HomeMy WebLinkAbout023318 ORD - 05/19/1998AN ORDINANCE
AMENDING THE CODE OF ORDINANCES, CITY OF CORPUS CHRISTI,
CHAPTER 13, BUILDING; CONSTRUCTION AND RELATED
OPERATIONS; HOUSING AND HOUSING PREMISES STANDARDS AND
THE CORPUS CHRISTI PLATTING ORDINANCE REGARDING THE
ADOPTION OF A STORM WATER MANAGEMENT GUIDANCE
DOCUMENT FOR ASSESSMENT OF WATER QUALITY IMPACTS FROM
NEW FLOOD CONTROL PROJECTS; PROVIDING FOR SEVERANCE;
AND PROVIDING FOR PUBLICATION.
WHEREAS, the NPDES Municipal Separate Storm Sewer System Permit issued to the City of
Corpus Christi by the United States Environmental Protection Agency requires the City to develop
and adopt a guidance document that provides technical criteria for the assessment of water quality
impacts from new flood control projects, and requires that the technical criteria be applied during
the planning and design phases of new flood control project;
AND WHEREAS, the City Manager, or his designated representative, will review this ordinance
and work with the affected parties to determine whether any amendments are needed to ensure that
this ordinance does not impose unnecessary burdens or undue hardships.
BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY OF CORPUS CHRISTI,
TEXAS:
SECTION 1. Section 13-200, Adoption of the Storm Water Management Guidance Document, in
Chapter 13, Buildings; Construction and Related Operations; Housing and Housing Premises
Standards, of the Code of Ordinances, City of Corpus Christi, is amended to read as follows:
"ARTICLE VI. STORM WATER QUALITY MANAGEMENT PLANS
"Sec. 13-200. Adoption of the Storm Water Management Guidance D.,eument Documents.
"The Storm Water Management Guidance Document for Development Planning and
Construction Activities, . - .: - :. - - . . - ... - - - , dated May 28, 1997, and the
Storm Water Guidance Document for Assessment of Water Quality Impacts from New Flood
Control Project nrne ared bathe storm water department dated May 1998 a true copy true
copies of which is -are on file with the city secretary, i -s -are adopted to ensure storm water
entering the navigable waters of the United States from the city's municipal separate storm sewer
system does not violate the terms of the city's storm water national pollution discharge
elimination system permit. The Guidance Due,.me„ t cot/tains Documents contain suggested best
management practices that owners, developers, and contractors should consider adopting to help
control and reduce pollutants that are transported by storm waters and technical guidance related
to erosion and sediment controls and other measures to reduce pollutants from new flood control
projects."
R83408A2.125
niltaU.ElLMED
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SECTION 2. Section IV.C.2. of the Corpus Christi Platting Ordinance is amended to read as
follows:
"SECTION IV - DESIGN STANDARDS
* * * * *
"C. EASEMENTS.
"2. Where a subdivision is traversed by a water course, drainage way, channel,
or stream, there shall be provided a storm water easement or drainage
right-of-way conforming substantially with the lines of such water course,
and such further width or construction, or both, as will be adequate for the
purpose. The width and construction of any drainage way. ditch, or
channel shall conform the Storm Water Management Guidance Document
for Assessment of Water Quality Impacts from New Flood Control
Projects adopted by Section 13-200, Code of Ordinances. Parallel streets
or parkways may be required in connection therewith."
SECTION 3. Section V.10. of the Corpus Christi Platting Ordinance is amended by revising
subdivision a) and adding a new subdivision d) to read as follows:
"SECTION V - REQUIRED IMPROVEMENTS
"10. STORM WATER QUALITY CONTROL MEASURES TO REDUCE
POLLUTION AFTER CONSTRUCTION.
"a) Storm Water Management Guidance Document for Developmental
Planning and Construction Activities. Prior to submission of a
preliminary plat or final plat for review, the Responsible Party shall
review the City of Corpus Christi's Storm Water Management Guidance
Document for Developmental Planning and Construction Activities and
Storm Water Management Guidance Document for Assessment of Water
Ouality Impacts from New Flood Control Projects, adopted by Section
13-200, Code of Ordinances, and determine whether it is feasible to
incorporate cost effective Best Management Practices that will reduce
pollution of the receiving waters by storm water runoff from the property
being platted."
R83408A2.125
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Required Design Criteria Relating to Water Quality Impacts from
Flood Control Projects. The following design criteria shall be
incorporated into new flood control projects'
I,w
A vegetated buffer strip of at least 10feet in width or other
comparable stabilization feature shall be established along the
banks of a City drainage channel.
An outfall structure. which empties into a earthen ditch or natural
drainage area. must provide scour protection to prevent erosion
including the use of rubble berms. geotextile reinforcement. or
velocity dissipators.
Channels designs shall include provisions for velocity control
structures in order to limit the velocity of stormwater to five (51
feet per second in clay soils and three and a half (3.5) feet per
second in sandy soils.
Control berms with outfall pipes shall be provided along the edge
of channels where more than 200 feet of overland flow will drain
into the ditch."
SECTION 4. If for any reason any section, paragraph, subdivision, clause, phrase, word or
provision of this ordinance shall be held invalid or unconstitutional by final judgment of a court
of competent jurisdiction, it shall not affect any other section, paragraph, subdivision, clause,
phrase, word or provision of this ordinance, for it is the definite intent of this City Council that
every section, paragraph, subdivision, clause, phrase, word or provision hereof be given full
force and effect for its purpose.
SECTION 5. Publication shall be made in the official publication of the City of Corpus Christi
as required by the City Charter of the City of Corpus Christi.
R83408A2.125
That the� foregoing ordinance was read for the first time and passed to its second reading on this
the _n� Day of %Yl4 , 1998, by the following vote:
Samuel L. Neal, Jr.
Javier D. Colmenero
Melody Cooper
Alex L. Garcia, Jr.
Dr. Arnold Gonzales
That the f re oing ordinance was read
Day of Q.ti
Samuel L. Neal, Jr.
Javier D. Colmenero
Melody Cooper
Alex L. Garcia, Jr.
Dr. Amold Gonzales
Betty Jean Longoria
John Longoria
Edward A. Martin
Dr. David McNichols
t
11.602
he second time and passed finally on this the / QUI
, 199 by the following vote:
Betty Jean Longoria
John Longoria
Edward A. Martin
Dr. David McNichols
PASSED AND APPROVED, this the /4N1 Day of / 1998.
ATTEST:
Armando Chapa
City Secretary
Samuel L. Neal, Jr.
