LOMR Modeling
LOMR Modeling
(OP)
I have some questions that maybe some of the more experienced hydraulic engineers can help me with. I have a farm approximately 2 miles downstream of a LOMR which was just approved by FEMA which has resulted in a increase in the floodway BFE by a foot. In reviewing the modeling done on the channel it appears there are several discrepancies.
1) The LOMR is downstream of a major obstruction in the form of an interstate highway ROW and passes through 5 5 x 6 culverts. The stream channel crossection is modeled upstream of the interstate with a gradually increasing bed. The model results in a water surface about three feet above the starting elevation of the beginning station. However, the actual topography has a slight rise adjacent to the stream bed but a culvert parallel to the interstate and a ditch along the service road all appear to slope slightly the other way. This is to a ~120 ft wide underpass for a State Highway under the Interstate. It would appear the model for the 100 year flood is taking credit for an elevated water surface profile since it shows the water going over the interstate in the 100 year flood event. It ignores the reality of the water bypassing under the overpass and down the four lane divided road bed. (As everyone knows in town the area under the interstate floods during the flood events and the water runs down the road to where the bridge over the same stream is perched over the stream. This is downstream of the LOMR. Would such an error in modeling cause and increase in discharge rate downstream.
2) The floodway has been narrowed slightly with a bridge added over it in front of a new super retail center. All the drainage from the parking lot has been diverted to a retention pond. The model had the 100 year flood discharge flow rates inputted at each RS in the reach. The boundary conditions were defined by setting the starting and ending BFEs for a portion of the reach. The reach studied did not include all the SFHA in which my farm is in downstream of the modeled area. I am concerned the model was constrained to match the original BFE so that the only degree of freedom allowed was to allow the BFE to increase at the interior RS areas of the small area studied.
3) I have attecmpted to model the entire SFHA in the original FIS using the HEC-RAS 3.1.3 modeling software. It appears that over a one to three foot increase had occured downstream. I have tried to argue with FEMA that I am an affected party that should have received a letter of the proposed change prior to its being filed rather than just a public notice in the local newspaper (whcih I never saw). FEMA's position appears to be that only landowners adjoining the LOMR area is an affected landowner.
Any suggestions?
1) The LOMR is downstream of a major obstruction in the form of an interstate highway ROW and passes through 5 5 x 6 culverts. The stream channel crossection is modeled upstream of the interstate with a gradually increasing bed. The model results in a water surface about three feet above the starting elevation of the beginning station. However, the actual topography has a slight rise adjacent to the stream bed but a culvert parallel to the interstate and a ditch along the service road all appear to slope slightly the other way. This is to a ~120 ft wide underpass for a State Highway under the Interstate. It would appear the model for the 100 year flood is taking credit for an elevated water surface profile since it shows the water going over the interstate in the 100 year flood event. It ignores the reality of the water bypassing under the overpass and down the four lane divided road bed. (As everyone knows in town the area under the interstate floods during the flood events and the water runs down the road to where the bridge over the same stream is perched over the stream. This is downstream of the LOMR. Would such an error in modeling cause and increase in discharge rate downstream.
2) The floodway has been narrowed slightly with a bridge added over it in front of a new super retail center. All the drainage from the parking lot has been diverted to a retention pond. The model had the 100 year flood discharge flow rates inputted at each RS in the reach. The boundary conditions were defined by setting the starting and ending BFEs for a portion of the reach. The reach studied did not include all the SFHA in which my farm is in downstream of the modeled area. I am concerned the model was constrained to match the original BFE so that the only degree of freedom allowed was to allow the BFE to increase at the interior RS areas of the small area studied.
3) I have attecmpted to model the entire SFHA in the original FIS using the HEC-RAS 3.1.3 modeling software. It appears that over a one to three foot increase had occured downstream. I have tried to argue with FEMA that I am an affected party that should have received a letter of the proposed change prior to its being filed rather than just a public notice in the local newspaper (whcih I never saw). FEMA's position appears to be that only landowners adjoining the LOMR area is an affected landowner.
Any suggestions?
Talk To Other Members
RE: LOMR Modeling
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: LOMR Modeling
Post this question in the Storm Flood Engineering forum where you are much more likely to get expert help. This forum deals pretty strictly with HydroCad.
Try contacting your FEMA Regional Office and ask who did the flood study which resulted in the LOMR. That is likely to be a consultant / contractor, rather than FEMA staff.
It sounds as though you can make a strong case that you are, in fact, an affected party. As such, you can ask for an independent review of the modeling work. FEMA will probably not let you do that review yourself but they may agree to a review if they think you might sue them and their contractor.
good luck
RE: LOMR Modeling
I heard these same things before... before two of my close relatives homes (Mom/Dad/divorced) both received 4 feet of water attributable to upstream development due to flash flooding by Mingo and Joe Creek in Tulsa Oklahoma in the 70's. And I was run out of one of the homes and only got out because my VW front end floated but the back end kept contact with the high center of the road bed. Emotion aside. I am an licensed engineer. Just not in civil. But have studied transient hydraulics under the likes of Ben Wylie at UM in Ann Arbor. Something about this does not seem right based on my understanding of the basic equations of energy, and conservation of mass and momentum.
I plan to get to the bottom of this even if I have to get a few congressmen and US Senators involved. I don't want to be caught in my FEMA 320 safe room that was built out of the established Flood Plain protecting me from storm winds only to be trapped and drowned by rising flood waters. I have posted my query on the other site. Thanks RWF3437. Didn't realize this forum was the private domain of Peter Smart.
RE: LOMR Modeling
RE: LOMR Modeling
You were right my comments with regard to Peter were not called for and he is obviously a significant contributor to the forum. Peter: my apologies. I had done a search on floodplain modeling and it had only given me this forum as a result. I appreciate the direction to the other forum site.