If you want to evacuate a large amount of surface water in a short period of time, trench drain is the best way to do it. And, where is one of the best places in the world to find an abundance of surface water? The rain forest in northern Brazil. I went to Brazil’s northern states of Amazon and Para’ to see firsthand how trench drain is used to help drain surface water. I visited Manaus, the capital of Amazonas state, and flew to the small mining community of Porto de Trombetas, in Para’ state. Though only 7000 live here, Trombetas has the largest metallurgical grade bauxite mine in the world and is the first ISO 14000 city in the western hemisphere. Besides being in the heart of the rain forest, they are ground zero for big daily rainfalls and surface water evacuation.
Trenches without Grates
Maybe the best trench for water evacuation is one without a grate. Creeks, streams and rivers don’t have grates, why should trench drain? Of course, in urban areas, we have safety issues that force us to use grates so we can walk and drive over the trench. Grates also help separate large debris from entering the water ways. However, all grates can constrict flow by design or by snagging trash. In rural areas, like Trombetas, grate-less trenches seem to be common. They are used around the yard to collect water from the roofs (below, center). They are used at the carport and at the driveway entrances (below, left). I guess if you make the trench narrow enough, you minimize the risk of having someone hurt themselves from stepping in the trench. I suspect, the folks in the rain forest are more concerned over living with water than they are living by lawsuits. Near the city square in Manaus, I found an 8” wide trench drain (above, far right) that looked as if it once had a cover of some sort. With the cover gone, it is easier for trash to collect in the trench, thus making water quality more difficult to maintain.
Trenches with Concrete Covers
Concrete is an inexpensive and “low tech” material. Concrete commonly finds itself as a trench covering material in regions of the world where metal products are precious or where labor rates are low. (Has anyone counted missing manhole covers in Detroit lately?) However, I was impressed how large surface drainage was accomplished without the obvious use of metal grates.
In most cases, the areas that used concrete drainage covers were predominantly for foot or light vehicle traffic (Class A or B). Though the drainage trench may have been covered by a low open surface area concrete cover (sometimes 0% openings), grate sections existed that accepted larger flows.
One exception to this observation was found at the Airport at Manaus. The trench covering was made of individual concrete “plates” which stood on end, packed together as if on a book shelf. The individual concrete sections made up the veins of the grate which supported aircraft traffic over the trench drain. I was sufficiently impressed.
Crude Metal Trench Coverings
If you need a trench covering in a high traffic area where bulky concrete grates will not work, an inexpensive metal covering is the next option. Sometimes, you need to just keep people from stepping in the animal drippings off the butcher’s table or make the entrance to your store a little smoother. A simple flat metal or rebar grate may be all you need. However, if you need a trench grate strong enough for high foot or automobile traffic, bar grates are a good option. Just make certain that the bars are thick enough to support the load that is being supported. Solid metal trench coverings were found in automobile traffic areas. The best covers seemed to be made with thick plate and with a reinforced edge . However, a poor frame design may cause premature failure leading to the drain covering to dismount.
Engineered Metal Grates
Lastly, I was able to find engineered bar grates – ones that had cross supporting metal bars! These are usually more difficult to manufacture and show a little more sophistication than a simple bar grate. These products were found only in the mining town of Trombetas. Examples of this grate type were found at the swimming pool, at the bauxite processing plant and at the mine’s green house. In this final photo from this trip you see me (author) at the tree farm that the mine owns. I am standing on an engineered metal grate made of aluminum (this is a metallurgical grade bauxite plant). I am wearing traditional Gringo attire — camo shorts and a flowered shirt. A wonderful point to make about this photo is the purpose of the tree farm. Prior to the mining of the rain forest, an inventory of the vegetation is made. After mining (and the return of the original top soil), trees are again replanted with the species and in the proportions that were taken from the original forest. This is an integral part of the mining operations at Trombetas and a testimony to Brazil’s respect for the world’s last great wilderness they hold.
Trench drain is alive and well in the rain forest. The drain channel bodies are cast-in-place concrete. The trench coverings, if they exist, are made from concrete and bar stock steel. No cast iron grates were seen in this region during my visit. To the best of my knowledge, no preformed or pre-engineered trench drain systems are available.