Initial Environmental Evaluation, Northeast Regional Water Management Plan, Bangladesh Flood Action Plan 6

CHAPTER 2: REGIONAL PLAN ALTERNATIVES

2.1  FUTURE-WITHOUT-PLAN SCENARIO

2.2  REGIONAL PLAN ALTERNATIVES AND SELECTION OF 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).

3.1  PLAN OVERVIEW

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 MEASURES

3.3  BASIC PRINCIPLES OF FCD AND RIVER IMPROVEMENT INTERVENTIONS

3.3.1  Introduction

3.3.2  Flood Control and Drainage

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 projects

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 projects

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 projects

3.3.3  Channel Re-Location and Loop Cuts

Some past projects that have utilized loop cuts include:

• Kalni River, six loop cuts made between 1978-1983, shortening the river by 6 km;
• Kushiyara River near Sherpur, two loop cuts made between 1952-1963;
• Baulai River, eight loop cuts made between 1978-83, shortening the river by 12 km;
• Surma River between Sylhet and Sunamganj, isolated loop cuts made to reduce bank erosion at bends;
• Khowai River between Chunarghat and Shaistaganj, 6 km of loop cuts and channel re-locations made between 1970-1981 for flood control purposes.

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 SCHEDULING

3.5  SUBREGIONS

3.6  STATUS OF PLAN COMPONENTS DURING IEE STUDY

• One initiative (Improved Flood Warning), is documented in a NERP concept paper.
• Three initiatives (Ground Water Investigation, Institutionalize Public Consultation) are documented in Specialist Studies or the Training Plan.
• Two initiatives (Bhairab Bazar Erosion Protection and Jamuna Floodplain Floodproofing) are included in work to be carried out under other FAP projects, FAP 9B and FAP 23 respectively.
• Three (Urban Potable Water, Urban Sanitation, Village Water Supply) are endorsements of ongoing DPHE/UNICEF projects.
• All sixteen of the FCD initiatives are described in separate pre-feasibility studies. The measures described in these reports are non-overlapping, with the exception of the Baulai River Improvement pre-feasibility study, which includes drainage improvement measures also mentioned in the Surma-Kushiyara-Baulai Project and Jadukata-Rakti Project studies.
• Four non-FCD initiatives (Pond Aquaculture, Applied Research for Improved Farming, Fisheries Engineering, and Fisheries Management) are described in separate pre-feasibility studies.

• Flood- and Erosion-Affected Villages Development documents two initiatives (Improvement of Homestead Platforms and Village Afforestation);
• Water Transport Study documents two initiatives (Dredging for Navigation and Support to Country Boats); and
• Northeast Regional Environmental Management, Research, and Education Project documents the remaining 11 initiatives.

3.7  PROJECT ACTIVITIES

3.7.1  Purpose

3.7.2  FCD Projects

Preconstruction activities

• Surveying,
• Land acquisition, and
• People's participation activities.
• Construction activities

• Site preparation (vegetation removal; infrastructure removal/relocation; removal/resettlement of inhabitants/squatters; levelling; installation of temporary structures such as access roads, godowns, accommodations, garages and parking sites, cooking and eating facilities, waste disposal sites, water supply, drainage, sanitary facilities)
• Manual canal excavation (labour and material mobilization, topsoil removal, soil taking and transport, compaction, turfing, paving)
• Mechanical dredging (dredger operation, spoil deposition)
• Embankment construction (labour and materials mobilization, topsoil removal, soil taking and transport, compaction, turfing, paving)
• Structure (sluice gate, culvert, pump house, etc.) construction (labour and material mobilization, de-watering, excavation, pile driving, foundation works, structure construction, earthwork filling turfing, paving)
• Tube well installation (boring, distribution facilities, electrification)

• Suspension of construction before completion, construction delays
• Incorrect construction practices or techniques
• Infrastructure components

• Full flood embankments
• Submersible flood embankment
• Channel closures (in embankments)
• Regulators
• Weirs
• Re-excavated and new drainage/flushing rivers and channels
• River diversion structures and channels
• Roads
• Bridges
• Culverts
• Irrigation inlets
• Fish passes
• Boat passes

Operation and maintenance activities

• Pre-monsoon flood protection
• Monsoon flood protection
• Surface water irrigation
• Ground water irrigation
• Drainage
• Water retention behind regulators
• Agricultural activities resulting from project
• Water management activities

• Unintended pre-monsoon flooding due to extreme event, infrastructure failure
• Unintended monsoon flooding
• Embankment overtopping
• Under- and over-drainage
• Improper operation (public cuts, mistiming of scheduled O&M events such as gate opening/closure)
• River bed aggradation/degradation

Abandonment activities

• Re-occupation of infrastructure sites
• Reclamation of materials

3.7.3  Other Types of Projects

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