Phreatic surface or Water table or Groundwater level - The free surface of seeping ground water at atmospheric pressure.

Unit hydrograph - A hydrograph with a volume of one inch of runoff resulting from a storm of a specified duration and areal distribution. Hydrographs from other storms of the same duration and distribution are assumed to have the same time base but with ordinates of flow in proportion to the runoff volumes.

Probable maximum flood (PMF) - The most severe flood that is considered reasonably possible at a site as a result of meteorologic and hydrologic conditions.

[In american usage, distinction is made between the “ Standard Project Flood ” (SPF) and the “ Probable Maximum Flood ” (PMF) Both are flood discharges that may be expected from the most severe combination of critical meteorological and hydrological conditions that are reasonably possible in a region The difference is that extremely rare combinations of meteorological and hydrological conditions are excluded for SPF The “ Design flood ” may be either greater or less than the SPF]

Design flood - The flood used in the design of a dam for sizing the spillway and for determining the flood storage or the flood surcharge.

Flood routing - Calculation of the attenuating effect on a flood passing through a valley, a channel or reservoir by reason of a feature acting as a control, e.g. a reservoir with a spillway capacity less than the flood inflow, or the widening or narrowing of a valley. A flooded village (by a flood, i.e. temporarily).

Bed load or Bed load sediment (Am.) - Material moving on or near the stream bed by rolling, sliding, and sometimes making brief excursions into the flow a few diameters above the bed.

Density current - A highly turbid and relatively dense current which usually moves along the bottom or a body of water.

Aggradation - The geologic process by which streambeds, flood plains and the bottom of other water bodies are raised in elevation by the deposition of material eroded and transported from other areas. It is the opposite of “degradation”.

Degradation - The geologic process by which streambeds, flood plains and the bottom of other water bodies are lowered in elevation by the removal of material from the boundary. It is the opposite of “aggradation”.