On 4 October 2019, I had the opportunity to accompany Assoc. Prof. Dr. Suttisak Soralump and two other Thai colleagues for the investigation of soil erosion problem and storm water management at Ichinoseki, Iwate prefecture, Tohoku region Japan. Here, I shall try to depict the general context along with major findings, and proposed solutions through words and pictures.
The site is a solar farm owned by Global Power Synergy Public Company Limited (GPSC) and is distributing electricity to Tohoku Electric Power. The project area is situated in the southwest of Ichinoseki city, close to the border between Kurihara city, Miyagi Prefecture. It is a hilly landform ranging its altitude from around 250 to 360 m. The nearby area mainly includes forest and cropland. The study area is divided into zone A and zone B which was further divided into smaller regions B1-B9 and A1-A9 making it easy to pinpoint the problem occurring in a particular region.
Stormwater management (SWM) and Erosion sediment control (ESC) for solar farms present an interesting challenge because they usually involve an impervious surface elevated above a pervious surface. This generally results in the localized concentration of surface water from the panels and the reduction of immediate ground infiltration area caused by the individual solar array support structures.
We first performed a drone survey to identify general pattern of the erosion at the site. The areas consisted of sheet, rill and gully erosion.
The excessive erosion at the site was due to the concentrated flow of water dripping from the ends of solar panels. When rainwater hit the solar panel, it gained momentum before hitting the ground. The dripping water always hit at the same place and eroded the surface which in time was accumulated & concentrated forming a channel (Picture 3). Apart from such behavior, a distributed type of flow was also observed depending upon the topography of the area (Picture 4). Distributed flow contributed to less erosion but when remained un-intercepted by drainage it also gathered in a long run to erode the soil.
The nature of the ground contributed significantly to worsening the soil erosion problem. The ground surface was inclined and the running water forming a channel began to take the form of gully erosion. Vegetation would have been an effective solution but in most of the areas, the soil was found to be acidic which inhibited the growth of general plants and grasses. We performed a simple pH litmus paper test in different areas and categorized them on the basis of the degree of acidity. In some places, the soil was highly acidic such that the corrosion at the foundation of the solar panel was observed. In order to prevent the corrosion of the steel, and save it from foundation failure, enamel was recommended to be applied at the contact surfaces. Moreover, in some areas, it was suggested to opt for Vetilever, a type of vegetation that can even be grown at a certain range of an acidic soil.
Also, the absence of interception drainage was hugely felt. Many of the erosion would have been controlled with a well-placed interception drain avoiding the concentrated flow to grow. For instance, as shown in the picture 5, there hasn’t been a concentrated flow of water to form a gully erosion due to the absence of guided water through the solar panel, and only rill erosion is dominant at this site. If proper interception drains are managed in this section, then it would prohibit the further increase of erosion, preventing the formation of gully erosion.
As for the solution, it is highly recommended to use an energy-dissipating structure like a combination of geotextile and gravel along the drip line of the solar panel. With the use of such structures, the initiation point for erosion is highly controlled. Another form of geosynthetic that can be used here is geocell, structures produced from High-Density Polyethylene (HDPE) and has three-dimensional cells that can reinforce a variety of fill materials including local fill or granular.
In some areas where erosion is not severe, a simple mitigation measure of re-compacting the soil with a mixture of cement (4-8%) can stabilize the soil. Soil-cement has been used in the protection of erodible stream banks during flood events.
For the deep gully erosion area, one of the ways that can fill the eroded area is through the use of sandbags that consists of a bag or sack filled with sand or soil. The use of it can reduce the velocity of water too. Such composition has been used for many purposes such as flood control, military fortification in trenches and bunkers, counterweight etc. The major advantage is that it is inexpensive.
One of the common ways to reduce the acidity of the soil is through the use of pulverized limestone which acts as a soil acid neutralizer and consists of either calcium or magnesium carbonate or calcium carbonate. However, it is very important to make sure the correct amount of limestone to be added which generally depends upon the current pH value of the soil and the type of plant we would want to cultivate. Generally, the soil lab test determines the needed amount of limestone to be added in the soil. If we add limestone in excess then it would result in an alkaline soil making the soil deficient in iron, manganese and zinc. It would also support an uncontrollable increase of bacteria that would harm the plants especially the ones that spends longer time underground like potatoes.
Proper mitigation measures as outlined briefly here were suggested for the soil erosion control in the solar farm. With such measures the situation can be improved drastically in a short period. If neglected, in the long term, the situation might get worsened and primarily affect the foundation of the solar panels. This will in turn affect the whole system resulting in losses of economy; affecting the distribution of electricity to the Tohoku Electric Power.