Floodwater depth

Floodwater depth (Figure 8.33b) Periphyton activity is higher in wetlands with shallow water depth and decreases with an increase in water depth. In shallow water depths, mixing and currents increase the rate of exchange of ammonia between the water column and the... 

Figure 2.14 Effect of mixing by tubificids on fiux of P between soil and floodwater calculated with Equations (2.37) and (2.40). Numbers on curves are depths of mixing...

Figure 5.6 shows profiles of NO3 concentration measured with microsensors in soil cores taken from ricefields by Revsbech and co-workers (Liesack et al., 2000). During illumination of the cores, O2 generated in the floodwater penetrated to a depth of 2-3 mm and a clear peak of was apparent, produced in... 

Figure 8.9 Profiles of urea-N, ammoniacal-N and pH with depth following broadcast application of urea on ricefield floodwater, and the corresponding rates of NH3 volatilization (calculated with the model of Rachhpal-Singh and Kirk, 1993a,b)...

In soils, the pH of soil pore water tends to decrease with depth. The pH values are usually above 7 at the soil-floodwater interface and decrease to native soil pH at a lower depth. Some examples of soil pore water pH as a function of depth in selected Florida s wetland soils are shown in Figure 4.15. High pH values at the soil-floodwater interface are due to the photosynthesis activity of algae. 

Distinct Eh gradients are present (i) at the soil-floodwater interface, (ii) at the root zone of wetland plants, and (iii) aronnd the soil aggregates in drained portions of wetlands dnring low water table depths. 

The thickness of the aerobic layer can be determined by measuring Eh as a function of depth redox potentials show sharp gradients at the soil-floodwater interface. Laboratory studies have indicated complete disappearance of Oj at Eh <300 mV (pH = 7.0). This Eh value was used as a boundary between aerobic and anaerobic layers. A simple technique to determine redox profiles as a function of depth is described by Patrick and DeLaune (1972). This method involves a special motor-driven assembly that advances a platinum electrode at a rate of 2 mm h" through a soil profile. Redox potential is recorded continuously on a recorder or a data logger. Examples of Eh profiles are shown in Figure 6.22. 

Nitrate flux from the aerobic portion of the soil is controlled by (1) labile organic carbon supply in anaerobic portion of the soil, (2) thickness of aerobic soil layer, (3) water column depth, (4) mixing and aeration in the water column, (5) nitrate concentration, and (6) temperature. The flux of nitrate from the floodwater to underlying soil increases with an increase in temperature (Figure 8.59). At low temperatures, nitrate can diffuse to deeper layers into anaerobic zones. Under these conditions, it is likely that nitrate may play significant role in ANAMOX reactions as temperature optima for this reaction is between 10 and 15°C, as compared to denitrification that has temperature optima around 30°C (see Figures 8.39 and 8.47). 

Types of Floods. Damaging floods occur in varying locations. Flash floods are associated with quickly developing thunderstorms in mountainous areas. The rapid downslope movement of water, even if relatively shallow, can quickly move vehicles and their occupants to the extent that 50 percent of the flood deaths in the United States are caused by flash floods. If floodwater is flowing at a depth of only 2 feet, a trapped vehicle experiences a lateral force of 1,000 pounds and a buoyant force of 1,500 pounds, more than enough to flip the vehicle over and drown the passengers. 

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