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Delaware: Floods and Droughts

By G.N. Paulachok and R.H. Simmons, U.S. Geological Survey; "General Climatology" section by J.R. Mather and J.A. Skindlov, Office of the State Climatologist of Delaware; "Water Management" section by L.A. Sprague and P.J. Cherry, Delaware Department of Natural Resources and Environmental Control

Precipitation is the source of all freshwater in Delaware and results from movement of moist airmasses over the region, ascent and cooling of the airmasses, formation of clouds, and condensation of water vapor. The climate of Delaware is affected chiefly by airmasses of tropical maritime and polar continental origin. Meteorological extremes are moderated by nearby large water bodies. Depending on location, average annual precipitation ranges from about 41 to 46 inches. Extensive flooding, although infrequent, generally is caused by tropical cyclones such as hurricanes or tropical storms, whereas local flooding results from intense convective rainfall. Droughts occur when the Bermuda High, the principal moisture-delivery system for the State, is displaced from an oceanic to a stable continental position.

Major floods in Delaware have been widespread and have resulted in substantial loss of property and, in some instances, human lives. Documented severe flood occurred in 1846, 1933, 1947, 1955, 1960, 1962, 1967, 1972, 1979, and 1989. Major droughts in the State persisted at least several years and materially affected water supplies and agricultural activities. Documented severe droughts occurred in 1930-34, 1953-57, 1961-71, 1979-83, and 1984-88.

Local governments in Delaware administer flood-plain management and regulation programs, with technical and financial assistance from State and Federal agencies. All 41 communities having designated flood plains have passed local ordinances that comply with the requirements of the National Flood Insurance Program and receive flood insurance through the Federal Government. Flood and flash-flood warnings, which are issued by the National Weather Service office in Wilmington, advise that flooding is imminent or in progress at a particular location or area. Hydrologic conditions are monitored by several State agencies for indications of impending drought. During drought, voluntary or mandatory water-use restrictions may be implemented.

General Climatology

The climate of Delaware, which is moderated considerably by the Atlantic Ocean, Delaware Bay, and nearby Chesapeake Bay is classified as modified continental. Climatic conditions in the State are affected chiefly by airmasses of tropical maritime or polar continental origin. In summer, the subtropical Atlantic high-pressure cell, or Bermuda High, develops over the Eastern United States. The southerly winds that commonly result from these conditions bring warm, humid, tropical maritime air into the area. Occasionally, the Bermuda High weakens and cool, dry air flows out of Canada. These conditions commonly bring temporary relief from the typical sultry summer weather. In winter, airmass changes are more frequent than in summer, and cold, dry Canadian air flows over the State. About one-third of the days from December to February are affected mainly by cold, dry, polar continental or arctic air. Another one-third of the days are affected principally by milder airmasses, such as those of polar maritime, tropical maritime, and modified Pacific origin. The remaining one-third of the days are transitional periods between airmasses and are affected commonly by frontal systems, migrating wave cyclones, and high-pressure cells moving away from the State.

Two major processes cause precipitation: (1) ascent and cooling of a parcel of warm air, or convective precipitation, and (2) meeting of airmasses having substantially different temperatures and water content, or cyclonic precipitation. The principal sources of moisture and delivery patterns are shown in figure 1. In summer, warm, moist air from the Atlantic Ocean and Gulf of Mexico is circulated into the State by the Bermuda High, and causes scattered, convective showers and thunderstorms that account for most of the precipitation. Because of their typically localized occurrence, however, storms of this type commonly affect only small geographic areas. On the average, 18 thunderstorms pass over Delaware from June through August. Much of the precipitation in spring, fall, and winter is caused by more extensive wave cyclones or frontal storms that originate over the Atlantic Ocean or Southern United States and migrate northward along the Atlantic coast. In winter, wave cyclones that originate in the Gulf of Mexico and migrate northeastward are important precipitation-producing systems. On the average, three to five wave cyclones per month pass directly over or near the State from November through April (Whittaker and Horn, 1982). Although infrequent, tropical cyclones including hurricanes occasionally cause intense rain during late summer and early fall.


Principal sources and patterns of delivery of moisture into Delaware. Size of arrow implies relative contribution of moisture from source shown

Figure 1. Principal sources and patterns of delivery of moisture into Delaware. Size of arrow implies relative contribution of moisture from source shown. (Source: Data from Douglas R. Clark and Andrea Lage, Wisconsin Geological and Natural History Survey.)

