Flood Zone Calculation
Flood Zone Calculation
(OP)
I am trying to set a Base Flood Elevation for a subdivision. This is my first endeavor so please bear with me. A few questions:
1. Hydrologic method? I am using the SCS method. The peak that I am getting is a lot higher than the USGS regression equation (due to storage effects?). Should I use the more conservative?
2. Software? I have XPSWMM which I think is approved. But I also downloaded QUICK 2. The drainage area is 979 acres. At the lower end of the drainage area are (4) 36" RCPs which will cause the area to pond.
any help as far as outlining a solution would be most helpful.
1. Hydrologic method? I am using the SCS method. The peak that I am getting is a lot higher than the USGS regression equation (due to storage effects?). Should I use the more conservative?
2. Software? I have XPSWMM which I think is approved. But I also downloaded QUICK 2. The drainage area is 979 acres. At the lower end of the drainage area are (4) 36" RCPs which will cause the area to pond.
any help as far as outlining a solution would be most helpful.





RE: Flood Zone Calculation
I have never used SWMM but I believe I have read that it is FEMA approved software. I have used HEC-RAS which is also FEMA approved and free.
RE: Flood Zone Calculation
As for software, FEMA's website has a list of all approved software for flood studies. Go to their website. My own preference would be HEC-RAS. It is free and also available for download at the Hydraulic Engineering Center (HEC) website.
Google these things to get exact web addresses.
Good Luck
RE: Flood Zone Calculation
RE: Flood Zone Calculation
RE: Flood Zone Calculation
I have used both SCS method and regression equation method for determining flows; it depends on the basin size. You're not trying to create a FIRM, you're trying to set a flood elevation (or a finished floor elevation) that is going to keep water out of the houses and prevent you being sued or losing your license. Use FEMA methods as a minimum, and be as conservative as your judgement tells you to be. I used SCS where I was sure of my land use, soil types and TR-55 times of concentration. I had recent aerial photos, a county-wide soil map and contours accurate to 5'. I used regression equations when I had a basin that was of the order of square miles, across a state boundary and where topo maps were my only source of contours for basin delimiting and slope estimation.
I have done this exercise several times over the past year and I recommend HEC-RAS. You will need to be very careful with the road; the culvert calculations are tricky in HEC-RAS and you need to read the Hydraulic Reference Manual (or the help!) to make sure you are assuming the right variables for the right calculation method. Make sure you check the "pressure flow" option if the road is being overtopped, which it almost certainly is.
HEC-RAS needs a few extra downstream cross-sections to allow the assumed downstream water depth to settle to the "true" value. (For instance, if you assumed normal depth, but the velocity is low, then the "true" depth will be deeper; HEC-RAS requires a few extra downstream cross-sections to settle the water elevation calculation.) If the road is your downstream limit, you might consider including a few cross-sections downstream of the road. The water surface elevation upstream is controlled by the road/culvert hydraulics only, but HEC-RAS will need at least one downstream cross-section, so you may as well give it a few more. It's much easier to plan for more cross-sections now than to add them later when you've set your stationing, etc. (Adding extra cross-sections within your modeled area is easy thanks to the interpolation feature.)
My final bit of advice is this: do not show the flood zone beyond your property boundary. You may set an elevation (or series of elevations), but don't show the area on the opposite side of the creek unless the land belongs to the same developer. Make sure your subdivision won't increase the flood level (local storm water runoff rate may increase, but will be gone before the flood peak arrives); i.e. don't fill in the flood plain, but don't set the flood plain for any property but your client's.
RE: Flood Zone Calculation
1. Property in question is on one side of the creek.
2. Lower end of property is at the road/culvert. Road is at elevation 192.
3. Upper end of property is 1000' upstream of the property. The elevation at the centerline of the creek here is 188.
4. Culvert will not pass flow without overtopping.
5. I have field surveyed cross section at the lower and upper ends of the property and some culvert info.
6. As gbam has stated the creek will be flowing over the road and not act like a pond.
7. I can find the 100-year elevation at the culvert by using the weir equation and culvert nomographs.
8. I need to find the elevation of the 100-year flood at the upper cross-section.
If this is simple enough to outline on this forum I really would appreciate the help. If not, please let me know, and Ill start reading HEC-RAS manuals tonight. Thanks.
RE: Flood Zone Calculation
Good Luck
RE: Flood Zone Calculation
RE: Flood Zone Calculation
RE: Flood Zone Calculation
I guess you could do some quick-and-dirty estimates. You can always replat after doing the HEC-RAS, assuming some lots aren't wiped out by the flood zone. Since you know your flow and your cross-section, and can calculate your average bottom slope so you could come up with a velocity and use Q = v * A to get the depth. (Exclude all area below the elevation of the road, as it's an "ineffective flow area" because the water will be more-or-less stationary.) Be overconservative in your Manning's coefficients, because the slower the water, the deeper the it is.
