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Reservoir routing - hydrological problem
7

Reservoir routing - hydrological problem

Reservoir routing - hydrological problem

(OP)
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!

RE: Reservoir routing - hydrological problem

2
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
www.hydrocad.net
 

RE: Reservoir routing - hydrological problem

(OP)
Thanks Peter.
This is precious informations

 

RE: Reservoir routing - hydrological problem

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

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

RE: Reservoir routing - hydrological problem

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
www.hydrocad.net
 

RE: Reservoir routing - hydrological problem

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.

RE: Reservoir routing - hydrological problem

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
www.hydrocad.net
 

RE: Reservoir routing - hydrological problem

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

RE: Reservoir routing - hydrological problem

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
www.hydrocad.net
 

RE: Reservoir routing - hydrological problem

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

RE: Reservoir routing - hydrological problem

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
www.hydrocad.net
 

RE: Reservoir routing - hydrological problem

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

Mike McCann
MMC Engineering

RE: Reservoir routing - hydrological problem

(OP)
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.


 

RE: Reservoir routing - hydrological problem

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.

RE: Reservoir routing - hydrological problem

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
www.hydrocad.net
 

RE: Reservoir routing - hydrological problem

(OP)
Thanks!

RE: Reservoir routing - hydrological problem

(OP)
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.

RE: Reservoir routing - hydrological problem

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

Failure is catastrophic.

Mike McCann
MMC Engineering

RE: Reservoir routing - hydrological problem

(OP)
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.

RE: Reservoir routing - hydrological problem

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
www.hydrocad.net
 

RE: Reservoir routing - hydrological problem

3
A couple of comments...

Regulators will let you get away with a CN of 98 for the lake area itself, but the appropriate thing to do is set it at 100 if it doesn't break your software.  The evaporation from a lake for design storm is negligible over that time scale, especially during a storm.  But as has been mentioned above, the differences aren't really worth arguing about.

Always always always start your reservoir routing at the control elevation of the dam.  (That'd be your lowest orifice or your spillway / etc)  There are two exceptions I can think of, and they're both driven by the regulatory agency you're doing your work for, so you'll know if they're meaningful.

Some regulatory agencies like to have stormwater treatment ponds that capture and infiltrate water into the ground.  (e.g. Florida WMDs)  In these cases, you're usually required to prove that this dead storage retention volume is recaptured between storms, by showing infiltration calcs and water table elevations and whatnot.  Some regulatory agencies (some munis in Georgia) like you to start higher than the lowest control, if the lowest control is a long term bleeder device for water quality and they don't trust that you'd have all the volume available during the flood routing.

The elevation you start your reservoir routing at is important.  Folks get sued over this kind of stuff.  I was an expert witness against another engineer who screwed this up badly and was exposed for a lot of damages because of it.

On backwater, there's two issues.  One is whether folks upstream of the reservoir get flooded by the reservoir level increasing.  For any reasonably sized application just assume the water is flat across the reservoir's surface.  The other issue is one engineers also screw up quite a bit in some areas of the country, and that's tailwater on your outlet control structure.  This is a big deal in Florida since the discharge from series of connected ponds can be effected by the downstream pond, because the driving head is not just upstream head, it's head differential.  You need time series routing tools for these, such as ICPR or XP-SWMM.  In regions of the country where you aren't likely to have tailwater on your outlet control system, engineers have grown used to using simpler tools that only consider upstream head. (Hydraflow ... ugh, what a piece of crap)  Engineers who don't realize the difference, and use the latter to design the former situations can get into big big big trouble, because their pond doesn't draw down like they expect it to, and stages higher than they expect.  Again, fodder for lawsuits.

My comments are largely for small reservoirs in land development, or regional reservoirs that are not directly managed.  I'm not talking about municipal drinking reservoirs or other things that are managed by some dude at a control desk.  How you do hydrology for those sorts of applications is much more complicated, and I assume the top poster isn't asking about those on a forum such as this.

As always, go with widely accepted practice for your region (if such a thing exists) because hydrology is very, very regionally influenced.  

 

RE: Reservoir routing - hydrological problem

(OP)
Very interesting comments.

HEC HMS doesn't support 100... I guess 98 and specifying 0 inches of Initial Infiltration is something suitable.

"The elevation you start your reservoir routing at is important."

I'd tend to start it to the normal pool elevation.
I think that assuming full storage prior to a, let say, 1000-year storm is really conservative.

Indeed this is related to a small reservoir where 2d hydraulic modeling would be... too much.

