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CHAPTER 6: OVERVIEW OF KEY BIOPHYSICAL RESPONSE AND IMPACT PROCESSES
6.1 INTRODUCTIONThis chapter discusses in general terms how key biophysical systems are thought to respond to and be impacted by the kinds of measures included in the Regional Plan portfolio of initiatives. The terms `respond' and `response' are used here to capture the dynamic nature of some of the processes. The systems discussed here are: river morphology; regional surface water; water quality; fisheries; and grazing lands, wetlands, and threatened ecological communities.
This discussions picks up from the description of basic principles of FCD and river improvement projects (Section 3.2), and leads into the presentation of assessment methodologies (Chapter 7) and the impact assessment (Chapter 9).
6.2 HYDROLOGY, HYDRAULICS, AND RIVER MORPHOLOGY RESPONSES TO FCD
6.2.1 IntroductionThis section reviews past findings and experience concerning physical impacts of flood control works. These comments are intended to illustrate some of the main physical processes that take place when FCD projects are constructed. The main topics in this section include hydrologic, hydraulic, and morphologic responses. These physical responses are important because they may govern the nature of many other environmental impacts such as changes in habitat characteristics and water quality.
Potential hydrological, hydraulic and morphologic impacts have been considered for four types of interventions:
6.2.2 Full Flood Control EmbankmentsRiver discharges are normally confined to the main channel until flows approach or exceed bankfull conditions. In most naturally formed river systems unaffected by downstream controls or backwater effects, the channel's bankfull discharge capacity tends to correspond to a relatively frequently occurring annual flood. Although conditions vary according to local hydrology and geological setting, bankfull conditions on many rivers throughout the world have been found to correspond to an annual recurrence interval of between two to five years. Similar conditions have been found on several rivers in the Northeast Region outside the deeply flooded Central Basin which is affected by backwater from the Meghna River system.
Once bankfull conditions are approached or exceeded, water and sediment will spill onto the floodplain by several mechanisms, including direct overtopping of banks, through breaching of banks, or as spills through distributaries or khals that connect the main channel with the lower-lying flood basins. A portion of the fine sediment load will be deposited overbank in flood basins where velocities are low, while some of the sand load is deposited as natural levees or crevasse splays near the main channel.
Floodplains are important hydrologically because they provide overbank conveyance and storage which can reduce peak flows and attenuate flood hydrographs. The magnitude of these floodplain flows can be substantial. Discharge measurements were made in the Kushiyara floodplain in 1993 at three locations where the floodplain discharges are reasonably confined; at Sheola, Fenchuganj, and Sherpur. These measurements demonstrate that the floodplain discharge in a major flood can be as large as that in the main river channel (at Fenchuganj), or, stated another way, that the floodplain can carries as much as one-half of the total discharge. Floodplain flows in the Central Basin are more difficult to measure, but model results indicate that as much as three-quarters of the total flow is carried on the floodplain during the monsoon season and very little in the main channel.
An illustration of the storage effect is provided in Figure 4, which compares the 1991 water year total inflow to the Northeast Region with the outflow at Bhairab Bazaar. The inflow hydrograph includes all the border stream inflows, which were mostly determined by direct measurement, plus local runoff from ungauged areas within Bangladesh as calculated with the NAM rainfall-runoff model. From these hydrographs, it can be seen that inflow and outflow volumes are equal over the year, but that peak outflows are considerably damped and delayed. Up to 25 km3 of water was stored on the floodplain during the peak of the monsoon season during 1991. Runoff peaks are attenuated by storage effects, such that the individual flood events are not apparent in the outflow hydrograph. It can also be seen that without floodplain storage, the peak outflow from the region would be approximately 50% higher that at present.
Cutting off floodplain spills by means of embankments and khal closures will increase main channel discharges due both to direct flow confinement and to reduction of floodplain storage. Examples of such project impacts can be provided from several past developments in the region.
