Workshop 4 – Multipurpose Reservoirs

WS4_Unsolved.7z

WS4_Solution.7z

Fact Sheet

LAKE TED HILLYER

Reservoir Information

Location: On the Purple River above the confluence with the Orange River.  

Purpose: Water Supply, Hydropower, Flood Control

Lake Data: Based on current sedimentation survey

Model Information

Networks

1. Basic Network – Contains a single reservoir (Upper) with a diverted outlet
2. Basic - DS Diversion – Contains a single reservoir (Upper) and a downstream diversion

Operations Sets

1. WS Only – Empty set for use modeling water supply as the only purpose

Rules

                None

Alternatives

1. WS-FromRes – Water supply only run, using Basic Network
2. DS Div – Run using the Basic – DS Diversion network

Simulations

1. Full Period – Runs the extended historical period (1943 – 2015)

Workshop #4 – Multipurpose Reservoirs

In this workshop you will model a reservoir with more than one purpose. You will learn how to add a rule representing a second purpose, and how to model a diversion directly from the reservoir pool. You will also model a diversion downstream of the reservoir. You will evaluate the firm yield impacts of these different modeling options and of the additional reservoir use.

Part 1 – Diversion from Reservoir Pool

• Open the HEC-ResSim watershed saved as '\Workshop4\Workshop4.wksp'.
• Go to the Network module and open 'Basic Network'.
  • Note the blue/black arrow pointing out of 'Upper Res' that indicates the reservoir now has a diverted outlet included.

• • A diverted outlet allows the model to pull water out of the reservoir without sending it downstream into the channel. In this model, the diverted outlet will represent the water supply demand for which we are trying to determine the firm yield. Previously, you were able to model the demand using a simple minimum reservoir release rule because it did not matter where the water went as long as it left the reservoir. However, now that you want to model more than one purpose, you will have to model releases and diversions more precisely.
• Open the Reservoir Editor and go to the Physical tab. Note that the Diverted Outlet now appears on the left side of the editor. The outlet is parameterized with a controlled outlet with an arbitrarily large capacity.

• Go to the Operations tab and make sure the 'WS Only' Operation Set is loaded. There are no rules in this set yet, and you will need to create them. This set will operate with just water supply, similar to previous workshops.
• Create a new rule in the 'Conservation' zone by highlighting the zone name and right clicking, then selecting Add New Rule…

• • This rule will specify the minimum required diversion through the diverted outlet. Later, you will maximize the size of this rule to determine the firm yield.
  • Call the new rule 'Min Diversion', set it to release from 'Upper Res-Diverted Outlet', and make the rule type a Release Function.

• • First, you will need to define the function the rule will follow. There are many options, but for your simple model you can define the release as a function of the date. Click the Define button, make sure the dropdown box is set to Date, then click OK.

• • This rule will set the diversion each time step. Diverting less than the desired value is considered a failure during the yield analysis. Therefore, your rule should be set as a minimum limit.

• • Choose an initial guess for the firm yield and add it to the table as the value for 01JAN. This value will apply for the full year since there are no other rows in the table.

• • This minimum diversion should always apply, no matter what the state of the reservoir is. Right now, the rule is only active between the top of the Buffer zone and the top of the Conservation zone. Add the same rule to the rest of the zones by right-clicking on the rule and selecting Add Rule to All Zones.

• • Note that it is the same rule in each zone. Any change made to the rule will apply to all zones. This same rule could also be used in other Operation Sets.
• At this point, the model is set up the same way as the model from Workshop #3 (but now diverts through the diverted outlet) and should result in the same firm yield value. We can check this by running a quick analysis.
  • Open the Alternative Editor and select the already-created 'WS-FromRes' alternative. This alternative is mostly identical to what you used in the last workshop, with a few differences. The Lookback tab has an additional row for the diverted outlet where the lookback value has been set to 0. The inputs on the Time-Series tab are the same as the end of Workshop #3 and include evaporation separate from the inflow.
  • On the Operations tab, make sure the 'WS Only' operation set that you just modified is selected. 

• • Go to the Yield Analysis tab and set it up the same as past workshops. Choose the option to Maximize Water Supply Rules, and select your new minimum diversion rule. Note that the fact that this rule releases through the diverted outlet is not visible here.

• • As before, you can leave the tolerances at their default values. Set the Lower Limit of Con Pool as the 'Inactive' zone, and save the alternative. 
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  • Switch to the Simulation module and open the 'Full Period' simulation. 
  • Right-click on the alternative name on the right side of the screen, and update the simulation's copy of the 'WS-FromRes' alternative using the Replace From Base Directory… menu item.
  • Run this alternative and note the resulting firm yield. It should be close to the 1508 cfs we saw in the last workshop after extending the data. Using the diverted outlet instead of releasing the water through the dam does not affect the firm yield in this case.
• Now, make the reservoir multipurpose by adding another rule. Go back to the Reservoir Network module and open the 'Basic Network' network, if it isn't open already.
• Create another operation set to model water supply and hydropower as multiple purposes. With the 'WS Only' operation set open, select Duplicate… from the Operations menu. This will copy the operation set with a new name. Name this set 'WS and HP' and click OK.

• In this new operation set, create another rule in the Conservation zone named 'Hydropower'.  Define this rule to release from 'Upper Res-Dam at Purple River' as a Release Function. Releasing from 'Upper Res-Dam at Purple River' instructs the model to move the water requested by this rule through the dam into the downstream channel where it can be routed to downstream points.
  • The 'Hydropower' rule represents an agreed minimum amount of water that will be released through the dam to generate power every day, so set the Limit Type to Minimum. The power demand in this area peaks in the summer due to air conditioning, so the minimum release will vary over the year. Set the rule to have a minimum release of 500 cfs in the summer, and 250 cfs the rest of the year. Note, HEC-ResSim can model hydropower using more complex rules based on details of the powerhouse and energy demand, but for this class we are using a simple example that just specifies a release.