In addition to the ocean and bays, important moisture sources include local and upwind land surfaces, as well as lakes and reservoirs, from which moisture evaporates into the atmosphere. Typically, as a moisture-laden ocean airmass moves inland, it is modified through interactions with the terrestrial and vegetative surfaces to include some water that has been recycled one or more times through the land-vegetation air interface.

Average annual precipitation in Delaware ranges from about 41 inches in the central part of the State to about 46 inches along the southern Atlantic coastal margin and in topographically higher areas in the northern part of the State. Precipitation measurements at Wilmington, dating back to 1895, indicate that 1965 was the driest year of record, with a total of 24.9 inches, and that 1945 was the wettest year of record, with a total of 61.0 inches. The distribution of precipitation is fairly uniform throughout the year, although the greatest quantities generally are received during the summer from frequent thunderstorms and occasional tropical cyclones. Despite the relatively uniform distribution of long-term precipitation, short-term precipitation surpluses and deficiencies are common.

Various atmospheric conditions and processes can cause flooding on a variety of scales. Storms of low-latitude origin, including tropical depressions, tropical storms, and hurricanes, pass Delaware mainly during late summer or early fall and constitute the most serious flood threat. Such storms commonly cause flooding over broad geographic areas. From 1891 to 1986, 88 tropical cyclones, or an average of slightly less than 1 per year. passed directly over Delaware or near enough to cause substantial precipitation. Of these, 74 occurred from August to October, 33 have been observed in September alone. Regional floods also may develop in late winter and early spring, when a combination of intense rain and melting snow causes streams to overflow their banks and inundate low-lying areas. Flooding of coastal and tidal areas, particularly those localities adjoining the Delaware estuary, commonly results from runoff produced by intense rain on tributary areas, combined with high tides driven by strong easterly or southeasterly winds. More localized flooding, particularly in urbanized areas, generally is caused by intense convective rainfall such as that from summer thunderstorms.

The Bermuda High has a major effect on weather patterns in Delaware. When in its usual location over the western Atlantic Ocean, it produces thunderstorms that deliver substantial moisture to the State. However, when the Bermuda High moves to a new, stable position over the southeastern or midcontinental United States, drought commonly results, as the long, overland flow path allows the airmass to acquire considerable heat but little moisture. In its displaced position, the Bermuda High acquires moisture only in lower levels of the atmosphere and not throughout the entire air column; consequently, little precipitation results. During droughts, which can occur in any season, frontal convective storms are the principal means of moisture delivery. Because theses storms commonly are intense and of short duration, the resultant precipitation may not add materially to water supplies.

Drought can be defined as an extended period of time without sufficient precipitation. However, because of the variety of needs for water, a drought cannot be defined in terms that apply to all situations. For example, a period of a few weeks without rain may be a serious matter for agricultural activities, particularly if the temperature is high and the air is dry. In comparison, a water-supply project may operate for several months without rain, provided the supply of water in storage is adequate. Hydrologic drought can be defined as a period during which streamflow is inadequate to supply established uses under a given water-management system. (Linsley and others, 1982, p. 374). During drought, streamflow is affected by several factors in addition to the lack of precipitation. The most important of these are (1) quantity of water in storage as surface water, Groundwater, and soil moisture; (2) rates of evaporation and transpiration; and (3) rates of withdrawal.

Unlike extreme rainfall and floods, which can occur several times in any year, a hydrologic drought may require several years to develop. Droughts differ greatly in their extent, duration, and severity; these differences make quantitative analysis and comparison difficult. Furthermore, because of the lengthy time period involved in droughts and the large number of weather sequences that can lead to protracted dry spells, the worst possible drought conditions that might develop in a particular area cannot be estimated reliably.

Major Floods and Droughts

The floods discussed in this article were widespread and resulted in substantial loss of property and, in some instances, human lives. The droughts persisted at least several years and materially affected water supplies and agricultural activities. These major hydrologic events are listed chronologically in table 1: rivers and cities are shown in figure 2. To characterize floods (fig. 3) and droughts (fig 4), discharge records for representative streamflow-gaging stations were analyzed to determine recurrence intervals for the extreme events. Streamflow data are collected, stored, and reported by water year (the 12-month period from October 1 through September 30 and is identified by the calendar year in which it ends).

