Why can't government prevent flooding altogether?
Among its many attributes, Harris County's tropical climate also brings with it an established pattern of violent weather. In just the last decade, six major rain-producing storms have flooded nearly 80,000 homes - including 70,000 just from Tropical Storm Allison. Another contributing factor to flooding is the regions clay soils, which drain poorly and only absorb an inch of rain when dry. Despite the fact that Harris County's 22 watersheds performed at maximum capacity during Allison - Greens Bayou, for example, drained a Reliant Astrodome's worth of water every 16 minutes - the sheer intensity and volume of the rainfall still overwhelmed surrounding neighborhoods. It is not economically or physically possible to prevent all flooding in the Harris County region.

What is a regulatory floodway?
The regulatory floodway is the area on either side of a creek or bayou including the main channel that is needed to discharge or move all the water associated with a 1% chance flood downstream, and for Harris County, into Galveston Bay. This area has strict regulations on building that does not allow for any increase in the base flood water surface elevation of more than one foot at any point downstream or upstream.

How are flood hazard areas and flood levels determined?
Flood hazard areas are determined using statistical analyses of records of river flow, storm tides, and rainfall; information obtained through consultation with the community; floodplain topographic surveys; and (hydrologic and hydraulic analyses. The Flood Insurance Study (FIS)covers those areas subject to flooding from rivers and streams, along coastal areas and lake shores, or shallow flooding areas.

What does Water Surface Elevation mean?
Water Surface Elevation refers to the height of the water above the North America Vertical Datum of 1988. Galveston Bay would have a water surface elevation of 0 and all the Bayous in Harris County would have a higher water surface elevation as they flow down hill into the bay.

Will the impact of Tropical Storm Allison and other major storm events get incorporated into the new hydrological and hydraulic analysis?
Several past historical storms including Tropical Storm Allison were used to calibrate the computer models to make sure that the models are accurate. Passed storms are also used to help determine the amount of rain that represents the 1-percent chance storm event.

Will the Base Flood Elevation increase in Harris County?
The hydrology and hydraulic analysis are not completed yet to determine if the Base Flood Elevation will increase in Harris County. However, an updated 1% rainfall is being used that is slightly larger than what was used in the past. The largest affect on the Base Flood Elevations will be from having more accurate topographic information, which could increase or decrease the Base Flood Elevation.

Will all the areas that flooded during Tropical Storm Allison be included in the new floodplains?
No. The new 1% (100-yr) and 0.2% (500-yr) floodplains do not directly result from a single rainfall event, but reflect conditions anticipated to occur with a 1% and 0.2% chance, respectively at that location in any given year. However, information from Allison will be used to verify computer simulations.

Will the floodplain increase after the study is completed?
The floodplain will not change as a result of this study but our understanding of its location may change as a result of using better information and technology to define and map the floodplain.

What is LIDAR?
LIDAR refers to Light Detection and Ranging, which is the most economic and accurate method to obtain elevation data for large areas. LIDAR was first used in Harris County in 1995, and this most recent LIDAR study will produce an even more accurate Digital Elevation Model (DEM)s for Harris County - a two-foot contour topography.

Will the maps be on the same datum?
Yes, all the flooding data will be referenced to the 1988 North America Vertical Datum and adjusted to 2001 for subsidences.

How will the maps be maintained?
Geographic Information System (GIS) tools allow the maps to be easily updated with new information. This will allow the maps to be better maintained and extend their useful life.

What else is being done to reduce local flood damage in the future?
The same day that Tropical Storm Allison formed 80 miles off the coast of Galveston, June 5, 2001, the Harris County Commissioners Court approved a new funding approach for the District's Capital Improvement Program (CIP). This includes an initial commitment of $590 million in county and federal dollars over five years to implement flood damage reduction projects, conduct further studies, generate engineering designs and acquire land for future projects. It merits noting that the budget level for the 2001 five-year CIP is five times the budget commitment of any previous program. For more information about other projects that the District is currently working on to reduce flooding in Harris County please visit www.hcfcd.org.