Our farm is hilly and has a rolling terrain. There are a few flat lands, but the bigger area is hilly. It's in the middle of the flat lowlands, and the mountainous upland.
Uplands and hilly areas naturally have strong current of floodwaters during heavy rains by the simple explanation of gravitational pull. In order to protect hillsides, we facilitated pathways of rainwater by constructing canals. What I observe however, is that constructing canals also facilitate more soil erosion, aside from facilitating strong floods. Over the last rainy season alone last year, about 2-3 inches deep of soil along a canal path that we constructed have been eroded by the rain waters and occasional flash floods. Many shallow roots of trees previously covered by soil are now exposed.
So this year, we're changing track: instead of creating canals, we put up strong stone terraces, several "humps" along the waterpath, to (1) trap some of the eroded soil and organic matter (fallen leaves and branches, etc.), and (2) slow down the speed of water and flood, thereby help minimize the damages of flash floods.
Repeating this process through time, by constructing ever-higher terraces as more eroded soil and organic matter are trapped and deposited along those stone terraces, we expect the formerly low and depressed areas to be gaining height, and hilly areas should become flatter through time.
As to how strong those terraces should be so they can withstand strong flash floods, it's an experimental project that should vary from one place to another.
Utilizing this practice and similar soil erosion control technologies, the height or elevation of agricultural lands should be rising through time, as more organic matter are deposited in the soil every year. A "bonus" could be the emergence of new water springs in your area since those organic matter deposit in your land are trapping more rainwater, releasing the water hours, even days, after the rains have stopped.