## Why So Much Flood Damage?

Part 1 of a three part series – We can’t stop flooding because we can’t control rainfall. But we can control flood damage – by regulating where and how we build. So, why do we experience so much flood damage?

If our scientific and regulatory systems always worked, homes would never flood. But they do flood. Why? Root causes include:

1. Inaccurate predictions of future rainfall
2. Conflicting standards and building codes
3. Building too close to threats
4. Upstream changes that undermine downstream assumptions
5. Difficulty of adapting to those changes downstream
6. Historical unwillingness to fund flood mitigation at meaningful levels

To keep this post from getting too long, I’ll consider the first three today, the second three tomorrow, and what we can do about them after that.

#### Inaccurate Predictions of Future Rainfall

Drainage studies start with worst-case rainfall estimates. When engineers design a community, these estimates form the design basis for:

• Channels
• Detention basins
• Subdivisions
• Building codes
• Drainage

How much rain you predict determines how wide the channels need to be, how high buildings should be elevated, etc.

Worst-case estimates are based on a branch of mathematics called Extreme Value Analysis (EVA).

By definition, EVA uses extremely small data sets. That limits their reliability.

EVA has one other huge limitation. It assumes stationary, underlying processes. And many believe climate is changing.

So, estimates get updated periodically – especially after major storms such as Tropical Storm Allison in 2001 or Hurricane Harvey in 2017. A 1,000-year storm before Harvey is now considered a 100-year storm! But some areas still use estimates from the 1980s. As a result…

To add to the confusion, different areas within a watershed may use different rainfall probabilities. Right now, engineers throughout the region are designing subdivisions that appear to be outside of floodplains but actually are inside of them.

That’s because of inconsistent adoption of new rainfall probabilities. Some areas use lower standards as a way to attract new development. It’s a competitive tool that reduces developer’s costs.

As if the hodgepodge of standards weren’t complex enough for elected officials, regulators, engineers and residents to understand, the environment around us is constantly changing. The population of the Houston region grew 75% since 2000.

#### Conflicting Standards, Building Codes and Enforcement

That makes it difficult for engineers designing a new subdivision to know what to do.

But a standard has evolved in the industry called “no adverse impact.” The idea: a new development should not cause flood peaks to rise higher or faster downstream.

Engineers estimate adverse impacts by comparing pre- and post-development runoff rates, and recommending flood-mitigation measures such as stormwater detention basins to compensate for any increase in runoff caused by developments. They don’t always get it right.

In addition to the uncertainty discussed above:

• Local officials may not want to adopt new higher standards because it could make them less attractive to developers and reducing tax revenue.
• Contractors may not always build what engineers designed or regulations specify.
• Local regulators may not have the staff to monitor compliance with regulations.
• Business people rarely go beyond the minimum that regulations specify because it raises their costs.
• A few may cut corners by falsifying drainage studies to reduce their costs.

Whatever the reason(s), consequences can be devastating.

Even when regulations agree between different jurisdictions, outcomes may differ radically. Harris and Liberty Counties, for instance, have virtually identical regulations for the construction of drainage ditches. The regulations are designed to reduce erosion that can, in turn, reduce the conveyance of rivers downstream.

In the example above, the 3-mile long ditch on the right has expanded almost 80 feet between 2017 and 2023.

All that erosion contributed to the formation of a 4,000-foot sand bar where the San Jacinto East Fork meets Lake Houston. Sediment drops out of suspension where water slows down as it meets a standing body of water.

We frequently see similar runoff from sand mines and developments in Montgomery County.

Such sedimentation reduces conveyance at unnatural (accelerated) rates and creates blockages in the river.

Sixteen thousand homes and 3,300 businesses flooded in the Lake Houston Area, according to the Lake Houston Chamber.

Sedimentation is a maintenance issue everywhere at every scale. The montage below shows representative images of roadside ditches in Harris County that have filled in. During storms, blocked storm pipes trap water in neighborhoods.

