The two sets of questions on these pages are intended to highlight many of the issues related to: 1) the regulation of Houston’s floodplains and 2) the flow of stormwater into streams and their floodplains. This document  is intended to be a “living document” that will be revised to reflect comments and input and it is hoped that it will assist in formulating a clearer set of City Ordinances and Policies.

City of Houston Floodplain Management Policy Questions:

Note: These questions are typical of information requested by property owners during meetings about neighborhood drainage improvements and flood hazard reduction projects. They generally relate to the issue of placing fill inside the designated special flood hazard zone shown on FEMA’s Flood Insurance Rate Maps (FIRM Panels).

In order to clarify terminology, we will call compensation for floodplain fill or other storage-reducing activities that negatively affect the floodplain’s capacity to store flood waters Floodplain Displacement Mitigation. (Placing fill or building levees inside a floodplain reduces its capacity to store water and can displace and elevate flood water onto nearby properties and result in increased flood damages.)

1.       Who determines the need for floodplain displacement mitigation?

2.       Who is required to mitigate displaced floodplain capacity?

3.       Where is it spelled out who is required to mitigate displaced floodplain capacity?

4.       Where is it spelled out how to go about mitigating for displaced floodplain capacity?

5.       Is there a minimum amount of fill or displacement below which mitigation is not required? (Cubic feet, cubic yards or acre-feet?)

6.       Is there a minimum size of tract or property below which mitigation is not required? (Square feet or acres?)

7.       When someone creates floodplain displacement, when does the mitigation for this impact need to be completed? (Prior to the impact being created? Immediately after the impact is created? At some other, later moment in time?)

8.       Are there any exemptions from mitigating floodplain displacement? (If so, what are the exemptions?)

9.       What criteria determine who can create mitigation off-site?

10.   What criteria control the design of off-site mitigation?

11.   What criteria determine who can pay an impact fee in lieu of actually creating floodplain fill mitigation?

12.   What are the procedural or administrative steps for giving a floodplain displacement permit?

13.   Who checks to see that the displacement mitigation plans actually mitigate the displacement impact?

14.   Who checks to see that mitigation is complete and meets the terms of the permit?

15.   Is there a Design Standards Manual for design of floodplain fill mitigation?

16.   Are there any criteria to evaluate the cumulative effect of many small displacement impacts on the floodplain of a given watershed? Do these criteria have any effect on the requirements Displacement Impact mitigation?

17.   If the Planning Commission can only rule on whether or not a property plat submittal meets the subdivision rules of Chapter 42 and cannot consider any other issues in their deliberations and approvals, then why does plat approval grandfather a project in terms of all other governing ordinances, such as floodplain development regulations or non-point source pollution regulations?

Floodplain Displacement Case Study Questions:

1.       Residential property owner A is building a room addition to an existing house in the floodplain and will need to place 30 cy of fill for the new slab to match the existing house elevation (which is above the 100 year Base Flood Elevation or BFE). Is property owner A required to mitigate this fill?

2.       Industrial property owner B is going to build a large, heavy fabrication plant inside the outer edge of a shallow floodplain. To set his slab elevation 18” above the 100 year BFE he need only pour a grade beam, place some leveling sand and pour his structural slab. Is property owner B required to mitigate for the structural concrete he has placed in the floodplain?

3.       Commercial property owner C is going to build a shop on two acres of land that he just purchased. This property is part of a commercial subdivision that was platted in 1965, before the floodplain had been identified, but which is now shown as 3 feet below the 100 year BFE. Property owner B needs to place approximately 7,500 cy of fill to bring his shop floor 18” above the 100 year BFE. Is property owner C required to mitigate this fill?

4.       Commercial property owner D purchased property and submitted it for subdivision plat approval. At the time of plat approval, the property was not designated as being within the floodplain, but before the property owner acquired a building permit for commercial construction on the property, the floodplain maps were revised to include the property in the floodplain. Is property owner D required to mitigate fill placed within the floodplain that is necessary to bring the floor elevations above the new 100 year BFE?

5.       Institution E is located deep inside a regulatory floodplain and is planning to construct a 6’ tall floodwall and floodgates around its existing property improvements as a protection against flooding. Is Institution E required to mitigate for the floodplain displacement that will occur when the floodgates are closed during a flooding event?

6.       Property owner F owns 25 acres in the floodplain adjacent to a major bayou where the BFE is about 3 feet above natural ground. He intends to develop 5 acres of the property and intends to excavate most of the remaining 20 acres of the property as mitigation for the fill on the 5 acres (about 15 acre/feet) and to sell the excess capacity in the excavated area (about 135 acre/feet) to other developers in the area. Is property owner F allowed to sell excess capacity in his excavated area? Who keeps track of the capacity that is created and sold to various projects? If multiple projects have mitigation in one site, who owns the underlying land and who is responsible for its maintenance (strictly from a storage capacity standpoint)?

