The Effects of Tide Gates on Estuarine Habitats and Migratory Fishish
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The Effects of Tide Gates on Estuarine Habitats and Migratory Fishish by Guillermo R. Giannico and Jon A. Souderouder ikes and tide gates have been used the flow of upland water through creeks or worldwide for several centuries to sloughs into estuaries or rivers. A flood box D drain wetlands, both in estuaries might be as simple as a single culvert running and in the lower sections of rivers, which through a dike wall or as complex as a small, are influenced by tides. Wetland draining bridge-sized, concrete structure that includes has been carried out either to convert lands two or more culverts, deflection wing walls, into agricultural use, to control populations and upstream and downstream pilings (see of mosquitoes and other insects, or to allow figures 1a and 1b). In all cases, doors or lids urban development on low-lying coastal zones are attached to the discharge ends of the (Daiber 1986; Middleton 1999; Doody 2001). culverts to control water flow. These doors In the Pacific Northwest region of North are commonly referred to as tide gates, or America, the draining of estuarine wetlands flap gates (figure 1b). Tide gates close during began approximately two hundred years ago incoming (flood) tides to prevent tidal waters (Dahl 1990). Tidal marshes close to seaports from moving upland. They open during and urban centers have been particularly outgoing (ebb) tides to allow upland water to vulnerable to conversion, with losses of 50 flow through the culvert and into the receiv to 90 percent reported for many estuaries in ing body of water. Figure 2 illustrates the Oregon and Washington (NRC 1996). Many entire gate cycle as water levels change. of these marshes have been isolated from Tide gates tend to be effective at main the adjacent estuaries by dikes (Frenkel and taining low water levels on the upland side Morlan 1991) and in some cases completely of dikes. Unfortunately, by altering water or partly filled in to accommodate a variety flow they have some undesirable side effects of land uses (for example, agricultural, recre that can be classified into three main—but ational, residential, and industrial). In areas interrelated—categories: physical, chemical, like Coos Bay, Oregon, almost 90 percent of and biological. tidal marshes have been permanently lost to The physical effects of tide gates in dikes and landfills (Schultz 1990), and in parts clude elimination of upland tidal flooding of Puget Sound, Washington, over 95 percent and changes in the velocity, turbulence, and of tidal wetlands have been lost (Gregory and pattern of freshwater discharge that fluctu Guillermo R. Giannico Bisson 1997). ates between water stagnation and flushing is on the faculty of the Department of Fish Dikes are elevated earthen embankments flows. In turn, these changes in the circula eries and Wildlife at raised along tidally influenced channels in tion of water between both sides of a dike Oregon State Univer estuaries and coastal sections of rivers to keep cause alterations in water temperatures, soil sity, and Jon A. Souder low-lying lands from being flooded during is executive director moisture content, sediment transport, and of the Coos Watershed high tides. Structures known as flood boxes, channel morphology (Vranken and Oenema Association. or tide boxes, are installed in dikes to control 1990; Charland 1998). © 2004 by Oregon State University small “pet doors” (figure 5) (Char land 2001). Tide gates open and close because of water level differ ences between the downstream and the upstream sides of the gate. Water a level differences are caused by (1) tidal cycles (and magnitudes), (2) inflow into the reservoir pool that forms on the upland side, and (3) the extent to which this reservoir pool has been drained during previous gate-opening cycles. The amount of water level b difference required to open a tide gate is determined by the “effective weight” of the gate. For top-hinged gates, both the amount of time and the degree of opening are functions of the weight and area of the gate and the pressure force generated by the difference in water level between both sides of the gate (Raemy and Figure 1. Flood box (culvert with tide gate) installed in dike’s wall: (a) top view; Hager 1998). Thus, the difference (b) side view. in water level needed to open a gate is greater the larger the gate and the more it weighs (USACE 2001). The chemical effects of tide passage. Thus, the effect that flood Conversely, for side-hinged gates, gates consist of upstream increases in boxes have on the aquatic environ the effective weight of the gate is water nutrient concentration, tur ment depends on a combination of a function of the resistance of its bidity, and heavy metal suspension things: culvert diameter relative to hinges