Flooding results when rain and/or snowmelt creates water flow that exceed the carrying capacity of rivers, streams, channels, ditches, and other watercourses. In Oregon, flooding is most common from October through April when storms from the Pacific Ocean bring intense rainfall. Flooding can be aggravated when rain is accompanied by snowmelt and frozen ground. The frequency of flooding combined with concentrated development along rivers and streams caused millions of dollars in damage to Jackson County over the past several decades. The growing population and development activity in the floodplain can increase the risk of flood-related damages.
Anticipating and planning for flood events is an important activity for Jackson County. Federal programs provide insurance and funding to communities engaging in flood hazard mitigation. The Federal Emergency Management Agency (FEMA) manages the National Flood Insurance Program (NFIP) and the Hazard Mitigation Grant Program (HMGP). The NFIP provides flood insurance and pays claims to policyholders who have suffered losses from floods. The HMGP provides grants help mitigate flood hazards by elevating structures or relocating or removing them from flood hazard areas. These programs provide grant money to owners of properties who have suffered losses from floods, and in some cases, suffered losses from other natural hazard events.
The principal types of flood that occur in Jackson County include:
Riverine flooding typically occurs on larger rivers and streams when water levels overflow their banks, and this type of flooding usually results from large storms or prolonged wet periods. Portions of the county that are located along water bodies have the potential to experience riverine flooding after spring rains, heavy thunderstorms or rapid runoff from snow melt. Riverine floods can be slow or fast-rising, but usually develop over a period of days. The danger of riverine flooding occurs mainly during the winter months, with the onset of persistent, heavy rainfall, and during the spring, with melting of snow. Riverine flooding is the most common type of flooding in Jackson County. The Federal Emergency Management Agency (FEMA) has mapped the areas subject to riverine flooding for the National Flood Insurance Program (NFIP). The most severe flooding conditions generally occur when direct rainfall is augmented by snowmelt, like the 1964 Christmas Day flood and the 1997 New Year’s Day flood.
Flash floods are a major cause of weather-related deaths in the United States. Flash floods usually result from intense storms dropping large amounts of rain within a brief period. Flash floods occur with little or no warning and rivers can rise in a manner of minutes. Flash floods are most common in arid and semi-arid areas where there is steep topography, little vegetation and intense but short-duration rainfall. Jackson County, located in a semi-arid region, is prone to this type of flooding. Steep topography combined with clearing of vegetation for development and timber production causes rapid runoff of rainwater.
Flash floods occur in both urban and rural settings in Jackson County, principally along smaller rivers and drainage ways. Covering land within cities with non-permeable surfaces and the construction of storm water drainage systems compound the effects of flash flooding. Storm water systems are designed to move the rainwater quickly out of the city, and into the local drainage way. This additional rapid infusion of water can push rivers over their banks, and literally create a wall of water moving downstream. In flash flood situations, waters rise rapidly, move at high velocities, and often contain large amounts of debris. Occasionally, floating debris or ice can accumulate at a natural or man-made obstruction and restrict the flow of water. Water held back by ice jams or debris dams can cause flooding upstream. Subsequent flash flooding can occur downstream if the obstruction suddenly releases. Additionally, manmade structures like dams that retain water in reservoirs can fail and create flash floods downstream.
In 1918 Jackson County experienced a flash flood that was caused when the Fish Lake dam failed. The earthen dam was washed away by water flowing over the top. It took three hours and ten minutes for the wall of water and debris to reach the town of Medford. No human lives were lost in this flood, but the buildings and infrastructure of the town suffered significant damage. Dams, or impoundments, can mitigate the effects of some types of flood events by storing runoff from large storms and releasing it slowly. Conversely, dams can cause flooding as well, by failing and releasing a flash flood down the river channel.
Jackson County has a large number of impoundments and the Environmental Protection Agency notes that Jackson County has a high volume of impounded water. There are over 75 dams in Jackson County, including twelve high hazard dams.
Shallow Area Floods
Shallow area floods are a special type of riverine flooding. FEMA defines a shallow area flood hazard as an area that is inundated by a 100-year flood with a flood depth between one to three feet. Such areas are generally flooded by low velocity sheet flows of water. This type of flooding causes the most damage within Jackson County.
Urban flooding occurs where land has been converted from fields or woodlands to developed areas consisting of homes, parking lots, and commercial, industrial and public buildings and structures. In such areas the previous ability of water to filter into the ground is often prevented by the extensive impervious surfaces associated with urban development. This in turn results in more water quickly running off into watercourses, which causes water levels to rise above pre-development levels. During periods of urban flooding streets can rapidly become swift moving rivers and basements and backyards can quickly fill with water. Storm drains and smaller creeks can back up due to yard waste and debris. Clogged storm drainage systems often lead to further localized flooding.
Factors that contribute to flooding in Jackson County:
Heavy rainfall is the most common cause of flooding in Jackson County. The rainy season is from October through April, when Pacific storms from the ocean, 60 miles away, bring intense rainfall to the area. Jackson County receives approximately 20 inches of rain on average each year, however about 80% of the precipitation occurs during the seven wettest months of the year, October through April. It is during this “wet season” that flooding is most likely to occur. Snowmelt can also cause flooding, particularly when combined with new rainfall. Rain falling on top of snow causes the snow to quickly melt and river levels to rise rapidly. The two most severe flood events in Jackson County were the result of rain falling on snow pack.
