city of houston public works and engineering july 17, 2008
TRANSCRIPT
CITY OF HOUSTON
PUBLIC WORKS AND ENGINEERING
July 17, 2008
General Topics
Chapter 9 Changes –Infrastructure Design Manual
Chapter 13 Changes – Infrastructure Design Manual
Proposed Chapter 19 Changes – City Code of Ordinances
Abbreviations
IDM – Infrastructure Design Manual LID – Low Impact Development FEMA – Federal Emergency
Management Agency NFIP – National Flood Insurance
Program BFE – Base Flood Elevation
Infrastructure Design Manual (IDM) vs.Code of Ordinances vs.International Building Code
IDM issued by Director of Public Works & Engineering
IDM provides design standards for public infrastructure
Code of Ordinances adopted by City Council to govern City, for example Chapter 19 – Flood Plain Chapter 40 – Streets and Sidewalks Chapter 42 – Subdivisions, Developments & Platting
International Building Code administered by Building Official for private property permitting
2008 Changes to Infrastructure Design Manual
Chapter 9 – Storm Water Design Chapter 13 – Storm Water Quality
Design Culmination of 1 year of reviews,
comments and discussions. Part of Five Year PWE Technical
Review Cycle
Chapter 9 – IDMStorm Water Design Requirement Changes
Allows for consideration of alternative drainage designs for Evacuation Routes and Emergency Services Routes
Chapter 9 – IDMStorm Water Design Requirement Changes
Allows for use of another pipe material, HIGH DENSITY POLYETHYLENE (HDPE) SOLID AND PROFILE WALL PIPE, for storm sewer installations in public right of way.
Storm Water Technical Specifications
Specifications for Backfill for Utility Excavations – Updated Industry References
Specifications for Frames, Grates and Covers – Updated Industry References
Included specifications for HDPE pipe material
Chapter 9 – IDMStorm Water Design Requirement Changes
Requires stormwater detention for INCREASED impervious cover on all sites Less than 1 acre: 0.20 acre-feet per
acre Over 1 acre: 0.50 acre-feet per acre Removes 15,000 sf Single Family
Residential Exemption for detention of increased impervious cover
Chapter 9 – IDMStorm Water Design Requirement Changes
Changes are now effective October 1, 2008
Designs submitted prior to October 1, 2008 will be approved using current or proposed requirements
Reviewing additional filters/screens for smaller, single family improvements with limited increases in impervious cover
Chapter 9 – IDMStorm Water Design Requirement Changes
Development of series of “how to” documents for various stormwater detention techniques including LID
Goal is not to increase engineering requirements for single family residential property owners
Goal is to address additional stormwater runoff from increasing impervious cover
Chapter 13 – IDMStormwater Quality Design
Included information for use of Low Impact Development (LID) techniques for stormwater quality/quantity applications
Include: Bioretention, Infiltration Trenches, Porous Pavement, Vegetative Swales, Green Roofs, Hard Roofs, Rain Barrels
Bioretention Basin
Bioretention Basin
Using in-situ or new soils, design the bioretention area to empty within 48 hours. This may be accomplished through infiltration, evapotranspiration, and/or the design of a subsurface drainage system.
Mitigating detention volume requirements can be reduced by the volume in the bioretention area below its maximum design water surface.
Porous/Pervious Concrete
Porous/Pervious Concrete
The pavement surface may either be soft (grass) or hard (concrete, asphalt or advance gravel systems). The underlying permeable layer serves as a storage reservoir for runoff and/or infiltration.
Porous Pavement is applicable for both water quality and water quantity control practices.
Shall be limited to lightly traveled surfaces such as parking pads in parking lots, trails and sidewalks.
Porous pavement is not permitted for residential driveways (area of pavement likely to be coated or paved over because of a lack of awareness) and commercial areas designed for heavy traffic volume and/or vehicles.
Infiltration Trenches
Trenches or basins that temporarily detain a design water quality volume while allowing infiltration to occur over a prescribed period of time. Trenches are applicable for both water quality and water quantity control practices.
In-situ subsoil shall have a minimum infiltration rate of 0.5 inches per hour.
Subsurface drainage systems are required where the in-situ subsoil rate is less than 0.5 inches per hour or where the project is constructed on fill soils.
