The first step in understanding erosion is knowing what is causing the erosion. What is the problem stemming from. There's so many options! Poorly placed downspouts, bad grading, no sunlight to allow vegetation to grow, steep grades, landscape upgrades, tree removal, and on, and on. I typically try to get customers to start by thinking back to when this issue started, or maybe just identify where the water is coming from. Maybe the source can be differed into a safer corridor.
Typically, if major erosion is caught quickly enough the damage can be mitigated. When issues are left unaddressed and ignored is when the major problems begin.
Document the erosion
After establishing the source of the erosion, take the next step: know how long it has been an issue. If you just bought the house and inherited a drainage or erosion issue, you may not be able to tell how quickly it has escalated or been a problem. Start taking pictures, document the erosion right off the bat. Create a log or just have pictures to look back on. After a period of time you will be able to determine how serious of an issue it is.
If the erosion has happened over 15 years, then the ground is slowly eroding and the issue may just be poor soil conditions. These conditions could be caused by your lawn not having enough sunlight to grow thick grass. We often forget or fail to notice how big the trees were when we first moved to a property and how much they have grown. The solution may be to elevate the trees, bring in topsoil, and start the lawn over. You may just need to divert a few drains and the erosion problem is over.
If you just noticed this issue and it has become extreme in under 24 months, then it is probably time to be concerned. Continue documenting the erosion, check for any changes in the landscape during this period, grab a jacket during a downpour and see what the cause is. Water will always follow the path of least resistance; trace is back to the source, and typically the solution will present itself.
Create a solution based on the findings
Remember that water will always follow the path of least resistance. If you have identified excess water causing major erosion or slow soil erosion from a maturing landscape; both have solutions. Solutions may range from building a retention wall to creating a french drain to allow the water to seep into the soil or it may be as simple as putting an extension on your downspout.
If you are not sure if you have found the problem, create a temporary solution. You can link drainage pipes together above grade, capture your downspouts, surface drains, and reroute the water to a better location. If water is moving too fast or eroding soil rapidly, you can also create a temporary wall by using sandbags as a quick fix.