South Dakota OEM - Hazard Vulnerability

Floods are among the most serious, devastating and costly natural hazards that affect South Dakota. The greatest impact of this phenomena has been to the eastern half of the state, principally, the Big Sioux, Vermillion, and James River basins which have recurring problems.

The Big Sioux River basin is the eastern most major river pattern in South Dakota. It is formed within a topographic feature known as the Coteau de Prairie Highlands. This glacial formed feature rises about 800 feet above the bordering Red River lowlands of Minnesota. It is also bordered on the west by the James River lowlands. The Coteau has what is known as a flatiron shape lying in a general northwest to southeast direction. It is about 200 miles long and 80 miles wide at the widest point. It has a variation in elevation from 2,050 feet at the highest point to 1,090 feet at the lowest point.

The northern part of the Coteau has geologically developed features of pot holes, sloughs, and lakes. During periods of low precipitation, these features tend to hold back water and not contribute to the drainage of the Big Sioux River. Conversely, during wet years, this area can accumulate enough moisture so as to greatly increase the water supply to the drainage basin. There are about 1,970 square miles of land within the basin that is designated as non-contributing to the drainage system. The portion of the basin that does contribute to the Big Sioux River registers about 7,280 square miles. A total of 4,280 square miles of the figure is located in South Dakota.

The head waters for the Big Sioux River are found in the Coteau Lake Region of Roberts and Day counties. The river flows in a southerly direction to its junction with the Missouri River near Sioux City, Iowa. Contained within the drainage pattern are Lake Kampeska and Pelican Lake in Codington County and Lake Poinsett in Hamlin County.

The variation in elevation from the head waters to the mouth greatly influences the movement of water through the basin. The elevation decreases from 1,826 feet near Waubay to 1,381 at Sioux Falls. The Granite Falls formation of Sioux Falls has a 100 foot drop in elevation. Below the Falls, the elevation varies from 1,281 feet to 1,098 feet at its mouth near Sioux City, Iowa.

Associated with the elevation is the slope profile of the river. The slope varies from 1.83 feet per mile near Watertown to 1.50 feet per mile at Sioux Falls, to 0.5 feet per mile at the junction with the Missouri River. The Big Sioux River has a steeper gradient than either the James or Vermillion rivers. This steep slope causes water to move quickly down the drainage system and thus shortens the time of peak flooding in any given portion of the basin.

The Vermillion River Basin is the smallest of the East River systems. It has its head waters in the lake country of Kingsbury County. The river flows through McCook, Turner, and Clay counties to join with the Missouri River near Burbank, South Dakota. The west branch originates in Miner County and connects with the main stem near Parker in Turner County.

The Vermillion River Basin is formed in the Dakota Valley or what is more commonly called the James River Lowland. This area is more than 200 miles long and about 60 miles wide and occupies a portion of the lower half of the basin. The gradient of this river system is approximately 400 feet throughout the length of the river. The east branch elevation is 1,518 feet and the elevation near Vermillion is 1,119 feet. The slope profile is approximately 4.0 feet per mile.

The drainage system is supplied with water from both the east and west portion of the basin. The major tributaries are the Little Vermillion River, Turkey Ridge Creek, and Saddle Creek. There are also a number of very small tributaries contributing to its drainage pattern.

The James River Basin is the largest of the East River Basin Systems. It is bordered on the east by highlands of the Coteau de Prairie and on the west by the high ground of the Coteau de Missouri. The valley is a nearly flat stretch of land about 216 miles long and averaging 60 miles wide. It is only in the southern portion that the topography becomes steeper. There is little variance in the elevation of the basin. At Columbia, where the river basin forms in South Dakota, the elevation is 1,290 feet. At the southern terminus of the basin near Yankton, the elevation is 1,162 feet.

The James River drainage area encompasses all or part of 23 counties. It drains 12,609 square miles or over eight million acres of land in South Dakota. This represents 16.3 percent of the total land in the state. The river valley is about 400 miles long, 25 to 75 feet deep and varies in width from a few hundred feet to three miles. The slope of the valley is .493 feet per mile and the average slope of the river is .280 feet per mile.

There are seventeen contributing streams within the James River Valley. These streams drain 10,606 square miles. The majority of the basin lacks good drainage features. This is due to the slight variance in elevation and limited slope of the river. Much of its drainage is non-contributing and remains in small swales and basins.

The western most drainage system is found in the Black Hills region. The Black Hills lie within the states of Wyoming and South Dakota with the majority of the area within the western border of South Dakota. It is described as being 125 miles in length and 60 miles in width. The general shape of the Black Hills is elliptical. This formation presents a startling contrast to the surrounding topography. Its eastern side rises from the prairie to a height from 2,600 to 3,500 feet. The western part of the Black Hills varies in elevation from 3,500 to 7,200 feet at Harney Peak.

The major drainage creeks of Alkali, Battle, Bear Butte, Beaver, Box Elder, Elk, French, Rapid, Spearfish, Spring, and Whitewood are all capable of causing heavy flooding and flood related damage. These eleven creeks drain about 7,500 square miles of land.

Tornadoes mostly occur in South Dakota during the months of May, June, and July. The greatest period of tornado activity (about 82 percent of occurrence) is from 11 am to midnight. Within this time frame, most tornadoes occur between 4 pm and 6 pm. Unlike other disaster producing events, early information on tornadoes in South Dakota is limited. Possible explanations for historical warning times being limited are sparse population and the vast amount of agricultural and ranch land which reduces the chance of sightings during the pioneer and westward migration decades.

