Sediment Capture Research
Dr. Jon Witter, Ohio State University water management specialist shared research on how farmers and landowners may capture sediment in runoff, reducing water quality problems. Dr. Witter has been conducting long-term research on two-stage ditches and several other ditch modifications for several years. He found ways to capture sediment that save landowners money on clean out, are faster, more convenient, and are over all cheaper to maintain.
A typical man-made ditch in Ohio is called a trapezoidal ditch with 3:1 side slope which is common throughout Ohio. The bottom is flat, and the sides are fairly steep. Northwest Ohio has at least 10,000 miles of these ditches. When they were constructed to drain the drain the black swamp, the spoils were used to make roads. Often, steep trapezoidal ditches are located next to roads. They tend to erode, and they tend to fill up with sediment and nutrients from runoff. They have to be cleaned out frequently, and often the spoils (sediment in the bottom) are dipped out and piled along the ditch banks. This also may cause increased drainage problems and possibly more erosion.
There are several two-stage ditches located in Northwest Ohio. Several are half mile to a mile long. The problem with two stage ditches is they more expensive to construct. They use about 7% more land which is permanently taken out of production and it takes more money to construct. The two-stage ditch has a wider bottom and more capacity. On a two-stage ditch, at low flow, the water meanders but at high flow, it covers the benches. The wider bench area cause the water to slow down, and then sediments, nitrates, and phosphorus in the water drops out and accumulates on the bench. On two-stage ditches, clean out is needed less often (3X less) and that saves money! Farmers do not need to construct .5 to 1 mile two-stage ditches, possibly only much shorter distances (100-300 feet) to capture the sediment, which reduces costs of construction and cleanout.
Dr. Witter also researched the nutrient content of the spoils (sediment) that was applied to the land next to the ditch. He found that about 2 cm per year of sediment (.83 inch/year) accumulates in a typical trapezoidal ditch. That equivalent to a 5# sack of flour spread over an 8” X11.5” sheet of paper. The sediment is 11% soil organic matter, 5% carbon, 6-12 parts per million (PPM) phosphorus, and about .5% nitrates and has a nutrient value of about $8 per cubic yard, which is about what it costs to remove it. Dr. Witter found that this sediment has more weeds, 27-38 weed species per cubic yard with about 80% being broad leaf and 20% grass weeds. Most of our common herbicides will take the weeds.
Research on corn growth, showed that the corn grown in the spoils had thicker stalks and were 1-2 stages faster in growth. However, an insecticide may be needed in these areas due to increased seed corn maggot infestations. Dr. Witter researched the yield effect of the sediment. No treatment on conventional tillage yielded 188-bushel corn and 69-bushel soybeans. Adding sediment, increased the yield to 211 on corn and 78 on soybeans. Sometimes, sediment is mostly subsoil. A trial with .5 inch of subsoil added to the top soil, reduced corn yields to 179 bushel and soybeans to 70. However, if commercial fertilizer is amended and added to the subsoil, the yields recovered to 230 bushel corn and 75 bushel soybeans.
Another new concept is the idea of cascading waterfalls. On a traditional grass waterway, small retention cells or small dams are constructed to retain the water to form mini-wetlands. On a 100-acre watershed, about 65% of the sediment, and 50% of the nutrients can be maintained in these cells. Water flows over the small water falls, but some water is retained, reducing the velocity of water, capturing sediment and nutrients, and reducing soil erosion. Research shows that on 10-acre watersheds in Williams County, about 72# nitrates, 3.6# phosphorus, and 2.4 ton of sediment are saved per year. Water in the cells or mini-wetlands either evaporates or infiltrates. Most mini-cells have less than 12 inches of elevation difference.
On a 1.3 inch rain, there was no flow out, no discharge, so no nutrients or sediment left the cells. On larger rain fall events, there is discharge but the flow was reduced 20-24%, and it decreased nutrients and sediment losses significantly. Each watershed is different, and the larger the rainfall event, the effectiveness decreases. All these new innovations show promise for reducing costs and improving water quality in Lake Erie.