Reservoir routing - hydrological problem 7

[SMIAH]

Hi,

I have a project involving the determination of a 100-yr storm peak flow. The watershed has the following characteristics:

Area = 8.7 km2 (3.35 mile2)
Average slope = 13.1%
% woods = 67%
% pasture = 3.4%
% residential areas = 12%
% swmamps = 2%
% lake = 15%

I have a CN number for every subbasin.
I'm thinking of using a SCS Type II storm (6 hour, 12 hour and 24 hour) and compare the results.

My problem is the 15% of lake... Actually the lake is at the downstream end of the watershed (see the file attached).

I have the volume of the lake (1 250 000 m3.)
But regarding the reservoir routing... I'm quite lost.

What should i do to get a good estimate of the peak flow at the downstream end of the lake (where there is a small dam)?

Thanks for any clue regarding the method i should use for the reservoir routing!

 

[psmart]

1) You should be able to use a standard reservoir routing, such as the Storage-Indication method, which is part of all H&H programs.

2) The dam would normally be modeled as a weir outlet on the pond.

3) You will need to have some stage-storage information above the weir crest (such as contours) in order to do an accurate routing.

4) If there is a pre-storm outflow from the lake, you will need to incorporate this as a base flow and/or starting elevation. The exact mechanism varies depending on the software you use.

You also need to allow for the direct precipitation on the surface of the lake. This is generally done by including the surface "runoff" using a curve number of 98+


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[SMIAH]

Thanks Peter.
This is precious informations

 

[SMIAH]

One thing i'm not familiar with is the lag time for the lake?

I would tend to use a very short time.
Correct?

 

[psmart]

If you wanted to calcualte the Tc all the way to the dam, you could include the lag time for the lake. But since you're going to perform a separate resevoir routing for the lake, the Tc path for the contributing watershed would generally end at the water's edge.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Drew08]

You should also use a CN=100 for the reservoir area so S=0 and P=Q, i.e. no infiltration, storage, or inital abstraction losses.

 

[psmart]

Drew08 - Since there are still some losses (such as evaporation), a lower CN of 99 or 98 can be appropriate for direct rainfall on the water surface. In most cases, it won't make much difference to the final results. If in doubt, try a range of values and compare the results.

I beleive there was a recent thread on this topic...


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[msquared48]

I would assume the worst case here with no available extra storage in the lake and, depending on the length of the flow through of the lake, tend to set the time through the lake to zero, neglecting any upstream effects.

Mike McCann
MMC Engineering

 

[psmart]

Depending on the exact storage and discharge characteristics, the lake could provide significant attenuation of the peak inflow (runoff). To estimate this effect you need to perform a reservoir routing.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[msquared48]

Yes, but then you are assuming that when the design storm hits, the reservoir will be drawn down to some degree. Unfortunately, most major storms happen when the reservoirs tend to be higher, if not full.

If the structure is in the FERC network of dams, there could be enough time to draw the reservoir down prior to the storm, but that could be a luxury here. Even the FERC and Corps models are not perfect as the recent storms in the Northwest here proved recently. I would be conservative and design as if there is no storage. There could be a backwater effect, and I might yield on that issue, but not the available storage.

Mike McCann
MMC Engineering

 

[psmart]

I was not assuming any draw-down prior to the storm. In fact, I was assuming that the initial water surface was already overtopping the dam, in order to produce a pre-storm discharge, as you would commonly see for a natural lake.

Even when the initial WSE is above the spillway, the storm will increase the WSE even further, and this increase in storage will cause attenuation of the peak flow.

This effect can be simulated with a reservoir routing, based on the dam's stage-discharge relationship, and the stage-storage relationship as the water exceeds the spillway. A narrow spillway will "hold back" more water and produce more storage and attenuation than a wide spillway.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[msquared48]

OK.. I agree - backwater analysis as I said.

Mike McCann
MMC Engineering

 

[SMIAH]

This conversation is interesting.
I am assuming that the reservoir is full (i.e at the top of the crest of the small dam at the downstream end).

But combining a 100-year/6-hour SCS II storm + a full reservoir might be too conservative?

We don't have any legislation in here (Canada *sigh*) regarding those issues.

 

[cvg]

100-year storm plus full reservoir starting condition is the typical procedure here. Level pool routing is often used to route through the reservoir. For large reservoirs or long dams, a two dimensional routing method may be used which may do a better job of estimating water surface elevations than level pool methods. Also, if this is a larger reservoir or dam, then the spillway generally must also be large enough to pass the PMF.

 

[psmart]

A backawater analysis is usually not required to evaluate the peak-flow reduction in this type of scenario. As long as the outlet control (dam) is relatively small in relation to the lake, it will create a zero-velocity level-pool, which can be modeled using a reservoir routing, such as the storage-indication method.

On the other hand, if the level-pool assumption is not met, then a backawater analysis would be required. This would be the case with a dam across a river, but that's not how I interpreteed the question at hand.

"But combining a 100-year/6-hour SCS II storm + a full reservoir might be too conservative?"

If the lake is normally filled to the spillway, this does not seem overly conservative. But it's really a matter of the standards you are trying to meet, and the downstream consequences of any increase.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[SMIAH]

Thanks!

 

[SMIAH]

Back on the initial water surface.
Taken from the FERC:

http://www.ferc.gov/industries/hydropower/safety/guidelines/eng-guide.asp

2-4.1.2 Guidelines for Initial Elevations

Specific guidance for establishing the initial reservoir elevation during the PMF is provided
in Section 8-3.1 of Chapter VIII of these Guidelines. This criteria should also be applied to
routing the IDF when the IDF is less than the PMF. In general, if there is no allocated or
planned flood control storage (e.g. run-of-river), the flood routing usually begins with the
reservoir at the normal maximum pool elevation. If regulation studies show that pool levels
would be lower than the normal maximum pool elevation during the critical IDF season, then
the results of those specific regulation studies would be analyzed to determine the
appropriate initial pool level for routing the IDF.

 

[msquared48]

When it comes to floods, don't be conservative.

Failure is catastrophic.

Mike McCann
MMC Engineering

 

[SMIAH]

I would like to have more informations about what msquared48 pointed out about the fact that: "then you are assuming that when the design storm hits, the reservoir will be drawn down to some degree".

Starting with the reservoir at full storage capacity (or in terms of highest elevation) and using the SCS type II or constant intensity repartition of the storm, the lake is emptied quickly after the reservoir routing analysis starts as the discharge of the dam at this full storage capacity is quite high (could be something else in another projet).

So... when it comes to calculate the Water Surface Elevation related to this flood, the starting elevation used in the routing analysis is the critical one (highest).

Am i missing something here... It really depends on this elevation choice.

 

[psmart]

My comments were NOT predicated on the reservoir being drawn-down.

A reservoir will generally produce peak attenuation even if it is filled above the spillway prior to the event. Depending on the surface area, there can be considerable storage as the level rises just a few inches above the spillway, and this storage will result in peak attenuation. You will see this effect with a standard level-pool routing.

Sure, you'll get more attenuation if the reservoir is drawn-down prior to the event, but I was assuming that it is full, and in fact overflowing, prior to the event in question.

To model this in HydroCAD, you can set a starting elevation above the spillway, and then select "Automatic Base flow". This will calculate and apply an additional base-flow as required to keep the pond at equilbrium with this pre-storm outflow.



Peter Smart
HydroCAD Software

http://www.hydrocad.net