There are many possible flood scenarios. Too much debris (such as leaves, tree limbs, and trash) can clog street drains or ditches that cause flooding after a heavy rain. In such cases, water collects on paved surfaces instead of being absorbed by soil, fills water retention ponds, or goes downstream. Water rises either because the drains cannot handle the volume of water being channeled into them or the water cannot get to the ditches. When the rain stops, the rainwater will recede from roads and parking lots within minutes, provided the drains and ditches are clear and free of debris. During the rain or shortly after, the water rapidly seeks lower ground, which floods basements or lower buildings. Floods also happen when a river’s or creek’s natural bank or levee cannot contain the water within its channel. Such floods often recede slowly, because the floodwaters cannot drain back into streams, creeks, or ditches until the water level downstream drops to pre-flood levels. In this type of flood, waters tend to rise slowly and predictably, so emergency responders often have more time to warn the public.
Flash floods develop quickly and without much warning. For instance, an area might receive 8 to 10 inches of rain in an afternoon, and quickly saturate the soil. Once this happens, water will start to run off, filling low-lying areas first. As the water flows downhill, it eventually passes through, or concentrates on, just a few acres of ground, causing deep water in those areas. The deluge also can overwhelm the ability of creeks and streams to carry the water away. The water accumulates, and the ditches or creeks overflow their banks. Walls of water can roll across farms and towns and wreak havoc in their wakes. To make matters worse, floods can weaken levees and cause them to fail, releasing billions of gallons of water into already flooded areas. Broken levees often produce historic 100-year floods where the water is so deep that only the rooftops of buildings are seen. Flash flooding from levee breaks happens so quickly that the only recourse may be immediate evacuation. Floods can happen even when it doesn’t rain in your area.
Debris-clogged ditches and drains can cause floods after heavy rains. Let’s say an area upstream receives a heavy downpour or large amount of rain in a short time. This will cause flash flooding in that area and raises the potential for flooding in areas and communities downstream. Runoff from the area that was initially affected will eventually drain into creeks, streams, ditches, or rivers and continue downstream. As the mass of water moves downstream, flood watches and warnings are posted to alert those living along the stream. The bottom line is that flooding can and does occur anywhere — even in deserts — given the right circumstances.
Another form of flooding that plagues buildings is reverse flow flooding. This happens to buildings tied into public sewer systems. When heavy water loads on and around these public systems are too great, water pushes into sewers and works its way back into low-lying outlets in basements or into buildings with low grades. This type of flooding is often nasty because it can include sewage and all other types of foreign materials. Sometimes, you will hear about 100-year or 500-year floods. These terms, which will be described later, are being reconsidered by the experts because flooding has been so severe in recent years. Land-use (such as a lack of retention ponds) has such an influence on flooding that experts don’t feel the 100-year and 500-year terms apply anymore. If that’s the case, you may one day see experts use different terms to describe floods, such as a 1-5 or color-coded rating scale.