The eastern, western, and southern boundaries of Jackson County are mountainous, with a valley in the center of the county. These mountains collect rain and snowfall and deliver it into the Bear Creek and Rogue River valleys. Slopes of the surrounding mountains are relatively steep and have shallow, rocky soils. These types of soils have low absorption properties and quickly transport the rainwater to the river system.
The type and relative amount of vegetation cover dramatically affect how quickly rainwater moves into waterways. Heavy vegetation cover slows the movement of rainwater into the river. When vegetation is removed or reduced, rainwater moves more rapidly into the river system, and contributes to higher water levels. Logging, clearing for development, and agricultural practices can all contribute to rapid water level fluctuations in Jackson County’s rivers.
Location of Development
When development is located in the floodplain, it may cause floodwaters to rise higher than before the development was located in the hazard areas. This is particularly true if the development is located within the floodway. When structures or fill are placed in the floodplain, water is displaced. Development raises the base- flood elevation by forcing the river to compensate for the flow space obstructed by the inserted structures. Over time, when structures or materials are added to the floodplain and no fill is removed to compensate, serious problems can arise. The Jackson County Comprehensive Plan prohibits development in the floodway, but under certain circumstances may allow development in the floodplain.
Floodway development is currently regulated and Jackson County (and FEMA) require engineering (“no-rise”) certification that the proposed developments will not cause the base flood (100-year flood) elevation to raise more than 1.0 feet.
Displacement of a few inches of water can mean the difference between no structural damage occurring in a given flood event and the inundation of many homes, businesses, and other facilities. Careful attention must be paid to development that occurs within the floodplain and floodway of a river system to ensure that structures are prepared to withstand base flood events.
In urbanized areas, increased pavement leads to an increase in volume and velocity of runoff after a rainfall event, exacerbating potential flood hazards. Storm water systems collect and concentrate rainwater and then rapidly deliver it into the local waterway. Traditional storm water systems are a benefit to urban areas, by quickly removing captured rainwater. However, they can be detrimental to areas downstream because they cause increased stream flows due to the rapid influx of captured storm water into the waterway. It is very important to evaluate storm water systems in conjunction with development in the floodplain to prevent unnecessary flooding to downstream properties.
Terms Related To Flooding:
A floodplain is land adjacent to a river, stream, lake, estuary or other water body that is subject to flooding. These areas, if left undisturbed, act to store excess floodwater. The floodplain is made up of two areas: the flood fringe and the floodway:
The floodway is the portion of the floodplain that is closer to the river or stream. For National Flood Insurance Program (NFIP) and regulatory purposes, floodways are defined as the channel of a river or stream, and the over-bank areas adjacent to the channel. Unlike floodplains, floodways do not reflect a recognizable geologic feature. The floodway carries the bulk of the floodwater downstream and is usually the area where water velocities and forces are the greatest. NFIP regulations require that the floodway be kept open and free from development or other structures, so that flood flows are not obstructed or diverted onto other properties. The NFIP floodway definition is “the channel of a river or other watercourse and adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than one foot (See Figure FL.1).” Floodways are not mapped for all rivers and streams but are typically mapped in developed areas.
The Flood Fringe
The flood fringe refers to the outer portions of the floodplain, beginning at the edge of the floodway and continuing outward. This is the area where development is most likely to occur, and where precautions to protect life and property need to be taken.
Figure FL.1 Floodplain Schematic
Source: FEMA, Flood Insurance Study: Jackson County, Oregon and incorporated Areas, May 2011.
History of Floods in Jackson County:
Flooding is a familiar occurrence in Jackson County and generally occurs when unusually warm weather mixed with heavy rain melts snow in the higher elevations and flood local streams. Over the past 100 years, the county has experienced major flood events on a regular basis. Table FL.1 lists some of the major flood events that have occurred in Jackson County and the Pacific Northwest.
The landmark flood event for Jackson County in the last century was the flood of 1964. This flood set most of the record high-water marks for the region. The trigger for this flood was warm rain on a substantial snow pack. The rain quickly melted the snow, and caused the rivers to overrun their banks. All subsequent floods have been compared to this event. The 1964 flood was characterized as a “100-year” flood event. A “100- year” flood has a 1% chance of occurring in any given year, or a 26% chance of occurring during the life of a 30-year home mortgage.
In late December 1996 and early January 1997, Jackson County experienced another devastating flood event, known as the New Years' Day flood. In the weeks preceding the flood, the county received abundant rain and snowfall. A warm and heavily moisture-laden storm front, known as a “Pineapple Express,” followed the abundant snowfall. The warm rain quickly melted the snow pack, and county streams and rivers rapidly filled their channels and exceeded their banks. Bear Creek’s waters reached 17,600 cubic-feet-per-second. The mean daily flow for January 1st is 262 cubic-feet –per-second. This particular flood event caused over $12 million in damages to Jackson County homes, businesses and infrastructure. More than 1,500 people were evacuated and over 1,000 properties damaged. In January 1997, President Clinton declared fourteen Oregon counties, including Jackson County, eligible for disaster assistance due to damages resulting from these severe winter storms (DR-1160).