Mayor, The City of Corpus Christi
APPROVED: 614 DAY OF ev 1998:
James R. Bray, Jr., City Attorney
By:
R83408A2.125
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STORM WATER MANAGEMENT
A
%10%0001•01
City of
Corpus
Christi
GUIDANCE DOCUMENT FOR
ASSESSMENT OF WATER QUALITY IMPACTS
FROM NEW FLOOD CONTROL PROJECTS
MAY, 1998
TABLE OF CONTENTS
1.0 INTRODUCTION
1.1 BACKGROUND 1
1.1.1 Clean Water Act 1
1.1.2 City of Corpus Christi MS4 Permit and Flood Control Projects 1
1.2 PURPOSE OF THE GUIDANCE DOCUMENT 2
1.3 ACKNOWLEDGEMENTS 2
2.0 RECEIVING WATER QUALITY IMPACTS OF FLOOD CONTROL PROJECTS
2.1 SOURCES OF WATER QUALITY PROBLEMS 3
2.2 WATER QUALITY ASSESSMENT METHODS 4
3.0 CITY OF CORPUS CHRISTI COMPREHENSIVE MASTER PLAN
POLICIES AFFECTING THE WATER QUALITY OF FLOOD CONTROL
PROJECTS
3.1 SOURCES FOR EXISTING DESIGN CRITERIA 5
3.1.1 Ordinances 5
3.1.2 Design Manuals 6
3.1.3 Master Plans 7
3.1.4 Other Criteria Sources 8
3.2 EXISTING DESIGN CRITERIA
3.2.1 Flood Protection
Design Rainfall Event
Level of Protection
3.2.2 Drainage System
Open Channel Design
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Culvert Design
Storm Sewer Design
Street Flow Design
3.2.3 Easement/Right-Of-Way Dedications
3.2.4 Modeling Standards
Hydrologic Standards
Hydraulic Standards
3.2.5 Storm Water Pollution Control
3.2.6 Erosion Control
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3.3 STRUCTURAL ALTERNATIVES FOR IMPROVING WATER
QUALITY FOR FLOOD CONTROL PROJECTS
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3.3.1 Wetland Detention Ponds 19
3.3.2 Dry Detention Ponds 20
3.3.3 Regional Detention Ponds 20
3.3.4 Wetland Treatment 21
3.4 PROPOSED DRAINAGE DESIGN CRITERIA POLICIES RELATING
TO WATER QUALITY IMPACTS FROM FLOOD CONTROL
PROJECTS
3.4.1 Channel Side Slopes 22
3.4.2 Bank Stabilization 23
3.4.3 Vegetated Buffer Strips 24
3.4.4 Outfall Structures 25
3.4.5 Velocity Control Structures 26
3.4.6 Berms for Overland Flow Control 26
3.4.7 Project Review 27
4.0 REFERENCES
1.0 INTRODUCTION
1.1 BACKGROUND
1.1.1 Clean Water Act
The 1972 amendments to the Federal Water Pollution Control Act (also referred to as the
Clean Water Act) prohibit the discharge of any pollutant to waters of the United States from
a point source unless the discharge is authorized by a National Pollutant Discharge
Elimination System (NPDES) permit. Efforts to improve water quality under the NPDES
program traditionally focused on reducing pollutants in discharges of industrial process
wastewater and from municipal sewage treatment plants. Efforts to address storm water
discharges under the NPDES program have generally been limited to certain industrial
categories with effluent limitation for storm water.
Congress amended the Clean Water Act (CWA) in 1987 to require the Environmental
Protection Agency (EPA) to establish phased NPDES requirements for storm water
discharges. To implement these requirements, on November 16, 1990, EPA published (55
Federal Register 47990) the initial permit application requirements for (a) 11 categories of
stone water discharges associated with industrial activity, and (b) discharges from municipal
separate storm sewer systems (MS4) located in municipalities with a population of 100,000
or more.
1.1.2 City of Corpus Christi MS4 Permit and Flood Control Projects
The November 16, 1990 regulation mentioned above also established requirements of a two-
part permit application designed to facilitate development of site-specific permit conditions
for municipalities with a population of 100,000 or more. The CWA requires that NPDES
permits for discharges from an MS4 include a requirement to effectively prohibit non -storm
water discharges (including construction activities) into the storm sewers. The CWA also
requires NPDES permits for discharges from an MS4 to include controls to reduce the
discharge of pollutants to the maximum extent practicable by implementation of
management practices, control techniques, engineering methods, and other provisions
appropriate for the control of such pollutants.
In response to the EPA municipal storm water permit requirements, the City of Corpus
Christi; Texas Department of Transportation -Corpus Christi District; Corpus Christi Junior
College District; Port of Corpus Christi Authority, and Texas A&M University -Corpus
Christi submitted a Joint Part 1 permit application to EPA in May 1992 and Joint Part 2
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application to EPA in May 1993.
The City of Corpus Christi along with its co -permittees, was issued NPDES permit
No.TXS000601 in April 1995, which became effective on June 1, 1995. The permit
provides the opportunity to propose appropriate management programs to control pollutants
in discharges from the MS4. One of the management programs required by EPA is to
implement and maintain structural and non-structural best management practices to reduce
pollutants in storm water runoff from flood management projects.
1.2 PURPOSE OF THE GUIDANCE DOCUMENT
This handbook has been prepared to provide technical guidance related to erosion and sediment
controls and other measures to reduce pollutants from new flood control projects. The document
is to be used as a guidance manual to implement a local storm water quality management program
for new flood control projects such as excavated ditches, Tined channels, or major drainage mains.
The purpose of the document is threefold:
(1)
To provide best management practices for the reduction of pollutants from flood
control projects.
(2) To provide criteria for assessing the water quality impacts of flood control projects.
(3)
To provide necessary revisions to City of Corpus Christi's Comprehensive Plan
policies that will include the assessment criteria to minimize water quality impacts
from new flood control projects and result in the implementation of the appropriate
best management practices for flood control projects for the reduction of pollutants
in storm water.
1.3 ACKNOWLEDGEMENTS
This handbook contains information provided from handbooks developed in other cities and
states. The manuals prepared by Harris County, Harris County Flood Control District, the
City of Houston, the City of Austin, Department of Environmental Protection were
extensively used. In addition, information contained in the preliminary draft of the Storm
Water Management for Construction Activities prepared by Turner Collie & Braden, Inc.
for Harris County, was also used in preparing this handbook. J. R. Thompson, P.E., served
as major writer and technical consultant to the City of Corpus Christi in the preparation of
this guidance document.
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2.0 RECEIVING WATER QUALITY IMPACTS OF FLOOD CONTROL PROJECTS
2.1 SOURCES OF WATER QUALITY PROBLEMS
In the past, flood control efforts have focused primarily on containing the volume of water that
abruptly enters water bodies. Traditional methods to reduce flooding include using dry detention
basins that temporarily store excess runoff, constructing channels, streambank lining, and floodplain
restrictions that limit development along or in flood -prone areas.
These flood control measures were not originally designed to control pollution caused by increased
urbanization. The historical focus was on the quantity control - not quality control. Communities
now recognize that these issues must be incorporated into an overall strategy that assesses both
flooding and pollutant removal. Existing flood control BMP's can be retrofitted to protect water
quality. New flood control BMP's should be built with the dual purpose of handling storm water
quantity while maintaining storm water quality. A comprehensive watershed protection strategy,
which uses nonstructural and structural BMP's, will reduce the long-term costs of both controlling
floods and protecting water quality.
Identifying and preserving buffers and natural systems is an important component of a watershed
management plan. The areas serve as nonstructural controls, filtering out many pollutants in urban
runoff that might reach the water bodies. Numerous controls are in place with the City of Corpus
Christi that identifies environmentally sensitive areas such as wetlands, flood plains, etc. and
discourages the development of these areas.
Controls are also required to be utilized during the construction of sites within Corpus Christi.
These controls address erosion, trash, hazardous wastes, concrete washdown, and revegetation. The
Guidance Document for Development and Significant Redevelopment in the City of Corpus Christi
contains the requirements for site development.
Flood control projects receive storm water from the drainage system associated with site
developments, in a concentrated form and convey it downstream to the receiving waters. Flood
control projects can be designed to reduce pollutants in several ways. The primary means of
reducing pollutants in the Corpus Christi drainage system is in the reduction of erosion from the
sides and bottoms of the earthen channels. The erosion occurs due to steep side slopes that cannot
maintain vegetation, excessive velocities that scour the channel bottom, excessive overland bank
flow that cuts into the sides of the channels, and scour from turbulence due to abrupt changes in
velocity or direction of the storm water.
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Another feature of flood control projects that can be utilized to improve storm water quality is the
ability of detention facilities to filter out pollutants that may be suspended in free flowing storm
water. If certain pollutants are identified in the storm water discharge that should be reduced, wet
or dry retention/detention structures should be considered.
Retrofitting existing storm water management systems requires modifying runoff control structures
or conveyance systems, originally designed to control flooding to also control water quality.
Modifications might include enlarging structures, changing the inflow and outflow patterns, and
increasing detention times. Retrofitting costs are a major hindrance in improving water quality in
developed areas. Therefore, communities need to identify the most insidious pollutants and then
select the most cost-efficient and effective solutions to deal with them, thus improving water quality
in urban runoff.
