Proposal for Fish Pass Structures in the Northeast Region 1. General Requirement The requirement for fish pass facilities in aid of fish migration in the Northeast Region appears to be limited to the pre-monsoon period when adult fish need access from rivers to haors (in-migration) and from haors to rivers (out-migration). At existing flow control structures as well as at locations were new regulators are planned between rivers and hoars, migrating fish are and would be encountering adverse hydraulic conditions in both upstream and downstream directions and at the same time period. This unique situation would require separate facilities of different design for each direction. Fish passage facilities could be incorporated in the design of new flood control structures. Existing control structures would likely require major modifications. The main and important biological parameter in the design of fish pass structures is that the facility must accommodate the weakest fish within the weakest species requiring passage. A further requirement is that the size of the facility should be based on the expected numbers of fish that need passage and the expected hourly or daily rate of ascending migrants. 2. Hydrology Each of the existing flood control structures and new structures planned are all individually unique as to site hydrology. Information needed for design of fish pass structures include site specific data such as forebay and tailwater rating curves, local topography, magnitude of flow and velocities and water surface profiles along each bank upstream and downstream of the proposed fish pass location. Combined with biological requirements, hydraulics and physical parameters will determine the type of fish pass facility that will suit a specific dam structure at flood control projects in the Northeast Region. 3. Hydraulics Thorough knowledge of the hydraulics at existing and new flood control structures are imperative for planning and design of fish pass facilities. Theoretical principals combined with actual site observations and possible modeling are tools used to appropriately locate fish pass entrances and exits and determine the scale of the facility and passage flow in combination with biological requirements. Observing the behaviour of fish and the flow pattern downstream of existing structures are often the best method of locating a successful fish entrance to a bypass. Fish entrances based on theoretical (hydraulic) considerations alone often need modifications for improvements after construction. Model studies can be a valuable tool in helping the designer understand the fish pass and fish entrance setting. The outflow from the fish entrance should be void of excessive turbulence and have adequate depth for fish to readily enter. High velocity sluice water or hydraulic jumps adjacent to fish entrances make fish seek the acceptable velocities through a bypass. 4. Conceptual Layouts of Fish Pass Facilities Fish passes or fishways are channels or series of pools installed to aid fish in overcoming natural or artificial obstacles to migration. A small amount of water is diverted-from the river or forebay of a dam upstream of the obstacle. The diverted flow is controlled throughout the length of the fish pass by means of baffles, weirs or orifices to reduce water velocities to the point where the fish migrating upstream can swim without too much exertion. Allowance for pools within the fish passage provide rest areas so that fish can recover after darting or bursting through baffle slots or up across weirs. The main concern in the hydraulic design of fish passes is to determine the best way to control the water flowing through the structure to dissipate its energy most efficiently and without hindering the swimming ability of the fish. There is however, no rational approach to the problem of energy dissipation in a fish pass and most existing structures have been copied from original designs made with the aid of hydraulic model studies. The ultimate design would be a large-scale model where flow pattern, energy dissipation and surface profiles can be observed with live fish to allow improvements when required. Other than acceptable hydraulics, the main requirements when planning a fish pass are: 1. Sediment bedload, debris and polluted water must not interfere with the flow 2. The fish pass should be designed so that the least amount or at best no manual control would be required for maintenance of flow through the structure 3. The operating water level range must satisfy fluctuations in headwater and tailwater 4. The flow through the fish pass must be adequate to attract fish at the downstream end (fish entrance) 5. The upstream end of the fish pass (fish exit) must be located so that fish leaving will not be swept down over obstructions or dams. 6. The size of the fish pass must be based on the numbers of fish expected to use the facility. 7. This last and probably most important aspect of fish pass design is the location of the fish entrance. Other than adequate attraction flow, the entrance should be located and aligned so that the migrating fish will not be delayed unduly by turbulence which confuse direction and cause "traffic jams". See Figure 1 for examples of poorly and well located fish entrance locations at dams. In general, fish pass designs are of four main types as illustrated in Figure 2, Vertical Slot Fishways will operate over a wide range of water levels without adjustments. Upright vertical panels and vertical slots full height placed at intervals between fish entrance and exit areas dissipate flow energy, allow resting in pools between vertical panels. Short jets of water at low head through the slots enable the fish to dart or burst very short distances (about 1 m) from pool to pool. Vertical slot fishways are used extensively in Europe, North America and