Subject: Preventing maximum flow in diversion being exceeded after inflow downstream of original diversion Posted: 2/8/2017 Viewed: 12512 times
I have a number of canals represented by diversions. I have set the maximum diversion to the design capacity. However there are a number of water sources that flow into the canals but the capacity of the receiving canal does not increase. Unfortunately WEAP allows the capacity to be exceeded if canals/rivers flow into a canal downstream of the original diversion. Does anyone have a work around to prevent the original design capacity being exceeded?
Ms. Stephanie Galaitsi
Subject: Re: Preventing maximum flow in diversion being exceeded after inflow downstream of original diversion Posted: 2/8/2017 Viewed: 12505 times
So the short answer is that if you have diversions flowing into the canal with nowhere else to go, WEAP will accept them and the capacity will be exceeded.
This begs the question, what happens in the actual watershed? Who decides how much water goes into the canal? In WEAP, the canal should not take more water than is demanded by any demand sites withdrawing from it. If these withdrawals happen before the inflows into the canal, the flow in the canal could be zero downstream. This begs the question - if the other flows coming into the canal are so large, why is its capacity so small? Is flood a regular occurrence in this area? If these constraints exist in the real world, what is the real world's response to these large (exceeding) flows in the canal?
Mr. Oliver Priestley-Leach
Subject: Re: Preventing maximum flow in diversion being exceeded after inflow downstream of original diversion Posted: 2/9/2017 Viewed: 12467 times
I understand what you are saying but maybe I was not clear enough. The capacity of the canal is 50m3/s at the headflow of the diversion. Along the canal there are many offtakes for irrigation plus a number of rivers that cross the canal in an aqueduct/weir system where the operator can decide how much water continues in the river or is diverted to the canal. The total demand along the length of the canal is more than 50m3/s but because of the inflows and outflows along the main canal its discharge rises and falls. However I don't want it to rise above 50m3/s in any Reach.
For each of the joining rivers I have added a diversion from the river to the main canal. As we agree, one would only divert enough water from the joining rivers up to the maximum capacity of the receiving canal (to avoid flooding), the rest of the water would continue in the river. So my question is what work around (with expressions and/or additional diversions) could one best use to replicate that decision process to ensure the main canal's capacity is never exceeded? Without any such constraints WEAP simply adds enough water from the joining rivers to the canal in order to meet the calculated demand which in reality would lead to flooding of the canal. I know that the canal will not be able to provide sufficeint water to meet demand so I want to show that with the existing constraints there will be an unmet demand.
I have tried adding the following expression to each diversion from the additional contributing rivers:
If(PrevTSValue(Supply and Resources\River\CH_C_Atbashy_Canal\Reaches\Below CH_C_Alamedin2Atbashy Inflow:Streamflow[m^3])/(3600*24*Days)<CH_C_Atbashy_Canal[CMS],(CH_C_Atbashy_Canal[CMS]-PrevTSValue(Supply and Resources\River\CH_C_Atbashy_Canal\Reaches\Below CH_C_Alamedin2Atbashy Inflow:Streamflow[m^3])/(3600*24*Days)),0.00001)
where "Atbashy Canal" is the main canal and "Alamedin2Atbashy" is the link from the joining river.
However because the expression refers to the previous timestep there is the potential for the discharge to exceed the minimum for one month. Am I missing a trick somewhere?
Ms. Stephanie Galaitsi
Subject: Re: Preventing maximum flow in diversion being exceeded after inflow downstream of original diversion Posted: 2/9/2017 Viewed: 12460 times
Using the previous time step value is the best you can do it WEAP - it assumes that you do not have perfect knowledge of changes downstream at all times (though of course you also write expressions based on estimates, for example, of monthly variation throughout the year. This might be a better bet).
I'm beginning to see this a little more clearly now. Are the demands on the canal very frequent? What types of flows cause the canal to carry more than 50 m3/s - is it return flow?
One thing you can do is program WEAP to flood any extra water outside of the canal to the surrounding catchments. This will effectively set a limit on the canal's capacity and enable the demand sites to show unmet demand. This again leads to the question - what is happening in the real world? Is water limited at the various input, or is there flooding?
Mr. Oliver Priestley-Leach
Subject: Re: Preventing maximum flow in diversion being exceeded after inflow downstream of original diversion Posted: 2/9/2017 Viewed: 12453 times
Demands are basically over the summer months for irrigation, which is when discharge in rivers is highest (snowmelt), but precipitation low. There are literally dozens of canals coming off the main canal and three or four rivers that can divet water into the canal along its length.
With regard to setting the flooding limit to show unmet demand, I would rather this water is kept in the system for downstream use (there are transboundary demands to be met as well).
Sometimes the canals are used to prevent flooding in the rivers and also used to divert water into reservoirs (short and long term storage ones). The point is we need to use WEAP for planning; by ensuring the flow is correctly constrained to capacity we can show either increase the canal capacity, do not increase irrigated area, use different crops or find other water resources. A great demonstration of WEAP in action for scenario modelling!
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