Tag Archive for: Chapter 19

New Flood-Map Timetable and How It Affects New Development

Because of low interest rates, new developments seem to pop up weekly around the Lake Houston Area. The question often arises, “How will the development of new flood maps affect the development of new subdivisions?” Most people by now have heard that City building code revisions now require elevation at least two feet above the .02-percent-annual-chance flood (formerly known as 500-year flood). But does that mean two feet above the old floodplain or the new? Due to a little-known provision in the City’s floodplain regulations, it means the new floodplain even though the new floodplain maps are not official yet.

Developments in various stages in the northeast Houston Area and the City’s ETJ (extra territorial jurisdiction). From City of Houston Plat Tracker Plus.

Timetable for Updating Flood Maps

Here’s what you need to know if you’re concerned that someone may be building future buyouts next to your neighborhood. First the timetable for new flood maps.

Remaining timetable for new Harris County Flood Maps

Even though FEMA won’t release the new flood maps officially for approximately another five years, developers should still be building to the higher standards associated with newly acquired data (in other words, the data on which the new maps are being built). See below.

Floodplain Regs Authorize City Engineer to Use Data Behind New Flood Maps

Section 19-4 of the City’s Floodplain regulations address Use of other flood hazard data to supplement the effective Flood Insurance Rate Map (FIRM). It states, “New elevation and flooding studies are undertaken by or under the auspices of FEMA and local political subdivisions, such as the Harris County Flood Control District. Upon determination that the data generated by such a study appears to be reliable and based upon sound engineering and surveying practices and further that the study’s data indicate that the effective FIRMs are FIRM is materially inaccurate, the city engineer may cause the study data to be administered for purposes of this chapter as though it were a part of the effective FIRM. Any such determination shall be issued in writing and a copy shall be placed on file in the office of the city secretary. The city engineer is authorized to utilize updated information from FIS and floodplain models in administering this chapter.”

Basically, that means even though the new maps have not yet been adopted, the City Engineer can require developers’ plans to reflect the new underlying data as though it were part of the current map. In this case, the new underlying data is already in hand.

MaapNext Website Describes New Data Improvements

Current floodplain maps will change greatly according to Harris County Flood Control. The district has already started releasing information on a new website called MaapNext (Modeling, Assessment and Awareness Project).

Components of the MaapNext program. A $3.5 million FEMA grant made MaapNext possible.

According to the MaapNext site, we now have updated data based on:

  • County-wide impervious data developed from 2018 aerial imagery
  • Completed flood risk reduction projects
  • NOAA’s recently-released Atlas 14
  • Updated terrain data

We also have new and better ways to model that data since the last survey after TS Allison:

  • 2-Dimensional Hydraulic Modeling
  • New hydrology method that better accounts for a watershed’s conveyance capacity
  • Rain-on-Grid analysis that identifies previously unmapped urban flood risk.

And, we are beginning to develop better maps:

  • Modeling results in GIS (Geographic Information Systems)
  • New flood-risk data sets describe results in a variety of useful ways

Reportedly, the City is already requiring developers to act on the new “best available data” instead of waiting five years for the next maps.

Chapter 19 of the City Ordinances deals with dozens of other requirements for building in floodplains. But more on those in future posts.

Posted by Bob Rehak on 8/29/2020

1096 Days (Three Years) since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.

Flood Regs: What County Wants City to Do as Part of Woodridge Village Purchase Deal

Through a Freedom of Information Act (FOIA) request, ReduceFlooding.com has obtained details of Harris County’s request to the City of Houston to revise its flood regs. Complying with the request is one of two conditions the City must meet before the County will purchase Perry Homes’ Woodridge Village.


Woodridge Village twice contributed to flooding in Elm Grove and North Kingwood Forest last year. Ever since, flood-weary residents have pled with Harris County and City of Houston officials to buy the property and build regional floodwater-detention facilities there that would protect them. The City initially declined. The County agreed, but with two conditions.

  • First, the County wanted the cash-poor City to pay for half the purchase through the donation of land that the County could then use to help offset costs for other HCFCD projects.
  • Second, the County wanted the City update to its stormwater and floodplain ordinances to make them consistent with the County’s.

Harmonizing the regulations would provide consistency between the three largest governmental agencies tasked with drainage projects in the our area: Harris County Engineering Department, Harris County Flood Control, and the City of Houston. 

This is the first step in getting all municipalities and County governments that drain to Harris County to adopt similar standards to help reduce flooding risks and protect the billions of dollars of drainage infrastructure investments currently being made in the area.

A Houston Chronicle article (that didn’t even mention Elm Grove) said, that in the future, the county would not share flood bond money with any of 34 different municipalities within its jurisdiction that have not updated and harmonized their regulations with Harris County’s.