Japan to bypass river obstructions, diversion dams and power dams. Pool and Weir Fishways resemble staircases of water and are usually built when water levels at upstream and downstream ends do not fluctuate widely during the period that fish are migrating. A series of partitions or weirs are installed across the fish pass channel with water cascading over the crest from pool to pool. The head across the weirs is designed to suit the fish type(s) which require passage assistance. The overflow could be free fall or submerged. Fish will jump over the weirs from pool to pool or orifices could be provided in the baffle wall for swim-through. The pool and weir type fishway could be adapted for variable head and tailwater levels by telescoping weir crests or adjustable hinged weirs. Adjustable weirs require continuous mechanical or manual operation and are very costly. Denil Fishway or Steep Pass Fishways are narrow channels with closely spaced baffles on both sides and bottom and set at an angle to the axis of the aligned structure. The baffles form secondary channels at an angle with the main central flow and the energy is dissipated by intense mixing. This fishway type is generally used with small numbers of migrating fish or on a temporary basis and is not conducive to variable head or tailwater levels. Fish Locks, as the name implies will move fish in and out as well as up or down over obstructions and dams. The structure requires entry, exit and bypass gates or sluiceways by mechanical or manual operation. The main drawback is interrupted passage and lack of attraction flow. At specific locations, a design, that incorporates a combination of some of the above types of fish passes, could be adapted. At embankments and regulator dams in the Northeast Region the forebay levels normally vary independently of tailwater (haor side) during the pre-monsoon period. For these conditions a vertical slot fishway appears to be the logical option for covering the fish pass problem and will be discussed in further detail. Design of vertical slot fishways is normally based on hydraulic considerations combined with the expected number of migrants and their rate of arrival at a river obstruction or at dams, embankments & flood control structures as in Bangladesh. For conceptual and possible pilot project purposes in aid of fish passage across embankments or dams in the Northeast Region a copy of the pool and slot configuration for a fishway at Seton River Dam in British Columbia, Canada has been adopted. The Seton River Dam Fishway layout was originally model tested and has proven very successful. A similar fishway built at Hell's Gate in the Fraser River, British Columbia in 1989 (P.B. Saxvik, 1990) proved by visual count to pass about 12500 migrating adult salmon (each weighing about 2.2 kg) per hour up through a 9 pool structure with a total head of 3.0 m. The single vertical slot fishway proposed as a pilot project is a 10 pool structure with 12 baffles, each with a max head drop of 0.25 m to overcome a total forebay to tailwater difference of 3.0 m. The basic configuration of a suggested vertical slot fishway pool and slots are shown in Figure 3 and Figure 4. Each resting pool between baffles is 2.44 m wide x 3.05 m long. The clear width of each vertical baffle slot is 0.41 m. The discharge through the vertical slot varies with depth and is calculated by Equation 1: Q = C(D+h)W√2gh. Where; Q = discharge in cubic metre per sec (m3/s) C = coef. for discharge through orifices = 0.75 W = total with of slot (m) D = depth of water at downstream side of baffle (m) h = drop in water surface from the upstream to the downstream side of a baffle (m) (√ denotes square root) The calculated discharge for the proposed structure is approximate only since approach velocities are not accounted for. The velocity of the water jet through the vertical slot in the baffle is determined by Equation 2: V = C√2gh. Based on Equation 1, the discharge (Q) through the vertical slot fishway for depth (D) of one metre, head (h) of 0.25 m is 0.85 m3/s. The velocity of the water jet through the slot is independent of water depth and based on Equation 2, equals 1.66 m/s. The minimum volume of water through the fishway or min. depth of water depends on specie of fish that migrate. For carp and catfish the minimum depth is assumed as one metre. For this min. depth for migrating fish, the water volume within one fishway pool would be 7.44 m3. The migration capacity of a fishway depends on the acceptable density of fish within a pool during ascent and the swimming ability of the fish (rate of travel in m/minutes) through the structure. The pool (water) volume recommended for fish is about 0.025 m3/kg. For large carp or catfish at 30 kg weights the minimum pool volume per fish would be 0.75 m3 and the acceptable density at minimum operating depth of one metre would be 10 fish. For the average size carp or catfish weighing about 7.5 kg, the acceptable density would be 40 fish at minimum flow through the structure. Without actual tests or recording of swimming abilities of carps or catfish through a vertical slot fishway, the estimated rate of travel was conservatively assumed based on values for other fish. The rate of travel for Sockeye Salmon through a fishway has been recorded at approx. 4 m per minute (P. Saxvik, Dec. 20, 1990) equivalent to passing through a 3.05 m long pool in 0.76 minutes. Earlier data (C.H.Clay, 1961) indicated travel time for salmon in the range 2-5.8 minutes per pool. Adopting the conservative speed of 6 minutes of travel time for carp or catfish through the pool, the capacity in the proposed fishway at min. depth (1.0 m) would be 100 fish per hour based on large (30 kg) fish and 400 fish per hour based on the average size (7.5 kg). At periods of maximum river flood levels when forebay/tailwater differences at regulator dams would be approximately 3 m, and depth of water flowing through the fishway, about 5 m. The capacity for large and average size carp or catfish will be 500 and 2000 fish per hour respectively. A poorly located fish entrance, which is avoided by, or delaying the migrants could reduce these theoretical capacity numbers. Conversely good location and alignment choices could possibly improve the capacity. 