Low flow hydraulics in rivers for environmental applications in South Africa
Date
2009-03-24T08:21:54Z
Authors
Jordanova, Angelina Alekseevna
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Abstract
Implementation of the National Water Act in South Africa requires that an
ecological Reserve be determined for all significant resources. The ecological Reserve
determination is the estimation of the amount of water required to maintain the system
in a particular ecological condition. Because aquatic habitats are defined in terms of
local hydraulic variables rather than amounts of water, hydraulic analysis provides a
crucial link in relating hydrological conditions and river ecosystem integrity. Over the
last decade, considerable effort has been devoted to developing hydraulics for the
Reserve determination. The hydraulics needs for Reserve determination are primarily
for low flow analysis, and appropriate methods still need to be developed.
This thesis deals with hydraulics under low flow conditions. Its emphasis is on
developing appropriate methods for describing the hydraulic characteristics of South
African rivers under conditions of low discharge, and the influence of vegetation and
large bed roughness. The following methods have been developed:
· A new equation for prediction of overall flow resistance under large-scale
roughness, and a new approach for estimation of intermediate-scale roughness
resistance that distinguishes the influences of large and intermediate scale
roughness components.
· Prediction methods for velocity distributions with large roughness elements.
Under low flows, rocks and boulders may control the local velocity and depth
distributions. Distributions of velocities and depth are related to rapidly
spatially varied flow caused by the boundary geometry rather than flow
resistance phenomena. With increasing discharge, the multiple local controls
become submerged and the flow tends towards a resistance controlled condition.
Available information addressing the distinction between resistance controlled
and multiple local controls conditions is limited. This thesis contributes to
understanding the transformation between multiple local controls and the
resistance controlled conditions.
· Practical conveyance prediction methods for three situations pertaining to the
occurrence of vegetation in rivers and wetlands. In-channel and riparian
vegetation makes an important contribution to the creation of physical habitats
for aquatic animals, but also has significant effects on flow resistances that need
to be predicted.
Description
Keywords
Low flow, Flow resistance, Velocity distributions, Vegetation resistance