Mitigating detention volume can be reduced by the amount of infiltration into the subsoil and the volume of voids within the trench area.
Dry Swale
Dry Swales
Flow depth should be less than 4 inches for water quality treatment.
Flow velocity should be less than 1 fps for water quality, less than 5 fps for 2-yr storm (non-erosive velocities for grass and soils).
Length should yield a 10 minute residence time.
Side slopes should be flatter than 3:1. Maximum ponding time should be < 48
hours. Mow dry swales as required during growing
season to maintain grass heights in the 4 to 6 inch range.
Wet Swale
Wet Swales Flow depth should be less than 4 inches for
water quality treatment. Flow velocity should be less than 1 fps for
water quality, less than 5 fps for 2-yr storm (non-erosive velocities for grass and soils).
Length should yield a 10 minute residence time.
Side slopes should be flatter than 3:1. Maximum ponding time should be < 48
hours. Wet swales, employing wetland vegetation
or other low maintenance ground cover do not require frequent mowing.
Rain Barrel
Rain Barrels Gutters and downspouts carry water from
the rooftops to rain barrels Rain barrels should be equipped with a
drain spigot. Overflow outlet must be provided to
bypass rain barrel from large rainfall events.
Rain barrel must be designed with removable, child resistant covers and mosquito screening.
Empty rain barrel after each rainfall event.
Rain barrel should be inspected annually.
Hard Roofs/Green RoofsHard Roofs Detention volume can be controlled in several
ways, Typically, simple drain ring is placed around roof
drains. Flow into the roof drains is controlled by orifices
or slits in the drain ring. The roof deck must be designed to withstand the
live load and be properly waterproofed.Green Roofs A green roof, in simplest terms, is a vegetated
roof. Installation generally consists of a waterproof
membrane installed over a suitably constructed roof deck.
Chapter 19 – FloodplainsCity of Houston Code of Ordinances
Implemented for participation by City of Houston in National Flood Insurance Program (NFIP)
Floodplain studies managed by Harris County Flood Control District
Floodplain maps prepared and issued by Federal Emergency Management Agency (FEMA)
In effect since 1985 with revisions in 2006
Project Location and the SFHA
Address/Property Location Tools for determining location in the
SFHA zone: http:www.cohcdp.swmp.org/Website/
ProView/viewer.htm http://pwegis.pwe.ci.houston.tx.us/
viewer1.htm Call the Floodplain Management Office at
713-535-7666 Value of Proposed Improvements Market Value of Structure before
Improvements
WHAT DO YOU NEED?
Basic Floodplain Requirements
Basic Permit Requirements Floodplain Fill Mitigation Plans Conveyance Zone Requirements Floodway Requirements Variance Requirements
Basic Development Permit Application Requirements
New construction, additions, substantial improvements lowest floor/all utilities elevated or floodproofed
(non-residential only) ≥ 12 inches above the BFE Mitigate Loss of Floodplain Storage Volume Special requirements for projects in
floodway and conveyance zones Development permit expires if development
has not commenced within 18 months of issuance
Improvements to Existing Structures
Substantial vs Non-Substantial Single improvement project Value of proposed improvements to
market value of structure before improvements
Market value by HCAD, RSDE or independent appraiser (Texas Certified)
Substantial means Full Compliance
2008 Revisions to Chapter 19
o Define an area of Lake Houston where boathouses/piers can be permitted without calculation requirements of floodway zone designation
o Removes “10-year accumulation of flood related improvements” from definition of “substantial improvement”
Codifies requirements for permitting of reconstruction, renovation and repairs of existing structures in the floodway.
Currently, requirements are included in the Chapter 19 Guidelines
2008 Revisions to Chapter 19
Allows for issuance of development permits to vertical construction on vacant land in the floodway.
Establishes performance standards and criteria necessary for compliance to build on vacant land in the floodway
2008 Revisions to Chapter 19
Contact Information
Mark L. Loethen, PECity Engineer
City of Houston611 Walker, 19th Floor
Houston, TX 77002713-837-0724
City of Houston Floodplain Management Office
3300 Main
Houston, TX 77002
713-535-7666
Duane Hengst, PEManaging Engineer – Floodplain Management
Office
3300 Main Street
Houston, TX [email protected]
Questions/Answers