Environmental Engineering Reference
In-Depth Information
stage
, the depth of water passing a flume or weir of known dimensions and water
velocity (Plate 14.4), since it is virtually impossible to collect and measure total flow
except at very small volume and time scales. Other techniques measure the dilution of
known quantities of injected salts or dyes passing a downstream point, or estimate
discharge based on channel parameters. Continuous records of discharge, provided by
automated stage-discharge recorders, are the most useful and permit the construction of a
variety of
hydrographs
.
Flood
or
storm
hydrographs measure responses to single
meteorological events (although they usually involve neither over-bank floods nor
storms!), whereas annual hydrographs chart the discharge regime over a water balance
year (Figure 14.7). It is also sometimes useful to forecast the
unit hydrograph
response
to a fixed precipitation input. Hydrographs are essential to the prediction and
management of river flow and water resources, and we return to this application in the
box on pp. 292-3.
For present purposes, hydrographs summarize outcomes of the catchment hydrological
system and processes, which generate river flow as follows. Assuming dry antecedent
conditions, initial precipitation infiltrates spare interception and soil stores. Vegetation
intercepts the first 1·0 mm and 20 per cent of subsequent rainfall. Surface water flow
occurs wherever precipitation + net inward transfer (input) exceeds evapotranspiration
Figure 14.6
Some relationships between the particle size of
Earth materials (BSI) and water retention and transmission.
Table 14.3 Base flow indices (BFI) for typical rock types in
Britain
Rock type
Principal characteristics
Typical BFI
range
Permeability
Storage
Chalk
Fissure
High storage
0·90-0·98
Oolitic limestone
Fissure
High storage
0·85-0·95
Carboniferous limestone
Fissure
Low storage
0·20-0·75