In 2009, Harris County revised its floodplain development standards. The new code affected things such as:

• Elevation above the 100-year floodplain
• Acceptable foundation types in flood- hazard areas
• Building in floodways including width, depth, bracing and other requirements for piers
• Where fill can and cannot be used
• Detention pond requirements
• Wind resistance
• Elevation above street level

After Harvey, Harris County Engineering compared damage found in subdivisions built before and after the new standards.

However, “grandfathering” of new developments under older regulations often contributes to insufficient mitigation that can affect property owners downstream. For instance, one recently completed 77-acre RV park in the Kingwood area has a detention basin that holds half the stormwater required by new regulations, even though it was built after they went into effect.

#### Building, Buying Too Close to Threats

The goal of many engineering studies is to build “safely” near water. What do you have to do to protect a family from a hundred-year flood?

These scientific-looking studies are incomprehensible to most people. Yet the engineering seals on them create a sense of security. So does the availability of nationally subsidized flood insurance.

Given those reassurances and given that some people will pay a premium to live near water despite the threats, some developers push the envelope into areas prone to flooding. That’s how we get high-density development in floodplains.

Similarly, some developers pave over wetlands and ponds that are nature’s stormwater detention systems.

Loss of these wetlands reduces floodwater storage. Before development, this area was a 10-year flood zone.

It’s sometimes possible to fix flood threats after the fact. But building on properties like you see here leaves little room for mitigation.

#### More to Follow

Come back tomorrow when I’ll elaborate on three more root causes of flood damage.

• Upstream changes that undermine downstream assumptions
• Difficulty of adapting to those changes downstream
• Historical unwillingness to fund flood mitigation at meaningful levels

Posted by Bob Rehak on 11/19/23

2273 Days 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.

## Northpark South Will Drain Through Sand Mines

11/18/23 – It was a busy week for the new Northpark South development. A Drainage Impact Analysis submitted to Montgomery County for the development at the west end of Northpark Drive shows that the developer intends to drain its property through adjacent sand mines owned by other entities. The analysis does not address potential sedimentation issues.

The developer also applied for several variances from the Houston Planning Commission.

Meanwhile Century Land Holdings of Texas, LLC continued clearing land at the west end of Northpark Drive that borders the sand pits along the West Fork San Jacinto River. In the last week, contractors have cleared approximately one third to one half of the 54.4 acre development.

#### Drainage Analysis Shows Routing

A drainage impact analysis obtained from the Montgomery County Engineer’s office reveals that:

• Drainage from 107.2 acres outside (upstream from) the development flows through the development on its way to the West Fork.
• The remaining 43.2 acres will contain 236 homes on narrow lots for an average of 0.18 acres/home (1/6th acre) with impervious cover of 66%.

Drainage from the development and areas upstream will be routed into an on-site 11.2 detention basin (above) and, from there, into sand pits and the West Fork (below).

The drainage analysis claims the outfall from the proposed detention basin will not increase flow to the sand pits.

The analysis by RG Miller also indicates that the on-site basin will provide a storage rate of approximately 1.26 acre-feet per acre. That exceeds the minimum of .55 acre-feet per acre recommended by Harris County Flood Control District for sites this size draining into Harris County.

RG Miller claims that the water surface elevations in the sand pits will decrease during both 25-year and 100-year storm events.

The engineering firm also claims that the proposed development lies outside the 100-year floodplain of the West Fork. However, that claim is based on old data.

New flood maps, expected to be released next year, will likely show the floodway and floodplains expanding by 50- to 100%, according to preliminary guidance from HCFCD and FEMA.

However, RG Miller makes no mention of the shifting floodplains. Nor do its engineers mention any wetlands on the property which the US government clearly shows.

