Stormwater Flood Issues in the Cypress Creek Watershed Harris County, Texas
Preface: This information is provided to assist the non-technical reader in understanding basic facts and issues about stormwater flooding in the Cypress Creek watershed, in particular, and in Harris County, Texas, in general.
- A “watershed” is the land area that drains into a specific water body such as a tributary, stream/bayou, river or ocean bay. Its “floodplain” is that portion of the watershed within and adjacent to the stream banks where storm/rainwater run-off builds up when it falls faster than it can flow downstream. Floodplains are dynamic. It is well-known among professional floodplain managers that today’s floodplain is not necessarily tomorrow’s floodplain. Floodplains commonly change/increase in size due to the effects of urban development and natural events. Current federal floodplain management regulations do not consider the increase in future flood levels caused by such new development. Therefore, if more appropriate mitigation is not provided at the local level in advance of ongoing urban development, flooding and flood damage will likely increase.
Stormwater flooding in Houston and Harris County can be categorized into two main groups: 1) properties flooded by out-of-bank rising water coming from the area’s twenty-two natural bayous, and 2) properties flooded by sheet flow (water flowing across the land surface to drainage channels but temporarily blocked by obstructions or overburdened conveyance systems). Typically, Harris County’s stormwater sewer lines are sized to handle a three-year or less storm; thus, during heavier rains, the water run-off “ponds” in subdivision streets inadequately equipped with sewers or ditches to convey the excess water out. According to government sources, one-third of flood loss claims are from property outside the 100-year flood plain.
The Cypress Creek Watershed is a 320 square-mile drainage collection basin, the largest in Harris County. Because of its size, it has the potential for receiving greater volumes of floodwater than other watersheds in Harris County. According to information contained in Harris County publications, it is almost as large as the watershed areas of Brays Bayou, Buffalo Bayou (below Addicks and Barker Dams) and White Oak Bayou combined (Source: Data published in the Capital Improvement Plan, HCFCD, submitted to and accepted by Harris County Commissioners Court on February 20, 2003).
- Climatic conditions: Harris County’s natural climatic conditions serve as a catalyst for creating heavy rainfalls, often within short durations. Warm fronts pushing inland from the coast carry massive amounts of moisture accumulated while passing over Gulf and Caribbean waters. Storms often stall over coastal communities after colliding with northern cold fronts. These weather patterns trigger localized flooding in the watershed…flooding caused by a combination of the following variables, all interacting with each other: 1) the intensity of rainfall at any given moment, 2) the duration of rainfall at that intensity, 3) the area covered at that intensity, 4) the watershed’s topography including roughness, slope, and porosity of the land surface (clay/sandy soils vs. asphalt or concrete), 5) the type of vegetation growing on the land, 6) the sinuosity (bending and winding) characteristics of the stream, and 7) the capacity of the watershed’s drainage channels (both natural and man-made) in terms of the volume of water the channels can transport within the stream banks.
- Topography: Harris County’s topography is extremely flat. The elevation of the 52-mile Cypress Creek watershed slopes in an east-northeast direction from an elevation of 310 feet at its Waller County headwaters to sixty feet at the mouth (i.e., roughly five feet per mile or one inch per 100 feet). Because this flat topography is not conducive to effective drainage, intense rainfall often exceeds the bayous’ carrying capacity resulting in out-of-bank flooding.
- 4. Development: As defined in National Flood Insurance Program (NFIP) regulations, development as used herein is “any man-made change to improved or unimproved real estate, including but not limited to buildings or other structures, mining, dredging, filling, grading, paving, excavation or drilling operation or storage of equipment or materials.”
- Loss of trees: In 2003, using satellite imaging techniques, the Texas Forest Service conducted an environmental analysis to quantify the effect on flooding and air pollution attributable to the loss of trees in the Cypress Creek watershed. This analysis determined the effects of a typical undeveloped acre being clear-cut and covered with asphalt. Key findings:
- Stormwater peak runoff from forested land during a 5-inch rainfall (Rainfall type III) in 24 hours was 1.98 cubic feet
per second (cfs). When paved, it increased to 5.15 cfs, a 162 % increase.
b. The total volume of stormwater runoff doubled, increasing from 61,570 gallons to 128,672 gallons.
c. The detention storage capacity required to mitigate the change in peak flow was calculated as 2,738 cubic feet.