5. Proposed Fishways Fish passes to be planned for at regulators and embankments between rivers and haors in the Northeast Region would be required to accommodate fluctuations in head water levels of up to 3-3.5 m during the pre-monsoon period. Fish migrating from the haor to the river (out-migration) would ascend from low to high water levels whereas in-migrating fish would move downstream in opposite direction from river to haor. Out-migration (upstream moving fish) can be accommodated by a vertical slot fishway which operates at variable water depths as proven on many occasions. However, downstream passage of fish (in-migration) through at the same time is questionable. Small fish, fry and fingerlings have been known to enter upstream ends of fishways and either get flushed or swim with the flow. No records are available indicating adult fish will do the same or if they would be adversely delayed and possibly injured by moving down a fishway. Because of this unknown factor it is suggested that fish migrating from the river to the haor be provided an alternate bypass through a fish friendly underflow sluiceway structure. Vertical slot fishways and sluiceway passage facilities will be assessed and incorporated in the same layouts. Plans and general details for an; 11 pool, 12 baffle fishway structure, fish sluiceway passage and fence for guiding the migrants are shown for two configurations. Alternate Layout I, Figure 5 and Figure 6, is a straight alignment with the fish entrance and exit a distance away from the regulator dam. In-stream fencing using bamboo, steel or other suitable material would guide out-migration from the haor at the downstream end and in-migration from the river at the upstream end (down fish exit).. A fish sluice gate is proposed to be incorporated with the regulator control and would be used if adult in-migrating fish will not enter the fishway and move down with the flow. The fish sluiceway layout include allowance for a stilling basin at the tailwater end. The stilling basin or deep pool at the tailwater end of the fish sluiceway as detailed in Figure 7 will dissipate the high velocity flow under the gate and provide a transition for the migrants to swim down the khal into the haor. The fish sluice gate or any other regulator sluiceway gate when fish are present must be opened fully to avoid injuries to the migrants. Flow out of the stilling basin across slots in the wall must be adjusted by stoplog control. After initial setting to suit tailwater conditions no further adjustments should be needed. Upstream (and downstream) passage of migrant fish through the fishway depends on adequate depth of water over the forebay and fishway invert (floor) levels. Adequate passage is expected with submergence of one metre. Ideally for a vertical slot fishway, which is self regulating the forebay (fish exit) and tailwater (fish entrance) levels change by equal amounts and the water depths and flow are the same at each baffle slot. However, at regulator dams for flood protection around haors in the Northeast Region, the forebay in the initial flood stages rises faster than the tailwater. To prevent hydraulic drawdown in the upper pools when water starts flowing through the fishway the water levels at the fish entrance need to be adjusted. The adjustment can be made by telescoping weir(s) at the fish entrance operated by float control at the forebay or otherwise by manually correcting using stoplogs in the fish entrance slots. The stream channel (khal) downstream of the fish entrance might have to be trenched and/or provided with berm weirs to facilitate a channel with adequate depth of water for fish to swim to or from the fishway. The flow through the fishway would be 0.85 m3/s at one metre of depth at the forebay of the regulator dam. At 5 m depth the flow would be 3.57 m3/s. The flow down the fish sluice way through the 0.7 m wide x 2.0 m, gate fully open would be approx. 8 m3/s. If not used for passing fish the sluice gate could be used to provide attraction water at the fish entrance. Such additional flow would also benefit fish migrating up or down the khal within the haor. Fishway Layout, Alternate II, Figure 8 and Figure 9 depict a folded alignment of the vertical slot structure shown for Alternate I. Hydraulically and in performance as fish passes, Alternate I & II are similar. Other than structurally being more integral with the regulator dam, Alternate II provides downstream fish pass (river to haor) exclusively through a fish sluiceway on the river side opposite the vertical slot fishway. The stilling basin at the downstream end of the fish sluice are similar for both layouts, for general details see Figure 7. The fish migrants are guided by fences constructed using bamboo, steel or other suitable material and with "finger gate" openings in either direction. It is apparent that the fish entrance and exit areas, the pools within the fishways and the finger gate openings in the fences upstream and downstream of the structure would be attractive locations for catching fish. Therefore, it will be imperative that rules and regulations are adopted to prohibit any commercial or subsistence fishery or fishing by individuals in the vicinity near the fish pass facilities. Such rules and regulations, however, will only be effective with adequate policing and enforcement and distribution of information about the project to the fishermen and general public.