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Definition A weir of such crest length in the direction of flow that critical flow occurs on the crest of the weir. Consist of a pair of suspension cables strung across the river and used in conjunction with gauging winch when deploying a current meter The condition downstream from a gauging station that determines the stage discharge relation. It may be a stretch of rapids, a weir or other artificial structure. In the absence of such features, the control may be a less obvious condition such as a convergence of the channel or even simply the resistance to flow through a downstream reach. A shifting control exists where the stage-discharge relation tends to change because of impermanent bed or banks. Is an emergency medical procedure for a victimof cardiac arrest or, in some circumstances, respiratory arrest. This is associated with the application of mouth-to-mouth ventilation, combined with chest compressions based on the assumption that active ventilation is necessary to keep circulating oxygenated blood in the lung. A gauge, usually vertical, used to indicate a peak stage that has occurred since the previous setting. The flow in which specific energy (depth of flow + velocity head) is a minimum for a given discharge; under this condition a small surface disturbance can not travel upstream. The ratio of inertia to gravity forces (Froude Number) is equal to unity. A specified vertical plane through a stream bounded by the wetted perimeter and the free surface. Instrument to measure discretely velocity of water flow in the water column. Can be of propeller, electromagnetic or acoustic type. Is an electronic device that records data over time and is usually integrated to either a built in instrument or sensor or connected to external instruments and sensors. A multi-parameter data logger has many channels to accommodate a repertoire of measurements e.g. temperature, humidity, rainfall, water level etc.
8.1 STREAM ECOLOGY Streams like springs and rivers are also known as lotic habitat. A lotic habitat and its ecology is primarily characterized by unidirectional flow, constant state of physical change and a high diversity of microhabitats, where the fauna and flora has adapted themselves to the flow conditions. The ecology of lotic habitat is governed by abiotic and biotic factors, the former being flow, light, temperature, chemistry and substrate. Flow is the most influential abiotic component in determining a stream ecosystem as it is responsible for producing riffles, pools and gliders via erosion and deposition. Light is the main source of energy in a lotic environment and is required for primary production which provides the foundation for the trophic web (Figure 8.1) in an aquatic system. Aquatic plants and algae including periphyton (filamentous and tufted algae which clings on rocks) need light for photosynthesis and other animals depend on them as part of the food chain. The varying degree of light available in the water column as well as the stream surroundings also help dictate the community structure of both fauna and flora. Likewise differential temperatures resulting from varying light intensities in a stream affects the biological population in a stream ecosystem. Deeper waters tend to host fish with preference for colder temperature while shallower exposed streams are dominated by species with tolerance for higher temperatures. Stream water temperature usually varies diurnally and for some localities seasonally e.g. lower temperatures are normally associated with wet season and vice versa.
Normal organisms found in a stream biota or biological community are bacteria, tiny plants or phytoplankton, zooplankton, fishes, insects and other invertebrates including those living within the substrate (also known as benthos). Zooplanktons are tiny animals including fish larvae (ichthyoplankton) which predate on smaller planktons. All these organisms interact with each other and play a key role in driving the trophic (or energy) relationship in a stream ecosystem. Bacteria are found everywhere in a stream and they play a large role in recycling energy in the system. Phytoplankton and other aquatic plants tie up food or carbon via photosynthesis which in turn is passed on to zooplankton and other grazers including fish when they get eaten. The zooplankton then is fed upon by juvenile fishes which in turn are preyed upon by larger fishes. Aquatic plants also offer shelter and food source to other organisms thus forming microhabitats (Brown, 1987). Insects can make up to 90% of a lotic system invertebrate population and are found in various microhabitats or niches in the system. They feed on plants, decaying matter and other smaller insects and pass on energy when they get eaten by a fish or any organism on higher trophic level. Benthic invertebrate such as polychaetes (worms), mollusks and gastropods are common denizens of the stream bed and banks and are also fodder for higher positioned animals within the trophic web. Fishes are usually near the top of the food chain in a stream ecosystem and usually consist of planktivores, herbivores/detritivores, omnivores and carnivores. They are flexible in their feeding role and their diet may vary according to food availability and their respective development stage (young of different species usually require different prey sizes).
Most if not all aquatic communities whether micro- or macroscopic, may provide information on the quality of its environment. The community normally used in evaluating lotic systems condition are large, readily visible invertebrate animals colonizing the substrata of all rivers. These animals are collectively referred to as macroinvertebrates of which the main constituents are young aquatic stages of insects.