- Land use: Land use is a very significant determinant of stormwater flooding within a watershed. Some uses (i.e., timber production) have a beneficial effect. Others (i.e., urban development) make flooding worse. Until recently, one example unique to Harris County’s Cypress Creek watershed was the benefit derived from rice farming, a prominent early crop in the western upper reaches of the watershed. Rice fields are subdivided by dikes or levees which range from one to two feet high and four to six feet wide. They hold two to six inches of water until the grain begins to ripen. This holding capacity significantly slows the stormwater run-off rate thereby lowering the peak flood elevation unless the field is already flooded to capacity by the farmer.
Cypress Creek watershed land devoted to rice farming has dropped dramatically during the last twenty years due to sales to real estate developers. According to CCFCC research, this 30,000-acre reduction, when developed, will be equivalent to the loss of forty-eight square miles of stormwater detention (ultimately contributing to flood potential in the downstream watershed). For comparative purposes, the land areas (government and private combined) of the Addicks Reservoir and the Barker Reservoir are 45 square miles each (Source: U.S. Army Corps of Engineers, Galveston District, August 27, 2002).
- Effects of urban development: The Cypress Creek watershed is one of the fastest growing urban areas in Harris County. In the 1960s, Houston rose from the fourteenth to sixth most populous U.S. city (the largest in the Sun Belt), and Northwest Harris County’s quiet countryside evolved into a suburban area of nearly 75,000 people. The population subsequently skyrocketed to 200,000 by 1977, and the Wall Street Journal declared the watershed’s FM 1960 area the fastest growing residential community in the United States.
Left in their natural state, the watershed’s pastureland and forests absorb significant rainfall before draining into Cypress Creek and its tributaries. But urban development covers much of the surface with an overlay of impervious concrete/asphalt, which prevents its natural absorption of rainfall. These manmade changes to the land significantly increase both the volume and rate of water run-off flowing into the drainage channels, resulting in deeper and more frequent flooding.
“The primary source of damaging floods along Cypress Creek is increased development of the floodplain” (Source: General Reevaluation Report Environmental Assessment and Appendices. San Jacinto Rive and Tributaries, Texas Cypress Creek Flood Damage Prevention, U.S. Army Corps of Engineers, July 1998, Chapter 3, page 11). This fact should be carefully noted when considering the most recent (2006) population projections by local authorities: that the Houston-Galveston area will increase by 3½ million to an astounding 9 million people by 2035, equivalent to the population of Los Angeles.
Turner Collie & Braden, Inc., in their 1984 Cypress Creek Watershed Master Drainage Plan, the official (and only) plan for this watershed adopted by Harris County Commissioners Court, reported on this critical element in effective land-use planning. Their calculation of flooding during a l00-year storm event when the watershed is fully urbanized determined that the peak-flow flood level into the main channel would rise by as much as 500% with a resulting damage per incident of $412 million (in l984 dollars) if effective flood mitigation infrastructure is not built.
- Flood maps: These vital maps, which guide effective land-use planning, are published by FEMA as a product of the National Flood Insurance Program (NFIP). They show the floodplains and “Special Flood Hazard Area” (SFHA) along each stream and bayou in Harris County. The “Special Flood Hazard Area (SFHA), commonly called a “base flood” or “100-year flood,” would be the area inundated by a flood having a one percent chance of occurring in every given year. The floodplain shown on these maps means any area susceptible to being inundated by water from any source. Detailed flood hazard maps for Harris County streams were not completed until l985. (Source: HCFCD).
Later, the catastrophic flood damages suffered throughout the Houston area from Tropical Storm Allison became the impetus for the government to recalculate the floodplain areas for all watersheds with the county revealing that the floodplain sizes are significantly greater than shown on the 1985 maps.
Urban development is one of the many factors taken into consideration when floodplain areas are calculated using sophisticated U. S. Army Corps of Engineers computer simulation programs. But federal regulations prohibit future development from being used in the calculations of peak run-off. Rather, the regulations mandate that the SFHA be calculated based only on “then existing” development. The maps are simply point-in-time “snap shots.” As forested areas are destroyed, making room for new subdivisions, shopping centers, roads, streets and other urban development, the most recent “snap shots” are rendered more and more inaccurate relative to flood levels and locations. This problem will not occur if proactive flood mitigation projects are constructed ahead of urban development with sufficient capacity to handle the increased run-off.
- Cost-to-Benefit ratio: This is a basic criterion in determining whether or not federal assistance is provided for a proposed flood reduction project. Federal regulations stipulate that if the cost of a recommended project exceeds the flood protection benefit to be derived, the U.S. Army Corps of Engineers (USACE) is prohibited from participating. Regulations also prohibit including “avoided cost damage estimates” (the expected cost to repair flood damage incurred by yet to be constructed development) in the calculation to determine flood protection benefits to be derived from a project. If the cost-benefit ratio criterion is met, the federal government’s share of the project cost is 75% based on the cost-sharing provision of the Water Resources Act of l986.
