## Why Rivers Move

Have you ever flown over a winding river and wondered why rivers move? Why do they wander across the landscape and evolve the way they do? The lower Mississippi River and its delta form a spectacular example. But others are all around us.

Take, for instance, the Trinity River where it enters Galveston Bay. Or look on any map. You will likely see landscapes carved by rivers that leave evidence of their former paths behind.

But why do rivers change course? The brilliantly simple YouTube Video by Practical Engineering below describes the basics of “fluvial geomorphology.” That fancy phrase describes the science behind the shape of rivers.

#### The Mathematics of Geological Change

Back in 1944, a geologist named Harold Fisk, Ph.D., then a professor at Louisiana State, produced a report for the Army Corps of Engineers called “Geological Investigation of the Alluvial Valley of the Lower Mississippi River.”

Fisk produced gorgeous historical maps of the river snaking across miles of river valley.

A decade later, Emory Lane, a civil engineer and hydrologist at Colorado State University, went on to develop a unified theory of sediment transport. His theory explains the movement and shape of such rivers in an “equation” that uses just four variables:

• Quantity of sediment carried by the water
• Median sediment size
• Quantity of water
• Slope of the landscape (length divided by elevation change)

Lane’s “equation” looks like this.

That funny symbol in the middle means ‘is proportional to.’ Scientists use it to show something that varies in relation to something else.

If you change one variable, one or more other variables change to bring the river back to its “normal” state. Scientists and engineers still use this formula today.

It means that in a stable stream, the flow of water multiplied by the slope is proportional to the amount of sediment being transported times the size of that sediment. (But don’t let that scare you!)

#### From Stream-Table Models to Real World

In the abstract, that may be a lot for average people to wrap their heads around. So the video uses a “stream table” and balance-scale model to illustrate what happens when you change each variable. They bring the formula to life and make it easy to understand. For example…

This can threaten roads, pipelines, and property. The eventual deposition of all that sediment can also choke a channel and contribute to flooding. Or fill up reservoirs and reduce water supply.

Sound familiar? All those things happened in the Lake Houston area.

To restore balance, the river changes its slope by increasing its length. This explains the meanders found in most rivers in this region. A meandering river wanders back and forth across the landscape like a snake instead of making a straight line through it.

Wherever slight bends occur, the river scours the outside of the curve (called the cut bank). That’s because the water moves faster on the outside of a curve. The river then deposits larger particles of sediment on inside curves farther downriver (called point bars) where water moves slower.

Eventually these curves in a river become so exaggerated, that they cut themselves off, leaving oxbow lakes behind.

This National Park Service page contains an excellent series of illustrations that show the evolution of meanders over time plus their migration across the landscape.

Lane’s equation predicts that there’s no such thing as a stable river. All rivers change all the time in response floods, drought, development, dams, sand mining, farming and more.

#### When Natural Systems Lose Balance…

At every point along a river or stream, erosion and deposition are constantly balancing each other.

But Lane’s equation can’t predict exactly where or when a river will move. Nor can it predict the rate of change. The Practical Engineering video points out that the rate and volume of change depend on other factors not in the equation, such as vegetation and the “pulsing” of flows as you might see downstream of a dam like the one on Lake Conroe.

The screen capture below shows what happened in models comparing a steady flow and a pulsed flow.

The pulsed flow creates much more erosion and faster movement of channels. And that has many real world implications.

#### Who Should Watch This Video?

This 16-minute video is a real eye opener for a variety of audiences. It’s suitable for students from late middle school and up. You don’t need to be a math or science whiz to understand it. Its power is its simplicity.

Among students, the video may stimulate curiosity in earth sciences, engineering, math, economics, history and urban planning. And for adults, it shows how four variables tie them all together.

It makes a great tutorial for policy makers struggling with issues such as setbacks from rivers for homes and businesses.

In addition, everyone who lives near or is considering buying property near a river, stream or channel should view this.

The producers say the next video in the Why-Rivers-Move series will show how human changes affect the flow of rivers. Can’t wait!

My thanks to Dr. Matthew Berg, CEO of Simfero Consultants for bringing this to my attention.

Posted by Bob Rehak on 3/24/23

2033 Days since Hurricane Harvey

## Erosion: Sometimes Sudden

Erosion can sometimes be sudden. It’s not always a slow process of water grinding away at dirt and dissolving it, or wearing down rocks. This post will examine several examples around us and look at their implications. I intend it as a continuation of yesterday’s post about ditch maintenance.

There are four main types of erosion.

• Hydraulic action – When rapidly moving water churns against river banks and scours or undermines them.
• Abrasion – Caused by small pebbles moving along a river bank or bed and knocking other particles loose. Think of sandpaper.
• Attrition – When rocks carried by the river knock against each other. They break apart to become smaller and more rounded. This is how boulders turn into gravel.
• Solution – When water dissolves certain types of rocks, for example limestone. We often see this in Florida, where sinkholes frequently develop.

Most of these processes happen slowly. But the first can be sudden. One storm. One flood. And boom. That river bank where you used to sit and quietly contemplate nature is gone.

#### Now You See It; Now You Don’t

Sometimes large slabs of a river bank or ditch suddenly slump into a river, almost like mini landslides. One flood expert commented on the picture above; he said “The owners of those new homes may suddenly find the ditch in their backyards.”

At other times, the size of a flood forces a river to widen. We saw this during Harvey and Imelda. The relentless pounding of flood waters carries away everything in their path. Cutbanks (the outside of a river bend) are especially vulnerable. Water slams directly into them like a firehose and washes them away. This action actually changes the course of a river over time.

Most of the time, it happens so slowly, we barely notice it. But during large floods, it’s sometimes sudden, large, and devastating to homeowners or businesses near rivers.