City of Houston Watershed Management Questions

Note: These next questions relate to stormwater runoff leaving property anywhere inside or outside regulatory floodplains.

Conventional drainage design increased the rate at which stormwater leaves a given property. (This is called “better drainage” and results in a reduced “time of concentration” of the runoff.) The cumulative effect of improved drainage on many individual properties is excessive stormwater discharges into a receiving stream or bayou, which causes the Base Flood Elevation to rise and the floodplain to get wider. In an urban region, this will generally result in increased flood damages. To prevent an increase in flood induced damages, the effect of improved drainage must be mitigated.

In order to clarify terminology, we will call attenuation of the discharge rate of runoff from a property Discharge Increase Mitigation.

1.       Who determines the need for Discharge Increase mitigation?

2.       Who is required to mitigate for Discharge Increases?

3.       Where is it spelled out who is required to mitigate?

4.       Where is it spelled out how they are required to mitigate for Discharge Increases?

5.       Is there a minimum property area below which it is not necessary to mitigate for Discharge Increases?

6.       Is there a minimum change in flow rate below which it is not necessary to mitigate for Discharge Increases?

7.       Are there any exemptions from mitigating Discharge Increases?

8.       Under what circumstances are discharges from a property allowed to increase?

9.       What scientific or engineering criteria are used to establish the base level of runoff on a given property against which discharge changes are measured?

10.   Are there alternatives to mitigating Discharge Increases on-site?

11.   Can a property owner pay an impact fee in lieu of actually creating Discharge Increase mitigation?

12.   What procedural or administrative steps insure that projects do not increase discharge rates?

13.   Who checks to see that mitigation is constructed per plans?

14.   Who checks to see that mitigation features are functional after the project has been in place a while?

15.   Is there a Design Standards Manual for design of Discharge Increase Mitigation?

16.   Does the City of Houston have a basis for overruling the opinion of the HCFCD as to whether a project needs to be mitigated?

17.   Are there any criteria to evaluate the cumulative effect of many small impacts on the overall discharge a given watershed? Do these criteria have an effect on the requirements for Discharge Increase Mitigation.

18.   Who checks to see that the Discharge Increase mitigation plans actually mitigate the project impact?

Discharge Increase Mitigation Case Study Questions:

1.       Residential property owner A adds significantly onto his house and builds new driveways and patios, changing the development coverage of his property from 30% to 80%. Is he required to mitigate for the change in stormwater discharge from his property?

2.       Developer B buys 3 contiguous 16,000 s.f. lots each with one old bungalow, re-plats them into 12 lots, and builds 12 new townhouses, changing the development coverage of his property from 15% to 80%. Is he required to mitigate for the change in stormwater discharge from his property?

3.       Commercial used car dealer C buys an old house on a corner 20,000 s.f. lot. He tears the old house down and builds a small office and new parking lot on his property in order to sell used cars. Is he required to mitigate for the change in stormwater discharge from his property?

4.       Commercial property owner D buys an abandoned 15,000 s.f. warehouse and two acres of truck parking on 40 acres of land. He tears down the warehouse and builds a new shopping center filling the full 40 acres with the retail center, pad sites, parking and the minimum required landscape planting areas. Is he required to mitigate for the change in stormwater discharge from his property?

5.       Neighborhood E has been asking City Council for improved drainage in their low-lying area for years, and finally the City is going to replace the rural streets and roadside ditches with new curb and gutter streets and underground drainage. Will the city be required to mitigate for the change in stormwater discharge from this neighborhood?

6.       Neighborhood F is undergoing rapid in-fill development as the City plats and permits hundreds of lot splits and new inner-city homes. Will the City be required to mitigate for the change in stormwater discharge from all these newly platted and permitted projects if they individually fall below the criteria requiring individual mitigation property by property?

7.       Corporation G is building a gas station at the corner of two major thoroughfares. They over-size their underground storm drain pipes to contain the discharge mitigation. To restrict the discharge, the outlet pipe needs to be 4” in diameter.  However, the City has a requirement that no storm drainage pipe shall be less than 8” in diameter. What should Corporation G do?

8.       Property owner H has acquired 500 acres of heavily forested undeveloped land. Property owner H clears the forest and dense understory, levels the land and cuts a number of drainage ditches across the property to dry it out, sows the acreage with coastal Bermuda grass for pasturage, and places 50 steer on the property for an agricultural exemption. Is Property owner H required to mitigate the increase in stormwater discharge from his newly improved pastures?