Appendix A: A Practical Guide to Effective Discharge Calculations
for flood flows. More recently, steps have been taken under the Blackwater Catchment Plan to improve
stream habitats by softening the impacts of engineering works, modifying the maintenance regime, and
enhancing the channel environment.
Under plans to restore the environmental function of the river, a geomorphological study was
performed to establish how the existing, engineered channel differed morphologically from a natural, regime
channel. An effective discharge calculation was performed using data from a nearby gaging station and the
"expected" morphology for a natural, regime channel was established by applying the hydraulic geometry
equations of Hey and Thorne (1986). Stream reconnaissance was performed to establish the morphology
and bankfull dimensions of the existing, engineered channel using the method reported by Thorne (1998).
The effective discharge calculation showed that the dominant discharge was 3.65 m3s-1, compared
to a bankfull capacity observed in the current channel of 16.4 m3s-1. The main morphological parameters
calculated and observed are listed in Table 2. Contrasts between the regime and engineered channel
parameters were used to support the conclusion that the engineered channel was over-large in width, depth,
and cross-sectional area, and that in-channel velocities were insufficient to transport the sediment load
supplied from upstream. This explained its tendency for siltation and requirement for frequent maintenance.
On the basis of the geomorphological assessment, initial recommendations were proposed for
morphological restoration of the channel to support the enhanced river environment envisaged in the
catchment plan. It was further proposed that the viability of these initial recommendations should be
examined further to determine their feasibility for a restoration project.
Table 2 Regime and Engineered Morphology of the River Blackwater, UK
Channel Parameter
Regime Channel
Engineered Channel
(1)
(2)
(3)
Width (m)
6.3
12
Depth (m)
0.58
1
X-section area (m2)
3.7
12
Mean velocity (ms-1)
1.0
0.3
In this application, the effective discharge was found to be the key to deriving values for the
dimensions of a natural, regime channel appropriate to the current flow regime. This information was
useful in highlighting the problems with the engineered channel and it indicated the starting point for
detailed design of a restored channel.
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