Table 1. Chronology of major and other memorable floods and droughts in Delaware, 1846-1989

[Recurrence interval: The average interval of time within which streamflow will be greater than a particular value for floods or less than a particular value for droughts. Symbol: >, greater than. Sources: Recurrence intervals calculated from U.S. Geological Survey data; other information from U.S. Geological Survey, National Weather Service, State and local reports, and newspapers]
Flood or
Drought
Date Area affected
(fig. 2)
Recurrence
interval
(years)
Remarks
Flood Oct. 13, 1846 Lower Delaware River. Unknown "Great Hurricane of 1846." Severe storm-surge flooding near New Castle.
Drought 1930-34 Statewide. Unknown Most extensive drought since 1894 in humid parts of United States.
Flood Aug. 23, 1933 Coastal parts of Delaware. Unknown Severe tidal flooding and widespread damage to resort areas.
Flood May 1, 1947 Christina River basin. 25 to 50 Record rainfall intensity. Until exceeded in 1989, this was the largest discharge recorded since April 1943 on Christina River at Coochs Bridge.
Drought 1953-57 Statewide. 10 to 25 Agricultural operations affected substantially.
Flood Aug. 18-19, 1955 Christina River, White Clay, Red Clay, and Brandywine Creek basins. 25 to 50 Hurricane Connie (August 12-13, 1955) and Diane (August 18-19, 1955). Many lives lost and extensive property damage.
Flood Sept. 12-13, 1960 Statewide. 10 to >50 Hurricane Donna. Largest discharge recorded since January 1958 on St. Jones River at Dover.
Flood Mar. 1962 Coastal parts of Delaware. >2 Record high tide at Lewes. Severe storm-surge flooding.
Drought 1961-71 Statewide. 10 to >25 Longest and most severe drought in Northeastern United States.
Flood Aug. 3-27, 1967 Statewide. 25 to 50 Record monthly rainfall at Bridgeville. In central Delaware, 3 lives lost and extensive property damage.
Flood June 22, 1972 Christina River, Blackbird Creek, Brandywine Creek, and Smyrna River basins. 10 to >50 Hurricane Agnes. Greatest flooding and damage in adjacent Middle Atlantic States.
Flood Feb. 25-26, 1979 Southern Delaware. 25 to >50 Intense rain on about 20 inches of snow cover. Lives lost, 1.
Drought 1979-83 Statewide. 10 to 25 Decreased crop yields. Water rationing in effect.
Drought 1984-88 Statewide. 10 to 25 Decreased crop yields. Temporary restrictions on nonessential water use in northern Delaware.
Flood July 5, 1989 Christina River, White Clay, Red Clay, and Shellpot Creek basins. >100 Tropical Storm Allison. Lives lost, 3. Property damage, $5 million.

Figure 2: Selected geographic features, Delaware.
Click to view larger image.

Figure 2. Selected geographic features, Delaware.

The comprehensive Federal-State streamflow-gaging program in Delaware was instituted in the early 1940's, consequently, accurate determination of recurrence intervals for hydrologic events before that time is not possible. However, several major floods before 1940 have been documented and warrant mention here.

Floods

On October 13, 1846, flooding was caused by a storm termed "The Great Hurricane of 1846." The center of the storm passed west of all principal ports of the Southern Atlantic States, then moved into tidewater areas of the Chesapeake Bay. Low-lying areas along the Delaware River near New Castle were inundated in the greatest storm surge in 70 years. The floodwater rose high enough to extinguish the fire in the boiler of a locomotive that had been stalled by the rising water (Ludlum, 1963, p. 94)

The hurricane of August 23, 1933, produced severe flooding and caused widespread damage to resorts on the Delmarva Peninsula. Damage from this storm in Delaware and Maryland was estimated at $17.5 million (National Oceanic and Atmospheric Administration, 1987). According to Truitt (1967):

Farm crops were laid waste and boardwalk, cottages, and many other buildings, together with a new marina and machine shop at Ocean City [Maryland] were carried away. While this destruction was vast, the accompanying winds were barely, if at all, of hurricane strength. Rather, over a long haul, or fetch, the winds built up waves and tides that were highly devastating. The Ocean City inlet was gained by the outflow of pent-up high water in the Assawoman and Isle of Wight Bays, and later made permanent.

At Bethany Beach, the boardwalk and streets were damaged severely by the storm, and drainage systems were filled with sand. Rehoboth Beach suffered substantial property damage and beach erosion. At Lewes, high water driven by the hurricane reached 6.4 feet above sea level (U.S. Army Corps of Engineers, 1963).