RE: Reservoir routing - hydrological problem

Assuming "normal pool" = the elevation of your lowest control, then yes, start it there unless there's a compelling and documented reason to do otherwise.

RE: Reservoir routing - hydrological problem

I have a similar situation with drainage areas and a lake, but I have 3 separate subcatchment areas that outlet to the lake at 3 different points.  How can I model the lake runoff through an existing outlet dam with HydroCad?  Thanks.

RE: Reservoir routing - hydrological problem

orshii - I beleive your question is a double-post.  Please see answer(s) in the HydroCAD forum.
 

Peter Smart
HydroCAD Software
www.hydrocad.net
 

RE: Reservoir routing - hydrological problem

(OP)
Back on this subject:

The regulatory agency in charge of reviewing a similar project is asking me to find another way to calculate the outflow from a dam.

I did use HEC HMS and reservoir routing.

They say that they don't get the same outflow using flow rate recorded at a gage station located on a river near the study area and transposing it for the basin related to this study.

I really think that such a comparison is not possible considering the important storage of the lake...

Performing my analysis, the WSE of the lake increased of almost 2' during the simulation. The outflow from the dam was then calculated with standard weir equation = CLH^1.5 = 3.1(8.2)(2)^1.5 = 72 cfs.

Someone have any idea of another method to calculate the outflow from the dam?

Please don't say "Rational Method".

RE: Reservoir routing - hydrological problem

First: What rainfall are you using in he model?  And what rainfall produced the gauge reading you are being asked to match?  Unless you are using the exact rainfall distribution that produced their gauge reading, your results will be different.

Second: Since you are modeling a complex 3.5 SM watershed, the margin of error will be significant.

How large is the difference?  I would verify all the input data (including the rainfall depth and distribution) to look for errors or reasonable adjustments that would bring the model into closer agreement with the gauge.
 

Peter Smart
HydroCAD Software
www.hydrocad.net
 

RE: Reservoir routing - hydrological problem

(OP)
They are using recorded flow rate at a gauge station located outside of the watershed.
And a statistical analysis (e.g. using log-Pearson II distribution) to determine a 100-year flow rate.
Then they are using the ratio of basin areas to transpose this flow rate to the area of study.
 
I'm using a 24-hour SCS-II 100-year rainfall.

This particular method of comparaison doesn't make any sense to me (!?)


 

RE: Reservoir routing - hydrological problem

you may also have discrepancies with the outflow for the dam. I assume you used a rating curve for your outlet, not just a single flow rate? Also, where did you get the weir coefficient? It seems high for a broad crested weir and low for a sharp crested one. In fact, most weir coefficients vary with the flow rate depending on the width, depth of flow, contraction, entrance conditions and submergence. This could have a big effect on the outflow from the reservoir.

RE: Reservoir routing - hydrological problem

(OP)
3.1 in SA would equal to 1.7 in SI which is critical depth.
Yes the rating curve take in account effective length as state in Design of Small Dams (USBR, 1987). No Submergence effects.

Anyway, my question was more related to a validation method fur a project like this one? Another method than using HEC HMS and reservoir routing.

RE: Reservoir routing - hydrological problem

(OP)
1.705 and 3.09 for purist.   

RE: Reservoir routing - hydrological problem

I think if you were comparing apples to apples, HEC-HMS reservoir routing would be fine. HEC-1 should also work. But it sounds like your hydrograph doesn't compare favorably with the agencies hydrograph. Obviously SCS vs statistical manipulation of a similar (perhaps) watershed could yield entirely different results. You may also want to use an antecedent moisture condition III instead of II.  Perhaps starting water surface elevation of the reservoir don't match either and who knows what outlet rating curve they are using.

Perhaps you could ask them to provide their inflow hydrograph and plot the two and compare the differences.  

RE: Reservoir routing - hydrological problem

(OP)
Thats the point! They're not using any reservoir routing method. They're just using statistical basin transposition and comparing peak flows only (no hydrograph).
 

RE: Reservoir routing - hydrological problem

well duh, I thought they were reconstituting a hydrograph. They are measuring flow on an uncontrolled watershed and you are modeling a controlled watershed. Obviously the flows can never match. I'm surprised they are able to understand the statistical analysis and basin transposition and yet don't understand basic reservoir routing...

RE: Reservoir routing - hydrological problem

(OP)
This is such a relief to read someone thinking the same thing than i do.

What to do now (apart from teaching them basic hydrology).
A way to validate the max increase of 0.5 m in the lake.

  

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