The angle of friction is the angle at which a material begins to slide down an inclined plane. While often confused with the angle of repose, the angle of friction specifically relates to the interface between the material and the surface. In soil mechanics, this is a fundamental parameter used to determine the stability of slopes and the design of retaining structures.

Surface runoff refers to the flow of water occurring on the ground surface when excess rainwater cannot infiltrate the soil. While 'surface runoff' is the general term, 'true surface runoff' is sometimes used in hydrological literature to distinguish it from interflow or subsurface flow, specifically referring to the immediate overland flow component.

A deltaic river is one that deposits sediment at its mouth as the water velocity decreases upon entering a larger body of water like the sea. This deposition causes the river to branch into smaller channels, forming a characteristic triangular or fan-shaped landform known as a delta.

Meandering is the process by which a river forms winding curves or loops. As water flows around a bend, the velocity is higher on the outer (concave) bank, leading to erosion or scouring, while the velocity is lower on the inner (convex) bank, leading to the deposition of sediment or silting.

In hydraulic engineering, groynes are structures built into a river to control flow and prevent erosion. A groyne pointing upstream (towards the flow) is known as a 'repelling' groyne because it deflects the current away from the bank, thereby protecting the shoreline from scouring and erosion.

Bund construction, such as contour bunding, is a common soil and water conservation technique used on agricultural lands. It is generally recommended for areas with moderate slopes ranging from 2% to 10% to effectively reduce runoff velocity and prevent soil erosion.

Stream frequency is defined as the total number of stream segments of all orders per unit area. Since area has dimensions of length squared (L^2), the frequency, being a count per unit area, is expressed as 1/L^2 or L^-2.

In hydraulic engineering, groynes are structures built perpendicular to the river bank. An attracting groyne is oriented downstream, which directs the flow of water towards the bank it is attached to, thereby protecting the bank from erosion by guiding the current along the structure.

The Rational Formula, Q = CiA/360, is termed 'rational' because the runoff coefficient (C) is a dimensionless ratio representing the fraction of rainfall that becomes runoff. While the formula itself is not dimensionally homogeneous in its standard empirical form, the logic relies on the dimensionless nature of the coefficient to relate rainfall intensity and drainage area to peak discharge.

A marginal embankment is a structure built along the banks of a river, set back at a specific distance from the main channel. Its primary purpose is to contain floodwaters within the designated floodway during high-flow events, thereby protecting adjacent agricultural lands and infrastructure from inundation and erosion caused by river overflow.