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.
1. 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).
2.
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.
3. 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.”
5.
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:
a. 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.
6. 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).
7. 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.
8. 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.
9. 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
10. 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).
11.
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
12. 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.
13.
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:
a. 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
14. 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.
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