The majority of the area of the watersheds that drain into the Corpus Christi storm water system and
are considered "undeveloped," are actually agricultural. The erosion associated with agricultural
practices has occurred over a period of the last century and is probably the greatest contributor to
storm water pollution in the area. New construction has also caused temporary increases in storm
water pollution until the sites are stabilized. In the Corpus Christi storm water system, pollution
from soil erosion remains the greatest problem in maintaining and often improving the quality of
storm water after development has occurred.
Additional pollutants are introduced into storm water runoff from developed areas of the MS4
drainage basins. Runoff from parking lots and roadways contain oil and grease, while runoff from
landscaped areas frequently contain fertilizers and grass clippings.
2.2 WATER QUALITY ASSESSMENT METHODS
The decision as to which BMP's to use in a particular area depends upon the topography of the
region, types of native soils, regional adaptability of vegetation, and on any particular pollutants
that may need to be removed from the storm water.
In areas such as Corpus Christi where the predominant problem is erosion prevention and sediment
removal, the methods of water quality assessment should include a review of each project to include
velocity control, revegetation of slopes, scour protection, and in limited cases, detention systems for
sediment removal.
The following sections detail the existing criteria concerning drainage that affects water quality, and
the steps that should be used to improve these criteria. All new flood control projects should be
assessed to determine that the project meets as many of the additional criteria as practical.
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3.0 CITY OF CORPUS CHRISTI COMPREHENSIVE MASTER PLAN POLICIES
AFFECTING THE WATER QUALITY OF FLOOD CONTROL PROJECTS
3.1 EXISTING POLICIES AND PRACTICES
The following section provides an overview of local design criteria and policies applicable to storm
water management in the Corpus Christi/Nueces County area. The Nueces County area is included
because it covers much of the extra territorial jurisdiction of the City of Corpus Christi. Local
ordinances, drainage criteria, design manuals and storm water master plans have been compiled in
order to determine: 1) current standards for the design and construction of storm water management
systems; 2) current floodplain management policies; and 3) current storm water runoff pollution
management policies. Recommendations have been made to enhance current criteria to provide
adequate levels of storm water pollution management.
As part of this review, the following issues were addressed:
Design rainfall event specifications
Roadway and structural flood protection
Easement/right-of-way dedication
Hydrologic/hydraulic modeling standards
Storm water pollution control
Erosion control
3.1.1 Ordinances
Nueces County
For Nueces County, some drainage related criteria are found in the platting ordinance. The
Nueces County Platting Ordinance, recorded in Volume 23, Page 181 of the County
Records, details the required standards for the subdivision and platting of land within
Nueces County but outside the extraterritorial jurisdiction of any incorporated city or town.
Regulations contained within the platting ordinance establish minimum requirements for lot
sizes, road rights-of-way widths, and ditch slopes. The ordinance requires drainage plans be
prepared and submitted by a registered professional engineer to the County Engineer to
determine compliance with the platting ordinance.
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To maintain eligibility to participate in the National Flood Insurance Program administered
by the Federal Emergency Management Agency (FEMA), Nueces County revised its Flood
Damage Prevention Ordinance in 1987. This ordinance was based on "The Flood Insurance
Study for Nueces County, Texas" dated September 18, 1984 with accompanying Flood
Insurance Rate Maps and Flood Boundary - Floodway Maps (FIRM and FBFM). The
ordinance requires the acquisition of development permits to ensure compliance with
provisions of the ordinance.
Ciry of Corpus Christi
City of Corpus Christi Platting Ordinance No. 4168 (adopted 1955 with numerous
subsequent amendments) details similar requirements to the County, for establishing criteria
for design and construction of subdivision improvements. Minimum design flows for
drainage, acceptable limits of street flooding, gutter and inlet construction standards are
addressed in the City's platting ordinance.
Similar to Nueces County, Corpus Christ passed an ordinance authorizing the enforcement
of a Flood Hazard Prevention Code in compliance with FEMA requirements. The
ordinance also includes provisions for development permits for construction within the City.
3.1.2 DESIGN MANUALS
Nueces County
In conjunction with the development of the 1986 Nueces County Storm Water Master Plan,
the Nueces County Drainage Criteria and Design Manual was prepared (Ref. 1). The
manual contains detailed drainage design criteria for the calculation of storm water runoff
and the subsequent design of open channels, culverts, bridges, storm sewers, inlets and
streetflow. Though the manual has not been formally adopted by Nueces County, the County
Engineer uses the manual as a guide for acceptable drainage design practices for
development within the County.
City of Corpus Christi
The City of Corpus Christi does not have a single consolidated design criteria manual which
contains all of the City's storm water related technical criteria.
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3.1.3 Master Plans
Nueces County
Nueces County utilizes the floodplain mapping element of the Nueces County Storm Water
Management Plan (1986) to identify the extent of the 100 -year floodplain for major creeks
throughout the County. These include Oso Creek, Nueces River, Petronilla Creek, Pinitas
Creek, Agua Dulce Creek, Banquete Creek, Quinta Creek, San Fernando Creek, and Correta
Creek. When development is proposed near these creeks, the County Engineer consults the
master plan's maps to determine if the project lies within a designated floodplain. If it does,
then appropriate design measures may be required to prevent the flooding of structures or
impediment of flood waters.
City of Corpus Christi
In October 1946, an engineering study entitled "A Report on a Storm Sewer System" (Ref.
3) was prepared for the City of Corpus Christi. This report is the earliest engineering study
available for Corpus Christi which describes, in detail, the hydraulic parameters and criteria
used in the design of a large drainage system. The system comprised 15 areas in the north
side of and along the bayfront of Corpus Christi which needed a coordinated design. The
selection of design criteria for this system became the defacto criteria for many years of
future design which interconnected into the main drainage system. This study and report,
though not on an official plan, provided an early example of reasonable design which was
incorporated into Corpus Christi's Master Plan documents.
The City of Corpus Christi later prepared a series of drainage master plans beginning in
1961. These plans cover specific areas of Corpus Christi, as follows:
A. Southside Master Plan, 1961 (Ref. 4)
B. West of Clarkwood and Flour Bluff Master Plan, 1970 (Ref 5)
C. Five Points Master Plan, 1982 (Ref.6)
D. South of Oso Creek Master Plan, 1988 (Ref. 6)*
Master Plan for area south of Oso Creek remains unadopted at the date of this report.
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These Master Plans, when adopted, are used by the City to determine appropriate design
criteria for expanding the storm drainage system. Contained within these master plans are
recommended design criteria such as storm design frequency, location, size, and hydraulic
losses based on standard hydraulic methodology. During drainage system design review,
City Engineering Department staff addresses site specific issues using standard hydraulic
principles and assure compatibility with existing drainage master plans (Section 3.1.3).
3.1.4 Other Criteria Sources
Federal Emergency Management Agency (FEMA)
The Federal Emergency Management Agency (FEMA) has studied the major creeks and
drainage ways within the Corpus Christi area. As a result, FEMA has established floodplain
elevations and floodplain widths for various design storms. Additionally, FEMA has
specified floodways which comprise the minimum areas of the main stream channel which
must remain open and free from future land development improvement in order to pass the
100 -year storm with no greater than a one foot rise in flood waters. This effectively prevents
the placement of any fills or structures within this area along the main channel. In order to
participate in the Federal Flood Insurance Program, the City and County are required to
maintain FEMA's criteria for construction within the designated flood hazard areas. The
criterion requires structures to be elevated above the 100 -year flood elevation (or flood
proofed), and to be located outside of the floodway.
