Ir. Joko Nugroho, S.T., M.T., Ph.D., a lecturer in the Water Resources Engineering Research Group, Faculty of Civil and Environmental Engineering, Institut Teknologi Bandung (FTSL ITB), explained that the environmental impacts of volcanic material depend on its particle size and distribution. Fine volcanic ash is of particular concern because it can be carried by the wind over considerable distances.

“Its effects on the environment can vary. From a water resources perspective, we are particularly concerned with relatively fine ash particles that can spread over long distances, depending on their size,” he said.

Volcanic Ash Can Reduce the Soil’s Ability to Absorb Water

According to Joko, the extremely fine particle size of volcanic ash means that it can potentially fill or clog the spaces between soil particles and other materials already present on the ground.

When volcanic ash accumulates on the soil surface, it can block some of the pore spaces through which water normally infiltrates. As a result, rainwater may have greater difficulty entering the soil and instead flow over the surface, particularly during heavy rainfall.

Joko explained that substantial ash accumulation can alter the soil’s capacity to absorb water. “Ash covering the surface can make it more difficult for water to enter the soil, causing greater surface runoff,” he said.

This condition requires particular attention in areas with high rainfall and steep topography. In mountainous regions, rainwater tends to flow more rapidly over the ground because the slope accelerates its movement. The runoff then concentrates in lower-lying areas, potentially increasing discharge as it moves downstream.

Under certain conditions, this increase in runoff can contribute to flooding.

In addition to increasing runoff, surface water can transport volcanic material that has been deposited on the ground. The material may then be carried into drainage channels, rivers, and lower-lying areas.

Joko explained that the extent of the impact depends heavily on the amount of material available, rainfall intensity, and how much material can be transported by surface runoff.

Volcanic material carried by water at high sediment concentrations can form a debris flow. In volcanic settings, such flows are known as lahars.

According to him, sediment concentration is one of the key factors distinguishing ordinary floods from debris flows. When the concentration of transported material is relatively low, the flow more closely resembles an ordinary flood. However, when a substantial amount of erupted material mixes with rainwater and reaches a high concentration, the flow can develop into a lahar, often referred to as a cold lahar.

Lahar Hazards Differ from Ordinary Floods

^{Ir. Joko Nugroho, S.T., M.T., Ph.D., a lecturer in the Water Resources Engineering Research Group, Faculty of Civil and Environmental Engineering, Institut Teknologi Bandung (FTSL ITB)}

Joko added that these differences are also evident after the water recedes. Ordinary floods generally leave behind finer sediment or mud, whereas debris flows or lahars can deposit larger quantities of coarser material.

The remaining deposits can also make recovery more difficult. Under certain conditions, debris-flow deposits may accumulate to heights that complicate the cleaning and restoration of affected environments and residential areas.

“When the water recedes, a lahar or debris flow leaves behind coarse material that is difficult to clear and restore to its original condition. In some cases, when debris-flow deposits reach nearly the height of a roof, removing them can be extremely difficult,” he explained.

Eruption Ash Can Affect Water Quality

The impacts of volcanic material are not limited to the quantity and movement of water. They can also affect the characteristics of the water with which the material comes into contact.

Joko explained that volcanic ash can influence water quality both physically and chemically. Physically, fine particles entering a water body can increase turbidity. This becomes particularly important when the water is intended for community use, as specific treatment processes may be required to remove suspended material.

Chemically, changes in water quality may be influenced by the chemical composition of volcanic material entering water sources. One parameter that may change is the water’s acidity, or pH. Therefore, water that has experienced changes in acidity and is intended for particular uses may require further treatment to ensure that it meets the necessary quality standards.

Volcanic Material Can Reach Coastal Environments

The movement of volcanic material does not necessarily stop near the eruption site. Material entering river channels can be transported downstream and subsequently deposited according to its size and characteristics.

Coarser material tends to settle more quickly along river channels, while finer particles can travel farther downstream and eventually reach coastal environments.

“When material is carried downstream, finer particles are generally more easily transported and may eventually be deposited in coastal environments. Such deposition can affect the morphology of rivers and coastal areas and, under certain conditions, may even contribute to delta formation,” he explained.

Volcanic Material Has Both Positive and Negative Impacts

Although volcanic material can cause various environmental impacts, Joko emphasized that its presence is not always detrimental.

“One advantage is that volcanic material is relatively fertile because it contains inorganic materials or minerals needed by plants. The disadvantage is that many community activities can be affected,” he said.

The impacts of volcanic material must therefore be understood in relation to the processes that occur after it reaches the ground, including its interactions with rainfall, soil, water, and the surrounding environment.

Monitoring Should Integrate Ash, Rainfall, and Flow Data

To reduce post-eruption risks, monitoring should begin as volcanic material is dispersed. Volcanological and meteorological information can be used to assess ash distribution, while hydrological conditions are needed to evaluate potential impacts following rainfall.

In areas with volcanic deposits located near river channels, rainfall and flow data are particularly important for anticipating erosion, increased runoff, and possible lahar flows.

“What can be done is to identify the rainfall intensity at which erosion or lahar flooding may occur. This requires observations from rainfall stations, as well as monitoring of surface runoff,” he explained.

According to Joko, these data should be supported by a monitoring system connected through telemetry so that field conditions and parameters can be observed promptly. The information obtained can then support early monitoring and warnings of potential hazards.

Communities Should Remain Alert to Surface Water Conditions and Heavy Rainfall

Following an eruption, public attention should extend beyond volcanic activity itself to the environmental conditions affected by erupted material. Areas near river channels require particular attention, especially during heavy rainfall, which can increase surface runoff and carry volcanic material downstream.

“People need to be careful when using surface water because of the potential presence of volcanic ash. If they have water storage containers, these should be covered,” he advised.

Joko also emphasized the importance of exercising caution when receiving disaster-related information, particularly following natural phenomena that may generate widespread public discussion.

“Make sure to rely on official sources. We need to improve disaster literacy so that we can evaluate and filter the information we receive,” he said.

Reporter: Chysara Rabani (Mining Engineering, 2022)