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Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -

C. The optimisation and control of impoundment hydrology

On the basis of the above relationships, wetlands should be designed to retain the mass of nutrients and other contaminants moving into aquatic ecosystems during the nutrient-condensing stage of high/moderate floods, having the highest concentrations of nutrients. While it is not generally feasible to totally contain these flows during all precipitation events, designs should cover the low and moderate flow events. Wetland designs can accommodate a range of flow conditions by utilising inflow control devices that select specific levels at which inflow to the wetland can occur. These levels are commonly linked to flood stage, or a specific water level in the river at which water flows into the wetland. Outflow from the wetland can be similarly controlled using fixed crest or fixed diameter outflow structures that restrict the outflow and maintain a predetermined water level in the wetland. The outflow structure should be located opposite the inflow. Alternatively, the inflow structure could provide an outlet that would become active during the falling limb of the hydrograph. Any water remaining in the wetland would decrease over time as a consequence of evapotranspiration and groundwater outflows.

Flash floods of very high discharge and short duration commonly result in short reservoir retention times. In these situations, the nutrient concentration transported into the reservoir is often diluted and flushed out of the reservoir by the high volume of water entering and moving through the reservoir. This reduces the risk of eutrophication and lessens the likelihood of formation of toxic algal blooms. An high concentration of humic substances, transported in the runoff, could also reduce water clarity and limit the formation of algal blooms. In such situations, wetlands can be used for flood control and the reduction of flood-induced hydro-peaking (Figure 7.3).

D. Sediment trapping using hydrodynamics

The rate of nutrient and suspended solids retention in a wetland depends mostly upon the water retention time (WRT) in the wetland.

The theoretical water retention time in a wetland is a function of the wetland volume to water inflow volume ratio.

For a specific wetland area, the water retention time may be optimised by regulating the hydrological parameters of the wetland. This may be achieved by means of controlling the rate of water inflow to, and outflow from, the wetland.

The calculation of the theoretical total phosphorus reduction rate in a small wetland with a given water retention time may be calculated using the formula of Tomlinson et al. (1993). The rate of total phosphorus reduction (TPR) in a reed wetland within an urban catchment can be estimated as:

TPR = 42.41 (WRT) 0.147

where: TPR - total phosphorus reduction [%],
  WRT - water residence time [days].

Calculations based on the above equation may vary significantly depending on the land structure, plant cover, water retention time during particular flood phases, volume of water directed into the wetland area and initial concentration of nutrients. The theoretical reduction in TP load transported by a river into a reservoir increases logarithmically with time.

  • The larger the wetland area, and hence the longer the water retention time, the more effective are the sedimentation processes and phosphorus trapping;
  • Water retention times of up to about 15 days increase the efficiency of total phosphorus retention;
  • A network of intermediate-sized wetlands, with shorter water retention times, yield better results than with a single large wetland with a longer water retention time;
  • Water retention time may be controlled by controlling the volume of water flowing into or out of the wetland (Figure 7.4).



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