The engineering firm concludes that their design will have “no adverse impact” on downstream properties. Nor will it “unreasonably”:

• “Impede the natural flow of surface waters from higher adjacent properties”
• “Alter the natural flow of surface waters so as to discharge them upon adjacent properties at a more rapid rate or in a different location…”

One hydrologist I interviewed about this plan said, “Well, this isn’t how I would do it.  He is draining directly to the sand pit and showing that he isn’t increasing flows going to the sand pit.  Nowhere in the report did he document that he had permission from the sand pit owner to drain to the sand pit. Also, he didn’t have any field survey to back up how he knew the topography and details on the hydraulic connections between the sand pits and the river.  So I don’t know how he can be certain of his results.”

The drainage analysis did not address what effect the proposed routing would have on sedimentation. The outfall to the drainage ditch is already severely restricted by sediment accumulations.

#### Variance Requests

A request for variances obtained from the Houston Planning Commission shows a different outline for the property. It omits the areas designated as commercial at the entry.

The MoCo Appraisal District (MCAD) shows that Hanover Estates owns the parcels that bracket the entry. MCAD also shows that Hanover and another company called Maryfield LTD jointly own the sandpits. The drainage analysis does not address the ownership issues or permissions.

#### Requests for Variances from Houston Planning Commission

This property lies within the City of Houston’s extra territorial jurisdiction. The developer requested three variances from the Houston Planning Commission:

• To allow lots less than 5000 square feet.
• To exceed intersection spacing by not providing a southern stub street (outlet).
• To exceed intersection spacing by not providing an east/west street

The planning commission has not yet responded to an information request about whether it granted the variances. Meanwhile, clearing continues.

#### One Third to One Half of Site Now Cleared

In the last week, contractors have taken down an estimated one half to one third of the trees on the site. See below.

Compare that to this previous post.

At this rate, the entire site could be cleared by early December.

#### A Closing Thought

The goal of most drainage studies is to figure out how to develop property safely. If all of the studies were always correct, we would never flood. But we do. Why?

And why is property that was too dangerous to develop before Harvey now safe after Harvey … when we now realize how much greater the risk is?

More on that in my next post.

Posted by Bob Rehak on 11/18/23

2272 Days 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.

## Northpark Expansion Project Struck Oil

The Northpark Drive Expansion Project struck oil this week. But this was no Jed-Clampett moment. Oil in the context of a construction project is not good.

Workers discovered what appears to be oil dumped some years ago on the site of what will become the north pond at the 59 entrance to Kingwood.

Ralph De Leon, project manager for the Lake Houston Redevelopment Authority/TIRZ, said that contractors are currently trying to discover the extent of the spill to avoid future contamination on the site. Although the photos below certainly look like excavation, the work is definitely not part of the pond-excavation project, according to De Leon.

De Leon also said the budget contained a line item for contingencies such as this. It should not affect the overall cost of the project.

#### West Sidewalks Complete

De Leon also said contractors have completed the sidewalks west of 59 that connect the main part of the project to the Kingwood Place West and Kingwood College.

This morning, the last cement pour was drying and contractors were removing forms from areas completed earlier.

Dr. Katherine Persson, retired head of Kingwood College, requested the sidewalk extension to fill in a critical gap for her students who rode their bicycles to school.

#### Other Highlights

• Extension of the drain pipe to Ditch One behind Public Storage is nearly complete. However, contractors still need to install a concrete headwall around the pipe to prevent future erosion.
• Contractors are preparing an estimate to install the city-approved, water-main detour under Northpark to the new Parkwood Baptist Church. The original water main was too high and interfered with the box culverts being installed in the median.
• The City of Houston Mayor has signed the inter-local agreement with Union-Pacific (UP) Railroad. UP is now expediting the agreement through its system.
• Centerpoint has nearly completed relocating its utilities that interfered with the box-culvert installation. As soon as CenterPoint completes that work, installation of the box culverts will resume.
• The pole-mounted traffic signal at Northpark and Russell-Palmer must come out before contractors can install the box culverts under that intersection. A wire-mounted, temporary signal will replace the pole-mounted system. The wire will stretch across the road. As construction advances, that will enable the signals to be moved left or right as lanes change.
• Tree transplantation should ramp up in the next couple weeks.