Hydrologic Model for solar farms 1

[DenverJ]

I'm looking for any guidance/advice for preparing a stormwater model for a 200+ acre solar farm. We're at the very early stages of this project but I would like to get some input on how I should approach the project. The site is currently in agricultural use with rolling crop fields and a stream channel flowing through the center. The developer has said the ground under the solar arrays will be maintained as pasture with sheep grazing. My concerns are with runoff from the arrays forming concentrated flows that will need to be controlled. I also worry about erosion under the drip edge. Are these valid concerns? Any guidance or advice would be greatly appreciated. My hope is I can use HydroCAD for the majority of my calculations. Thanks.

 

[swazimatt]

depending on the terrain, if the land under the panels remains as pasture there should be no change in total runoff from the site - rain runs off the panel and is able to soak into the pasture under the next downstream panel. It is a good question and i am keen to hear other peoples opinion.

If the above statement is true, you have rightly identified the main concern being management of the runoff direcltly from the panels. The simplest would be to have a stone filled trench under the panel dripline and possible topsoil bunding to slow the runoff and encourage infiltration at source.

I found this report on a quick google NZ solar stormwater report

a quick skim read shows no need for dripline protection as the panels tilt to follow the sun. NZ is quite far south so the tilt may differ from your location, but when the panel is at its steepest the plan catchment area of the panel is at its smallest

interesting topic, i am keen to hear other opinions

 

[1503-44]

Typically erosion is of no real concern, unless you have pre-existing erosion problems from erosion prone soils and bad, rapid runoff slopes. If no previous site erosion has occurred, probably nothing to worry about.

However the panels may produce more runoff. Their runoff coefficient would be 1.0, in which case you may experience more runoff making it quicker to the stream and increasing stream flow. It might be required to temporarily store that increased runoff volume in a small retaining pond and reduce its potential max flow by keeping runoff flows in the stream leaving the site to a maximum of its previous flow rate value and water level.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[TigerGuy]

The challenge would be to keep the water running off each panel, within the panel footprint on the ground to then infiltrate or runoff as it would have prior to installation of the panels.

 

[bimr]

The average commercial solar panels in a solar farm has a surface area of 21.6ft², which means you can install about 1,500 solar panels per acre. That’s just a single panel size without considering panel spacing, racking hardware, inverters, or wiring.

If the typical panel is roughly 4 feet x 5 feet, the difference in erosion before and after solar panel installation should not be a concern. However, you still will have to prepare a stormwater management plan.

 

[DenverJ]

I wanted to thank everyone for your replies.

We recently had a meeting with PA DEP at the site. Their main concern from a stormwater standpoint was not with the solar arrays themselves but any concrete for footers and any stone under the panels. Our client stated that there shouldn't be a need to concrete footers as they plan to drive the posts.

There will still be a calculated increase based on Pennsylvania's calculation criteria. We have to assume any non-forested pervious area as meadow in the pre-construction condition regardless of what the actual land cover is. Going from meadow to short-grass pasture is going to result in an increase in both rate and volume. But we should be able to handle that with retentive grading under the arrays.

Our biggest concern is DEP wants us to complete infiltration testing at one test for every acre. On a 200 acre site, that's a lot of testing.

It's going to be an interesting project and I'm sure I'll learn a lot.

 

[1503-44]

Not much grass will grow under the panels, if they are fixed position. That's made worse by grazing.

Runoff from panel area will be 100%.
You should probably use a weighted coefficient based on area of panel to area between panels.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[swazimatt]

Runoff from panel area will be 100%.
You should probably use a weighted coefficient based on area of panel to area between panels.

I do not entirely agree with this as you are assuming that there will be no effect of the grass (or bare soil) under the panels on sheet flow. Perhaps the weighted average should be based on
(100% runoff from panels + 30% runoff from from total area)/ (total area +panel area)
This should account for the faster runoff from the panels and take into account the fact that this runoff will be able to soak into the soil under the next panel. If you feel the grass will not grow under the panels a slightly higher runoff coefficient could be used for this area

It is definitely an interesting question

 

[1503-44]