For example, full embankments and khal closures have been constructed along the Kushiyara and the Surma left bank since the 1960's. One effect has been to reduce floodplain discharge. This change can be seen in Figure 5, which shows the floodplain discharge and Kushiyara River at Sheola discharge (note that the Sada Khal floodplain gauge was discontinued in 1977 but has recently been reactivated). Floodplain discharge has declined while river discharge has increased.
Impacts on water levels are apparent in Figure 6 which shows Sheola water levels since 1949. It would appear from this graph that Sheola peak water levels have risen by approximately 0.5 m during that time period, mostly before 1978, which coincides generally with the history of embankment works along the river and changes in floodplain discharge.
Another example of embankment impacts can be illustrated from experience with the Khowai River Project near Habiganj. Full flood control embankments along the Khowai were constructed in the 1970's and early 1980's. Figure 7 and 8 show time series of historic water levels and discharges at various locations along the Khowai since 1964. In the un-embanked upper reaches, water levels do not show noticeable trends over time, whereas in the embanked reach levels and peak discharge have risen dramatically.
Once embankments are in place, river channel geometry, longitudinal profile, and morphology will respond, subject to various types of physical constraints, to the changed hydrologic regime. Figure 9 illustrates an idealized case of river responses following construction of embankments. Initial hydraulic changes include:
Increased main channel velocities and depths during large floods. Water surface slopes will increase through the embanked reach, producing an afflux, with the greatest water level rise occurring at the embankments' upstream end;
An M-1 type backwater profile upstream of the embankments due to higher water levels in the embanked reach. Levels will gradually converge to pre-project levels upstream of the embankment.
Increased sediment loads during high floods through the embanked reach due to higher velocities, depths, and slopes. The implications of this for river erosion can be considerable, since sediment transport in fine sand-bed channels is quite sensitive to small changes in velocity or stream power. As a result, the channel will tend to degrade through the embanked reach, since its transport capacity will exceed the sediment supply from upstream.
After embankments are constructed, most of the sediment will tend to be flushed through the confined reach, due to higher in-channel velocities. However, rapid aggradation may occur downstream of the confined reach, particularly if there are large spills onto the floodplain or if the slope decreases appreciably below the confined reach. In some situations, sediment deposition will take place primarily overbank on the floodplain, so that overall impacts to the main channel may be relatively minor. In the long term, downstream aggradation may initiate further slope adjustments along the river, eventually causing backwater effects to propagate upstream, leading to further increases in water levels in the embanked reach.
The preceding comments describe the response of the river bed in the main channel. Additional changes may occur to the berms, the overbank section of the river between the embankment and the top of the channel bank. It is commonly perceived that berms aggrade rapidly after embankments are constructed. However, it is difficult to verify this claim with the available historic survey data. In fact, berm elevations along embanked sections of the Surma River and Kushiyara River do not appear to have changed much over the last 20 years. Overbank deposition rates are probably highest when high-velocity main channel flows breach through inner banks and spill into low basins or slack water floodplain areas. On most past projects in the region, the embankment set-back distance has been very small leaving narrow berms. As a result, the overall hydraulic impacts of berm deposition tend to be relatively small in comparison to the initial confinement effect associated with embankment construction.
Flood embankments also affect the lateral stability of rivers. Natural rivers flowing through alluvium are found to adjust their channel geometry to accommodate a dominant discharge. Observations on many rivers throughout the world show the average top width (W) of an alluvial channel is related to the dominant or bankfull discharge, Q, by:
W Q1/2where dominant discharge Q is represented by a relatively frequently recurring flood discharge (typically, return period of 18 months to two years).
Full embankments lead to an increase in in-channel flood discharge, which effectively raises the dominant discharge in the confined reach. Consequently, if the banks are not protected, the channel will tend to widen over time as a result of bank erosion. Table 6.1 shows the expected change in width (expressed in percent) for various changes in channel-forming discharges. Observed changes in top widths after completion of embankment projects along the Upper Surma, Upper Kushiyara, Manu, and Khowai Rivers generally agree with estimates derived from simple regime-type equations.
