3. Table 3 lists a range of channel characteristics for which the same cross-sectional area and hydraulic radius were observed. Complete the Table by calculating the flow velocity and discharge using the Manning Equation. (6 marks)

TABLE 3: Observations on three hypothetical stream cross-sections

A	R	S	Channel Description	V	Q
110	0.9	0.08	Clean, straight, no rapids or pools
110	0.9	0.12	Clean, straight, no rapids or pools
110	0.9	0.12	Sluggish reaches, weedy, deep pools

 

 

4a. Referring to question 3, explain why the discharge (Q) in the second stream is higher than the discharge in the first stream. (1 mark)

4b. Why is the discharge in the third stream lower than that of the second stream? (1 mark)

Height of the hydrograph peak produced by a storm will vary with the storm character and the properties of the drainage basin that contributes runoff to the stream. The Rational Method is often used to estimate peak discharge for small (i.e. less than 0.5 km²) drainage basins as follows:

Rational Method Formula:           Q_pk     = (0.278)(C)(I)(A)

where:                                         Q_pk     = peak discharge (m³/s)

                                                    C        = rational runoff coefficient

                                                    I          = rainstorm intensity (mm/hr)

                                                    A        = basin area (km²)

Runoff coefficient (C) gives the proportion of precipitation that will flow over the land surface to become stream flow. The rest of the precipitation is retained by the basin in ponds and as soil moisture, or percolates deep beneath the surface to become groundwater. The coefficient (C) varies with the soil type and surface cover of the basin (Table 4). In urban areas, C increases as streets, sidewalks, and buildings cover more of the land surface. These kinds of surfaces have a low infiltration capacity, or ability to absorb water. In non-urbanized areas, the amount of precipitation that becomes streamflow varies with the infiltration capacity of the soil and the land use practice.

The Rational Method suggests that peak discharge will increase as storm intensity and basin area increase, and as the infiltration capacity of the basin surface declines.

 

TABLE 4: Values of the Rational runoff coefficient

Descriptio			C
		URBAN AREAS
Parks and cemeteries			0.1 - 0.3
Residential areas	Single family dwellings		0.3 - 0.5
	Apartments		0.5 - 0.7
Industrial areas	Light		0.5 - 0.8
	Heavy		0.6 - 0.9
Business areas	City core		0.8 - 1.0
	Neighbourhood		0.5 - 0.7
		RURAL AREAS
Sandy soils	Woodland		0.1
	Pasture		0.15
	Cultivated		0.2
Clay soils	Woodland		0.4
	Pasture		0.45
	Cultivated		0.5