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3.2 EXISTING DESIGN CRITERIA REVIEW
3.2.1 Flood Protection
Design Rainfall Event
In order to establish a uniform level of protection from flooding within any storm sewer
system, a rainfall event, i.e., 5 -year frequency events, 25 -year frequency events, etc., is
established to provide the level of protection desired at a cost that the community is willing
to afford. For instance, a 5 -year rainfall frequency event is the amount of rainfall which has
a 20 percent chance of occurrence in any given year while the 25 -year rainfall event has a
4 percent chance of occurrence in any given year. The storm water drainage systems are
designed to carry the peak discharge anticipated during the selected rainfall event. Thus, the
criterion for the level of protection provided by the storm water system becomes the design
rainfall event.
Nueces County follows the design frequency guidelines stated in the Nueces County Design
Manual, 1986 as follows:
"Storm drainage systems are usually planned to accommodate low levels of storm
influx. The initial drainage system handles a 25 year storm event with no disruption
of traffic flow or flooding outside the channels. The major drainage system handles
the 100 -year storm event, perhaps not carrying the load but at least preventing loss
of life and major damage. To provide for an orderly community growth, reduce costs
to future generations, and prevent loss of life and major property damage, these two
separate and distinct drainage systems should be planned and properly engineered"
(Ref. I)
Under these guidelines, Nueces County drainage systems are designed to carry the 25 -year
rainfall runoff within conduits or within ditch banks. Major drainage systems are designed
to contain the entire 100 -year storm where possible, but all systems are designed to
preventing major damage due to storms in excess of the 25 -year design frequency.
The City of Corpus Christi relies on its adopted Master Plans for the determination of area
specific design criteria. The Master Plans cited in Section 3.1.3 have been written and
adopted over a period of more than 25 years, from 1961 and 1988. Later Master Plans have
recommended improving the design standard due to the need to provide greater levels of
flooding protection to the urbanizing areas of the City where, for instance, major
commercial or residential centers would be damaged by flooding and where automobile or
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emergency vehicle traffic, to and from these areas, may be severely hampered.
Along with increasing design rainfall event frequencies, subsequent Master Plans have
recommended increasing design values for imperviousness due to the development of large
areas of the City and the greater density of development.
Table 3-1 lists the design frequencies and basic values for percent impervious used for the
design of drainage ways in the various Master Plans.
Additional criterion for drainage design is contained in the City of Corpus Christi Platting
Ordinance (page 28) - "The runoff factor used in design of storm sewers will be a minimum
of one and three -tenths (1.3) cubic feet (per sec.) per acre for a minimum time of
concentration of ten (10) minutes." These figures follow closely with the runoff tables
provided in the 1961 Master Plan for a 35% impervious surface due to a 5 -year rainfall
event frequency. The design criterion of 1.3 cfs/acre has thus been frequently used as a
minimum design criteria with site specific issues evaluated independently.
Based upon the Master Plans and Platting Ordinance, the City Engineer enforces the
following design criteria for storm water systems:
25 -year rainfall event - Bridges, channel structures, and major drainageways indicated on
Master Plan maps and generally serving areas greater than approximately 100 acres.
5 -year rainfall event - Closed conduit storm sewers and channels serving minor areas such
as residential internal drainage systems.
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TABLE 3-1
CITY OF CORPUS CHRISTI
MASTER PLAN DESIGN STORM PARAMETERS
Major Drainage
Master Plan Systems Design Percent
Year Area Frequency Impervinim
1946 Northside/Bayfront 5 -year 30% Residential/
70% Business
1961 Southside 5 -year 20%
1970 West of Clarkwood 25 -year 20%
(Oso Creek)
1970 West of Clarkwood 25 -year 35%
(Nueces River)
1970 Flour Bluff 25 -year
1982 Five Points 25 -year
1988 South of Oso Creek 25 -year
(Nueces River)
Level of Protection
20%
20%
45%
Nueces County requires special precautions for the construction of buildings above the
elevations of adjacent roadways and 100 -year flood elevations. The Platting Ordinances
require that building floor elevation be constructed above the elevation of the fronting road
(6 inches above a nearest roadway). Participation in FEMA's National Flood Insurance
Program requires the County to ensure construction to be above the FEMA 100 -year flood
elevation's roadway. Participation in FEMA's National Flood Insurance Program requires
the County to ensure construction to be above the FEMA 100 -year flood elevations. These
regulations are enforced by the County Engineer for approving plats and building permits.
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The City of Corpus Christi also requires construction to be above the 100 -year flood
elevations and outside the regulated floodway as explained in Section 3.1.4. Additionally,
the City requires that storm sewer systems be designed with the capacity to carry the 5 -year
design storm with street flooding limited to the street right-of-way. Minimum slope
requirements for subdivision lots effectively require that minimum floor elevations be
located above adjacent street elevations (normally eighteen inches for compliance with FHA
and VA construction requirements).
3.2.2 Drainage System
Open Channel Design
The Nueces County Engineer utilizes the design presented in the Nueces County Design
Manual (Ref. 1) as a guideline to determine the adequacy of drainage design. The following
is a summary of the design parameters used by the County:
Design Method: Uniform Flow/Manning's Formula for Headloss
Suggested Frictional
Coefficients ("n"): .027 (Short Grass ) to 0.50 (Brush on Banks)
Maximum Velocity: 6 feet per second (fps)
Maximum Depth:
No maximum depth specified. Recommended depth
to be as shallow as possible considering maintenance
cost and available right-of-way
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The City of Corpus Christi also has design criteria for channel parameters which have been
adopted with the Storm Water Master Plans. These criteria which have varied among Master
Plans, are summarized as follows:
Design "n" Value = .0225 for unlined straight channels
Maximum Velocity = 5 to 8 -fps
Maximum Depth = 6 to 8 feet
Maximum Side Slopes = 2:1
Culvert Design
Nueces County minimum design criteria for culverts are as follows:
Minimum Size: 18" Diameter
Maximum Velocity: 6 fps - Unlined Downstream
15 fps - Lined Downstream
Design Method: Sizing Based Upon Entrance, Exit & Frictional Loss
The City of Corpus Christi has not established specific criteria for culvert design, except for
the 25 -year design rainfall criteria which is included in later Master Plans. Design aspects,
such as consideration of minor losses and backwater effects, are selected consistent with the
design rainfall event on a case-by-case basis.
Storm Sewer Design
Nueces County has an established design criterion for storm sewer design based on the
Nueces County Drainage Design Manual. Few systems exist within the County's rural
jurisdiction as enclosed storm sewers generally accompany only urban development.
13
The following is a summary of the design criterion requirements of Nueces County:
Minimum Size: 18" Diameter
Minimum Slope: 0.40%
Maximum Velocity: 15 fps (Collectors) - 12 fps (Mains)
Design Method: Minor Losses & Friction Loss
In the absence of a specific adopted policy, the City Engineer evaluates storm sewer design
based upon site specific parameters and sound hydraulic principles. The City of Corpus
Christi requires that underground storm sewer systems be installed to drain the curb and
guttered streets in new developments. Inlets and conduits are designed for a 5 -year rainfall,
inlets having a 5' throat dimension. Conduits are a 15" minimum diameter. Hydraulic
gradients are calculated based upon Manning's formula for determining headloss.
Velocities are typically low due to flat slopes and therefore, velocity related headlosses are
usually not considered.
Street Flow Design
Nueces County has established the requirements for roadside ditches which are common in
areas of the County's jurisdiction. These requirements provided by the County's platting
ordinance are as follows:
Minimum Grade = 0.10%
Maximum Side Scopes = 4:1
Drainage Map Provided of System by Consultant Engineer
14
Additional criteria for curbed streets are found in the Nueces County Design Criteria and
Design Manual (Ref. 1).
Street Storm Maximum
Classification Frequency Encroachment
Local 10 -year Curb Line
Collector 10 -year Curb Line
One Lane Open
Arterial 25 -year Curb Line
Two Lanes Open
Expressway 25 -year No Encroachment
The City of Corpus Christi requires the following design criteria established by their platting
ordinance be utilized in street design as it relates to drainage:
Roadway Width = 28 ft. To 80 ft. as required by Master
Transportation Plan
Residential Section - 6" Roll Curb
Commercial Section - 6" L Curb
Minimum Slope - .20%
Maximum Level of Flooding - Back of Walk During 5 -year rainfall event
The City has adopted an administrative policy which requires that major arterials and
collector streets have adequate drainage to maintain two lanes and one lane open,
respectively, for vehicular access for the 10 -year rainfall event.