In fact, enlargement of channel cross section appears to be one of the principal responses in embanked reaches of the region. This enlargement may partially offset confinement effects from the embankments, particularly at moderate flow conditions. For example, most specific gauge plots in embanked reaches indicate that river stages have decreased slightly over time, when compared at any given discharge.
The long-term response of a river channel to increased flood flows due to confinement can be considerably more complex, particularly when local geomorphic controls are present. For example, long-term increases in flood discharge and sediment inflow can cause a change in channel type, not just size: a previously meandering channel can change into a predominantly split or anastomosing one, for example. A transformation of this kind produces major changes to channel geometry, channel migration pattern, and sedimentation patterns.
6.2.3 Submersible (Partial Flood Control) EmbankmentsSubmersible embankments reduce floodplain discharges and increase in-channel discharges, especially during the pre-monsoon period. They tend to concentrate floodplain discharges and overbank spills into fewer locations and more specific spill points, often at locations where embankments are eroded and channel erosion/deposition problems are occurring.
Further, while water level and discharge effects may be negligible for individual submersible embankment projects, several such projects occurring together within a drainage system can produce significant cumulative effects on water levels and flows.
In the past in the Northeast Region, this potential for cumulative impacts was not appreciated and numerous submersible embankment projects were built throughout the Central Basin without planning for systemic drainage and other requirements. As it has turned out, their potential for cumulative impact has been not been fully realized as a result of frequent embankment breeches, wave damage, public cuts, and incomplete structures and embankments. It is expected that if these projects became fully operational (as could happen if in the future they were rehabilitated), they would have significant impacts on pre-monsoon and in some cases monsoon water levels and flows.
The assumption that monsoon conditions are less affected by submersible embankments than by full flood embankments holds good where monsoon water levels are distinctly higher than pre-monsoon water levels. For example, the difference between a 1:10 year pre-monsoon flood and an average monsoon flood varies between one to three metres in the region. In locations with only one metre difference, submersible embankments having normal freeboard allowances will begin to encroach on the lower-magnitude monsoon flood flows. In these cases, the kinds of impacts associated with full flood control embankments will begin to be experienced.
Outside the Central Basin, morphologic impacts from submersible embankments are expected to be relatively minor in comparison to full flood control embankments since the structures are designed to be overtopped during the period of highest flows. Berm deposition could be accentuated since the submersible embankments will tend to trap fine sediments carried overbank during the monsoon season.
Impacts of submersible embankments on rivers flowing through the deeply flooded Central Basin may be appreciable. Most sediment transport on these rivers takes place during the pre-monsoon season (April and early May), when channel velocities and water surface slopes have their highest values. Later, during the monsoon season, backwater from the Meghna River drowns the rivers and reduces their slopes, channel velocities, and sediment transport capacity. Thus, through their effects on discharges and velocities in these rivers during the morphologically active pre-monsoon period, submersible embankments could conceivably have an appreciable effect on sediment transport regimes and channel responses similar to those described above for full flood embankments i.e. initial degradation and channel enlargement in the embanked reach).
6.2.4 Loop CutsLoop cuts reduce the effective length of the river, particularly during low-to-medium flows, thereby increasing river slopes and lowering upstream water levels. Hydrologic impacts of individual loop cuts depend on the initial river slope and the channel length reduction relative to original channel length. Impacts tend to be relatively small in the Northeast Region where river slopes are relatively low.
Impacts of loop cuts are reduced during flood stages when a large portion of the flow passes onto the floodplain and is therefore not affected by the channel changes. These changes become more significant when the loop cuts are combined with adjacent embankments which modify the overbank flows.
