The Cypress Creek Flood Control Coalition

Frequently Asked Questions (Flooding):

Apart from the Harris County Flood Control District, who has authority over drainage and flooding in Harris County?The City of Houston is one of several floodplain administrators for the community’s participation in the National Flood Insurance Program (NFIP). Houston develops its own criteria for the design of its storm sewer, street drainage, and stormwater detention storage systems. Other incorporated areas have floodplain administrators that develop their own drainage design criteria. In the unincorporated areas of Harris County, the County Engineer’s office is the floodplain administrator. In all, the county has thirty-four floodplain administrators, and the HCFCD is not one of them.

Who has authority over drainage and flooding in the Cypress Creek Watershed?  The Cypress Creek Watershed lies within both Waller and Harris Counties. Harris County Commissioners Court has the authority to construct flood mitigation projects and regulate drainage within the unincorporated areas of Harris County, and does so through the Harris County Flood Control District (HCFCD); thus, HCFCD is responsible for a majority of Harris County drainage, but not all of it.  Prior to development, drainage responsibility (and authority) lies with the landowner. Developers have responsibility for tracts covered by their plat.  Certain drainage responsibility also accompanies the authority granted to MUDs by legislation from which they derive their powers.  And a homeowner cannot divert the flow of natural drainage from his property onto that of another if it causes damage.

What legislation gave the Harris County Commissioners Court authority over drainage and flooding in the Cypress Creek Watershed?
The enabling legislation that created the Harris County Flood Control District includes the following: “The Commissioners Court of Harris County, Texas, is hereby designated as the governing body of such District and the agency through which the management and control of the District shall be administered, and it is hereby empowered to do any and all things necessary to carry out the aims and purposes of this Act.  In addition to the powers given to the Commissioners Court by General Laws and in addition to the general powers herein given it shall be authorized, in connection with the Harris County Flood Control District, to exercise the following added rights, powers, privileges and functions: 2(a)  To acquire land and rights and interest therein to carry out the work of flood control, and 2(b) To devise plans and construct works to lessen and control floods; to reclaim lands in the District; to prevent the deposit of silt in navigable streams; to remove obstructions, natural or artificial, from streams and water courses; to regulate the flow of surface and flood water; and to provide drainage where essential to the flood control project.”

What is a watershed?
A watershed is the land area that ultimately drains rainfall runoff to a common outlet point, typically a creek or bayou in Harris County. If your home is located in the Cypress Creek Watershed, rain that falls on your house will eventually end up in Cypress Creek. Mother Nature designs and builds watersheds, largely determined by the topography or “lay” of the land. Harris County has twenty-two major watersheds.

What is a tributary watershed?
The land area that drains to one of the smaller streams that flow to the main channel of the watershed (i.e., Faulkey Gully’s tributary watershed ultimately flows into Cypress Creek).

What is a floodplain?
As defined by FEMA, a floodplain is “Any land area that is susceptible to being inundated by water from any source.” In Harris County, a floodplain is generally defined as an area flooded due to either a channel’s capacity being exceeded or due to a tidal storm surge (See glossary for a more detailed explanation).

Why are the floodplains on the new FEMA maps (effective June 18, 2007) often much larger in size than those shown on existing maps?  Might these revised floodplains change again?
FEMA reports that this expansion of the floodplain is not necessarily an increase in its physical size; rather, the change is due to advanced technology which provides greater accuracy in delineating the actual floodplains than existed twenty years ago. But because rapid urban development has occurred in the Cypress Creek Watershed since the LiDAR topography data was obtained (late 2001), and because this development will likely continue, the floodplains are expected to change.  HCFCD and CCFCC are working together on engineering analysis to determine the extent to which this will occur under a variety of land-use conditions, drainage regulatory requirements, etc.

What is a floodway?
The strictest area of regulation along both sides of a bayou or creek (including the main channel) because it moves the 1% (100-year) flood downstream, away from homes or businesses that it may have flooded (See glossary for a more detailed explanation).

What is a channel?
A channel is the main portion of a stream that carries stormwater flow from the watershed. It can vary in size and shape and can be natural or man-made.

