PRIORITIZATION OF DEGRADED WATERSHEDS
4.5 ASSESSMENT OF SEDIMENT CONTROL CAPACITY OF SOME SELECTED COMPETITIVE EMP S
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1000 V
P 2 HCO M
3 − ×
− = (4.4)
Where, V= Volume of sample taken (ml)
Bicarbonate Alkalinity as HCO3- (mg/L) = 1.22 × Bicarbonate Alkalinity as CaCO3 (mg/L)
4.4.4.4. Determination of Hardness
The EDTA hardness of the water samples were determined by the titration method (APHA, WEF, AWWA, 1998). The hardness of the samples was determined as below:
S 1000 B ) A CaCO (
Hardness 3 × ×
= (mg/L) (4.5) Where
A= ml titrant for sample
B=mg of CaCO3 equivalent to 1 ml of EDTA titrant (=1) S= Sample volume (ml)
4.5 ASSESSMENT OF SEDIMENT CONTROL CAPACITY OF SOME
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species. Thus, this simulation study gave a qualitative estimation regarding efficiency of grass in controlling sediment yield.
Parajuli et al. (2008) applied the SWAT model to see the effectiveness of vegetative filter strips (VFS) (lengths 10, 15 and 20 m) in removing sediment and fecal bacteria in the Upper Wakarusa watershed (950 km2) in northeast Kansas. The following equation was used for finding the effectiveness of VFS in SWAT model:
trapef =0.367×(widthfiltstrip)0.2967 (4.6)
where trapef is the fraction of the constituent loading trapped by the filter strip, and widthfiltstrip is the width of the filter strip (m). Based on this, it was estimated that about 73% of sediment yield can be reduced with a 10-m, 82% by a 15-m and 89% by a 20-m VFS. Here also, the equation used is a generalized form and for accurate estimation, the coefficient and exponent of the equations may vary with type of vegetation and topographic and soil condition.
Knowing the effectiveness of vegetative measures in controlling non point source pollution, various field based studies were conducted on VFS by many investigators to see their efficiencies in controlling sediment yield. Gharabaghi et al. (2006) conducted a detailed field based study with six different filter strips having combination of different species and tested their efficiency in terms of type & width of strip, and runoff flow rate and inflow sediment characteristics with artificial water supply. The study concluded that sediment removal efficiency increased from 50 to 98% as the width of the filter increased from 2.5 to 20 m. In addition, grass type and flow rate were found to be significant factors.
Another field based study was conducted by Shiono et al. (2007) in Okinawa, Japan for centipede grass (Eremochloa ophiuroides) for different strip lengths (0.5,1.5 and 3.0 m) with a 4.0-m by 31.5-m bare source area under natural conditions. The study has reported
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that the sediment removal efficiencies of 24% for the 0.5-m strip, 36 to 54% for the 1.5-m strip and 73% for the 3.0-m strip. The study also found that strips trapped well the sediment aggregates larger than 0.02 mm in diameter, regardless of strip length. Also, the longer strip trapped more aggregates of the 0.002–0.02-mm size class, which were dominant in the eroded sediment runoff from the plots. The strips poorly trapped aggregates smaller than 0.002 mm. These field based experiments were conducted basically to find out the efficiency of vegetative strips in terms of sediment trapping form sediment laden water. However, the study did not investigate the capability of vegetation in minimizing sediment erosion from the area itself, which in fact is an important aspect of study.
To have an idea about erosion control efficiency of different EMPs, an experimental set up having facility of studying performance of 3 EMPs at a time was constructed in the study watershed (Figure 4.7). Three strips, each of length 4 m along the slope and of width 2 m across the slope were constructed for installing different EMPs.
Varieties of grass species and other type of herbs, apparently having similar erosion control capacities, were identified. Based on convenience one indigenous common grass locally called bon in Assamese language and buffalo grass in English (Scientific name: Paspalum conjugatum (Poaceae) ) was selected for the study(Figure 4.8). The other selected species is a herb called Golden Glory or Wandering jew, (Scientific name: Tradescantia zebrina (Commelinaceae) was considered for the comparative study (Figure 4.9). The reason of selecting this grass is that it is widely available and also grows easily even in undulated hilly terrain. The Golden glory is an ornamental herb having aesthetic view and thus this type of herb may be preferred for urban areas by some people. Another advantage of this species is that once grown, it requires minimal effort for maintenance and gives a good coverage within a short period.
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Figure 4.7: Experimental setup for performance analysis of EMPs
Figure 4.8: A close view of the grass used as EMP in the study
Figure 4.9: A close view of Golden glory used as EMP in the study
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Erosion control capabilities of these two vegetative measures were compared with that of bare land. All the strips were of equal area and similar slope condition were maintained in all the strips. Sediment traps were constructed at immediate downstream of each of these three strips. The three sediment traps were basically constructed by dividing a large chamber into three sub chambers, so that the sediment coming from these three strips can be collected separately in the three chambers and the relative efficiencies of these strips can be compared by weighing the amount of sediment that was collected in each of these three chambers. Each chamber was of 0.5 m depth, 2 m in length and 0.8 m in width.