Estimating impervious cover for a large (~20 square mile) watershed

[beej67]

I have an opportunity to work on a large stream restoration project, of the size where TR-20 methods aren't generally applicable, so I'll be defaulting to USGS regression equations and unit hydrographs for the hydrologic analysis. Project is in central Georgia. I don't currently carry a license for any GIS software, and my experience with GIS is limited. In order to do the watershed analysis, I will need to delineate basins and estimate impervious cover. As I see it, I have two options for the analysis:

1) Clip together a bunch of USGS rasters in CAD and do the basin delineation by hand, assigning a certain imperviousness to urban areas and calling the rest pervious, or

2) Obtain free/cheap GIS software, topo coverages, and land use coverages, and learn to do the procedure that way.

I'm leaning towards (1) simply because the learning curve for (2) might not be worth it for this one job, although (2) is certainly something I've always wanted to learn if I can get away with less than a grand in software purchases. If I go with (1), what imperviousness would you folks assume for urban areas? I would rather not spend time getting down to the gnat's posterior on different land uses. (commercial/industrial/etc) If I go with (2), what software can get the job done on the cheap and where can I grab the data?

Thanks in advance.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[gbam]

Beej - I agree, 1) will most likely be the best way to go, most efficient for you. I am in the same boat with GIS, I would like to learn it but initial cost is too great for any one project to absorb along with the delay in learning the software. As to the imperviousness based on land use, one could sample different landuse types to measure impervious area and assign a percentage. For instance, our local agency (city and county) have provided that information in their hydrology manuals. Here we use:
Natural/Rural - 5% +/-
Accounts for minor rock outcrops, compacted soils and etc...
Suburban (2RAC) - 15% +/-
Mod Urban (3-5 RAC) 35% +/-
Highly Urban (6+ RAC)60 % +/-
Commercial 90% +/-
This is for directly connected impervious areas. Otherwise use the nat/rural unless you can delineate grouping of rock outcrop.

 

[Drew08]

With watershed of 20 square miles, a regression analysis is a good option, but I'd look at doing a site specific regression analysis first before resorting to the published equations. The USGS has a pretty comprehensive coverage of stream gages throughout the southeast. Gage data can be downloaded directly, and analyzed for frequency flows within USACE HEC-SSP model. The USGS website for water data also has a new online map viewer for showing the location of filtered search results with options for various background data including imagery for reviewing land use. For instance, you can filter and map all stream gages in Georgia with 10 years of peak flow data or more. If gages exist upstream and downstream of your point of interest within the watershed you’re evaluating, than you’re golden. If not, you can likely still find gages nearby to analyze within the same physiographic/meteorologic region that would be a good fit. Plot watershed area vs. frequency flow rate for 5 or 6 gages with a spread of watershed area and best fit a power curve.

There is also freeware available that will convert a lat-long list of points (stream gage locations) to a Google Earth KML/KMZ file for landuse/location review. EasyGPS,MN DNRGarmin to name a couple. The later will also convert shapefiles to KML that you might download from the national map, like river data, basin data, etc.

 

[civilman72]

Have you thought about importing the terrain from Google Earth? I've found it to be acceptable for large drainage basin delineations and it has to be just as accurate as USGS topos (only 40' contours?). Plus it's free.

You can download the contours into the Civil3D terrain modeling and hydrology programs and then you don't have to "do the basin delineation by hand."

Let me know if you need more info.

 

[peter246]

A coworker recently showed me an incredible online mapping tool - unfortunately it looks like they don't have Georgia ready yet, but the USGS Streamset program can automatically delineate watershed boundaries:

http://streamstats.usgs.gov/

 

[beej67]

Good point Drew, I'll check on that to see if it's available. Last project I worked on was the Etowah River, much larger than this one, and gauge data wasn't available for that so I'm not sure it'll be available here. But it wouldn't hurt to check.

civilman72 -

I don't use Civil3D, and I've quite often found errors in its automated basin delineation routines, so I do all mine by hand. I've also found extensive errors in automated basins done by ArcGIS as well, by the way. Don't trust them a lick. Mostly I want to use GIS (if I go with that option) to do automated watershed characterization.

That said, is there an easy way to get Google Earth contours into a regular cad platform? Where is the option in GE to simply show them on a raster export? I haven't fooled with that much.

peter246 -

Seen that. It worries me. My experience with automated hydrology is it's quite often wrong, even when someone's checking it.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[civilman72]

I understand you concerns with checking the automated basin delineations.

Not sure if it's a Civil 3D only option, but in Civil3D I go to File-->Import-->Import Google Earth Terrain. You do this at the same time you have Google Earth up and running. Civil3D will grab all terrain information off the screen shot you are currently showing in Google Earth.