and the degree to which the and reductions in dissolved oxygen the upstream channel; culvert surface door is tilted downward. In general, and pH (that is, water alkalinity) roughness; channel bottom charac and because of the opening force (Portnoy 1987; Vranken and Oenema teristics both upstream and down caused by their tilt, side-hinged gates 1990). Soil salinity is also reduced stream from the flood box; culvert open wider and for longer periods because tide gates prevent brackish bottom (also known as invert or sill) with less upland water pressure than tidewaters from reaching past dikes, elevation relative to the channel equivalently sized top-hinged gates. and the freshwater that is allowed bottom and to tide levels; type of tide Top-hinged tide gates may have “pet to drain toward the estuary removes gate; wing wall orientation; presence doors” that are hinged at the top, salts from soils over time (Dreyer of pilings in the channel; and other side, or bottom. These smaller doors and Niering 1995). features of the flood box (figures 1a remain open for longer periods than The biological effects of tide and 1b). the large gates they are part of and, gates come in the form of obstruc There are many different types as a result, they facilitate fish passage tion to fish migration, changes to the of tide gates, but the traditional (Charland 2001). composition of aquatic plants, and configurations have either a heavy lid Unfortunately, tide gates can pulses of coliform bacteria into estu (made of treated wood) suspended easily be jammed by floating pieces arine waters during low tides (Eliasen from the top of the culvert by either of wood, and, over time, they tend to 1988; Charland 2001). a bar with hinges or chains (figure hang twisted from their hinges. This Although the tide gate itself is 3a), or a metal lid (usually of cast changes the way the gates operate what stops water and fish movement, iron or steel) double hinged from and creates problems in terms of it is important to recognize that the above (figure 3b). Alternative designs both fish passage and flood control. design of the entire flood box affects include side-hinged gates (figures Regular inspections are recommend the circulation of water and fish 4a–c) and top-hinged gates with ed to avoid this type of problem. 2 The Effects of Tide Gates in Estuarine Habitats and Migratory Fish Tide Flooding Tide Flooding a b Tide Ebbing Tide Ebbing c d Figure 2. Side view of tide gate: (a) closing, as upstream water level drops and tide level (see arrow) begins to increase; (b) closed, as tidal water level gets higher (see arrow) than upstream freshwater level; (c) opening, as tidal water level becomes lower (see arrow) than the freshwater level inside culvert; (d) open, during low tide (see arrow) before upstream water level begins to decline. is because water temperatures during Physical Effects Changes to Water Temperature Water temperature not only the spawning periods are below the of Tide Gates plays a critical role in affecting spawning temperature standard. Because tide gates cause freshwater Changes to Channel Morphology the speed at which many chemi cal processes occur (Richardson stagnation and restrict tidal inflow, Channel morphology is altered and Vepraskas 2001), but it is also they tend to increase upstream water by tide gates in two ways. First, up extremely important in determining temperatures. The increased peri stream scour tends to form an inlet the suitability of habitat for different od of water circulation allowed by pool, and the water jet through the aquatic organisms. Water tempera side-hinged gates may result in lower culvert forms a deep scour pool on ture criteria are established under water temperatures by reducing the outlet end. Second, if the flood the Clean Water Act for salmon and freshwater stagnation and augment box is replaced and the bottom of the trout spawning (55˚F) and nursery ing cooler estuarine inflow. culvert is set at a lower level, then (64˚F) habitats (ODEQ). In Oregon upstream erosion of the accumulated and Washington, limiting tempera sediments could result in changes to tures in coastal streams typically the channel morphology. affects summer nursery habitat. This The Effects of Tide Gates in Estuarine Habitats and Migratory Fish 3 a Figure 3. Top-hinged tide gates (a) suspended with chains, (b) held in place by metal arms, and (c) illustrated. b Chemical Effects the amount of tidal exchange (that is, the difference between high and of Tide Gates low tides for any specific cycle) and by the amount of freshwater entering Changes to Water the estuary from tributary streams. Salinity In the Pacific Northwest, freshwater Salinity in wetland flow into estuaries varies by season, soils determines their with high flows during the winter oxidation-reduction and spring and low flows during the potential, ultimately summer and fall.