What does the term confluence mean?
It is the intersection of two channels, where the outfall of one channel flows into another channel (i.e., where Cypress Creek flows into Spring Creek).

What is meant by the term 100-Year Flood, Base Flood, or 1% Flood?
A 100-year floodplain is an area of land that has a 1% chance of being inundated by floodwaters in a given year. The 1% (100-year) flood is a regulatory standard used to establish risk zones with varying risk probabilities which are then used by insurance actuaries to compute the flood insurance premium rate.  The Base Flood Elevation (BFE) is used as a benchmark to administer floodplain management programs, the National Flood Insurance Program (NFIP), and to set building requirements for new construction.

What are the terms used for some other Frequency Floods?
There are an infinite number of flood frequencies that can occur. The .2% flood is called the 500-year flood; the 2% flood is called the 50-year flood; the 10% flood is called the 10-year flood; the 50% flood is called the 2-year flood, etc.

Does flooding occur outside of a FEMA-mapped floodplain?
Yes. Some flood hazards simply aren’t mapped on FEMA Flood Insurance Rate Maps (see questions on “ponding” and “overland sheetwater flow”), nor is every small tributary in the county included. The mapped floodplain is only an estimate of where flooding is predicted to occur given a set of parameters which include a hypothetical rainfall that occurs over a watershed for an assumed amount of time. During an actual rainstorm, natural conditions can result in greater amounts of rainfall or runoff, resulting in flood levels deeper and wider than shown on the FEMA Flood Insurance Rate Maps.
On June 19, 2006, “sheet flooding” occurred outside the mapped floodplains in Harris County (principally in the Sims Bayou area). HCFCD estimated that of the 3,370 affected homes, ninety to ninety-five percent were flooded due to poor street drainage or from channel flooding by small tributaries. The June 19 event was the county’s second largest flood based on the number of inundated homes.  NFIP representatives indicated that approximately 50% of these homes did not have flood insurance coverage.

What is a stormwater detention basin?
Detention basins are excavated areas of land where potentially damaging excess flood water is temporarily stored and then drained over time as water levels recede.   Because Harris County is pancake flat, most of its detention storage must be excavated at substantial cost; still, the Harris County Flood Control District makes extensive use of detention basins to reduce the risk of flooding. They are typically large regional facilities, several hundred acres in size. Locally, two of the largest are Addicks and Barker Reservoirs.

Are “detention basins” and “retention basins” the same thing?
No. Detention and retention basins are different methods by which flood damage reduction can be accomplished.

Detention Basin:
As defined above, a detention basin is an area where excess stormwater is stored or held temporarily, and then slowly drains when water levels in the receiving channel recede. In essence the water in a detention basin is temporarily detained until additional room becomes available in the receiving channel. Detention basins are used extensively in the Harris County region. There are approximately fifty detention basins in operation within the District throughout Harris County (source: HCFCD).

Retention Basin:
A retention basin also stores stormwater, but retention storage implies a more permanent basis. In fact, water often remains in a retention basin indefinitely, with the exception of the volume lost to evaporation and soil absorption. This differs greatly from a detention basin, which typically drains after the peak of the storm flow has passed, sometimes while it is still raining. Retention basins, for the sake of flood damage reduction, are not common in the Harris County region; they are popular in parts of the country that have soils more amenable to this type of flood damage reduction measure (source: HCFCD).

Are land developers required to build detention ponds?
Yes, beginning in 2004, with few exceptions, area developers of more than ten acres are required to build stormwater detention ponds to mitigate the negative flooding impact a new development causes (i.e., buildings, roads, and parking lots cover virgin soil, which both increases the volume and accelerates the rate of water runoff).

My subdivision has a detention pond but we still experience flooding. Why?
If subdivision flooding is due to out-of-bank rising water rather than localized sheetflow, it almost certainly originates upstream.  This is why regional detention systems must be built to contain upstream water runoff due to urban development, enabling gradual floodwater release rates which are not damaging to downstream neighborhoods.

