Thursday, March 30, 2017

Local Flood Hazard

This page describes the physical shapes and forms which flooding takes in Yavapai County.

Flood hazard determination is a tricky business, particularly in a climate where rains and floods occur infrequently.  The following discussion is intended to provide the lay person guidance in identifying flood hazard signs and obtaining flood hazard information.  This discussion is not intended to replace the need for professional expertise.  Proper flood hazard determination should always include contact with the Flood Control District.

Visit Property – Various signs of flood hazard will often be obvious to the critical observer, such as; topography (washes, swales, direction of slopes, drainage paths, etc.); vegetation (dense or pronounced variations); soils (loose sand, silt, or cobbles on the land surface may indicate flood prone land); and debris (lines or accumulations of debris along the ground or in branches of bushes or trees may be material left behind by receding flood waters).

  • Riverine Flooding – Many visitors to this area are somewhat surprised to see what we call a “river” in Yavapai County.  The majority of the rivers in Yavapai County are usually dry and carry runoff only during infrequent storms.  For the purposes of this discussion however, riverine flooding includes any flow of stormwater in a watercourse which appears to be a well defined path for conveying flood waters.  These watercourses are locally known by various names: river, creek, stream, wash, arroyo, channel, drainageway, draw, and swale.
  • Sheet Flooding – Sheet flooding is flooding caused by comparatively shallow water flowing over a wide, relatively flat area which typically does not have the appearance of a well defined watercourse as described above.  Flooding of these areas can occur due to a number of reasons.  A defined channel may fail to form due to geologic or topographic constrains.  Sometimes water flows over the banks of a channel with insufficient flood carrying capacity, and the land areas nearby become inundated.  Other times water maybe flowing down slope from lot to lot, or down roadways.  Sheet flooding is especially dangerous because it is usually difficult to tell when an area is subject to this type of flooding.  Even when standing in an area subject to sheet flooding, it is not often obvious that the area could become inundated.
  • Flash Flooding – Flash floods in Yavapai County most often result from high intensity, short duration storms, typically in summer.  Although flooding has been experienced in every month of the year, July through September is considered flash flood season.  Attempting to cross a flooded wash or road can be very dangerous and should be avoided.  Even walking through shallow fast moving flood waters can prove fatal.  Dips, bridges, and low areas are particularly prone to flooding and are not always visible at night.

Listen to your radio and television for reports about local flooding and areas of danger.  If storms with the potential for flooding are apparent, the National Weather Service may issue the following;

  • FLASH FLOOD WATCH (There is rain in the area and flash flood is possible)
  • FLASH FLOOD WARNING (Flooding is occurring or is expected in the area)

Erosion Hazard Areas – A somewhat different and less obvious type of flood hazard is erosion of land by flood waters.  Although technically not part of the “floodplain”, these areas can be extremely hazardous because erosion can occur suddenly and without warning, undermining buildings or other structures which are located near a wash or channel, and which appear to be safe from flooding.  For this reason, flood hazard regulations in Yavapai County typically include minimum setback requirements from the edge of a watercourse.

Flooding has occurred along Oak CreekBeaver Creek and the Verde River, as a result of overbank flooding.  These watercourses have very large watersheds which can experience abnormally large amounts of rainfall.  The accumulated affects of the saturated soils and the continued rains, along with heavy storm surges, can cause flows in these watercourses to overtop their banks.

Special Flood Hazard Areas (SFHA) may have been identified in your area.  For more information regarding floodplains, or to discuss the flood hazard in your area, contact the Flood Control District office.

To obtain a flood hazard status report, visit our Flood Hazard Status Reports page and complete all the information requested, or call the District and request a report.  A report on that parcel will either be mailed or emailed to you.

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