 

[beej67]

Is there a way to just show the contours in GE? That'd save a lot of trouble going through USGS quads for simple projects.

I'm running BricsCAD, which I'm very pleased with both in terms of it's functionality, it's ACAD-like interface, and it's price. Wonder of someone's written a module to do this for it.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Drew08]

Click the "Earth Gallery" button on the "Layers" menu in Google Earth. You can load and view the USGS topo maps and USGS National Elevation Dataset from Google Earth Gallery.

Have you tried ESRI's ArcGIS Expolorer Online:

http://www.arcgis.com/explorer/?

It appears to have basic ArcGIS functionality including viewing and editing shapefiles, and has USGS topo and imagery base layers built in.

Check out the freeware section at:

http://www.esri.com/products.

There might be something useful there.

 

[froude]

QuantumGIS (QGIS) is an open-source GIS platform that is available for free. We have access to ESRI licenses at work, so I haven't tinkered with QGIS much. But from what I hear, it is very comparable and, well, free.

I believe that 10-ft interval USGS topography can be downloaded for free or nominal cost, as can the USGS Quad Maps. Lastly, the NAIP frequently flies 1-meter resolution natural color aerials which they also provide for free, or at nominal cost. I know for sure that the USDA provides free GIS files of their soil surveys. All of this should at least be a good basis from which to delineate basins and put together fairly comprehensive hydrograph calculations.

In Texas, the State provides a GIS clearinghouse where all this data is centralized and provided at no cost. You may check to see if Georgia has similar.

I'm currently away from the office so I'm unable to post in more detail. I'll post back in detail once I get in.

good luck.

 

[froude]

It looks like a bevy of data is available through [URL unfurl="true"]https://data.georgiaspatial.org/[/url]. This includes quad maps, imagery, soils, among other things.

 

[beej67]

Thanks for all the input. I bid the job presuming I was clipping together USGS rasters in CAD and I'll explore QGIS if the job moves forward. Thanks again.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[sam74]

I'm not sure this will help your situation but, you can get DEM files that were used to produce USGS quad maps for free just google it. I'm not familiar with BrisCAD but I know Civil3d and Geopak can translate these to .tin files which can then be written out in CAD so you have real data to work with. You can then reference a raster for comparison. Worked for me in the past rather well if USGS quality is all that is available.

I've found that where I'm currently located Bing Maps has more recent aerial photography for determining impervious areas than Google Earth. I would use most recent aerial I could find and try to back it up with some site visits for large descripancy verification and adjustment.

Also in the past on a DOE drainage project we looked at several available possible applicable methods of runoff determination. I believe this included State DOT regression equation, USGS regression equation, and NRCS method or rational method can't remember which but I think it was rational method because it is typically meant for larger areas. I'd do some research on applicable methods size (area) limitations and use a few methods and pick a conservative method to model. We were using this for HEC-RAS input so we didn't need a hydrograph only a peak flow.

Hope something I mentioned is of value.

 

[TerryScan]

I recommend using GIS Land Cover Data:

http://www.webgis.com/terr_pages/lulcutm_ga.html

or

http://viewer.nationalmap.gov/viewer/

for example.

It may be worth hiring a GIS consultant to work with if you have limited GIS abilities.

 

[debgallery]

Just like sam74 said, you can use DEM files for USGS maps. I found this post a while back that tells you exactly how to find them and use them. I refer to it everytime I need to delineate large basin, which is a lot lately for some reason.

[URL unfurl="true"]http://thecadgeek.com/blog/2009/03/acquiring-converting-dem-downloads/[/url]

The downside for you is that the directions are for creating surfaces in Civl3d but once you get the dem file you can figure out how to import it to BricsCAD (if you don't know already). I know nothing about BricsCAD but I'm assuming it has this capability

I also use The National Map Viewer to download hydrography and orthoimagry files that fit seamlessly in with the contours when everything is imported on the same datum. It's very important to set up your original drawing with a known coordinate system in order for all the files to import correctly. It doesn't have to be the same coordinate system as the imported files, just a known one. You can also get roads (transportation files), which helps with large basins in just knowing where you are. The hydrography and transportation files can be downloaded as shape files.

I agree with delineating by hand though - the catchment area function in Civil 3D never seems to really work for me. Maybe it works better on smaller watersheds or "designed" surfaces.

 

[SMIAH]

When using option number 2, I'd find someone with an ArcGIS sotfware License to perform the job.
They're going to make you save alot of time (money).
Plus, sometimes they got better data (ex.: LIDAR coverage might be available for your client).

There might be someone local that can do exactly what you want (e.g. telling you the median watershed slope) ?