Streamflow data for 44 continuous or partial-record gaging stations in Delaware and on the Eastern Shore of Maryland were analyzed to determine the extent and frequency of the more recent floods illustrated in figure 3. Data for six selected gaging stations show the spatial and temporal variations in peak discharge (fig.3).

Areal extent of major floods with a recurrence interval of 25 years or more in Delaware, and annual peak discharge for selected sites, water years 1932-89.
Click to view the entire image.

Figure 3: Areal extent of major floods with a recurrence interval of 25 years or more in Delaware, and annual peak discharge for selected sites, water years 1932-89. (Source: Data from U.S. Geological Survey files.)

On May 1, 1947, locally intense rain fell in northwestern Delaware and adjoining parts of Pennsylvania and Maryland. This rainfall, 4.2 inches in 24 hours, was at that time the greatest 24-hour quantity measured in Delaware and resulted in the most severe flooding on the Christiana River since that reported on July 5, 1937. The peak flow of the 1947 flood, measured on the Christina River at Coochs Bridge (fig. 3, site 1), was more that 4,000 ft³/s (cubic feet per second), or about 210 (ft³/s)/mi² (cubic feet per second per square mile) of drainage area. Until exceeded in 1989, this was the largest discharge recorded since the gaging station was established in April 1943. Recurrence intervals of the 1947 flood peak discharge on streams in the Christina River basin range from about 25 to 50 years.

The disastrous floods of August 18-19, 1955, were caused by Hurricanes Connie and Diane, which passed in short succession. Although Hurricane Connie produced intense rain, the resulting runoff generally was inconsequential because a drought of several years duration preceded the hurricane. The rainfall associated with Hurricane Diane, which passed over Delaware about 5 days later, was slightly more intense but resulted in record-breaking floods because the soil was still saturated from the previous storm. These hurricanes produced major floods in an area about 200 miles wide and parallel to the Atlantic Coast from North Carolina to Massachusetts and caused considerable loss of life and extensive property damage. The distribution of rainfall was such that the largest floods developed on small streams (Bogart, 1960, p.1)

In Delaware, the floods of August 18-19, 1955, were confined chiefly to the northern part of the State. The principal streams affected were Brandywine Creek, the lower reaches of the Christina River, and smaller tributaries of the Delaware River. Peak flow of Brandywine Creek at Wilmington was 17,800 ft³/s or 57 (ft³/s)/mi². Recurrence intervals of the 1955 flood peaks for streams in the affected localities range from 25 to 50 years.

On September 12-13, 1960, flooding was produced as Hurricane Donna moved northeastward over the coast of the Maryland peninsula, Delaware, and New Jersey. Winds of hurricane force extended as far as 30 miles inland from the Delaware coast, and rainfall in that area ranged from 4 to 6 inches. As far as 100 miles inland, rainfall ranged from 3 to 4 inches, producing peak discharges that, on some streams, exceeded the 50-year recurrence interval. At Wilmington, the rainfall total of 5.6 inches was the greatest recorded 24-hour quantity since 6.2 inches fell in July 1952 (Rostvedt, 1965, p. 122). Flooding was severe in the northern part of the State on White Clay Creek near Newark (fig. 3, site 2) and on Red Clay Creek at Wooddale. Peak discharges at these locations were the largest recorded since the gaging stations were established. Major floods also developed in central Delaware on Blackbird Creek at Blackbird (fig. 3, site 3) and St. Jones River at Dover (fig. 3, site 4). Peak flow of the St. Jones River at Dover was 1,900 ft³/s or about 60 (ft³/s)/mi², which is the largest discharge recorded since the gaging station was established in January 1958.

Occasionally, coastal and low-lying tidal areas are inundated by high water driven by strong winds from the east and southeast. In March 1962, a combination of high spring tides and winds having velocities as great as 72 miles per hour caused extensive flooding and damage to all coastal communities in Delaware. The storm resulted in the loss of 21 lives in Delaware and New Jersey and damage of $23 million in Delaware. (U.S. Army Corps of Engineers, 1963, apps. 4-10). On March 6, the high tide at Lewes was 7.9 feet above sea level, the maximum height recorded at that location. Farther up the Delaware estuary at Reedy Point, however, the maximum storm surge of 3.5 feet above sea level was considerably lower than that along other parts of the Atlantic, Coast (U.S. Army Corps of Engineers, 1963, p. 3-7)

Damage to waterfront in Rehoboth Beach, Del., caused by coastal storm
of March 1962 (Photograph courtesy of Delaware Department of 
Transportation.)