15
3.2.3 Easement/right-of-way Dedications
General criterions for both Nueces County and City of Corpus Christi are to obtain
sufficient right-of-way to contain the ultimate channel required to serve the drainage basin
as well as for maintenance operations. Nueces County requires a minimum 15 ft. wide
easement. The City of Corpus Christi has adopted the recommendations contained in the
Southside Master Plan (Ref. 4) and utilize these criteria wherever possible. This drainage
right-of-way dedication includes the ditch top width, maintenance easement of 32 ft. plus
area required for excavation stockpiles.
3.2.4 Modeling Standards
Hydrologic Standards
The Nueces County Drainage Criteria and Design Manual recommends the use of the
Rational Method for determining peak storm water runoff areas less than 400 acres. For
larger areas, the USGS Regional Flood Analysis Method is proposed. Other methods are
acceptable with the approval of the County Engineer.
The City of Corpus Christi has established several means of calculating runoff for design
storms. The accepted means of calculating design runoff quantities varies with the size of
the drainage basin, The rational method is used for areas less than 1000 acres. In smaller
areas, less than a few acres such as for single site developments, the values from the 1961
Master Plan (Ref. 4) have been used based upon an assumed minimum 30 minute time of
concentration. These runoff values for the 5 -year design storm were thus 1.30 cfs per acre
for residential development (35% impervious) up to 2.60 cfs per acre for commercial (100%
impervious). The design of areas up to 1,000 acres utilizes calculated times of concentration
and composite land use percentages to determine runoff. Areas greater than 1,000 acres up
to 22,000 acres utilize mid-range curves developed in the 1970 Master Plan for areas west
of Clarkwood (Ref. 4). Design for areas greater than 22,000 acres are seldom required, but
in this event, the regional curves in Texas Water Commission Bulletin 6311 (Ref. 8) are
utilized.
FEMA conducts their stream modeling for large areas and have utilized the regional curves
in the Texas Water Commission Bulletin 6311 (Ref. 8) for their studies. Therefore, data
submitted to FEMA generally follows this method.
16
Hydraulic Standards
The Nueces County Drainage Criteria and Design Manual provides extensive direction on
calculating channel and culvert capacities. Numerous nomographs are provided for
determining headlosses for various hydraulically controlling situations. Drainage designs
are expected to follow these guidelines though drainage calculations are not specifically
required to be submitted.
The City of Corpus Christi requires that storm sewer systems be designed with consideration
for the hydraulic grade line of the conduit or channel. Hydraulic grade line slopes are
calculated based upon Manning's equation for friction headloss. Minor losses related to
velocity are included where velocities are high. HEC -2 analysis is utilized where backwater
is expected to influence water surface profiles significantly.
3.2.5 Storm Water Pollution Control
Based on a review of the documents inventoried in Section 3.1, no specific design criteria
related to the control of storm water pollution were found.
There are no specific references to storm water pollution control in the County's design
criteria. Nueces County is authorized to prohibit disposal of any manner of waste on
property which may ultimately enter into local streams and water courses. Besides this
prohibition, Nueces County is limited by statute as to its authority to implement design
criteria aimed at the reduction of storm water pollution. In consideration for water quality
concerns, an environmental impact assessment was performed for each drainage project
proposed in the Nueces County Storm Water Management Master Plan. The assessment
focused primarily on expected impacts to downstream estuarine systems. No significant
impacts were predicted.
The City of Corpus Christi possesses a wide range of regulatory authority and has additional
authority available through the Texas local Government code and water code to address
storm water pollution issues. A review of the City's various master plans was conducted to
identify water quality related criteria.
In the "Storm Drainage and Sanitary Sewer Plan for the Area West of Calallen (Five
Points)," storm water quality and design criteria are referenced. Since the raw water intakes
for the local water supply are located on the Nueces River just downstream of the study area,
the quality of storm water runoff from this area was an important consideration. The
drainage plan presented a basic design concept for storm water quality treatment.
17
"Relatively shallow flow in broad channels with maintained grass cover," was
recommended to provide "overland flow" treatment capacity before discharging to the
Nueces River. It was also recommended that storm water runoff conveyed in confined
conduits (storm sewer pipes) should be permitted to enter the river only after passing
through a detention pond for water quality enhancement.
The plan identified three basic purposes served by a detention pond: 1) Reduce peak flow
rates; 2) provide for sedimentation of pollutants; and 3) provide additional control of
water quality. Design criteria suggested for detention ponds were given as follows:
1) Flow through velocity at design Q= 0.5 ft./sec.
2) Siphon outlet structure, baffled and grated, with overflow provision.
3) Provision for fully draining basin to facilitate removal of trapped sediment, debris
and other maintenance.
Multiple use facilities, obtaining easements for future detention facilities, maintaining grass
cover and increasing public awareness were also recommended in the drainage plan to
promote water quality protection.
3.2.6 Erosion Control
In the Nueces County Design Criteria Manual, guidelines for flow velocity, channel width,
slope and cover have been previously discussed. By controlling flow velocity and
promoting uniform flow in open channels, erosion is minimized. Similar recommendations
for flow velocity and channel dimensions are found in the City's master plans as previously
discussed. Again, these criteria serve to provide proper drainage while minimizing channel
erosion.
3.3 STRUCTURAL ALTERNATIVES FOR IMPROVING WATER QUALITY IN THE
CITY OF CORPUS CHRISTI STORMWATER SYSTEM
Numerous structural methods are being employed worldwide to improve the water quality of storm
water. Many of these methods are suitable only for certain soil conditions and topography. Others
are still in the experimental stages and treatment data is still being collected and assessed. This
document reviews the primary structural methods being utilized and each methods appropriateness
for employment within the City's storm water system. Structural methods reviewed included the
following:
18
* • Wet Detention Basins
* • Dry Detention Basins
• Retention/Treatment Ponds
• Onsite Detention Basins
* • Regional Detention Basins
• Combined Sewer Overflow Devices
• Infiltration Trenches
* • Wetland Treatment
* • Drainage Channel Retrofit with Underdrains
(* denotes recommended structural methods
for the Corpus Christi area)
Recommendations for use in the Corpus Christi area are made based upon this review. The
following discussions of the recommended structural methods also include various limitations on
these applications.
3.3.1 Wet Detention Ponds
Recommendation- Utilize wet detention ponds only where required for removal of identified
pollutants in storm water which can be removed by sedimentation.
Discussion - Wet detention basins have a potential for use in providing sedimentation of
heavy pollutants. Freeboard volume allows the storage of limited amounts of storm water
and reduces peak discharges downstream. Overflow spillways can be equipped to trap
floating trash. Means need to be provided for draining the pond to allow cleaning which
complicates construction of the facility. Maintenance of the facility is required on a
continual basis to prevent the slopes and open water from becoming a health hazard. Use
as a storm water treatment strategy will be limited to specific situations where an identified
pollutant needs to be removed by sedimentation.
19
3.3.2 Dry Detention Basins
Recommendation - Utilize dry detention basins for flood control, floating trash screening,
and pollutant removal in areas of steep topography such as along the Nueces River west of
Five Points.
Discussion- The utilization of dry detention ponds occurs primarily where the topography
is steep enough to allow draining by gravity. The storage capacity of the pond allows the
attenuation of peak discharges. Overflow spillways can be equipped to trap floating trash.
Sediment pollutants settle to the bottom and are removed periodically and disposed at the
landfill. In the flat coastal plains which describe most of the Corpus Christi area, the lack
of grades and the high water table limits the application of dry detention ponds. Therefore,
these types of structures are recommended only for areas of the city along the Nueces River
west of Five Points.
