, "Is it going to work?" to which we answered, "Yes." His reply was, "It sure better." Politics always seems to play a part.
Thomas Edison once said, "Genius is one percent inspiration and ninety-nine percent perspiration." The City of Longview staff learned that the same philosophy applies when attempting to control erosion.