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D

DABBLE—To bob forward and under in shallow water so as to feed off the bottom.

DAILY FLOOD PEAK—The maximum mean daily discharge occurring in a stream during a given flood event.

DAILY TEMPERATURE RANGE—The difference between the highest and lowest temperatures recorded on a particular day.

DAM—A structure of earth, rock, or concrete designed to form a basin and hold water back to make a pond, lake, or reservoir. A barrier built, usually across a watercourse, for impounding or diverting the flow of water. General types of dams include:

[1] Arch Dam—Curved masonry or concrete dam, convex in shape upstream, that depends on arch action for its stability; the load or water pressure is transferred by the arch to the Abutments.
[2] Buttress Dam—A dam consisting of a watertight upstream face supported at intervals on the downstream side by a series of buttresses.
[3] Cofferdam—A temporary watertight enclosure that is pumped dry to expose the bottom of a body of water so that construction, as of piers, a dam, and bridge footings, may be undertaken. A "diversion cofferdam" prevents all downstream flow by diverting the flow of a river into a pipe, channel, or tunnel.
[4] Crib Dam—A barrier or form of Gravity Dam constructed of timber forming bays, boxes, cribs, crossed timbers, gabions or cells that are filled with earth, stone or heavy material.
[5] Embankment Dam—A dam structure constructed of fill material, usually earth or rock, placed with sloping sides and usually with a length greater than its height. Types of embankment dams include: Earthfill or Earth Dam—A dam in which more than 50 percent of the total volume is formed of compacted fine-grained material obtained from a borrow area (i.e., excavation pit); Fill Dam—Any dam constructed of excavated natural materials or of industrial waste materials; Homogeneous Earthfill Dam—A dam constructed of similar earth material throughout, except for the possible inclusion of internal drains or drainage blankets; distinguished from a Zoned Earthfill Dam; Hydraulic Fill Dam—A dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing water; Rockfill Dam—A dam in which more than 50 percent of the total volume is comprised of compacted or dumped pervious natural or crushed rock; Rolled Fill Dam—A dam of earth or rock in which the material is placed in layers and compacted by using rollers or rolling equipment; and Zoned Embankment Dam—A dam which is composed of zones of selected materials having different degrees of porosity, permeability, and density.
[6] Gravity Dam—A dam constructed of concrete and/or masonry that relies on its weight for stability.
[7] Masonry Dam—A dam constructed mainly of stone, brick, or concrete blocks that may or may not be joined with mortar. A dam having only a masonry facing should not be referred to as a masonry dam.
[8] Weir—A dam in a river to stop and raise the water, for the purpose of conducting it to a mill, forming a fishpond, or the like. When uncontrolled, the weir is termed a fixed-crest weir. Other types of weirs include broad-crested, sharp-crested, drowned, and submerged.

DAMAGE-FREQUENCY CURVE—A graph showing the flood damages and their probabilities of occurrence. The total area under the curve represents the annual damage.

DAMAGES PREVENTED—The difference between the amount of damages without a particular water project and the damages with the project in place.

DAMP—Slightly wet; somewhat moist or wet.

DAP—(1) To dip lightly or quickly into water, as a bird does. (2) To skip or bounce, especially over the surface of water.

DARCY'S LAW—An empirically derived equation for the flow of fluids through porous media. It is based on the assumption that flow is laminar and inertia can be neglected, and states that velocity of flow is directly proportional to Hydraulic Gradient. For groundwater, this is equivalent to the velocity being equal to the product of the hydraulic gradient and the effective subsoil conductivity or permeability. See Specific Discharge (Specific Flux).

DATA—In its strictest sense, data may be defined only as the raw numbers (or descriptions, in the case of qualitative data), either in Time-Series format (data covering observations over specific periods of time), Cross-Sectional format (data consisting of a number of observations taken at a specific point in time or about a specific event or phenomenon), or a combination of these two. Also see Information.

DATA BANK—A well-defined collection of data, usually of the same general type, which can be accessed by a computer and may readily be used for further analysis, presentation, and forecasting. Also referred to as a Data Base.

DATA, CROSS-SECTIONAL—(Statistics) Data which describe the activities or behavior of individual persons, firms, or other units at a given point in time.

DATA MANAGEMENT—The act, process, or means by which data is managed. This may include the compilation, storage, safe-guarding, listing, organization, extraction, retrieval, manipulation, and dissemination of data. In its strictest sense, data may be defined only as the raw numbers for numeric or quantitative data (or descriptions, in the case of qualitative data), either in time-series format (data covering observations over specific periods of time), cross-sectional format (data consisting of a number of observations taken at a specific point in time or about a specific event or phenomenon without regard to its behavior over time), or a combination of these two. Information, on the other hand, deals more specifically with the manipulation, re-organization, analysis, graphing, charting, and presentation of data for specific management and decision-making purposes. Also see Information Management.

DATA, PRIMARY—Typically, data acquired by direct interaction, such as direct observation through measurements, tabulation, or surveys. Contrast with Secondary Data.

DATA, SECONDARY—Typically, data acquired from published sources as opposed to data acquired from direct observation or measurement such as a survey. Contrast with Primary Data.

DATA, TIME-SERIES—(Statistics) Data which describe the movement of a variable over time, e.g., monthly, quarterly, annually.

DATUM—Any numerical or geometric quantity or set of such quantities that may serve as a reference or base for other, comparable quantities. For example, Mean Sea Level (MSL) is the datum used on most topographic maps. However, most river gages use an arbitrary elevation above the National Geodetic Vertical Datum (NGVD) of 1929 for use as a zero datum (e.g., datum equals 3412.6 feet above NGVD of 1929). Datums are always chosen so there will never be negative stages.

DDT (DICHLORODIPHENYLTRICHLOROETHANE)—A colorless odorless water-insoluble crystalline insecticide C14H9Cl5 that tends to accumulate in ecosystems and has toxic effects on many vertebrates. DDT was used extensively prior to 1972 at which time the U.S. Environmental Protection Agency (EPA) banned its production and distribution. Although banned from usage for a number of years, the inert nature of such toxic chemicals and their low biodegradability (15-year half-life