FLOOD02929 - Laserfiche WebLink

Home Browse Search

Flooding usually involves a build-up of water in the channel followed by overflow that inundates the floodplain, Generally, the rise in water surface elevation is slow in large streams and more rapid in small streams, Several types of riverine flooding occur, each posing specific threats to manufactured homes, Flash flooding involves an extremely fast rise in water surface elevation and abnormally high water velocity, often creating a "wall" of water and debris moving down the channel and floodplain, Flash floods usually result from some combina- tion of intense precipitation, steep slopes, a small drainage basin, and a high proportion of imper- vious ground surfaces, They often occur in small streams that are normally shallow or dry and can cause extensive damage as shown in Figure 1,22, Figure 1.22 Flash Flood Damage The high water velocity characteristic of flash floods can significantly affect manufactured homes in terms of erosion or scour around the founda- tion, movement of the manufacfured home off the foundation. and impact on the manufactured home by debris carried in the floodwaters, Although in- undation is always a concern, damage sustained from a flash flood event will generally depend upon the strength of the foundation and anchoring system to resist movement of the manufactured home, Even in situations where floodwaters do not reach the floor of the manufactured home, the foundation and anchoring systems will have to resist high water velocity and debris impact forces, If these systems are not properly designed and installed, failure resulting in foundation collapse and manufactured home inundation will most likely occur, as shown in Figure 1.23, When greater flood depths are experienced, in- undation of the manufactured home is likely to oc, cur, and the forces on the foundation and anchor- ing systems are subsequently intensified, Shallow flooding includes unconfined flows over broad, relatively low areas; intermittent flows in Figure 1.23 Flash Flood Damage arid or semi-arid regions that have not developed _ _ _a_system_ol weJI-d~fjl1ed_ch!l.n_lleJs; .rmnoJ _sJ[e_alJ1. . bank flows that remain unconfined; overland runoff in dense urban areas; flows where heavy debris deposits cause constantly shifting channels; and "ponding" in topographic depressions, Flow direc, tion is extremely difficult to predict. Generally, shallow flooding areas are considered as those areas exhibiting depths of three feet or less, A con- ventional manufactured home installation may pro- vide adequate elevation during a shallow flooding occurrence, A cautionary statement is appropriate, however, calling attention to the substantial damage that can be caused by a relatively small amount of water within the manufactured home, The aforementioned flood. loads applied to the foundation and anchoring systems can also be ap- plicable to shallow flooding circumstances, Even very shallow floodwater can have high velocity suf, ficient to cause erosion, scour, and subsequent failure of the foundation system, as shown in Figure 1,24, Figure 1.24 Shallow Flooding Damage 9