ANCHOS DE ZANJA PARA SUELOS ESTABLES TUBERÍA

Introduction 11.3.1

A major source of erosion and sedimentation in channels, and sometimes detention basins, is pipe or box outfalls. (References to pipe outfalls shall include box outfalls, as well.)

Considerations 11.3.2

Factors to consider when designing and laying out a pipe outfall into a HCFCD maintained facility are:

• Exit velocity from the pipe.

• Alignment relative to the flow in the HCFCD facility.

• Location of the pipe relative to the HCFCD facility geometry.

• Location of the pipe relative to other structures in the HCFCD facility. Continued on next page

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Harris County Flood Control District Policy and Criteria Manual 11-6

11.3 Pipe Outfalls,

Design Criteria 11.3.3 •

Design and construct outfall pipes in grass lined channels in accordance with the HCFCD Standard Storm Sewer Outfall and Riprap Detail Sheet in Appendix D.

• Design and construct outfall pipes in concrete lined channels in accordance with the HCFCD Standard Concrete Channel Lining Detail Sheet in Appendix D.

• Install riprap erosion protection in grass lined channels for pipes 48 inches and larger, treatment plant outfall pipes, and where the design velocity out of the pipe exceeds the maximum for the soil type in the Section 4.4.1, Maximum Velocities table. Minimum riprap layout is shown on the Standard Storm Sewer Outfall and Riprap Detail Sheet in Appendix D.

• Use corrugated metal pipe (galvanized steel or aluminum) or HDPE pipe with a minimum 24” diameter for outfall pipes within a HCFCD facility right-of-way. (Storm sewer inflow pipes into detention basins can be reinforced concrete pipe. See Section 6.6.2 - Inflow Structures.)

• Place a standard manhole or junction box just outside the ultimate channel or detention basin right-of-way.

• Place all storm sewer inlets outside the HCFCD facility right-of-way.

• Locate storm sewer outfalls 48 inches and larger on the downstream side of bridges and culverts.

• Angle pipes and boxes downstream a minimum of 30 degrees starting at the last manhole and measured from line perpendicular to the channel.

• Pave the corrugated metal pipe invert of effluent outfalls from wastewater treatment plants with concrete, or use plastic pipe designed for wastewater effluent.

• Use the pipe adjustment details shown on the Standard Storm Sewer Outfall and Riprap Detail Sheet where applicable and include custom pipe adjustment details where the standard does not apply.

10/10/03 Draft

SECTION 12 – CHANNEL ENCLOSURES

12.1 Introduction

Overview

12.1.1

Channels normally maintained by the HCFCD can be enclosed provided the criteria and procedures in this manual are followed.

Analysis and Methodologies 12.1.2

Hydraulic analysis and submittal requirements that apply to channels also apply to channel enclosures. Hydraulic aspects specific to channel enclosures are presented in this section.

Maintenance Responsibility 12.1.3

Since the HCFCD’s primary responsibility is for open drainage facilities, convey the right-of-way and maintenance responsibility of an enclosed channel to a taxing entity that maintains underground drainage systems.

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Harris County Flood Control District Policy and Criteria Manual 12-2

12.2 Design Criteria

Application 12.2.1

The criteria in this section apply where the drainage area for the enclosure is greater than 200 acres or where the HCFCD maintained facility would normally begin.

Hydraulic

Criteria 12.2.2

Hydraulic criteria are:

• For new developments or new drainage facilities, design the pipe or box to contain the 1% exceedance probability, 24-hour storm event within the facility right-of-way for ultimate watershed conditions.

• Show how the 1% exceedance probability flow will get into the pipe or box.

• Accommodate extreme event flows (in excess of the 1% exceedance probability) where possible.

• Submit hydraulic calculations and profiles for review.

Structural Criteria 12.2.3

Use the criteria from the jurisdiction that is going to maintain the enclosed drainage facility.

Manholes and Inlets

12.2.4

Criteria for manholes are:

• Use the criteria from the jurisdiction that is going to maintain the enclosed drainage facility.

• Manholes can be combined with City of Houston Type "E" or Type "B" inlets to drain surface swales.

• Select inlets to minimize reduction in flow capacity due to clogging.

Right-of-Way 12.2.5

The right-of-way width for an enclosed channel shall be the outside width of the pipe(s) or box(es) plus a distance on each side equal to the flowline depth measured from natural ground or proposed fill elevation, whichever is higher and rounded up to the nearest 5 feet. The minimum width on each side shall be 10 feet.

Encompass all manholes within the right-of-way for the facility. Extend the right-of-way limit 5 feet from the edge of the manholes and inlets.

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SECTION 13 –EXTREME EVENT OVERFLOW

13.1 Introduction

Overview

13.1.1

Storm water runoff that travels on the surface trying to reach an open channel or detention basin is referred to in this manual as overland flow. Local jurisdictions such as Harris County and the City of Houston have regulations and requirements for accommodating extreme event overland flow within their streets and storm sewers.

Since extreme event overland flow eventually discharges into HCFCD maintained channels or detention basins, this chapter covers the design criteria for conveying the flow, or overflow, into the facility.

For swales or ditches carrying normal flows to a HCFCD maintained facility, see Section 11, Backslope Drainage Systems and Pipe Outfalls.

10/10/03 Draft

Harris County Flood Control District Policy and Criteria Manual 13-2