Newsletter and Technical Publications
Freshwater Management Series No. 5
Guidelines for the Integrated Management of
- 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
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
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
||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
- 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).