Damage to waterfront in Rehoboth Beach, Del., caused by coastal storm of March 1962
(Photograph courtesy of Delaware Department of Transportation.)


Intense rain associated with a succession of turbulent and persistent thunderstorms caused considerable flooding during August 3-27, 1967, throughout Delaware. At Bridgeville, the rainfall total of 17.7 inches in August 1967 set a new monthly record for the State. Rainfall intensities as great as 2.5 inches per hour and totals as large as 9 inches in 6 hours resulted from the storm of August 3-5, the most severe of the thunderstorms. In central Delaware, 3 lives were lost and 36 bridges and culverts were destroyed or damaged badly be the August 3-5 storm. Damage from this storm to highways and public property was estimated at $200,000 (Carpenter and Simmons, 1969, p.23). Subsequent storms on August 9-10, 24-25, and 26-27 were most intense in northern Delaware. The August 9-10 storm resulted in severe but localized flooding in the Little Mill Creek basin in suburban Wilmington; total damage in the basin from the floods of August 1967 exceeded $900,000 (Carpenter and Simmons, 1969, p. 25). Recurrence intervals of the 1967 flood peak discharges on streams statewide range from 25 to 50 years.

On June 22, 1972, major flooding was caused by torrential rains associated with Hurricane Agnes, which followed unusually wet weather in May in the Middle Atlantic States. The National Weather Service office in Wilmington recorded about 4.4 inches of rainfall in 24 hours as the storm passed through the area. Because Hurricane Agnes did not track directly over Delaware, flooding was less severe than elsewhere in the region. However, record-breaking floods developed on several streams in the northern part of the State, particularly those in the Christina River basin (Baily and others, 1975, p. 58). The peak discharge of White Clay Creek above Newark was more than twice as large as the previous maximum during 1953-59 and 1963-72. Peak flow of the White Clay Creek near Newark (fig. 3, site 2) exceeded 9,000 ft³/s, or slightly more than 100 (ft³/s)/mi² and established a new maximum discharge record for the gaging station. Peak discharge of Brandywine Creek at Wilmington was the largest since the gaging station was established in 1946. Peak flow of Blackbird Creek at Blackbird (fig. 3, site 3), which was 712 ft³/s or 185 (ft³/s)/mi², was the largest discharge recorded since the gaging station was established in October 1956 (annual maximum flow and base flow were measured from 1952 to 1956). In northern Delaware, runoff from Hurricane Agnes resulted in flood peaks having recurrence intervals of more than 50 years.

On February 25-26, 1979, extensive flooding in southern Delaware resulted from intense rain falling on about 20 inches of compacted snow cover. Rainfall from the storm of February 24-26 totaled about 4-6 inches at Milford and increased the monthly total to 7.4 inches. Because the soil was saturated, snowmelt runoff from the storm poured onto roads and highways and caused many of them to become impassable. One local resident, whose automobile stalled on an inundated county road near Frederica, drowned while attempting to reach safety. The floodwater damaged several bridges and caused the collapse of the bridge on State Highway 24 in Millsboro. Peak flows of 217 fti³/s or about 41 (ft³/s)/mi² in Stockley Branch at Stockley (fig. 3, site 5) and 3,020 ft³/s or 40 (ft³/s)/mi² in the Nanticoke river near Bridgeville (fig. 3, site 6) were the greatest recorded since 1943. Recurrence intervals of the 1979 flood peaks in the southern part of the State ranged from 25 to more than 50 years.

On July 5, 1989, intense rainfall from the remnants of Tropical Storm Allison, falling on nearly saturated soils, caused record-breaking floods in northern Delaware. The flooding claimed three lives and caused damage estimated at $5 million. Peak discharges at three gaging stations exceeded previously recorded maximums. Peak streamflow at Shellpot Creek at Wilmington was 8,040 ft³/s, or about 1,100 (ft³/s)/mi², Christina River at Coochs Bridge (fig. 3, site 1) peaked at 5,530 ft³/s, or 270 (ft³/s)/mi². White Clay Creek near Newark (fig. 3, site 2) peaked at 11,600 ft³/s, or 130 (ft³/s)/mi². Recurrence intervals for the 1989 peak flows at these gaging stations were greater than 100 years.