Note: The Corps of Engineers flood protection plan being developed for the Cypress Creek Watershed, as authorized by Harris County Commissioners Court in February 1994, was dropped in 1997 because it failed the cost/benefit test. The only flood reduction approach identified at that time, which met the cost-benefit criterion, was the acquisition and removal of 38 homes located low in the (5-year) floodplain
- Erosion and sediment: Cypress Creek is one of the few unchannelized streams left in Harris County. The areas along its banks contain a prominence of “sugar sand” deposits which erode from natural causes. This sensitive erosion process can be adversely impacted when urban development significantly increases both the volume and the velocity of stormwater run-off, resulting in increased (new) floodwater areas, depths, and frequencies.
If preventive measures to reduce the uncontrolled runoff are not constructed in advance of the urban development process, the resultant increased flooding causes greater erosion to the stream channels, filling them with silt. The greater volumes and velocity undercut the steam banks leading to the loss of valued trees which tumble into the stream bed further blocking the flow of water. The displacement by silt further reduces the channel capacity to carry stormwater and further worsens the flooding conditions.
Erosion is a major reason for emphasis being placed on detention as a “preferred” flood hazard mitigation approach in the Cypress Creek watershed in lieu of channel widening, dredging and/or concrete lining of long sections. Effective detention can be accomplished by creating a watershed system of: 1) small on-site detention ponds constructed by subdivision developers as mandated by recent Harris County land development regulations, and 2) mid-size to large regional detention basins constructed by Harris County Flood Control District (not a mandatory requirement).
- Land Subsidence: Harris County land subsidence is primarily the result of drawing well water from an aquifer at a rate exceeding the aquifer’s capacity to replenish it. As a result, the ground above the aquifer sinks to a lower elevation thereby changing the topography and its drainage characteristics. This sinking is one of the key elements contributing to stormwater flooding.
The Sixty-fourth Texas Legislature created the Houston-Galveston Subsidence District (HGSD) in 1975 “ for the purpose of ending subsidence which contributes to or precipitates flooding, inundation, or overflow of the district including without limitation rising waters resulting from storms or hurricanes.” HGSD mandated that underground water withdrawal in the area underlying the Cypress Creek watershed be reduced a minimum 20% of total demand by the year 2010, 30% by no later than 2020, and 80% by no later than 2030 (it is estimated that the northwest section of Harris County will sink another 1½ feet by 2030). TSARP maps account for subsidence through late 2001, when the LiDAR data was collected
- Flood hazard mitigation: Mitigation refers to activities that lessen the potential for future flood damages such as:
- Elevating structures above the predicted flood level.
- Enlarging the natural flood storage capacity of a floodplain, or preserving it by prohibiting development within its boundaries.
- Controlling the rate/volume of drainage with reservoirs and retention/detention basins, and
- Encouraging land-use restrictions which guide and/or limit urban development while promoting preservation and conservation in selected areas. The Katy Prairie Conservancy, which is partially located in the Cypress Creek watershed, is an example of the latter.
- Benefits of a natural floodplain: Construction anywhere in the watershed can increase the risk of flooding to other properties if drainage impacts are not appropriately considered. This is especially true in Harris County where almost all construction contributes to worsening flood losses. Some of the benefits derived from leaving the floodplain in a natural state are:
- Floodwater storage retention.
b. Enhanced stormwater management.
c. Reduced flood damages due to absence of urban development.
d. Improved water quality.
e. Recreational opportunities and aesthetics.
f. Preservation of wildlife and their natural habitat.
g. Sustained biological productivity
h. Enhanced erosion control
i. Opportunities for scientific study and outdoor education
j. Increased property values
k. Preservation of cultural resources
l. Sustained economic prosperity
- Floodplain management responsibility: Federal regulations (NFIP) require that each local community be responsible for the adoption and enforcement of floodplain standards in its area. Harris County has thirty-eight “communities”…incorporated cities…ranging in size from Houston to Hockley. Accordingly, there are thirty-eight independent flood plain administration bodies which are responsible for their respective communities. Additionally, Harris County is responsible for these requirements throughout the unincorporated areas of the county. With minor exceptions, the Cypress Creek watershed lies entirely within the unincorporated areas of Harris and Waller Counties; thus, its Harris County inhabitants are heavily dependent upon county government for effective flood hazard mitigation and flood management planning. In Harris County, the County Engineer or his designee is responsible for administration of regulations, and the issuance of permits required by flood regulations and enforcement.