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CHAPTER 2: REGIONAL PLAN ALTERNATIVES
2.1 FUTURE-WITHOUT-PLAN SCENARIOThe future-without-Plan (FWO) scenario is described in Chapter 8.
2.2 REGIONAL PLAN ALTERNATIVES AND SELECTION OF ALTERNATIVESA key step in the planning of individual projects often includes making a selection from among several more or less well-defined alternatives.
This was not the case in the development of the Regional Plan. The problem here was to identify ways to contribute to regional development, broadly defined, through strategic interventions in the internal planning environment (water), in the context of the external environment (everything else). The planning was undertaken by a multidisciplinary team over a two-year period, culminating in team strategy sessions to define and rank strategic thrusts, to which individual projects were assigned and ranked. Throughout this process, major and minor alternatives in many areas (strengths, weaknesses, opportunities, threats, issues, driving forces, and potential projects) were being continually sifted and resifted. The Regional Plan represents the residue of this process and the consensus of the team.
The planning process is documented in more detail in the Regional Plan (pp. 1-3).
CHAPTER 3: REGIONAL PLAN DESCRIPTION | previous section | next section |
3.1 PLAN OVERVIEWThe Plan is described in the concluding chapters of the Regional Plan document. The Plan consists of eight strategic thrusts which identify key developmental themes for water resources development in the region, plus a portfolio of 44 projects which address these. This IEE assesses the impacts of these 44 projects. Impacts associated with other elements of the strategic thrusts have not been characterized at this time.
Most of the projects in the portfolio are documented in pre-feasibility studies produced by NERP. A few are documented in other NERP reports or, in cases where NERP is endorsing the proposals of other agencies, reports by other entities.
The description of the Plan and the project portfolio are summarized in Figure 24 and Table 21 of the Regional Plan. Regional Plan Figure 24 provides, for each project in the portfolio, the primary strategic thrust, the implementation priority, the project type and objectives, presence of structural and non-structural measures, project area planning subregion, documentation status, implementing agency, and cost. Regional Plan Table 21 provides aggregate indicators of the scale of the FCD projects proposed, relative to existing FCD projects.
3.2 OVERALL SCALE OF PROPOSED MEASURESThe overall scale of the proposed measures is documented in Regional Plan Section 9.2.
3.3 BASIC PRINCIPLES OF FCD AND RIVER IMPROVEMENT INTERVENTIONS
3.3.1 IntroductionThis section provides background information on how FCD and river improvement measures are intended to function in a generic sense. This information leads logically into the description of how biophysical systems respond to FCD and river improvement (Chapter 6), the assessment methodologies (Chapter 7) and the characterization of these measures' biophysical impacts (Chapter 9).
3.3.2 Flood Control and DrainageFlood control, drainage (FCD) and river improvement projects produce desired social and economic outcomes through their impacts on, and the responses of, biophysical systems: for example, changes in water level set the stage for farmers to modify their choices of crops.
The purpose of flood control and drainage improvement projects is to reduce the depth or occurrence of water over a certain area. Both the flooding and the inadequate drainage conditions cause an excess of water and as such are interdependent.
Projects designed to control flooding throughout the year are called Full Flood Control and Drainage Projects; projects designed to delay flooding (that is, control pre-monsoon floods) are called Partial Flood Control and Drainage Projects.
Full flood control and drainage projectsFull flood control projects are associated with high flood embankments constructed above the annual flood level. These projects are designed to protect a certain area from the inflow of external floods, while allowing accumulated rainfall runoff to flow along internal channels to either gravity drainage structures or pumps.(1) Depending on topographical and flooding conditions, the area can be protected with a ring embankment, a linear river embankment, or a bottom-open embankment. The ring embankment encircles the area and is used in flatter areas; the linear river embankment runs along a river and protects the adjacent area from river spills; and the bottom-open embankment protects the upper sides of a sloping area affected by flood spills from above, while leaving the lower end open for drainage, fish migration, and navigation.
According to BWDB guidelines, high full flood control embankments are designed for the 1:20-year flood to protect agricultural land, and for the 1:50 or 1:100-year flood to protect infrastructure and populated areas. Internal drainage channels are designed to convey within banks the project basin flood from the 5-day, 1:10-year return period rainstorm. In anticipation of siltation, the channel section is usually designed to be 20 to 30% larger than this.
The capacity of the gravity drainage structures should match the design discharge of the drainage channels. Structures are designed for operation under a hydraulic head of up to 0.30 m. Pumping station capacities are determined from an economic optimization exercise that involves flood frequency, flood damages, and the station's capital and O&M costs.
FWO flood levels are determined mainly from existing river gauge records, usually located on the periphery or outside the project area. Flooding from ponded rainwater (flatter areas) and from flood waves from rivers (both sloping and flatter areas) are both considered. Sometimes temporary gauges are established in the project area. Existing hydrological data may be adjusted to reflect known trends, such as rising flood levels, increasing rainfall, or siltation. The flood levels so obtained are then verified by field observations and interviews with local people. The extent of flood damage to crops, homesteads, and infrastructure is initially estimated from the flood levels and then cross-checked in the field.
FW flood levels can be determined from a water balance analysis (inputs less outputs), in the case of ring or river embankments; or from backwater analysis, in case of bottom-open embankments.
High embankments always increase discharges and water levels outside the project area, by confining to the river channel flood discharges that otherwise would spill onto the floodplain. Full confinement (high embankments on both sides of a river channel) means that the total flow is contained within the river channel; if a significant proportion of the flow was across the floodplain before the project, water level increases can be dramatic.