Well ... I don't agree with that. First thing that will happen is the area between the panels will start forming troughs made by the impact of the water falling from the panels, which will collect that falling water in the trough until it finds the next adjacent low spot. I'm not a great believer in sheet flow anyway. Rain either soaks in and quickly finds low spots, ponds, then channel flows away to the next low spot. Sheet flow is for rooftops, solar panels and parking lots. You can see for yourself when you water your garden. Little puddles turn into tiny rivers. Not many natural surfaces are so flat as to allow a true sheet flow. Even wide areas of what looks like very flat "flat land", especially with any clay content at all, even with slopes of only 1 ft drop in a mile, apx 1/5000 slope, like 99% of the entire Gulf Coast, will form tiny riverlets to drain off any excess water as soon as the first inch of dirt becomes saturated.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[swazimatt]

but your method of weighted average assumes that the ground under the panel (weather grassed or bare) has no effect on the runoff. at an absolute minimum it will add length to the flow path
If a depression does form at the drip line this will slow runoff and encourage soakage

 

[TerryScan]

Land cover between panels will certainly vary from site to site. However, it is not necessarily going to be bare or eroding.

Here are a couple of sites in Delaware (flat) that appear to have decent vegetative cover.

 

[1503-44]

Lengths are not affected by what Cd is used. Distances to the solar panels do not change.

It does not seem reasonable to expect that sheltered areas will capture much rainwater. Maybe a little bit around the panel peripheries, but assuming any more than 10% or so doesn't really make sense to me. I already said that I doubt sheet flow occurs on natural land, but if uniformly graded to a constant sloping plane, maybe some will happen. But in any case, if you do have high rainfalls, especially with erosive soils, proper grading of drainage chanels or swales to detention ponds should be considered. The OP is already considering grading in drainage, so sheet flow does not seem to be a relevant topic.

The soak tests will resolve the runoff coefficients. However the area of land under the panels will not receives rain, so any water falling on the panels will go immediately to exposed areas. The soak test retained volume will have to be applied only to the exposed land, so all the water running off the panels should first be applied only to the area between panels. What does not soak into that area will runoff and apparently will go into the swales, or bladed channels and to wherever those lead.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[darthsoilsguy2]

It sounds to me like you don't need an engineering solution but need a bureaucratic solution. There is no way that the PA DEP representative's concern about concrete pedestals that you expressed is based on science or the intent of the PA legislative intent. You're dealing with a regulator who is trying to fit your solar farm square peg project into their commercial development permitting guidance round hole. The whole point of having stormwater regulations in the first place is the preservation of water quality in our streams and lakes.... and you know what can be REALLY BAD for water quality? Oh....I don't know....Just about EVERYTHING in fossil fuels projects that renewable energy projects offset, even when designed/permitted/constructed appropriately because... ya know....'stuff' happens.... The ONLY thing that should get any stormwater treatment in a solar farm is the driveway to service the transformer.

I don't understand what this infiltration testing program would be trying to prove? If the land is clay, then your little pedestals don't change existing runoff conditions. Permits and regulatory oversight is supposed to be only about the changes... if the land is sand, then you do change the runoff conditions very slightly.... BUT I suspect that people would perceive a testing program that shows highly infiltrative soils to be a 'good thing', and if that's the case... what is the point?

Without learning about PA stormwater rules and how there permitting works.... I would look and see if there is language for "disconnects" for the water quality and look for any outs on water quality treatment if you deem the pedestal a rooftop.... If the impervious amount is as small as I'm thinking, you could do a Weighted-CN HydroCAD of the drainage area both pre & post. HydroCAD has rounding in it so you'll probably find a way to get the same CN in pre & post and if you have the same CN, it's real easy to get no increase in multiple storms without doing anything. My state requires Weighted-Q to be used which is harder to tinker with.

Just know that DEP is not as handcuffed to their own procedures as they pretend sometimes. ALL the DEP's out there have jurisdictional discretion. All these states have different rules and procedures set up to comply with EPA Clean Water Act and for some reason they are all different, yet they all magically comply. Figure out your sales pitch and write a SHORT memo, mention the weighted-cn modeling results but don't encumber them too much data, and send it to your regulator and include a specific question they can answer. Hopefully, they'll let their boss look at it and they'll find a way to cut out some of the nonsense and give you a path forward

 

[1503-44]

My experience with gov all over the world is...
It's usually a lot less headaches if you just say yes sir, do your tests, blame the Biden administration, and get on with panel building. Resistance is futile. You will only create delays and do the tests later anyway.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."