Droughts

In Delaware, water shortages resulting from droughts commonly are most severe in summer or early fall when streamflow and Groundwater levels are lowest and demand for water is greatest. However, droughts that become apparent during the growing season may have developed from an antecedent precipitation deficiency; these are delayed or hidden droughts (Parker and others, 1964, p. 19).

Because relatively few data on historical streamflow in Delaware are available, accurate estimation of recurrence intervals for hydrologic droughts before the early 1940's is not possible. However, by using precipitation data and limited streamflow information. Hoyt (1936, p. 66) characterized drought conditions in 1930 as the most extensive since 1894 in humid parts of the United States. Drought conditions persisted through 1934, and effects were manifested chiefly as low streamflow and decreased crop yields. Economic losses probably were greater than those sustained during any previous drought, as use and demand for water had increased considerably by the early 1930's. By the end of summer 1930, several government and private agencies had implemented relief measures in many drought-stricken communities.

Streamflow data for 26 gaging stations in Delaware and on the Eastern Shore of Maryland were analyzed to determine the extent and frequency of recent major droughts. Data on the annual departure of streamflow from the long-term average flow at six selected gaging stations illustrate the spatial and temporal variations in streamflow deficiency and surplus (fig. 4). Droughts are represented on the graphs as extended periods of less than average streamflow; the length of the bar below the line of zero departure is proportional to the annual streamflow deficiency. The graphs show four severe droughts of considerable extent and duration: 1953-57, 1961-71, 1979-83, and 1984-88.

Areal extent of major droughts with a recurrence interval of 10 years or more in Delaware, and departure from average stream discharge for selected sites, water years 1932-89.
Click to view entire image.

Figure 4: Areal extent of major droughts with a recurrence interval of 10 years or more in Delaware, and departure from average stream discharge for selected sites, water years 1932-89. (Source: Data from U.S. Geological Survey files.)

The drought of 1953-57 had a substantial effect on water supplies in Delaware. Drought conditions developed statewide in early summer 1953 and were alleviated in the southern part of the State by fall 1956. However, drought conditions persisted in northern Delaware until fall 1957. The drought materially affected agricultural activities, as soil-moisture deficiencies resulted in decreased crop yields. Except for the temporary relief provided by Hurricanes Connie and Diane, flow in streams statewide was less than average during the period. In 1954, average annual streamflow of White Clay Creek near Newark (fig. 4, site 1) was about 38 ft³/s less than the long-term average flow. Streamflow increased by 1957 but was still about 24 ft³/s less than the long-term average. Recurrence intervals of the 1953-57 drought range from 10 to 25 years.

From 1961 through 1971, the longest and most severe-drought in the history of the region profoundly affected water supplies and agricultural activities in the northeastern United States. Although most water-supply facilities kept pace with demand, many communities utilized emergency supplies and accelerated construction of supplementary facilities. In 1965, however, more than 100 public water supplies in the Northeast were critically short of water or were faced with serious, impending water-supply problems. The President declared a limited national emergency in parts of Delaware, New Jersey, Pennsylvania, and New York, and directed all Federal agencies to assist communities with critical water-supply problems in the drought-stricken area. The U.S. Geological Survey assisted by locating, identifying, and estimating the capacity of sources of emergency supply for the affected communities. Although the accessibility and desirability of emergency supplies differed widely, suitable supplies were located in all instances (Barksdale and others, 1966).

Drought developed statewide in summer 1961 and persisted in southern Delaware until summer 1967, in the northern part of the State until spring 1970, and in central Delaware until summer 1971. In 1966, average annual streamflow at sites 1 and 3-6 (fig. 4) reached record minimum levels. Except for temporary increases caused by the August 1967 floods, streamflows throughout the State remained below average during the period. Minimum daily flows of record were observed in many streams, and, in several instances, streamflow ceased entirely. Recurrence intervals of the 1961-71 droughts ranged from 10 to more than 25 years.

During the drought of 1979-83, less than average streamflow materially affected water supplies and agricultural activities state-wide. Water-rationing programs were implemented at times to conserve dwindling supplies, and crop yields decreased substantially. Drought conditions developed in northern Delaware in winter 1979, progressed to central and southern parts of the State by summer 1980, and persisted statewide until early 1983. In 1981, average annual streamflow of Red Clay Creek at Wooddale (fig. 4, site 2) was about 31 ft³/s less than the long-term average flow. This is the largest deficiency in steamflow observed since 1944, when recordkeeping began at the site. Recurrence intervals of the 1979-83 droughts ranged from 10 to 25 years.