Partial flood control and drainage projectsPartial flood control is a special type of project designed to protect agricultural crops, usually boro rice, grown in the winter season and harvested during the pre-monsoon.
The crop protection is effected by lower, smaller-section `submersible' embankments intended to protect against pre-monsoon floods until boro rice harvesting is completed, and then to be overtopped by the higher monsoon floods, when the area is inundated and the monsoon floods pass over it. At the end of the monsoon, water from the basin drains back to the river system.
The current practice is to design submersible embankments to protection against 1:10-year return period external floods expected before the 15 May. Normally the crest of the embankment is 0.3 m above the design flood.
Flushing/drainage regulators are provided in the embankments to facilitate (1) filling in (flushing) of the project with water after the boro harvest is complete, so as to reduce the damage of embankments during overtopping, and (2) drainage in the post-monsoon period. Structure size is determined by:
Pre-monsoon flushing requirements. The capacity of the structures should be sufficient to allow the basin to fill in up to 0.3 m below the embankment crest by the time the embankments are overtopped, to limit damage to the embankment during overtopping, and
Post-monsoon drainage requirements. The head difference across the structure should not exceed 0.3 m, to discourage farmers from cutting the embankments to accelerate drainage.
Normally, the expected flushing flows are larger than drainage flows and therefore determine the size of the structures. The internal drainage system is designed for the pre-May 15, 5 day, 1:10-year return period rainstorm.
Partial flood control projects should not, in theory, reduce monsoon floodplain storage and thus should not increase monsoon flood levels. The recent trend at some locations, however, has been for pre-monsoon flood levels to increase, to the point where they are approaching monsoon flood levels. In parallel with this, embankments designed to protect from pre-monsoon floods are increasing in height; in fact they are becoming improperly designed full flood embankments, with insufficient set-back distance and weak cross sections. For example, raising a number of submersible embankments is planned under the Systems Rehabilitation Program to levels such that overtopping will occur only every second or third year (see also Section 8.4.2 below).
The local and systemic implications of this trend can be quite serious as projects increasingly produce full flood control impacts rather than partial flood control impacts: decreased monsoon floodplain storage, much greater magnitude negative impacts on fisheries, greater drainage congestion (greater water volume must be evacuated through structures), water quality deterioration, and so on.
Dry season water levels in flood control projectsHow much water is retained, given the physical parameters of the drainage channels and structures and external water levels, may depend on how structures are operated, which can be a sociopolitical matter between different interest groups (as it may be in the FWO condition if earth closures are being constructed and cut under local initiative). Low-lift pumps are widely used to redistribute water to allow boro plantation, for irrigation, and to facilitate fishing operations.
3.3.3 Channel Re-Location and Loop CutsTypically, loop cuts and channel re-locations are made to shorten channel length, thereby improving navigation, or to increase channel slope, thereby increasing flow velocities and reducing flood levels. Loop cuts are also made to arrest bank erosion associated with channel shifting or meander migration. Figure 3 shows an example of artificial loop cuts on the Baulai River.
Some past projects that have utilized loop cuts include:
Pilot channels will not enlarge satisfactorily if the water level difference across the neck of the bend is too small, or if the bank materials in the pilot channel resist erosion. Where this is the case, a local constriction may develop in the channel and water levels may increase rather than decrease as a result of the works, particularly if the former main channel becomes silted in. Local constrictions of this type have developed along the Kalni River and Baulai River at former loop cuts, probably as a result of cohesive bank materials in the channels. At such sites, excavation or dredging is required to enlarge the channel to its full cross sectional area. In the past, this has seldom been considered feasible, due to the large volume of excavation required and short length of the construction season.
Methods for designing the initial dimensions of pilot cuts so that they will enlarge to full cross sections are available (Petersen, 1986), but the geotechnical and hydraulic information required for this type of analysis is rarely available. As a result, loop cuts in the Northeast Region have not followed formal design procedures.
Whether loop cutting and channel re-location are beneficial depends on the specific setting. Planning of these types of intervention needs to include assessment of both local, short-term impacts and longer-term impacts upstream and downstream of the project.
3.4 IMPLEMENTATION PRIORITIES, PHASING, AND SCHEDULINGPrioritization and scheduling are documented in Section 9.3 of the Regional Plan. Scheduling is shown in Regional Plan Figure 22.
3.5 SUBREGIONSSection 9.9 documents the impacts received by each of six subregions: Eastern Seasonally Flooded Area, Northern Alluvial Fans, Central Basin, Western Seasonally Flooded Area, Southern Piedmont Floodplain, and Peri-Urban Area. This purpose of this presentation is to relate Plan impacts to particular settings within the region.
3.6 STATUS OF PLAN COMPONENTS DURING IEE STUDYThe status of each of the 44 initiatives in the Plan portfolio is:
3.7 PROJECT ACTIVITIES
3.7.1 PurposeThe purpose of this section is to identify any and all normal and abnormal project activities that could cause environmental impacts (project-on-environment impacts), or that could be affected by environmental processes (environment-on-project impacts). The activities listed here appear as row headings in the environmental screening matrix (Figure 2).
3.7.2 FCD Projects
Preconstruction activitiesPreconstruction activities on FCD projects can include:
Operation and maintenance activitiesOperation and maintenance activities can include:
Abandonment activitiesAbandonment activities for FCD projects can include:
3.7.3 Other Types of ProjectsProject activities for the non-FCD projects were identified on a project-by-project basis and are identified in the various project studies.
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