The main morphologic impacts from loop cuts arise from the change in slope through the shortened reach. These impacts are illustrated in Figure 10, which shows a simplified model of channel changes from a series of loop cuts on a piedmont river. In the artificially straightened reach, the slope will be increased, water levels will be lowered and the water surface will develop an M-2 type drawdown profile. As a result, the velocity and sediment transport capacity will increase through the straightened reach. This will initiate channel degradation, leading to further reductions in water levels. This secondary degradation will initiate further slope adjustments upstream, leading to a transient degradation wave progressing along the river.
If the natural channel slope flattens out downstream of the straightened reach, there can be appreciable aggradation immediately downstream of the loop cut. This aggradation occurs because the lower reach's sediment transport capacity will be substantially less than the transport rate in the straightened reach.
Loop cut impacts can be very unpredictable. If the excavated pilot channel runs near inerodible plugs or through highly variable bed and bank materials, the pilot cut will not enlarge uniformly and a channel of highly irregular width and depth may develop. If the bed and banks contain cohesive materials, the channel will not enlarge to a full cross-section, which will produce a local high-velocity constriction. This situation appears have occurred at loop cuts on the Kalni, Baulai, and Khowai Rivers.
Loop cuts may also impact lateral channel processes by modifying the channel pattern and sinuosity and by initiating new bank erosion.
6.2.5 Drainage ImprovementDrainage improvement here refers to widening and/or deepening drainage channels.
Hydrologic impacts are primarily limited to the provision of faster post-monsoon drainage which results in water levels falling more quickly after monsoon peaks have passed. Impacts on pre-monsoon and monsoon flood levels are generally smaller in magnitude, but this depends on the nature of the drainage constriction and the magnitude of the channel changes. Drainage works can also be designed to provide lower water levels to drain beels and other low-lying areas, but in such cases water control structures regulators are generally provided to allow control of drainage rates and water levels.
6.3 WATER QUALITY IMPACTSThe ecology of water quality is, of course, extremely complex. Important considerations include, but are not limited to: inputs of chemical and biological contaminants; flushing and dilution rates; in situ physical, chemical, and biological processes involving, among other things, micro- and macrophytes, sediments, and vector biology; and end users' requirements and water handling practices.
A number of the Regional Plan initiatives address water quality. The intervention point varies. Ground Water Investigation and Regional Surface Water Quality address the need to improve the management of water quality, starting with better information about water quality and water quality ecology. Urban Sanitation, Urban Water Supply, Village Water Supply and Sanitation, and Duckweed-Based Wastewater Treatment address the need to improve public health through safe domestic water supplies, improved management and treatment of human wastes, and rural hygiene education. Pulp and Paper Mill Effluent Treatment and Pollution Abatement at Smaller Industrial Facilities address the need to protect public health and environmental systems from industrial pollutants.
Many more Regional Plan initiatives indirectly affect water quality. Each of the water resources infrastructure development projects can be expected to change spatial and temporal patterns of water volumes and discharges, which in turn will affect flushing and dilution directly, and other processes indirectly. These latter include important water quality problems posed by the intensification of agriculture, which implies increasing use of fertilizers, pesticides, and irrigation water, and by the extension of agriculture (boro rice) into beel areas.
Habitat restoration programmes can be expected to improve water quality through biological and mechanical processes associated with or supported by them.
6.4 OPENWATER FISHERIES IMPACTSThe Fisheries Specialist Study describes the response of the openwater fishery to environmental processes, including the various types of water resources development interventions in a series of case studies of areas within and outside existing FCD projects in the Northeast Region (Section 3.1 and Appendices H through K). This information is very briefly summarized here.
Non-FCD negative factor complexOpenwater fisheries both inside and outside of FCD projects have significant problems. An analysis of numerous FCD and non-FCD case studies suggests that, independent of FCD-related stresses, the openwater fishery is under stress from a complex of negative, non-FCD, factors.
The major factors appear to be:
Flood controlFlood control projects in the region are of two types: full flood control and partial flood control. The very different fisheries impacts of existing projects of the two types are discussed below.
