Geoscience Reference
In-Depth Information
Table 14.3 Base flow indices (BFI) for typical rock types in Britain
Principal characteristics
Rock type
Permeability
Storage
Typical BFI range
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
Millstone grit
Fissure
Low storage
0·35-0·45
Permo-Triassic sandstone
Intergranular
High storage
0·70-0·80
Coal measures
Intergranular
Low storage
0·40-0·55
Cretaceous sands and silts
Intergranular
Low storage
0·35-0·50
Lias clays
Impermeable
Low storage at shallow depth
0·40-0·70
Old red sandstone
Impermeable
Low storage at shallow depth
0·46-0·54
Silurian/Ordovician shales/slates
Impermeable
Low storage at shallow depth
0·30-0·50
Metamorphic and igneous
Impermeable
Low storage at shallow depth
0·30-0·50
Oxford, Wealden and London clay
Impermeable
No storage
0·14-0·45
Source: After Institute of Hydrology (1980).
except at very small volume and timescales. 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 meteo-
rological events (although they usually involve neither
over-bank floods nor storms!), whereas annual hydro-
graphs chart the discharge regime over a water balance
year ( Figure 14.8 ). 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 p. 324.
For present purposes, hydrographs summarize out-
comes 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 + net
onward transfer by percolation, groundwater recharge,
seepage or abstraction (output). This occurs as transient
overland flow on slopes and intermittent , ephemeral or
permanent channel flow . Horton overland flow occurs
when precipitation intensity exceeds soil infiltration
capacity on non-vegetated surfaces. Water moves away
downslope at 10 1-2 mm s -1 , initially as sheet flow . This is
Plate 14.4 Flume for stream gauging in a steep catchment
on Plynlimon, mid-Wales. Its height, and baffles to dissipate
stream energy, reflect the 'flashy' nature of upland stream
flow.
Photo: M.A. Fullen
simulated by impervious urban surfaces when drain
capacity is exceeded briefly during rainfall. Much water is
detained in surface depression storage ,evaporates or
drains by percolation. Some reaches channels and
contributes to quickflow - augmenting direct channel
 
 
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