Like the 1979-83 drought, the drought of 1984-88 had a substantial effect on water supplies and agricultural activities throughout Delaware. In 1988, temporary restrictions on nonessential water uses such as vehicle washing and lawn watering were implemented to conserve diminishing supplies in northern Delaware. Agricultural yields decreased considerably, as many crops withered when soil moisture declined below the wilting point. Streamflow was much less than average in 1985-86 in northern Delaware (fig. 4, sites 1 and 2), and in 1985 and 1988 in the central (fig. 4, site 3) and southern (fig. 4, sites 4-6) parts of the State. Recurrence intervals of the 1984-88 drought ranged from 10 to 25 years statewide.

Water Management

Floods and droughts are of considerable economic importance because of their pronounced and commonly detrimental effects on human health and safety, water supplies, and agricultural operations. Agencies at various levels of government are responsible for developing and implementing regulations and management practices that minimize the effect of these extreme hydrologic events on the community. These official activities include flood-plain management, flood-warning systems, and water-use management during droughts.

Flood-Plain Management

Local governments in Delaware have primary responsibility for flood-plain management and regulation. These governments, in consultation with the State coordinating agency, develop and implement appropriate flood-plain management measures. Such measures, which are preventive in nature and designed to decrease future flood damage, generally include planning, subdivision and building requirements, zoning, and special-purpose flood-plain ordinances. Local communities identify their flood-prone areas chiefly by interpreting Flood Hazard Boundary Maps prepared by the Federal Emergency Management Agency. These maps are used in the emergency program of the National Flood Insurance Program for flood-plain management and insurance purposes. When a community joins the Program, it is obligated to require special permits for all construction and other development in flood-hazard areas and to ensure that the practices and materials used for construction are appropriate for minimizing flood damage. In return, flood insurance is made available through the Federal Government.

Currently, the State is not authorized by statute to regulate flood-plain areas directly and does not require local governments to adopt and administer such regulations. However, all 41 communities having designated flood plains have passed local ordinances that comply with the regular program requirements of the National Flood Insurance Program.

At the State level, the Delaware Department of Natural Resources and Environmental Control, Division of Soil and Water Conservation, is actively involved in flood-plain management. The Division, a nonregulatory agency, coordinates the Federal Emergency Management Agency's State-assistance program and provides technical assistance to local governments. Specific responsibilities of the Division include (1) assisting communities in developing and implementing flood-plain management measures and in revising local ordinances to comply with updated National Flood Insurance Program regulations, and (2) providing information and training on flood-plain management practices to local governments. Other related responsibilities of the Division include matters of dam safety, erosion and sediment control, drainage, and beach preservation.

Flood-Warning Systems

The National Weather Service office in Wilmington is responsible for issuing flood and flash-flood warnings for Delaware. These warnings advise that flooding is imminent or in progress at a particular location or area. On large streams, where the water-level rise is gradual, flood warnings can be issued from hours to days in advance of the flood peak. On small streams, particularly those in urban areas, water levels can rise quickly during periods of intense rain, and flash floods may develop before the rain stops. Commonly, there is little time between the detection of flood-producing conditions and the arrival of flood peaks on small streams. The National Weather Service issues flash-flood warnings when (1) flash flooding is reported, (2) flash-flood producing precipitation is reported or detected by radar, or (3) reliable reports of dam overtopping or failure, or other causative factors, make imminent the possibility of flash flooding. Flood warnings are disseminated to the general public by National Oceanic and Atmospheric Administration weather radio stations, commercial radio and television stations, and local emergency-assistance agencies. The warning message identifies the particular stream, the origin and timing of flooding, the anticipated extent of flooding, and the expected maximum water level at specific forecast points.