So what were the requested changes?

Below are revisions needed for the City of Houston to comply with Harris County Infrastructure Regulations (2019 version) and the HCFCD Policy, Criteria and Procedures Manual.

In all cases cited below, Harris County flood regulations exceed the City of Houston’s. The County does not ask the City to relax any guidelines.

Houston Chapter 9 (Stormwater Design Requirements – July 2019)

General Note – The City updated this Chapter in late 2019 to add Atlas 14 rainfall information for use in storm sewer design. The County also added Atlas 14 to its regulations.

However, the County requests that the City make additional changes as follows:

Section 9.2 Design Requirements:

  • 9.2.01(B)(3)(a)(1) Table of Rational Method Runoff Coefficients – Must be updated for lots greater than ¼ acre to be consistent with Harris County requirements. 
  • 9.2.01(C)(7)(d) Table 9-2 – Revise inlet capacities for Type A, D, D-1, C-2, C-2A, D, D-1, and E inlets to be consistent with Harris County requirements. 
  • 9.2.01(D)(3)(c) Relationship of Structures to Street – Revise finished slab elevation criteria to be consistent with Harris County requirements of 18” above the 100-year floodplain, one foot above the maximum ponding depth within a 10’ radius of the structure or at or above the 500-year floodplain, whichever is higher.
  • 9.2.01(H)(2)(d) Waiver of Detention Requirements – Remove this section; it would allow developments to be constructed without detention. 
  • 9.2.01(H)(3)(a-e) Calculation of Detention Volume – Revise to remove detention rates based on tract size, revise detention rates to be consistent with Harris County requirements of 0.75 acre-feet/acre for storm sewer outfalls and 1.0 acre-feet/acre for roadside ditch outfalls, or HCFCD requirements if outfalling to HCFCD facility.
  • 9.2.01(H)(3)(a-e) Tracts >50 acres – Refer to HCFCD requirements if outfalling to HCFCD facility, otherwise refer to Harris County requirements if outfalling to storm sewer or roadside ditch. 
  • 9.2.01(H)(4) Calculation of Outlet Size – Revise to be consistent with Harris County requirements,  remove minimum restrictor size, remove allowable discharge rates of 0.5 cfs and 2.0 cfs per acre and include calculated allowable rates.  
  • 9.2.01(H)(5)(a) Private Facilities – Include Harris County pumped detention information including detention rate, allowable drain times, and percentage that must be drained by gravity.  Add minimum bottom slopes and pilot channel slopes from Harris County requirements. 

Houston Code of Ordinances, Chapter 19 Floodplain (September 2018)

Under Article III: Standards for Flood Hazard Reduction:

  • 19-33(a) Base Flood Elevation Requirements Must also include a provision that no fill will be allowed to elevate structures proposed for the 100-year floodplain.  These structures must be on open foundations designed by a structural engineer.
  • 19-33(c) AO Zones Revise to require finish floor elevation of three feet above the depth number noted in the specific zone, or 6 feet if no depth number is specified.   
  • 19-34(a)(4)  – Remove this item that allows fill to be placed in the 0.2% floodplain without mitigating excavation. 
  • 19-34(d) Critical Facilities Add requirement for these facilities to have the lowest floor elevated 24” above the crown of the adjacent street if that is higher than 3’ above the 0.2% elevation. 
  • 19-43 (c)&(d) Floodways – Require an engineering report for the foundation in addition to the “no-rise” analysis and mitigation requirements. Add Harris County requirements for foundation design.
  • 19-43(e) Bridges – Add requirement that all bridge construction that modifies the base flood elevation or that modifies the geometry of the bridge or channel must submit a CLOMR and LOMR.
  • 19-75 Manufactured home placement in a floodway or coastal high hazard area – Remove this section that allows for manufactured homes to be placed in these areas.

The County also recommends that the Harris County Floodplain Administrator should review Chapter 19 for additional changes to ensure consistency with Harris County floodplain regulations.

Negotiations Still Reportedly Ongoing

City of Houston did not discuss conditions of the Perry purchase in last week’s City Council session. Neither are County Commissioners scheduled to discuss them this week. However, negotiations with Perry are reportedly continuing despite the passage of Perry’s extended deadline.

Meanwhile, with hurricane season less than two weeks away, Perry Homes’ new contractors continue to put the full-court press on construction of detention ponds. They have made more progress in two months than the previous contractors did in two years.

Posted by Bob Rehak on 5/19/2020 with keyframe from Jim Zura, Zura Productions

994 Days after Hurricane Harvey and 243 after Imelda