While reading the discussion that follows, it is important to bear in mind that the actual operation and impacts of many flood control projects differ from the intended operation and impacts. Most of these operational problems tend to reduce negative fisheries impacts: this is the case for premature overtopping of submersible embankments; embankment breaches; and public cuts. The only operational problems which tend to exacerbate fisheries losses are delay or failure to open water control structures, and failure to retain water (both of which commonly occur with the widely-used fall-board type of structure).
Full flood controlTwo-thirds of existing full flood control projects studied were found to have negative or mixed (part positive, part negative) impacts on the openwater fishery. Negative impacts centred on general reduction and disruption of aquatic habitat and area, and on interference with fish migration and reproduction. Case studies of 18 full flood control projects, with and without provision for pumped drainage and including four projects that channelized rivers, found seven projects with no impacts, three with mixed impacts, and nine with negative impacts.
Partial flood controlExisting partial flood control projects were found to be much less harmful to fisheries than full flood control. One-half had no impact. Of the rest, only one-third had mixed or negative impacts, while one-fifth were thought to have had positive effects, arising from higher dry season water levels, which improve katha fisheries, and from protection from siltation. Case studies of 19 partial flood control projects found four projects with positive fisheries impacts, eight with no impacts, two with mixed impacts, and four with negative impacts.
Drainage improvementOf the limited number of existing river and khal re-excavation projects in the region, all were reported to have had no or positive impacts. Benefits are probably due to greater habitat depth, better flow regime, and improved connections with other habitats and fish stocks. Case studies of six drainage improvement projects found two projects with positive fisheries impacts and four with no impacts.
A caution would be that, assuming these results are significant (they may not be, given the small number of projects), drainage improvement is benign but of limited benefit for enhancing fisheries habitat and for mitigating fisheries damage from FCD or other processes. To increase positive fisheries impacts, there may be a need incorporate specific fisheries elements in the detailed design of drainage improvements (e.g. variations in depth, non-straight channels, artificial habitat enhancement, etc.), to create preferred habitats -- assuming that this would be feasible, or could be made so through technical means.
Fish passes and beel embankmentsFish pass structures are intended to permit fish migration despite structural measures controlling or preventing flows of water. Beel embankments are intended to protect beels from sedimentation and increase water storage. Neither measure is currently in use in Bangladesh; they would be developed under the initiative Fisheries Engineering.
6.5 GRAZING, WETLAND, AND THREATENED ECOLOGICAL COMMUNITY IMPACTSWetland plant communities, including the three threatened ecological communities (swamp forest, reed swamp, and floodplain grassland), and grazing land are of interest because of the goods and services they provide, including and particularly benefits derived by local communities. The extent and condition of these biological assets changes in response to hydrologic changes (i.e. changes in land types) brought about by FCD projects; to shifts in cropping led by FCD or other factors, such as increasing availability of irrigation, new varieties, changing farmer preferences; to other changes in land use; and to direct exploitation and management actions.
Winter grazing area consists of winter fallow land of F0, F1, and F2 land type.
Winter wetland area, the area suitable for aquatic and semi-aquatic wild plant communities in winter, consists of winter fallow F3 land, F4 land, beels, and channels.
Summer wetland area, the area suitable for aquatic and semi-aquatic wild plant communities in the monsoon period, consists of summer fallow F1, F2, and F3 lands, plus F4 land, beels, and channels.
Threatened community areas refer to the actual areas occupied by swamp forest trees, stands of reed plants, and by flood plain grasses, not to the extent of areas suitable for these types of vegetation. Most of the surviving remnants of these threatened communities depend very much on hydrologic or other conditions which preclude conversion of the areas they occupy to agriculture. Thus, the likelihood of continued survival of these communities would be expected to decrease to the extent that FCD or non-FCD interventions shifts local conditions in favour of conversion to agriculture. In addition to this, of course, these communities are subjected to direct exploitation pressures and would benefit from management improvements and habitat restoration and afforestation.
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