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?