3.3.3 Regional Detention Ponds
Recommendation- Review the long term benefits of the regional detention facility on Oso
Creek as proposed by the Storm Water Master Plan of October 1991.
Discussion- The utilization of a few regional detention ponds throughout the City's storm
water system is recommended rather than the use of many onsite detention ponds. This
allows the City to properly inspect and maintain the ponds which can easily become hazards
if the slopes are not mowed, fences maintained, or the water becomes stagnant. The primary
reason for regional detention ponds is to provide flood control, but specific pollutants can
be removed by these facilities. The Regional Storm Water Master Plan, October 1991,
identified the possible location of a regional detention facility on Oso Creek. The 750 -acre
site begins at the Clarkwood Road Bridge and continues upstream for 3.5 miles. The
primary purpose of the detention facility is flood control which would effectively reduce the
peak discharge of the Oso Creek to the extent that approximately 795 acres downstream of
the facility would be removed from the 100 -year flood plain of Oso Creek. The pollutant
removal benefits of the facility could be determined by a study in order to assess the value
of this structural improvement in improving water quality.
20
3.3.4 Wetland Treatment
Recommendation- Utilize the wetland treatment method of storm water treatment only when
considerable amounts of specific pollutants are required to be treated or removed, since this
technology requires extensive planning and maintenance for proper functioning.
Discussion - This emerging technology can be combined with the wet detention pond and
will provide substantial amounts of treatment by sedimentation, biological interaction with
bottom sediments, and uptake of heavy metals into the plant tissue of the wetland plants.
Natural wetlands are not usually utilized due to environmental restrictions on the disruptions
of the natural processes. The constructed wetland facilities require a treatment train
consisting of pretreatment, pond settlement, a rock/reed filter, and a final naturalized
shallow marsh. Flow is controlled to occur in sheet flow wherever possible. Wetland plant
species need to be carefully selected and planted in order to achieve specific pollutant
removal goals. Thus construction, maintenance, and monitoring need to be well planned
and carried out.
21
3.4 PROPOSED DRAINAGE DESIGN CRITERIA POLICIES RELATING TO
WATER QUALITY IMPACTS FROM FLOOD CONTROL PROJECTS
Drainage design criterions are utilized by the City of Corpus Christi in designing drainage facilities
and in establishing the requirements of private development within the City's jurisdiction. The
effect of drainage criteria is that a level of flood protection be established, consistent throughout
the system, in protecting the health and property of the citizens. An important additional aspect
of design criteria is its effect upon water quality since storm water eventually flows into the
receiving waters of the area bays and estuaries. The critical issues of water quality in storm water
runoff are erosion prevention, sedimentation and percolation of contaminants, and the reduction in
floatable trash pollution. The Regional Storm Water Master Plan, October 1991, reviewed these
issues, summarized the result of previous studies, and made several specific recommendations. This
document reviewed and updated the recommendations contained in the Regional Storm Water
Master Plan. Current drainage criteria were reviewed and the following recommendations are made
to improve water quality in the storm water system of the City of Corpus Christi.
3.4.1 Channel Side Slopes
A demonstration project will be conducted by the City to determine and illustrate the
cost/benefits of utilizing 4:1 sideslopes in the design and construction of all new drainage
channels with the City storm water system.
Discussion - Channel geometry criteria effects water quality by reducing erosion and
increasing the sedimentation of pollutants and percolation at low flows which contain the
highest concentration of pollutants. A channel with flat side slopes will erode less and
maintain a better ground cover of vegetation which filters storm water and encourages
percolation into the channel bottom.
In order to assess the effect of changes in the drainage criteria for channel construction, an
economic analysis was performed on the cost impact of various ditch criteria (Reference
11). For instance, with the change in side slope of a channel to a flatter grade, the amount
of erosion decreases and the ability to maintain vegetation improves. These factors, in turn,
improve the hydraulic capacity of the channel - which is its primary purpose. Thus a
channel with flat side slopes, which is well maintained, can be constructed significantly
smaller in width and carry the same amount of storm water as a poorly maintained ditch.
Excavation amounts will also differ, as well as land requirements, and yearly maintenance
costs to the City.
22
In order to analyze the actual cost of various channel sideslopes, a numerical model which
contained the variables of water depth, bottom width, sideslope, frictional coefficients of
the sides and the channel bottom, channel slope, flow quantity, and velocity, was
constructed. From these variables, estimates were determined for excavation quantities,
right of way requirements, vegetative treatment areas, and maintenance factors. Five (5)
typical ditches in Corpus Christi were measured in cross-section to determine the geometry,
condition of the vegetation and frictional factors which control the hydraulics capacity.
Interviews were conducted with excavation contractors, real estate appraisers, vegetative
treatment application, contractors, and the supervisor of the City's channel maintenance
program, in order to determine appropriate costs for the elements of the model. The model
has been examined for a variety of different slopes, flow depth, and flow quantities. Table
1 in Appendix A contains the results of this analysis. The optimum channel geometry,
considering all of the above factors, are a flat bottom channel with 4:1 sideslopes. The
ability to maintain a channel of this geometry by regular mowing produces a channel with
less friction and considerably more hydraulic capacity than the normal channel with a
sideslope of 2:1. Rights of Way widths are not increased since a 4:1 ditch can be
maintained by mowing from within the banks of the channel and the additional 32 feet
maintenance easement dictated by the current criteria is not required. Thus, a channel with
4:1 sideslopes can be downsized to approximately 75% of the required width of a present
channel with 2:1 sideslopes. Based upon the cost factors of land, excavation, vegetation, and
maintenance, the lifetime cost for a channel with 41 sideslopes is 54% less than the cost of
a channel with 2.1 sideslopes For private developments the traditional developers cost is
reduced approximately 37% while the City's maintenance costs are reduced by 82%. The
benefits to water quality will be significant due to reduced erosion and the percolation of low
intensity and frequent rainfall runoff into the soil of the grassed bottomed channels.
Acceptance of a major change in design criteria such as this, will require a demonstration
project. The construction and maintenance costs will need to be verified as well as the
hydraulic carrying capacity of the improved channel section. By demonstrating the effect
of this policy change, the community of builders, developers, engineers, and City Council
policy makers can see the benefits of the changes.
3.4.2 Bank Stabilization
Criteria will be developed for a comprehensive policy to require the vegetation of all
slopes on drainage channel projects by application of hydro -seed or alternate method of
slope stabilization. Vegetation policy will initially be used by the City on municipal
projects in order to determine success rates of various methods.
23
Discussion - The stabilizing of slopes with a vegetative treatment greatly reduces the
amount of soil erosion which will occur before native grasses and weeds establish
voluntarily. The reduction of erosion reduces the amount of pollutants reaching receiving
waters and also reduces the regrading maintenance required of the slopes. The choice of
a manner of vegetative treatment requires consideration for acceptable levels of initial
erosion during establishment versus the cost of the treatment. The most effective
vegetative treatment is complete sodding but cost is high at approximately $11,000 per acre.
The next most effective method utilizing straw mats containing seed and fertilizer can be
installed for $7500 per acre. Hydro -mulch is another method frequently used for
applications such as the City landfill and consists of a mulch mixed with seed, fertilizer,
tackifier, and water. The application of hydro -mulch costs approximately $900 per acre and
also must be irrigated until established since the initial application causes germination of the
seed. Hydro -seed is an application of seed and fertilizer mixed with water for spraying.
The seed will not sprout until the first natural rainfall and thus will not combat initial
erosion, but the application is the least expensive at approximately $450 per acre.
Vegetative treatment of some manner is definitely a cost-effective pollution abatement and
maintenance reduction action. The relative cost of vegetative treatment is a very minor
portion of the total lifetime cost of a drainage channel, as illustrated in Figure 1 of Appendix
A. Therefore, the initial cost should not be the sole factor in determining the optimum
method for adoption as standard construction practice on channels within the City. The
Texas Highway Department has adopted various standards for differing slopes. The success
of different treatments will depend upon the practicality of maintaining the slopes after
application.
