r/Hydrology • u/big_bizniz • 20d ago

How to Model Impacts of Additional Water Storage on Catchment FLood Events

Hello Again to the helpful community of r/hydrology.

Rather than going into the subreddits of each software, I figured people on here would be able to point me in the right direction.

I am looking to model the impacts of adding additional areas of temporary water storage within a catchment on flood depths, extent, flows etc. I am researching the catchment-scale suitability of using additional temporary water storage to mitigate the impacts of climate change (increased rainfall) on flood risk. I'd be looking to do a "before and after" type comparison to show impacts of additional storage.

Do you know if/how this can be done?

Any input is appreciated. Thanks again guys!

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u/fishsticks40 20d ago

I do this kind of work all the time. 

First thing I'd ask is whether I can get existing hydrographs; if so compare the proposed volume of storage to the volume in the hydrograph. If it's 1% it's not going to achieve much. 

You can also just look at the contributing area and make some assumptions about initial abstraction to estimate runoff volume for a design event. 

If the outcome of these suggests it's worth pursuing further, you can use tools like HEC-HMS and HEC-RAS to simulate particular storage volumes, though neither of these are particularly accessible to a lay person.

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u/big_bizniz 20d ago

Thanks for the reply! 

The gaging stations on the river catchments considered only seem to show guard daily flow, and no hourly flow, so as far as I understand, I wouldn't be able to work out the water volume below the hydrograph for a flood event in this case. 

How would you recommend I consider your second point about the contributing area? I have land cover data, so could use approximate runoff coefficients, if that's what you mean?

What do you mean regarding access in your final paragraph there? Are you referring to access to HEC-RAS or HEC-HMS? 

Thanks for the help so far :)

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u/fishsticks40 20d ago

Daily data is fine for volumes, not so good for peaks. 

Contributing area is the catchment/watershed area. So for instance if the contributing area is 1000 acres and the 100-yr storm is 7", then you can assume an Ia of 1" and estimate that the 100-yr runoff volume is 500 acre-ft. If your available storage volume is, say, 1 ac-ft that's not going to do much for you. If it's 250 ac-ft it could do a fair amount, with lots of caveats about the geometry. 

I mean that HEC-HMS (probably the most sensible tool to use unless there's an existing RAS model) is not really something that a lay person can expect to use and get defensible results. It's very technical and it's perfectly possible to build something that runs but isn't accurate in ways you might not detect.

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u/ProfessorGarbanzo 20d ago

Agree with the other post.

There is nuance with storage (watershed position and watershed timing; active vs passive; draw-down times; etc.), but first establish that the available storage is non-trivial. Like u/fishsticks40 said, gage data would be ideal, or just take your storm of interest (10-yr, 100-yr, etc.) and make some very basic assumptions about runoff coefficients for a certain event. Then use GIS to find the total volume of potential storage you have. There's no amount of nuance or operation of storage that's going to have an impact if the total volume is miniscule compared to storm volume.

But assuming you do have some useful storage to work with, my next step would be HEC-HMS to model the watershed hydrology and how the reservoirs function for different events, and then if you're ultimately interested in riverine flood hydraulics (depth, velocity, extent), use HEC-RAS to model your before/after river discharges.

I don't do this that frequently, but when I have I've usually found that small, distributed storage in more headwaters areas has a better result in terms of reducing peak runoff. Once you get down to the main stem of a river, the volumes are often massive and there's less storage elevation to work with, so you need some very large storage areas (in valleys were pesky humans tend to already built homes, infrastructure, etc.).

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u/fishsticks40 20d ago

The other thing to remember that's a little tricky is that the shape of the hydrograph will matter a lot; basically if the storage fills before the flow peak it will have no effect (to a first order approximation), so a hydrograph with a steep rising limb is easier to abate than one that is slow to peak. 

This, of course, is part of why storage is more effective in the headwaters, where things tend to be flashier.

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u/big_bizniz 20d ago

Thanks for the response, I appreciate the help and detail you've gone into :)

The gaging station records only show gauged daily flow, therefore, as far as I understand, I wouldn't be able to calculate event volumes. I was thinking I could possibly use https://fehweb.ceh.ac.uk/ to calculate volumes, but I haven't used such software before. 

To calculate volumes of storage using GIS, would I be best to use DEM data? 

The areas identified as having water storage potential are distributed quite well across the catchment, with some large areas on tributary rivers and streams, and some areas of floodplain with limited development on perimeter, or where there is development, there is significant elevation above the floodplain. 

Thanks for your advice so far, I appreciate it!

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u/ProfessorGarbanzo 20d ago

If it's average daily flow, that should work well. Just subtract the base flow convert the flow rate to a volume and that should give your event volume. It's the peak flows you'd miss by not having more granular (15-min, hourly) data.

DEM data yes. If you are starting from scratch, and your watershed is sizeable, I'd look for a tool that can iterate across the entire DEM and find storages that meet certain criteria of size and depth. The best one I've used personally is in PCSWMM (commercial), but I believe ArcPro has some Storage Creator tool that does this well, if you have access to that. In QGIS there's a Hypsometric Curve tool that works fine if you're just looking at one (already known) depression, but I don't think it will work to "find" a bunch of storages across an entire watershed. It's been a while since I used it though.

It's possible HEC-HMS has some sort of storage delineator built into its watershed analysis setup. I'm a little off the pace on HMS