I wouldn't do that unless I were desperate and I could comfortably add 10' to the answer, however. Remember, it's your license and your reputation on the line, not to speak of potential law suits!
RE: Flood Zone Calculation
I would add at least one more cross section downstream of the culvert to define the expansion reach. More sections will increase the final accuracy of your result so don't be stingy. Also, make sure the cross sections are wide enough to include the entire flood plain. HEC-RAS will truncate sections when they are not wide enough leading to errors ( usually small) in the computed water surface elevation. Plot the cross sections to look for this possible error.
Good Luck
RE: Flood Zone Calculation
I know our surveyors have taken a published flood map, scanned to a .tiff, referenced into a .dgn, scaled to fit, and traced over the floodway boundary as provided. I imagine including a reference to the data source. That is what the published maps are for if your not disturbing the floodway.
Of course your out of luck if the stream has not been studied or you are filling/crossing in the floodway.
RE: Flood Zone Calculation
RWF7437: Does HEC-RAS use expansion reaches for culverts? It's been a few months, but I recall only doing expansion reach calculations for bridges. I can see how the principle would hold for bridges and culverts, and my failure to calculate expansion/contraction reaches for culverts would explain my lack of satisfaction with using HEC-RAS for culverts! I looked it up on the online help and I'm still not sure.
uncivilized: Presumably you have a fairly accurate DTM for your subdivision. Cross-sections drawn from your DTM will be accurate enough. The majority of flow will be on your overbanks, not in the main channel; in fact, much of your main channel may be "ineffective flow area" and all of it below the bottom elevation of your culverts will be, so accurate channel x-sections are low-priority.
Other tips:
* I've heard it said that "top of bank is about a 1.5 year storm" -- make sure you extend your x-sections a ways up the slope because it's infuriating to have to extend x-sections at a later date.
* Also make sure that your x-sections aren't straight, but in fact bend so that they're always perpendicular to the contours. A line drawn perpendicular to your x-section needs to point in the direction of flow (which is parallel to the bank contours and perpendicular to the creek bottom contours, assuming a U-shaped ditch contour).
RE: Flood Zone Calculation
Yes, of course, HEC-RAS uses expansion reaches for culverts, and bridges, and any other "obstructions" to the natural flow of the stream in question. As you have often said in this forum or these forums, visulize the flow. Then, look at the hydrologic and hydrology manuals and and you will "see" what must be happening.
Thanks for your comments.
Let's both wish "unicivilised" Good Luck
RE: Flood Zone Calculation
Franseca - We are in the flatwoods section of the country. The slope is so mild it is difficult to tell the surveyors to stay perpendicular.
I dont really have enough x-sections for HEC-RAS. I guess I could make my own based on DTM. Is is it typical to make your own, or do you get the surveyor to field survey the x-sections that you need?
I ended up using xp-swmm (fema approved) I used conservative run-off and conservative n values. However I dont quite understand the results. The water elevation at the road crossing is only .2' feet below the water elevation 1000' upstream. There is 1' of difference between the ground elevations in the bottom of the channel. Also the downstream channel cross-section is wider than the upstream which should cause the difference in water elevation to be even more pronounced.
RWF - thanks for the well wishes. Hope that I can you help you one day.
RE: Flood Zone Calculation
We found it a waste of time and money to send a survey crew out to conduct the cross-sections. We had a field crew surveying for nearly two weeks taking just stream channel x-sections, but the majority of the flow is on the overbanks. While of course it depends on the accuracy of your DTM, it is better to select cross-sections on paper than in the field and the level of accuracy is sufficient. HEC-RAS reduces your cross-section data to a single point, representing head loss at that location based on x-section geometry and Manning's roughness values.
RE: Flood Zone Calculation
RE: Flood Zone Calculation
RE: Flood Zone Calculation
I assume that you construct your DTM from a field survey and not USGS topo maps. Right?
RE: Flood Zone Calculation
I guess that should be past tense, because I went back to school in September, but they still work the same way.
RE: Flood Zone Calculation
The standard error is so large that they may be of limited value. I suggest you contact the USGS District Surface Water Specialist in your State and ask him to explain what is the error possible imbedded in applying the equations within your State.
I use the Tr 20 Methododolgy (HydrolCAD) and Hec 1 to get a range of flows for the same watershed. I then compare the results to the USGS regional equations. The answers can vary greatly .
RE: Flood Zone Calculation
The method we used is more art than science, but we used the 100-year elevation to set property boundaries (the developer sold off the land below the 100-year flood elevation to a farmer) rather than finished floor elevations, which will be on average 10' higher.
We did not plot the 100-year elevation for the opposite side of the creek, which belonged to a different developer.