After implementation of the vegetation policy to initially utilize the application of hydro -
seed on City projects, the success of this type of treatment should be monitored. Success
may be limited by the absence of topsoil or of erosion -preventing mulch in some cases. The
policy should be reviewed periodically and additional criteria should be added after review
of the success of completed projects.
3.4.3 Vegetated Buffer Strips
Vegetation zones with a minimum of 10 feet in width, or other appropriate stabilization
method, shall be maintained along the banks of City drainage channels.
Discussion - Reducing the slope of channel sideslopes and providing vegetative treatment
will greatly reduce the problems caused by soil erosion. A major part of the problem exists
in agricultural areas of the city where farming practices contribute to the erosion of channel
24
a •
banks. This problem already occupies a significant part of the storm water maintenance
crews' efforts to maintain the existing channel system. The problem is generally caused by
farming practices which cultivate too close to the top edge of the channel bank. The tilling
under of the soil -retaining vegetation, several times a year, allows numerous washouts to
occur when rainfall runs off the fields. The response of the land owner is to request that the
City repair the banks of their channel before the land owner loses the use of his property.
A line of vegetation along the top bank, in conjunction with a low berm which directs the
runoff from the fields to a structure, will solve this problem and is the remediation method
used by the storm water maintenance crews. But further plowing of the top bank area often
eradicates these improvements and allows the erosion to reoccur. In order to solve this
problem of water pollution and unnecessary maintenance expenditures, an ordinance should
be in effect requiring the maintenance of an undisturbed vegetation zone or other
stabilization method at least 10 feet wide along the top of all channels within the City's
jurisdiction. The 10' vegetation strip will occur within a drainage ditch right-of-way. The
developer will be responsible for stabilization only as a result of soil disturbing activities.
After stabilization is established, the City will maintain the vegetative stabilization. Some
regulations by the U.S. Soil Conservation Service require agricultural operations to maintain
and follow a plan for preventing soil erosion. The actions by the City are also limited by
the Agriculture Code which "limits the circumstances under which agricultural operations
may be regulated or considered to be a nuisance" for agricultural areas of the City annexed
after 1981, but the City can take the measures to ensure that these vegetated buffer strips
are maintained within their drainage easements and elsewhere in the City.
3.4.4 Outfall Structures
Outfall structures into earthen ditches or natural drainage areas shall provide scour
protection such as rubble berms, geotextile reinforcement, or velocity dissipators as
required to prevent erosion.
Discussion - Where storm water collection pipes outfall into earthen channels, the current
practice is to provide concrete plating around the outfall pipe to protect the outfall pipe
from being undermined by erosion. The concrete plating usually extends a minimum of 12"
around the sides of the pipe, and typically extends as a splash pad down to the flowline of
the ditch. Although this method reduces the greatest extent of erosion from the outfalling
storm water, there is still considerable erosion that occurs along the edges of the concrete
plating. This can usually be attributed to the erosive turbulence caused by the change in
velocity of the storm water as it transitions from a smooth concrete surface to a rougher
vegetated channel side or bottom.
25
a •
Several means can be employed to protect the adjoining soil from scour. Geotextile
reinforcement that will strengthen the holding ability of established vegetation and prevent
loss of soil can be used. Geotextile reinforcement can be substituted for the entire concrete
plating in appropriate locations, and will provide for a pervious, natural looking, and scour
resistant surface. Velocity can be controlled by the use of a rock rubble berm around the
bottom edge of the outfall structure. The loss in velocity will also cause suspended
sediments to drop out of the storm water behind the rock berm, preventing the dispersal
into the receiving waters.
3.4.5 Velocity Control Structures
Channel designs shall include provisions for velocity control structures in order to limit
the velocity of the storm water to S feet per second in clay soils and 3. Sfeet per second
in sandy soiL
Discussion - Excessive velocities in channels will cause the scour of the vegetative cover
and the rapid erosion of the channel bottom and sides. The maximum velocity that can be
maintained without erosion depends upon type of soil and vegetative cover.
The first aspect of channel design to be reviewed is the slope of the channel bottom. A
trapezoidal channel. In order to maintain velocities below the limits specified, channel
slopes should not be steeper than 0.25% in sandy soils and 0.50% in clay soils.
In areas where the terrain exceeds the maximum allowable slopes, concrete channel drop
structures can be used as velocity control structures which lower the elevation of the
channel flowline while maintaining the channel slopes maximums. Channel drop structures
shall incorporate velocity dissipators - mentioned in Section 3.4.4 above.
Where channel slopes will exceed these maximums, continuous concrete plating should be
utilized to control erosion of the channel bottom and banks.
3.4.6 Berms to Control Overland Flow
Control berms with outfall pipes shall be required along the edge of channels where more
than 200' of overland flow will drain into the ditch.
Discussion - Excessive overland flow into earthen ditches can cause erosion of the banks
of the ditch. This is frequently found in the case of agricultural land adjacent to a ditch.
26
a •
A low berm that is constructed with the material from excavation can divert the overland
flow, parallel to the ditch, to an inlet or pipe that drains into the ditch. Scour protection is
provided for the pipe that drains into the ditch. These outfall pipes can be spaced from 500'
to 1000' apart. The outfall pipes, as well as the height of the berm, should be sized to
contain the 25 -year frequency rainfall. The tops of the berms shall be graded flat to allow
normal maintenance of ditch.
3.4.7 Project Review
The City will provide formal review of the water quality impacts of any major flood
control project in the City of Corpus Christi.
Discussion - - Review of a flood control project for water quality issues during the design
stage will allow the designer to incorporate the recommendations and suggestions
contained within this document. The following items will be specifically reviewed to
determine compliance with the intent of not causing erosion or unnecessary degradation of
the receiving waters. Each project should be reviewed for the following items:
Will the channel side slopes be stable in slope, revegetated as part of
the construction, and protected from excessive overland flow?
Has velocity control been implemented by utilizing
flat channel slopes, protecting outfall structures, and
providing channel drop structures where required?
Have vegetated buffer strips along the tops of channel
banks been included in the layout of the proposed
land use?
Each major flood control project proposed by either the City or a developer will be
reviewed by City Engineering staff to determine compliance with these measures.
27
•
4.0 REFERENCES
"Nueces County Drainage Criteria and Design Manual," NEI, and HDR Infrastructure, Inc.
(HDR), Corpus Christi, Texas, July 1986.
2. Federal Emergency Management Agency, Federal Insurance Administration, Flood
Insurance Study for Nueces County, Texas 1985.
"A Report on a Storm Sewer System for Corpus Christi, Texas, Myers & Noyes Consulting
Engineers, Dallas and Corpus Christi, Texas, October 1946.
"Master Plan for Storm Drainage for the Area South and West of the City of Corpus
Christi, Texas," Blucher and Naismith Consulting Engineers, Inc., Corpus Christi, Texas,
1961.
"Master Plan for Storm Drainage for the Area West of Clarkwood Road and the Flour Bluff
Area of the City of Corpus Christi, Texas," Naismith Engineers, Inc. (NEI), Corpus Christi,
Texas, 1970.
6. "Storm Drainage Plan and Sanitary Sewer Plan for the Area West of Calallen (Five Points),"
Naismith Engineers, Corpus Christi, Texas, 1982.
7. "Master Nan for Storm Drainage for the Area South of Oso Creek," Naismith Engineers,
Corpus Christi, Texas, 1988.
8. "Floods in Texas - Magnitude and Frequency of Peak Flows," USGS and Texas Water
Commission, Austin, Texas, December 1963.
9. "Urbanization and Water Quality," Terrene Institute, Washington, D.C., March 1994.
10. "Regional Storm Water Master Plan," Camp Dresser & McKee, Inc., Archie Walker
Engineering Inc., October 1991.
