III. EMP area suitability constraint: The suitability conditions for the considered EMPs (suitable soil type, slope range and elevation) are taken from Sarma (2011) and
7.3 Results and discussions
The revised OPTEMP-LS model has been solved by using the Linear Model Solver Tool in Microsoft Excel application. The result shows that for the 2015 LULC scenario,
implementation of the considered EMPs makes possible to manage the peak runoff and sediment yield from the watershed within the permissible limits satisfying all the given constraints. However, for the 2025 LULC scenario, due to the projected increase in urban settlement, it is not possible to keep the peak discharge from the watershed within the maximum allowable limit and hence, the model gives an infeasible solution.
Sarma et al. (2015) mentioned that installation of rainwater harvesting system (RWH) along with the considered EMPs is a good alternative to bring the peak discharge within the permissible limit. It is found that by adopting roof top rain water harvesting, the runoff coefficient of the built-up area can be reduced by 20% in Guwahati city (Sarma et al. 2006). From last few years, GMDA is giving utmost importance to the adoption rainwater harvesting system and it is now made compulsory in building bye laws of the city. Therefore, it has been considered that till 2025, RWH schemes will be strictly implemented in Guwahati city and consequently, in 2025, the runoff coefficient of the impervious area has been expected to reduce by 20%. Under this consideration, a feasible solution is obtained for the 2025 LULC scenario. The model has been run for both with and without consideration of owner's choice constraints. Table 7.4 and Fig.
7.1 depict that the constraint "owner's choice for EMP" plays a vital role in determining the EMP combination and total costs of implementation. With respect to the 2015 LULC scenario, there is a 178.39% increase in total cost due to the consideration of
"owner's choice" constraint. This is because, with consideration owner's choice constraint, the model chooses all types EMP covers. Whereas, without consideration of this constraint (for 2015), it chooses the inexpensive EMPs like garden and detention pond as the optimal EMPs for the bare settlement area of hilly and plain portion of the watershed. On the other hand, for the bare steep hill cut area, "grass" has been chosen as the most optimal EMP as its cost is very less in comparison to another EMP (retaining wall) of steep hill cuts (Fig. 7.1). Another reason for the difference in total costs of these two cases is the owner's willingness to cover a minimum 50% of bare steep hill cut area by retaining wall, whose cost is the highest among all the EMPs.
Additionally, owners have shown a little interest for the less expensive EMPs like detention pond and forest. Table 7.4 also shows the increase in the total cost of EMPs with the increase of urban settlement in 2025. For a 36.73% increase in the urban settlement of hilly area of the watershed from 2015 to 2025, there is a 33.44% increase in total cost of EMPs to keep the sediment loss and peak runoff within the permissible limits.
Table 7.4: Results of revised OPTEMP-LS model
Computed parameters
2015 2025
With owner's choice
Without owner's choice
With owner's choice
Without owner's choice
Cost (×107 Rs)
Total=10.24 Plain= 0.85 Hill= 1.97
Steep hill cut=7.42
Total= 3.68 Plain= 0.80 Hill= 1.88 Steep hill cut=1.00
Total= 13.67 Plain= 0.85 Hill= 2.69 Steep hill cut=10.13
Total= 4.69 Plain= 0.80 Hill= 2.53
Steep hill cut=1.36
Sediment yield (t/yr) 2602.15 2602.15 2602.15 2602.15
Peak runoff (cumec) 4.00 4.00 3.851 3.998
Fig. 7.1: EMP areas for different scenarios
Again, sediment yield and peak runoff under different cases have been compared in Fig. 7.2. After the implementation of EMPs (Case 3), the annual sediment yield from the watershed becomes nearly equal to that in the natural land cover condition (Case1), which is only 7.6% of the sediment yield from the watershed without implementation of EMPs (Case 2). For all the cases of EMPs implementation (i.e. Case 3, Case 4, Case 6 and Case 7) sediment yields from the watershed are same unlike the dissimilar values of peak runoff. It is observed that peak runoff from the watershed has not reduced as much as sediment yield after the implementation of EMPs. It is 91.4% (Case 3) of the peak runoff generated from the watershed without implementation of EMPs (Case 2). This is because, here, the EMPs have been applied in the 40% of the settlement area (Sarma et al. 2015). Though EMPs applied in this 40% area reduces the sediments and peak runoff, the residual 60% of the urban settlement area, which is impervious, produces a high amount of surface runoff but no sediment. As a result, reduction in sediment yield is much higher than that of peak
0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000
2015 (with owner's
choice)
2015 (without owner's
choice)
2025 (with owner's
choice)
2025 (without owner's
choice)
Area ( sq. m)
Scenarios
Xh1 Xh2 Xh3 Xh4 Xp1 Xp2 Xp3 Xp4 Yh2
runoff. Again, after the implementation of EMPs, the lower peak runoff values for the 2025 LULC scenario than those for the 2015 LULC scenario indicates that the installation of RWH system along with the other considered EMPs is quite effective in reducing the peak runoff.
(i)
(ii)
Fig. 7.2: Comparison of (i) sediment yield (ii) peak runoff under different cases.
(Case 1: Natural land cover condition, Case 2: Without EMP in 2015 LULC scenario, Case 3: Application of EMP with the owner's choice constraint in 2015 LULC scenario, Case 4: Application of EMP without the owner's choice constraint in 2015 LULC scenario, Case 5: Without EMP in 2025 LULC scenario, Case 6: Application of EMP with the owner's choice constraint in 2025 LULC scenario, Case 7: Application of EMP without the owner's choice constraint in 2025 LULC scenario)
0 9000 18000 27000 36000 45000
1 2 3 4 5 6 7
Sediment yield (t/yr)
Cases
0 1 2 3 4 5
1 2 3 4 5 6 7
Peak runoff (cumec)
Cases
In this study, due to the consideration of soil loss from steep hill cuts, the watershed management (EMP) cost per unit settlement area in the hilly portion of the watershed is quite higher than that in the plain area (Fig. 7.3). Particularly, with consideration of owner's choice constraint in 2015 LULC scenario, the EMP cost per unit settlement area in the hilly portion becomes 4.77 times the unit cost for the plain portion of the watershed and it is 1.55 times without consideration of owner's choice constraint. Therefore, it can be said that a misinterpretation of the total cost of the EMP project can result from the application of OPTEMP-LS model for non-consideration of steep hill cut areas in soil loss estimation.
Fig. 7.3: Comparison of EMP costs per unit settlement area (2015) between hilly and plain area of the watershed