Adapting Floodplain Ordinances to New Mapping in Historically Modified Floodplains for Urban Areas By: Raymond J. Wilke, P.E., CFM City of Atlanta Watershed Director, Senior Bureau of Engineering Services Department of Watershed Management
Background Flooding is the most frequent and costly natural disaster in Atlanta Current riverine models of the older, developed urban core were simply not sufficient or they are no longer applicable As most of us are aware The multi-year project by the State to reexamine metro Atlanta s flood risks is nearly complete AP Photo/The Atlanta Journal & Constitution, Curtis Compton) Project goal: Develop and update digital flood hazard maps using data from new technologies and more recent rainfall data The availability of GIS based topographic maps, with their smaller contour intervals (2 feet instead of the 20 feet from quad maps), has greatly affected the accuracy of flood mapping, and thus the limits of flooding shown on maps
Updating Flood Maps Current re-mapping of the Upper Chattahoochee River Basin includes seven counties As part of its Floodplain Management Program, it is the responsibility of the City of Atlanta s Department of Watershed Management to disseminate the new information to city residents Dear Property Owner There have been Changes To Flooding Risks On Your Property Notify you of Increased Flood Risk - your property has been mapped into a higher risk zone known as a Special Flood Hazard Area (SFHA) by FEMA Notify you of Potential Increased Flood Risk - your property has been mapped into an area known as a Zone X (shaded) flood zone by FEMA Notify you that your property has been mapped into a lower flood risk zone by FEMA
Look Familiar? Looks CAN BE Deceiving Flooding in this picture was a result of September 2009 rainfall, but the cause was not necessarily what one might think Condition of hydraulics, or hydrology? A closer look at the cause reveals a unique quality of many older urban areas After running models of the stormwater conveyance system in this area, it was found that at least part of the flooding was caused by a Combined Sewer Overflow (CSO) condition in this area The red area to the left, and the muddy water to the right, are at the same location!
Combined Sewer Overflows Therefore, changes to the extent and elevation of the base flood are not the only issues which should be considered for updated mapping This is some type of sewer stormwater or sanitary but unknown Regardless, it is obvious that it cannot handle the volume of runoff! Older sections of Atlanta include combined sewer systems (CSSs) that are designed to collect rainwater runoff and domestic sewage in the same pipe Photo by Alan Cressler, USGS. Lest anybody forget These combined sewer systems not only contain stormwater and domestic sewage They also contain: untreated human and industrial waste illicit hazardous materials toxic materials, and all types of debris
Unaware of the Danger What if this scene occurred in a CSO area? What is this young lady being exposed to? Would she know? (AP Photo/The Atlanta Journal & Constitution, Curtis Compton - 2009) Did the policeman that told her she needed to get out of the car know?
Improving Urban Combined Sewers It is estimated that over 770 communities in the United States have combined sewers as part of their overall storm and sanitary systems The City of Atlanta is one of those communities Under a consent decree from GAEPD, the City is working to improve its sewer system Current projects include many sewer separation projects Once separation is complete, the previously combined units will be converted for use solely as storm drainage conveyance but we still do not have riverine conditions
Phased Approach A phased approach to basin and sub-basin separation has begun The City of Atlanta is spending about $1+ billion dollars to put in separate storm and waste systems Currently, 4 phases are planned The initial condition of the system included seven CS basins Three basins have been removed by separation With the added capacity, some of the CSO bowl areas have been reduced
Modeling the CSO Extensive modeling of the city s combined systems was performed Numerous areas of ponding and/or flooding were identified for the 100-year base flood event Ponding was primarily due to inadequate conveyance capacity, not poor inlet conditions The ponded areas were mapped and forwarded to the State s contractor who is preparing the revised FIRM and FIS for Atlanta and the other communities impacted by the Upper Chattahoochee River Basin study
Flood Classification How should CSO Areas be classified? If classified Zone A, residents affected would be required to purchase flood insurance Since CSO flooding is more often a hydraulic problem, and not necessarily a hydrologic problem, would this be fair? A CSO study is not necessarily connected and equivalent to flood plain models Thus, for the City of Atlanta, the CSO areas will be shown on the new flood maps as shaded Zone X, as they are not identified by conventional FEMA practices or riverine modeling By investing as necessary to improve the conditions in the CSO bowl areas, the adjoining properties will be properly protected These Historically Modified Floodplain (HMFP) areas will require a dual set of criteria within the City s Floodplain Ordinance for development and redevelopment
Why Historically Modified? Not all of the city drains to the Gulf of Mexico via the ACF (Apalachicola-Chattahoochee-Flint) River Basin. The Eastern Sub-continental Divide runs through the city Hence, some of the city drains to the Ocmulgee River, to the Altamaha River Basin and then the Atlantic Ocean. In the downtown area, the divide traverses Peachtree Street, through Five Points Gulf Five Points Atlantic Five Points, and the surrounding area, has undergone major modifications since the photo to the left was taken of the Atlanta Rail Station in 1869 Former Streams and Branches are no longer surface features, but pipes
Planning for Implementation What other steps is the City of Atlanta taking to relieve CSO problems? Flood Plain Ordinance revision WHY? Study indicates flooded areas at some CS inlets (CSO condition) Ma and Pa Kettle want to build a deck/addition Why must we buy insurance? Never been flooded lived here for 50 years Is study wrong? Are engineers wrong? Hydraulic Capacity versus Inlet Capacity Solution: Shaded Zone X other issues related to remapping, including interpretations of floodplain fill issues within CSO area in the urban core
Revised Ordinance Issues New Development vs Substantially Damaged or Improved SFR vs Multi-Family vs Non-Residential COA has had both a 2 ft vertical and 15 ft horizontal requirement from BFE/FP limit Desires to keep 15 ft cause issues with new mapping of HMFP areas
Proposed Requirements Being Considered LDP Permitted, but look at fill, struct type Fill ok if can demonstrate NAI to private property Allow garages, carports, HC ramps, decks If you cannot meet the 15 ft horizontal: Elevate 2 ft above high water for livable SF Rebuild/Relocate with No footprint restrictions Floodproofing Not an option, so shall relocate
But my town does not have Combined Sewers! How has your CBD developed? Do you have suburban hubs? Where were/are your large storm drains being placed? Did you/are you building radially from the ridge lines? Are you placing pipes at the headwaters and modifying the Historic Floodplains? Sizing them for Q10, Q25 or Q100??? You may only be dealing with relatively clean water and not combined sewers, BUT As good CFMs, don t box yourself in to the same issues that we are dealing with in COA
Correlation with SWMO Post Development Stormwater Management Ordinance Revision New houses and large additions - manage first 1.0 inch of runoff Promoting the use of Green Infrastructure BMPs Pervious Pavement BMPs Green Roofs
Questions/Discussion