Key Features

Slope-Area Streamflow Method

The Slope-Area method indirectly estimates stream flow and can be applied when the use of current meters or floats are not practical. The method is based on Mannings Equation

(Equation 1)

Q= 1/n(AR2/3S1/2)

which involves estimating the hydrological parameters of:

  1. The cross sectional area of the stream (A)
  2. Mannings n which is an index of the roughness of the stream bed (n), and increases with turbulence and flow retardance,
  3. the hydraulic radius (R) which is the ratio of the cross section area (A) of the stream to its wetted perimeter (which is the cross-sectional distance along the stream bed and banks that is in contact with the water)
  4. the energy slope (S) which is the change in elevation of the stream over a specified distance. This is assumed to be parallel to the top of the water surface or the base of the stream bed.

(Equation 2)

A similar approach is used in Chezy's Equation

Q=AC(RS)1/2

where a channel roughness index (C) is also used.

The procedure is to:

  1. Choose a stream reach that is relatively straight with uniform flows and where the slope of the water surface and the stream bed are relatively parallel. The stream reach should be greater than six times the mean channel width.
  2. Set up a measuring tape across the stream and determine the stream profile by taking a series of depth measurements along intervals along the tape. These depth-distance measurements are used to calculate the cross-sectional area and the wetted perimeter. The method can be applied to particular water levels of interest such as bankfull depth or recent flood heights rather than the existing water level at the time of measurement. These levels can be flagged or pegged along the reach and used in the analysis.
  3. Using a staff and a surveyors level, measure the stream bed elevations and water surface elevations for points about 20m upstream and downstream of the transect. The energy slope (S) can be calculated as the ratio of the difference in water surface elevations to the distance between these points.
  4. Estimate the roughness index (n) for the water level considered for the reach. Values for different settings have been suggested (Table 1). Also, by walking along the stream reach and observing and categorising the major factors that define channel roughness, these can be combined to estimate Mannings n (Table 2).
Table 1: Mannings n values for small natural streams (Gordon et al, 2004; after Chow, 1959)
Channel Description Minimum Normal Maximum
Lowland Streams
(a) Clean, straight, no deep pools 0.025 0.030 0.033
(b) Same as (a) but more stones and weeds 0.030 0.035 0.040
(c) Clean, winding, some pools and shoals 0.033 0.040 0.045
(d) Same as (c) but some weeds and stones 0.035 0.045 0.050
(e) Same as (c) at lower stages with less effective slopes and sections 0.040 0.048 0.055
(f) Same as (d) but more stones 0.045 0.050 0.060
(g) Sluggish reaches, weedy deep pools 0.050 0.070 0.080
(h) Very weedy reaches, deep polls or floodways with heavy stand of timber and underbrush 0.075 0.100 0.150
Mountain Streams (no vegetation in channel, banks steep, trees and brush on banks submerged at high stages
(a) Streambed consists of gravel, cobbles and few boulders 0.030 0.040 0.050
(b) Bed is cobbles with large boulders 0.040 0.050 0.070
Table 2: Calculation of Mannings n from Field Observations (Hauer & Lamberti, 1996) where n = (n0 + n1 + n2 + n3 + n4)m
Channel Condition Value
Additive Factors
Material Involved n0
Earth 0.020
Rock Cut 0.025
Fine Gravel 0.024
Coarse Gravel 0.028
Cobble 0.030-0.050
Boulder 0.040-0.070
Degree of Surface Irregularity n1
Smooth 0.000
Minor (slightly eroded or scoured) 0.005
Moderate (slumping) 0.010
Severe (badly slumped, eroded banks) 0.020
Variation in channel cross section n2
Gradual 0.000
Alternating occasionally 0.005
Alternating frequently 0.010-0.015
Effect of Obstructions (debris deposits, roots, boulders) n3
Negligible (few scattered) 0.000
Minor (<15% of area) 0.010-0.015
Appreciable (15-50% of area) 0.020-0.030
Severe (>50% of area, turbulent) 0.040-0.060
Vegetation n4
None or no effect 0.000
Low (grass/weeds) 0.005-0.010
Medium (brush, none in stream bed) 0.010-0.025
High (young trees) 0.025-0.050
Very High (brush in streams, full grown trees) 0.050-0.100
Multiplicative Factors m
Degree of Meandering
Minor (sinuosity 1.0-1.2) 1.000
Appreciable (sinuosity 1.2-1.5) 1.150
Severe (sinuosity > 1.5) 1.300

References