My home was flooded during Tropical Storm Allison. I read in government publications that it was a 100-year storm (500-year in some locations).  Based on this, can I assume that my home won’t be flooded again in my lifetime.
Absolutely not!  With the exception of a small Louisiana parish, Harris County has the most repetitive flood loss experience of any county in the United States.  During the last four decades, record rainfalls and flooding have repeatedly occurred within the Houston metroplex.

Why do HCFCD, Houston’s mayor, and CCFCC strongly recommend that all homes in this area be covered by flood insurance?
Due to the high risk of flooding and the low cost of insurance relative to the level of damages that might be incurred, flood insurance makes sense.  During a typical thirty-year mortgage period, one out of four homes is at risk of flooding. Prior to Tropical Storm Allison, the average flood insurance policy cost $409 and the average repair bill for homes damaged by flooding cost about $27,000. (Note: During the period 2005-06, construction costs in the Houston area reportedly rose another 35%).

Why does CCFCC warn Cypress Creek Watershed residents that the floodplains as shown on the new FEMA map will likely increase in size and depth?
The watershed is undergoing rapid urban development, especially in the western regions adjacent to and upstream of U. S. Highway 290. These big neighborhood developments will increase both the volume and peak runoff, which is likely to spill into downstream neighborhoods unless a more effective flood mitigation infrastructure is installed.

Is CCFCC taking action to avoid this increasing flood risk?
Yes, CCFCC has financed several on-going engineering studies to identify the location of flood mitigation needed but not existing throughout the watershed. In 2004, it employed engineering consultants to review selected regulatory requirements in HCFCD’s design criteria for new land developments, some of which were deemed unacceptable. And the coalition successfully overcame FEMA’s denial of an appeal it filed requesting that technical deficiencies in FEMA’s computer programming (used to calculate the floodplains) be corrected. CCFCC subsequently funded the efforts of an engineering consultant to work with a small HCFCD project team to recalibrate the programs.

Is CCFCC opposed to future development in the watershed?
No, CCFCC recognizes that urban development is inevitable. The Houston-Galveston area is projected to grow by 3 ½ million people by 2035.  For the past two years, CCFCC has been working with some of the watershed’s largest land developers.  A professional consultant has been hired to assist in this effort to collaborate with developers to achieve increased effectiveness in detention, floodplain preservation and government cooperation.

What does the term “hydrology” mean?
Hydrology is an engineering process used to convert a rainfall amount into a volume of water moving down a channel. This volume of water is then inputted to a hydraulic model and turned into a map of flooding areas using a computer model called HEC-HMS.

What is subsidence?
Webster’s defines it as “to sink; to fall to the bottom; to settle.” And according to the Harris-Galveston Subsidence District (HGSD), that’s exactly what some of the land in our area has been doing since the 1920s. More specifically, subsidence is when the elevation of the ground drops slowly over time as water is pumped from aquifers for commercial and domestic uses. Since 1973, most of Harris County has fallen one foot with some areas falling as much as five feet. The Harris-Galveston Subsidence District (www.subsidence.org) was created to monitor and address this problem. As a result of their efforts, subsidence rates throughout most of the county have been greatly reduced. For recent developments in this area, visit the North and West Harris County Regional Water Authorities’ websites (www.nhcrwa.com and www.whcrwa.com).

Can land subsidence be reversed? What progress has been achieved to date in reducing the area’s land subsidence?
Subsidence is irreversible. According to the Houston-Galveston Subsidence District (HGSD), in critical areas along Galveston Bay, the land surface has sunk as much as ten feet since 1906! With each hurricane the area weathers, subsidence and flooding problems worsen. A dramatic example of this was in the Brownwood subdivision, a coastal community of Baytown where almost continual flooding due to subsidence caused the area to eventually be abandoned. Here are some interesting facts provided by HGSD concerning land subsidence within their district:

The predicted subsidence is based on conversion to surface water required by the HGSD regulations mandating a graduating reduction in groundwater withdrawal until it reaches the 80% reduction mark by 2030.  Without this, predicted subsidence by 2030 would range as high as another three feet in the Southwest quadrant, four feet in the district’s area north of Loop 610 and five feet in the FM 1960 area (visit: www.subsidence.org)