Within this community each species tend to exhibit varying degree of sensitivity and tolerance to pollution. Some species are, for example, are very intolerant to high levels of silt i.e. turbidity thus will not be found where the concentration of suspended solids is high. A characteristic feature of polluted environments is a reduction in overall species assemblage and an increase in the density of tolerant species. Benthic macroinvertebrates generally inhabit their respective microhabitats in parts of a stream throughout their life cycle and tend to remain localized. Therefore they are continually exposed to any changes that occur in the environment. The composition of a macroinvertebrate community at any point in a stream or river then reflects the average water quality at that particular point. Hence the common objective in bio-monitoring projects is to detect stream and river degradation and the extent of it due to forest and agricultural practices, urbanization, or other controllable sources of impact.
i) Choose representative riffle-habitat (broken surface water) sampling of benthic macroinvertebrates, physical habitat, and water quality to describe biological community condition as a result of natural and human-induced disturbance. Normally, samples are collected from riffles to characterize the benthic macroinvertebrate community unless degradation is suspected in pool habitat (slow moving or eddying water). To distinguish natural versus human influence, data must be collected at reference sites and at degraded sites over a period of time to address spatial and temporal variability.
Minimally disturbed conditions reflect sites that have experienced very little historical activity that alters stream integrity. Least disturbed sites have been degraded historically, but exhibit some level of recovery. Reference sites are used to describe biological variability due to natural disturbances (e.g. precipitation, drought). Degraded sites are surveyed to describe a continuum of human influence on natural stream communities. Identification of what a degraded macroinvertebrate community is and the factor(s) that caused the resulting condition defines severity of impact
DID is in charge of river water management in Malaysia. A special case of water quality issue worthy of attention is the water quality downstream of reservoir or dam prevailing mostly in the upper reaches of the river basin. Examples are dams/reservoirs under the jurisdiction of the DID. These dams/reservoirs are mainly for irrigation as well as domestic water supply. The impacts of reservoirs/dams in the upper reach of a river basin are far reaching both in terms of temporality and spatiality. The impacts and repercussion on water quality downstream of the water retenting structures can be far reaching over time. Impacts such as relatively clear water outflows from the reservoir could erode and scour the river conveyance channels further downstream. Sometimes, releases from the dam/reservoir bottom will degrade the water quality downstream with its anoxic contents and could affect the fauna and flora and riparian community downstream.
Obviously the longer the lead time the more useful the forecast as it allows time for action to be taken to save lives and properties. To improve lead time, information on rainfall should be obtained in real-time and this is made possible through the establishment of telemetric stations and SCADA systems. In the case of the SMART Tunnel project where sufficient lead time is critical for its operation, DID is contemplating applying quantitative precipitation forecast (QPF) to further increase the lead time despite the uncertainty over the reliability of QPF. 9.8 FLOOD WARNING SYSTEMS The flood warning systems varies from sophisticated flood forecasting operated by DID, to community self help system such as the flood warning board system to simple flood siren system. Whilst ideally flood forecasting should be a component of a flood warning system, sometimes flood forecasting is not possible due to reasons such as:
Real-time data of the flood level at Kuala Krai (which is equipped with a telemetric water level station) is obtain at regular intervals (1-hourly to 6-hourly). When the level at Kuala Krai reaches alert level, the flood levels are broadcasted via the local radio stations so that residents in flood prone areas downstream are constantly informed of the Kuala Krai Levels. Though analyses of past flood records DID has established a reasonably reliable stage correlation of flood levels at Kuala Krai with flood levels at various target flood prone areas and are able to mark on the FWB the various flood levels that would be experienced at the FWB site corresponding to various Kuala Krai flood levels. At the flood prone areas, the FWB are located where the affected people congregates, i.e. near to a local coffee shop. The community affected by the flood is able to determine the expected flood level in their area by noting the flood level at the reference station Kuala Krai and the time of travel of flood is also displayed in the FWB as a guide to residents in planning their evacuation. After the 1983 flood in the east coast states of Peninsular Malaysia there was a review report prepared. The report concluded that FWBs are practical tools for delivering warnings direct to the public and some suggestions to improve the FWBs were made. A typical flood warning board is shown in Figure 9.5. The FWB system continues to be implemented and to date about 138 FWBs have been established.