• • Add the rule to all zones like you did previously with the 'Min Diversion' rule, and make it the highest priority rule in each zone.

• Click Apply to save the changes to 'Upper Res', then close the Reservoir Editor. Save the network, then open the Alternative Editor.

• • Create a new alternative to run this new operation set.  Do this by selecting the 'WS-FromRes' alternative and save with a new name using the Alternative → Save As… menu item. Call this new alternative 'WSHP-FrRes' (Water Supply and HydroPower – divert From the Reservoir).

• • Set the operation set to 'WS and HP' and ensure the Lookback and Time-Series tabs are still correct. Save the alternative using the Alternative menu so the new operation set is recognized, then go to the Yield Analysis tab.
  • Even though we now have a hydropower purpose, we are still going to maximize the firm yield for water supply, so set the tool to maximize Water Supply Rules. Once again use the 'Min Diversion' rule.  (Note: If the rule is already selected, deselect the rule and select it again!)  If we wanted to know how much water we could release for power, we could select the 'Hydropower' rule, which is just a minimum release, not an actual power generation rule.

• • Check the tolerances and other options and save the alternative.
• Save and close the alternative, then switch back to the Simulation module.
• Open the 'Full Period' simulation and edit the simulation parameters using the Simulation → Edit… menu option. Add the 'WSHP-FrRes' alternative to the simulation by checking it's box, select Run New Extract to ensure the necessary time series are pulled into this simulation, and click OK.

• Run the yield analysis for this multipurpose reservoir model. What firm yield do you calculate? Is it different from the yield when there was only one purpose? If so, why?

• Now change the model to have the 'Min Diversion' rule as the highest priority rule in each zone instead. Re-run the yield analysis.  (Note: before each run reset the min diversion demand to 1000 cfs). What yield do you calculate now? Is this what you expected?

• Now we will look at the impact of changing the guide curve in a multipurpose reservoir. With the 'WSHP-FrRes' alternative still active in the Simulation module, open the Reservoir Editor.
  • Select the 'Conservation' zone and create a seasonally varying guide curve that matches the 'Varying GC 2' operation set from Workshop #2, shown below.

                   
                     With just the water supply purpose, this guide curve had a firm yield of 1472 cfs, 63 cfs below the constant guide curve firm yield of 1535 cfs.

• • Run the analysis again. What is the firm yield? How does it compare to the firm yield from the multipurpose constant guide curve runs earlier in this workshop?

• What if this reservoir's operation plan specified stopping releases for hydropower below a certain pool elevation threshold? This will reserve all the water below that threshold for the water supply purpose. This operation could be a part of the project's drought contingency plan. We will model this scenario now.
• In order to make comparison between runs quick and easy, we will model this scenario with a Trial. Trials are available in HEC-ResSim to try out changes to a model without creating a new alternative. Trials should not be relied on to save any simulations you want to keep and should only be used for temporary runs.
  • Right click the 'WSHP-FrRes' alternative name and select New Trial…. Name the trial whatever you want and click OK.

• With the new trial active, open the Reservoir Editor. Remove the 'Hydropower' rule from the 'Buffer' zone. This will prevent the model from releasing for power below a pool elevation of 1054 ft.

• Run the yield analysis. What is the firm yield now? Does this make sense? You can plot the results or look at the Storage Yield Analysis Output Summary Report to better understand the analysis results. If you check the boxes next to 'WSHP-FrRes' and its trial, plots will show both runs at the same time for an easy comparison.

Part 2 – Downstream Diversion

• You saw that Hydropower competes with Water Supply for stored water when the water supply is diverted from the reservoir pool, and reduces the firm yield of the water supply diversion no matter its priority. Now imagine that during a flood the diversion infrastructure is damaged. The water supply user decides to rebuild the diversion below the reservoir where it can be better protected. They will now divert water after it has been released through the dam.
• Go to the Network module and open the 'Basic – DS Diversion' network. This network has the reservoir diversion removed and replaced with a stream channel diversion at 'DS Point 1'.

• • For this workshop, the diversion is included for clarity, but no water will actually be diverted to reduce the model complexity. Since there is no local flow or routing between the dam and the diversion, the outflow at the dam will match the flow available for diversion.
• Because this is a different network, the rules you created earlier are not available. Create the 'Hydropower' rule the same way you did for Part 1 of this workshop and add it to all zones, including the 'Buffer' zone.
• Create a 'Min Diversion' rule again, but this time have it release from 'Upper Res' instead of the diverted outlet. This will let the release pass downstream to the diversion point. You do not need to specify the Dam at Purple River this time, because there is no diverted outlet on the reservoir. With the dam as the only outlet, there is no difference between releasing from the dam vs. releasing from the whole reservoir. Give the rule a starting guess for firm yield and add it to all zones at lower priority than the 'Hydropower' rule.

• Edit the 'DS Div' alternative and update the Yield Analysis setup as you did before. Choose the rule that releases for the water supply purpose.
• Go back to the Simulation module and edit the 'Full Period' simulation to add the 'DS Div' alternative.

• Run the yield analysis. What do you think will happen? Were you right?

• Switch the priority so that water supply is higher than hydropower. Does this change the result?

• Now remove the 'Hydropower' rule from the buffer zone as before. Does the yield increase like it did last time you tried this?