Transcript Slide 1
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 – IDM Storm Water Design Requirement Changes Allows for consideration of alternative drainage designs for Evacuation Routes and Emergency Services Routes
Chapter 9 – IDM Storm 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 – IDM Storm 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 – IDM Storm 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 – IDM Storm 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 – IDM Stormwater 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 Roofs
Hard 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 – Floodplains City 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 WHAT DO YOU NEED?
Address/Property Location Tools for determining location in the SFHA zone: http:www.cohcdp.swmp.org/Website/Pro View/viewer.htm
http://pwegis.pwe.ci.houston.tx.us/view er1.htm
Call the Floodplain Management Office at 713-535-7666 Value of Proposed Improvements Market Value of Structure before Improvements
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 o Define an area of Lake Houston where boathouses/piers can be permitted without calculation requirements of floodway zone designation Removes “10-year accumulation of flood related improvements” from definition of “substantial improvement”
2008 Revisions to Chapter 19
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
Contact Information
City of Houston Floodplain Management Office 3300 Main Houston, TX 77002 713-535-7666
Mark L. Loethen, PE City Engineer City of Houston 611 Walker, 19 th Floor Houston, TX 77002 713-837-0724 [email protected]
Duane Hengst, PE Managing Engineer – Floodplain Management Office 3300 Main Street Houston, TX 77002 [email protected]