Technical Terms

A

  • Above – Upstream of
  • Attenuation – Flow reduction

B

  • Bed – Lowest natural level of a channel below any silt or debris
  • Below – Downstream of
  • Berm – Shallow earth bank. Often locally dredged material
  • Bund – Shallow embankment intercepting overland flow across the flood plain. Usually imported material
  • Bypass Sluice – High capacity sluice gate upstream of a mill taking excess wet weather flow around the bypass channel. Often augmented by side overfall

C

  • Catchment – Area of land draining to a river
  • Cill – Seating level of a sluice gate or formation level of a bridge aperture
  • Cofferdam – Temporary enclosure for dry working below normal river level. Often interlocked steel sheet piling
  • Crown – Highest point of an arched bridge aperture
  • Culvert – Box section or large diameter pipe taking flow beneath higher ground such as roads or embankments

D

  • Debris – Floating trash transported by flow
  • Debris Mark – Peak level of a flood taken from telltale signs. Essential to record promptly before evidence disappears
  • Discharge – Rate of flow, usually measured in cubic metres per second – m3/s

E

  • Eddy – Swirling current of water around obstructions in a river. Tendency to erode soft material

F

  • Flood Plain – Extensive low-lying area alongside a channel providing storage and taking overland flow. Occupied after banks overtop
  • Fluvial – Of or related to rivers
  • Freeboard – Distance between a water surface and the crest of whatever contains it

G

  • Gabion – Flexible wire mesh basket filled with random size stone. Often used for revetment
  • Gauge Board – Water level measurement in metres above Ordnance Datum. Witney readings will be between 79m and 82m
  • Gradient – Rate of fall for a river bed or water surface. Usually stated as a fraction, for example 1:200

H

  • Head – Upstream side of a mill, weir or other retention point
  • Head Drop – Difference between head and tail levels at a mill, weir or other retention point
  • Headwaters – Source or upper reaches of a river
  • Hydraulic Jump – Turbulence where higher velocity streamline flow changes to steadier channel flow. Caused by kinetic energy reverting to potential energy, for example at a weir

I

  • Invert – Lowest point of a pipe’s internal diameter or bridge aperture

L

  • Lateral Migration – Sideways movement of a river channel over time
  • Left Bank – Left side of a channel looking downstream

M

  • Meander – Continuing natural bends in a river
  • Mill – Ancient way of generating power by impounding flow. The miller’s right to regulate retention and flow prevails
  • Mill Pond – Impounded storage above a mill. Relies on perched banks 
  • Mill Race – Channel for turbine sluice flow
  • Mill Stream – Channel for waste sluice flow

N

  • Nappe – Water sheet flowing over the crest of a weir. Occurs with sluice gates and overfalls

O

  • Ordnance Datum – Mean sea level at Newlyn, the standard reference for water levels and hydraulic structures in UK
  • Outfall – Point at which a culvert or pipe discharges to an open channel. Usually includes a headwall for bank protection
  • Overfall – Fixed weir with continuous crest. May incorporate a notch for better dry weather level control

P

  • Perched Bank – Berm upstream of a mill or weir. Higher retention level increases the storage capacity of a natural channel
  • Permeability – Potential for water to pass through granular material such as sand or gravel
  • Pluvial – Of or relating to rainfall

R

  • Retention – Impounded water level. Determines the amount of storage at a mill or weir as well as head drop
  • Revetment – Hard form of bank protection to resist erosion or improve stability. Can be timber, gabions, concrete bagwork or steel sheet piling
  • Right Bank – Right side of a channel looking downstream
  • Riparian – of or relating to rivers
  • Runoff – Surface water draining to a watercourse either as overland flow or through an outfall

S

  • Scour – Channel erosion from high velocity or turbulent flow. Can adversely affect the bed and/or bank
  • Shoal – Settled material no longer carried in suspension once flow drops below its self-cleansing velocity
  • Sluice Gate – Variable weir with adjustable flow and level control
  • Soffit – Highest point of a pipe’s internal diameter or a clearspan bridge aperture
  • Spillway – Overflow system in an impounding embankment once storage capacity is exceeded. Can be a notch in the crest discharging to a stepped open channel or a low level pipe with flow control orifice

T

  • Tail – Downstream side of a mill, weir or any other retention point 
  • Trash Grid –  Debris trap above a weir. Reduces risk of obstructions otherwise difficult to remove
  • Turbine Sluice – Sluice gate at a mill for controlling flow through the turbine

W

  • Waste Sluice – Small sluice gate at a mill for trimming head water level in dry weather flow
  • Watershed – Catchment boundary