Statistical historical information was not developed until the early 1950's. It was at this time that the United States Department of Commerce began to systematically file tornado events throughout the US. This fact is exemplified by reported tornadoes before and after 1950. One source of information placed the number of tornadoes between 1915 and 1949 at an average of about three (3) per year. This amounts to 114 tornadoes for the 35 year period. National Weather Service (NWS) data places the number of tornadoes from 1950 to 1991 at 1,030 (30 related deaths). This averages to 25.12 tornadoes per year. The wide variation in reported tornadoes prior to 1950 appears not to be from an increase in such activity, but rather from better records of sightings and damages.

Winter storms are not limited to one portion of the state and historically occur from late fall to the middle of spring. They vary in intensity from mild to severe. Winter storms regularly destroy property and kill livestock and people. Such storms are generally classified into four categories with some taking the characteristics of several categories during distinct phases of the storm. These categories are: freezing rain, sleet, snow, and blizzard.

Freezing rain coats objects with ice. Although morning sunlight produces a spectacular landscape, the ice coating on sidewalks, roads, and etc. creates dangerous conditions.

Sleet does not generally cling to objects like freezing rain, but it does make the ground very slippery. This also increases the number of traffic accidents and personal injuries due to falls.

Blizzards are most commonly connected with blowing snow (resulting from storms) and low visibility. When such conditions arise, blizzard warnings or severe blizzard warnings are issued. Blizzard warnings take effect when wind conditions are at least 35 mph and temperatures of 20°F or less over an extended period of time are expected. Severe blizzard conditions exist when winds obtain speeds of at least 45 mph plus a great density of falling or blowing snow and a temperature of 10°F or lower.

The topography of South Dakota is such that no one area is immune from effects of winter storms. Prairie lands, which cover most of the state, offer little resistance to high winds and drifting snow. Even the Black Hills region, which presents some resistance to wind conditions, is not excluded from blizzard conditions. Blizzards in this region are often less severe than elsewhere in the state, but they still can produce heavy drifting snows. Early blizzards were so devastating that South Dakota had the dubious distinction of being called the Blizzard State.

South Dakota's climate is characterized by cold winters and warm to hot summers. There is usually light moisture in the winter and marginal to adequate moisture for the growing season for crops in the eastern portion of the state. Semi-arid conditions prevail in the western portion. This combination of hot summers and limited precipitation in a semi-arid geography places South Dakota in a potential position of suffering a drought in any given year. The climatic conditions are such that a small departure in the normal precipitation during the hot peak growing period of July and August could produce a partial or total crop failure. The fact South Dakota's economy is closely tied to agriculture only magnifies the potential loss which could be suffered by the state's economy during drought conditions.

Wild fires have caused major damage in South Dakota. Consequently, there is great concern for urban (or municipal) wild fires, but also for agricultural (or rural) wild fires. Fires involving grass, prairie, or timber have the potential to create mass destruction of property and vegetation -- both public and private. Grass or prairie wild fires are most commonly associated with the physical features of the state. Early writings by explorers, trappers, and settlers many times describe South Dakota as a sea of waving grass. These descriptions would not be valid today for the eastern half of the state. The more fertile and climatically desirable prairie of the eastern portion is under agricultural crop production.

It is in the western part of the state where the wild prairie still exists. South Dakota's portion of the Great Plains now exist from the foot hills of the Black Hills to the western boundary of the Missouri River. This amounts to nearly 35,000 square miles of land. Utilization of this land remains chiefly in livestock grazing with some wheat cultivation. The threat of fire is present throughout most of the year in this area. The nature of the ground cover and the limited precipitation makes this area very susceptible to fire.

The second major source of vegetation fires are forests. The largest timber development is found in the Black Hills region of western South Dakota. The Black Hills National Forest encompasses 1,524,164 acres of land in South Dakota and Wyoming. Over one million acres of the forest are exclusively in South Dakota. Of the one million acres about 80% is Federal controlled and only 20% is controlled by the state or private citizens.

In addition to the Black Hills forest, there are lesser size timber stands in Harding County, the Pine Ridge Reservation of Shannon County (unorganized), and the Rosebud Reservation of Todd County (also unorganized). These three counties are in western South Dakota.

"South Dakota is somewhat more seismically active than other areas in the northern plains states although the earthquake magnitudes have been relatively minor to date. A zone of higher earthquake frequency extends from the northeastern corner of the state and a generally higher frequency of earthquakes is recorded along the eastern flank of the Black Hills and in the southwestern corner of the state. The earthquakes occurring in South Dakota appear to be concentrated along the GLTZ [Great Lakes Tectonic Zone] and possibly along the boundaries of the structural provinces in the Precambrian, crystalline basement. At least two mechanisms may be important in generation of the earthquakes. These include initiation of movement along preexisting fractures due to crustal plate movements or movements due to glacial rebound. Ground motion accelerations can be calculated based upon historical seismic records, but the poor quality of the database does not allow great confidence to be placed in those calculations. These calculations show highs in ground motion acceleration that correspond reasonably closely with areas of greater earthquake frequency."

Historically, earthquakes cause relatively minor damage in South Dakota. Documented damages include: cattle stampedes, shaking buildings, falling or rattling dishes and pictures, stuck doors and windows, cracked window glass, foundations heaving or cracking, wall and ceiling plaster cracks, furniture moving, etc.

As the population grows near known seismologically active areas the potential for increasing amounts of "minor" damage becomes higher. South Dakota seems to be relatively geologically stable based upon the sparse data available. However, there is potential for larger earthquakes than the magnitude 4.4 (Richter Scale) quake which struck the Black Hills in 1964. One estimate places this risk as only a 10 percent chance of exceeding a 5.1 magnitude in any 100 year period.

1 W.M. Roggenthen, Earthquake Potential in South Dakota, Proc. S.D. Acad.Sci., VOL 69 (1990), Page 143.
2 Ibid, page 141.

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