Untitled Page
DWA Home Page  
Untitled Page


Untitled Page



Hydrology, Hydraulics and Fluvial Geomorphology

Hydraulics and geomorphology

The flow regime is regarded by many aquatic scientists as the master physical variable controlling ecosystem pattern and process. Poff et al. (1997) list five critical components of the flow regime that regulate ecological processes in river ecosystems: the magnitude, frequency, duration, timing (or predictability) and rate of change (flashiness) of hydrological conditions. The hydrology of the Berg River along its length should be described in these terms, for both low and high flows using historical DWAF data.
An important aspect of the hydrological analysis will be the naturalisation of the observed flow record for the past decade, since the last hydrological modelling for the river. Also critical is a description of the behaviour of the hydrograph downstream along the length of the river and through the estuary, including synchronicity of tributary inputs. These data will feed into the hydrodynamic model for describing floodplain and estuary dynamics.
Future refinement of the pattern of flood and fresh releases should be made on the basis of these analyses - for example defining the range of hydrograph shapes for each size category of flood event; timing and/or clustering of flood releases; inter-annual variation in flood flows. Such information could be used to set the operational procedures for Skuifraam Dam and /or any other water resource development (e.g. the Voelvlei/Lorelei scheme). In addition, information on the low-flow hydrological changes associated with irrigation releases should be described, for use in interpretation of existing possible geomorphological and biological impacts.

Data collection:
· Hydrological data from 7 DWAF recording stations (data collected and supplied by DWAF).
· Patch where necessary to provide naturalised virgin and present-day hydrological data, based on the full observed flow record.

· Modelling data for inclusion into the estuarine conceptual model workshop
· Report - analysis of data from main stem and tributary gauges to present a description of the hydrology of the river, including its variability - for inclusion in the baseline monitoring final report.

Hydraulics and geomorphology
Hydraulic surveys will be necessary to augment existing data on the natural variation in channel profile, and on velocity-discharge and depth-discharge relationships. This will allow for comparison in the post-dam phase to detect possible changes in channel shape due to e.g. channel degradation or aggradation. A control site will be established at Site 1, upstream of Skuifraam Dam, with five downstream sites established for monitoring. Cross sections will be established as far as is possible to utilise the existing transect information on channel profiles (e.g. at the sites corresponding to IFR Sites 1 and 3 from the 1996 IFR process), thereby providing an historical measure of the dynamics of the channel characteristics.
These data will also be used to extend the existing conceptual model of sediment transport dynamics along the river, to identify zones of sediment erosion and deposition. Suspended sediment sampling at sites will be necessary to determine sediment load - discharge relationships during floods, suspended sediment grading and tributary catchment sediment load contributions.
The major effect of potential increases in armouring of the river bed downstream of the dam will be on the architecture of the stream bed and thus the range of hydraulic conditions available to stream biota. Physical habitat morphology and architecture are well documented as one of the major determinants of invertebrate assemblage structure and biodiversity. Baseline conditions for the Berg River should be described therefore, for comparison with potential future changes.
The natural variation in the extent and quality of morphological units, which determine the availability of hydraulic biotopes (the largest spatial scale at which invertebrate assemblage composition is usually assessed), will be described for all sites, by mapping these and biotope characteristics within these morphological units. This will include quantification of the range in Froude number for different biotopes, as descriptors of hydraulic conditions at the scale of invertebrate fauna.. Mapping will be located within detailed (100 m) surveys of the river channel. Variation in biotopes will be related to discharge and channel geometry.
An additional aspect of this component of the study relates to one of the recommendations of the IFR Refinement workshop - for capping flows during summer not exceeding 3.1 m3 s-1 in the upper reaches of the river. This was to prevent a reversal of seasonality of flow-related biotope availability, and of armouring due to removal of fines. The baseline monitoring programme as proposed here will be able to describe the existing impact of increased summer flows (currently in the order of 4 m3 s-1) on habitat availability and quality.

Data collection:
· Detailed survey of 100-m reaches at six sampling sites to provide topographical information that will link biotope availability and characteristics to discharge. Each 100-meter section will be surveyed at 1-meter sections in the river with sparser data required for the riverbanks.
· Fixed transects using different morphological units to measure depth, velocity and wetted perimeter relationships to discharge; at low flows, depth velocity relationships will be established using stream gauging; at high flows, the slope-area method will be used.
· Channel profiles
· Mapping and measurement of spatial flow velocities, flow depth, wetted perimeter, hydraulic radius, Froude numbers, flow areas versus discharge. This will include identification of channel geometry / features, and flow types (hydraulic biotopes), linked with Froude number ranges for given discharges. This will include sediment distribution per biotope, armouring effects (also biotope specific), embeddedness and hiding and exposure of cobbles/boulders.
· Suspended sediment sampling at various points along the river and tributaries during flood flows.
· Re-survey of the river bed, banks and floodplain on an annual basis at 6 sites along the length of the river (3 sections at each site), and map hydraulic biotope proportions, to describe and monitor channel geometry, sediment distribution per biotope, armouring effects (also biotope specific), embeddedness and hiding and exposure of cobbles/boulders.

Monitoring frequency:
· Sampling will take place intensively during the first year, with repeat surveys in December / January to coincide with annual vegetation surveys.

· A conceptual model of sediment transport dynamics by means of a water and sediment mass balance simulation along the river, to identify zones of sediment erosion and deposition. This will include suspended sediment sampling at sites to determine sediment load - discharge relationships during floods, suspended sediment grading and tributary catchment sediment load contributions, and will feed into the Initialisation report.
· A description of the variability of and relationships between biotope characteristics, discharge and channel geometry.
· Final monitoring report to be included in the final baseline monitoring report.