11. "Economic Analysis Study of Drainage Channel Parameters for the City of Corpus
Christi," J. R. Thompson, P.E., June 1993.
28
a •
APPENDIX A
ECONOMIC ANALYSIS STUDY
OF
DRAINAGE CHANNEL PARAMETERS
FOR
THE CITY OF CORPUS CHRISTI
EXHIBITS
29
$$ HYDRAULIC PARAMETERS t4
NATER DEPTH
BOTTOM WIDTH
SIDESLOPE
SIDESLOPE "n"
BOTTOM 'n"
CHANNEL SLOPE
FLOW (0)
VELOCITY 01)
CORPUS CHRISTI DRAINAGE CRITERIA STUDY
LIFETIME CHANNEL COST
1:1
1:1 2:1 3:1 4:1 5:1 CONC
ft 8 8 8 8 8 8 8 8 8 8
ft 102.5 95.1 88.05 14.15 60.15 38.1 16.34 10.9 4 11.65
1 1.5 2 2.5 3 3.5 4 4.5 5 1
n1 0.090 0.086 0.082 0.012 0.062 0.046 0.030 0.028 0.025 0.012
n2 0.090 0.086 0.082 0.072 0.062 0.046 0.030 0.028 0.025 0.012
ft/ft 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005
cfs 1200 1200 1200 1200 1200 1200 1200 1200 1200 1200
fps 1.36 1.40 1.44 1.59 1.78 2.25 3.10 3.20 3.41 7.64
$$ COST FACTORS PER 1000 LF OF DITCH 44
TOTAL COST 5182,426 5169,147 5156,857 5139,929 $123,188 590,312 572,309 $15,111 $16,695 5118,291
UNIT COST
EXCAVATION COST 11.00 $50,889 $50,267 549,800
ROW COST $12,500 *45,483 $46,803 $48,224
VEGETATION COST $450 $1,631 51,685 51,136
PILOT CHANNEL PAVING 514,000 60 SO S0
MAINTENANCE COST $4,000 $84,417 $10,392 $57,091
(5200/yr@5%:$4000 Initial Worth / Lifetime Cost)
MAINTENANCE FACTOR 5.8 4.1 3.1
$46,289 542,133 $35,861
$41,678 $41,105 $35,210
$1,116 $1,696 $1,268
60 $0 SO
644,246 $31,654 618,028
2.9 2.1 1.6
$28,596 $28,844 $28,444 $10,511
532,231 $34,120 535,983 $19,413
$1,161 $1,228 $1,281 $699
$0 $0 SO $84,563
$10,316 $10,918 511,387 $3,106
1.0 1.0 1.0 0.5
TABLE 1
$200
1190
- $180
_ $170
$160
$150
$140
$130
m $120
t 1110
0 1100
O g$90
.., $80
$70
160
150
$40
$30
$20
110
$O
0 TOTAL
LIFETIME CHANNEL COST
(per 1000 I.f. of Channel)
A
11
A Y
21
.4
x
w
x
31
41
CHANNEL SIDE SLOPE
+ EXCAVATION 0 R.O.W. A VEGETATION
7 CONCRETE PLATING
51 CONC
X MAINTENANCE
FIGURE 1
10
REQUIRED CHANNEL ROW
10
50
CHANNEL RETROFIT
WITH UNDERDRAIN
ROUGHNESS C❑EFFICIENT "n"
100
EARTH CHANNEL .020
SURFACE IRREGULARITY ,000
CROSS—SECTION VARIATION .000
OBSTRUCTIONS ,004
VEGETATION .006
C❑MP❑SITE C❑EFFICIENT .030
FIGURE 2
10
10
REQUIRED CHANNEL ROW
PROPOSED 411 SECTION
EXISTING 2Q SECTION
10
\Tut
EXCAVATION TO
BALANCE FILL
10
50
U DERD
AIN
CHANNEL RETROFIT
WITH UNDERDRAIN
ROUGHNESS COEFFICIENT 'n'
100
EARTH CHANNEL .020
SURFACE IRREGULARITY .000
CROSS—SECTION VARIATION .000
OBSTRUCTIONS .004
VEGETATION .006
COMPOSITE COEFFICIENT .030
10
5
1 1 1 1 1 1 1 1 1 1 1 1
10 50 100
HOLLY ROAD
ROUGHNESS CO -EFFICIENT
EARTH CHANNEL .020
SURFACE IRREGULARITY .000
CROSS-SECTION VARIATI❑N .005
OBSTRUCTIONS .000
VEGETATION .005
COMPOSITE COEFFICIENT .030
10 ---
t
cU
1 1 1 1 1 1 1 1 1 1 1 1
10
50 100
CLUB ESTATES DITCH
ROUGHNESS COEFFICIENT
EARTH CHANNEL ,020
SURFACE IRREGULARITY .018
CROSS-SECTI❑N VARIATI❑N .006
❑BSTRUCTI❑NS .005
VEGETATI❑N .040
C❑MP❑SITE C❑EFFICIENT .089
5
10 -
50 100
MARY CARRDLL DITCH
ROUGHNESS C❑EFFICIENT °n°
EARTH CHANNEL .020
SURFACE IRREGULARITY .008
CR❑SS—SECTION VARIATION .004
OBSTRUCTI❑NS .006
VEGETATION .050
COMPOSITE C❑EFFICIENT .088
1
- 10
- 5
10 -
5--
50
BRAWNER PARKWAY
ROUGHNESS COEFFICIENT "n"
EARTH CHANNEL .020
SURFACE IRREGULARITY .000
CROSS—SECTION VARIATION .000
OBSTRUCTIONS .000
VEGETATION .005
COMPOSITE COEFFICIENT .025
-.- 10
5
10 - - 10
5
- 5
50
FLYNN PARKWAY
ROUGHNESS COEFFICIENT 'n'
100
EARTH CHANNEL .020
SURFACE IRREGULARITY .000
CROSS—SECTION VARIATI❑N .000
❑BSTRUCTI❑NS .004
VEGETATION .006
COMPOSITE C❑EFFICIENT .030
State of Texas
County of Nueces
}
}
PUBLISHER'S AFFIDAVIT
CITY OF CORPUS CHRISTI
ss: Ad # 2121482
PO #
Before me, the undersigned, a Notary Public, this day personally came Darrell
G. Coleman, who being first duly sworn, according to law, says that he is Vice -
President and Chief Financial Officer of the Corpus Christi Caller -Times, a daily
newspaper published at Corpus Christi in said City and State, generally
circulated in Aransas, Bee, Brooks, Duval, Jim Hogg, Jim Wells, Karnes,
Kenedy, Kleberg, Live Oak, Nueces, Refugio, San Patricio, Victoria and Webb
Counties, and that the publication of, NOTICE OF PASSAGE OF ORDINANCE
NO. which the annexed is a true copy, was inserted in the Corpus Christi Caller -
Times and on the World Wide Web on the Caller -Times Interactive on the 25TH
day(s) of MAY, 1998.
Cat . oa33(K
Corpus Christigaller-Times, Monday, May 25, 1998
1110 Legal Notices
— NOTICE
OFCE ASNOG 0F0W
023318
Amending the Code of Ordi-
nances, City of Corpus
Christi, Chapter 13, Building;
Construction and Related Op -
e.
W4 erations; Housing and
W Housing Premises Standfl
and the Corpus Christi Plat-
ting Ordinance regarding the
adoption of a Stormwater
Management Guidance Doan
mem for assessment of water
timpacts --from new
control projects, This
dinettes wa the
l Council
roved by
et 'tag ty o CQrPPus (Arias
Id Arnie SCimPa
ecrrryy
city of.Co s Chdeti cc
tF OF
fiilll n 111000 '01,0\
Vice-President and Chief Financial Officer
Subscribed and sworn to me on the date of
MAY 26, 1998.
Notary Public, Nueces County, Texas
CONNIE HARALSON
Print or Type Name of Notary Public
My commission expires on 5/14/01.