Water-Use Management During Droughts

Hydrologic conditions, including precipitation, streamflow, and Groundwater levels, are monitored routinely for indications of impending drought by the Delaware Geological Survey and the Department of Natural Resources and Environmental Control. When required by severe hydrologic conditions, the Governor's Drought Advisory Committee convenes to determine the appropriate water-supply management actions to be implemented during the drought. The Department, Division of Water Resources, in cooperation with local governments and law-enforcement agencies, is responsible for coordinating drought-related response actions. Actions include voluntary and, when necessary, mandatory water-use restrictions. The Department's Water Allocation Permitting Program requires drought planning, including development of water-supply contingency plans, by all suppliers producing more than 50,000 gallons of water per day. The Delaware River Basin Commission, a Federal interstate agency responsible for basinwide water-resources planning and management, also requires similar planning efforts by suppliers. In the event of drought or other conditions that may cause water shortages, the Commission may declare a water-supply emergency in all or part of the Delaware River basin, thereby activating special regulatory programs that temporarily supersede State and regular water allocation programs in the basin.

Selected References

Bailey, J.F., Patterson, J.L. and Paulhus, J.L.H., 1975
Hurricane Agnes rainfall and floods, June-July 1972: U.S. Geological Survey Professional Paper 924, 403 p.
Barksdale, H.C., O'Bryan, Deric, and Schneider, W.J., 1966
Effects of drought on water resources in the Northeast: U.S. Geological Survey Hydrologic Investigations Atlas HA-243.
Bogart, D.B., 1960
Floods of August-October 1955, New England to North Carolina: U.S. Geological Survey Water-Supply Paper 1420, 854 p.
Carpenter, D.H., and Simmons, R.H., 1969
Floods of August 1967 in Maryland and Delaware: U.S. Geological Survey Open-File Report, 98 p.
Holmes, S.L., 1987
Monthly streamflow and Groundwater conditions in the United States and southern Canada, water years 1945-85: U.S. Geological Survey Water-Supply Paper 2314, 250 p.
Hoyt, J.C., 1936
Droughts of 1930-34: U.,S. Geological Survey Water-Supply Paper 680, 106p.
Linsley, R.K., Jr., Kohler, M.A., and Paulhus, J.L.H., 1982
Hydrology for engineers (3rd ed.): New York, McGraw-Hill, 508 p.
Ludlum, D.M., 1963
Early American hurricanes, 1492-1870: Boston, Mass., American Meteorological Society, 198 p.
Maher, J.R., 1969
Factors of the climatic water balance over the Delmarva Peninsula: Elmer, N.J., C.W. Thornthwaite Associates, Laboratory of Climatology, 129 p.
National Oceanic and Atmospheric Administration, 1970-88
Climatological data annual summaries, Maryland and Delaware: Asheville, N.C., National Climatic Data Center (variously paginated).
___1987
National Weather Service hurricane information: National Weather Service Fact Sheet, 2 p.
Parker, G.G., Hely, A.G., Keighton, W.B., and others, 1964
Water resources of the Delaware River basin: U.S. Geological Survey Professional Paper 381, 200 p.
Rostvedt, J.O., 1965
Summary of floods in the Untied States during 1960: U.S. Geological Survey Water-Supply paper 1790-B, 147p.
Simmons, R.H., and Carpenter, D.H., 1978
Technique for estimating magnitude and frequency of floods in Delaware: U.S. Geological Survey Water-Resources Investigations Open-File Report 78-93, 69 p.
Truitt, R.V., 1967
High winds . . . high tides: College Park, University of Maryland, Natural Resources Institute, 35 p.
U.S. Army Corps of Engineers, 1963
Report on operation Five-High, March 1962 storm: New York. Civil Works Branch, Construction-Operations Division, North Atlantic Division, 8 chaps.
U.S. Geological Survey, 1986
National water summary 1985 - Hydrologic events and surface-water resources: U.S. Geological Survey Water-Supply Paper 2300, 506 p.
____ 1990
National water summary 1987 - Hydrologic events and water supply and use: U.S. Geological Survey Water-Supply Paper 2350, 553 p.
U.S. Weather Bureau, 1947-69
Climatological data, annual summaries 1947-69, Maryland and Delaware: Department of Commerce (variously paginated)
Whittaker, L.M., and Horn, L.H., 1982
Atlas of northern hemisphere extra-tropical cyclone activity, 1958-1977: Madison University of Wisconsin, Department of Meteorology, 65 p.

Prepared by G.N. Paulachok and R.H. Simmons, U.S. Geological Survey; "General Climatology" section by J.R. Mather and J.A. Skindlov, Office of the State Climatologist of Delaware; "Water Management" section by L.A. Sprague and P.J. Cherry, Delaware Department of Natural Resources and Environmental Control

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