#### Three More Examples of Hydraulic Action

##### Example C: River Aggregate Mine on West Fork in Porter

The third example comes from the abandoned River Aggregates sand mine beyond the new development in the first picture above. It’s a spectacular example of river migration.

In this case, the San Jacinto West Fork migrated 258 feet toward the mine’s dike in 23 years. When I first photographed the dike after Harvey, the river had eaten away an average of 12.4 feet per year. At the time, the dike was only 38 feet wide, and I predicted it could soon fail. It did. Within approximately a year.

Wait a minute, you say! What happened to the pond. After the river bank collapsed, the pond drained, exposing sediment already within it. And the action of draining concentrated more sediment in it, like all the remnants of food trapped in your sink drain after you’re done washing dishes.

#### History of Pond

The missing, shallow pond in the foreground above used to be the settling pond for River Aggregates.

Here’s how it looks today from a helicopter.

#### Lessons of Life Near a River

Most people never live long enough to see massive changes such as these in rivers. In most places, river change happens on a geologic time scale. But along the Gulf Coast, hurricanes can create floods that make rivers change on a human time scale, as these examples have shown.

What can we deduce from this?

1. Around here, we need to give rivers room to roam. Parks, green spaces, and golf courses, often represent the highest and best use of land near a river, bayou or ditch.
2. Building too close to rivers, bayous and drainage ditches can be costly. Disturbing wetlands and topsoil accelerates erosion. That, in turn, can threaten everything in its path. Be prepared to maintain anything you build near a watercourse, including the watercourse itself. And be prepared to fight what ultimately becomes a losing battle.
3. We need greater separation between mines and the San Jacinto riverKeep mines out of the meander belt. They worsen downstream sedimentation. And as we have seen, that can contribute to sediment build ups that require public money to remove. The alternative, leaving them in place, contributes to flooding.

Posted by Bob Rehak on 5/18/2020

993 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.

## West Fork Migrating Toward Proposed High-Rise Marina Project at 20 Feet Per Year

Steady northward migration of the San Jacinto West Fork could threaten the proposed new high-rise Kingwood Marina development – within the lifetime of many residents.

An analysis of satellite and aerial imagery in Google Earth shows that the river channel has shifted 758 feet north in 40 years – almost 20 feet per year – toward the site of proposed 25-50 story high rises. The proposed Kingwood Marina site is on the cutbank side of the West Fork. And the West Fork is definitely cutting.

#### Measuring River Migration Rate

These three images tell the story.

The migration of the river toward the high rises should continue. The river appears to be moving back toward one of its old meanders. The developer plans to build the high-rises in the old river bed. That’s a dangerous practice, because during floods, as residents all over Harris County discovered after Harvey, water seeks to return to old channels.

#### Floodway Shifting, Too

As the river moves closer to the high-rises, so will the floodway. Right now, the high-rises are built on the edge of the floodway that was mapped after Tropical Storm Allison in 2001. However, I believe that upstream development, river migration, and sedimentation are causing the floodway to expand and shift north. If current plans are approved “as is,” structures, people’s lives, and investor’s money will all be at risk.

#### Already at Greater Risk than Town Center

Harvey inundated Kingwood’s Town Center area. That’s a mile further from the river and on higher ground. About a year and a half later, approximately 25% of the businesses in Town Center still have not returned. That would certainly affect the economics of this development if it ever floods.

#### 50 Years or Bust

At the current rate of northward migration, the river could reach the marina in about 50 years; it’s currently about a 1000 feet away. If the river “captures” the marina (just as it captures sand pits), we could expect to see a rapid shift in river migration toward the high rises. See the demonstration in the video below.

Of course, before that happened, someone would try to prevent it. The owners would push to “shore up” the development with bulkheads or levees.

#### Futile Struggle to Combat Nature

Bulkheads didn’t work very well for these people on Marina Drive in Forest Cove.

Levees have their own set of problems. And anyway, how do you put a level around a marina? Seems like building this close to the river is just asking for trouble.

#### Planned Construction Level Likely to Flood Every 4-10 Years

The developer wants to build the foundations up to 57 feet. That’s asking for trouble, too.

If you go back and analyze the crest data for the West Fork for the last 90 years, you will see that the river has crested higher than 57 feet nine times – once a decade. But you will also see that it has crested higher than 57 feet six times in the 25 years since 1994 – about once every FOUR YEARS!

#### Rivers! Look pretty. Get ugly.

Sometimes rivers remind me of that classic 1983 teen flick called War Games staring Mathew Broderick and Ally Sheedy. The duo hacks into a Department of Defense Computer and starts playing what they think is a game. It’s called “Global Thermonuclear War.” They quickly discover it isn’t a game; they’ve triggered the real thing. In the end, they discover that “the only winning move is not to play.”

That’s certainly the case with the West Fork.

As always, these are my opinions on matters of public policy. They are protected by the First Amendment of the United States Constitution and the Anti-SLAPP statute of the Great State of Texas.

Posted by Bob Rehak on 1/26/19

515 Days since Hurricane Harvey

## River Migration: Another Reason for Greater Sand-Mine Setbacks

River migration can imperil sand-mine dikes and that can imperil people downstream.

In the case presented below, the San Jacinto river migrated 258 feet toward a dike in only 23 years and now threatens it. The river has eaten away at a dike by migrating an average of 12.4 feet per year. The dike is now only 38 feet wide. This a textbook case for why we need greater separation between mines and the San Jacinto river. Another dike failure could exacerbate downstream sedimentation and flooding, as it has before.

#### River Migration Raises Questions about Setbacks and Abandonment

This example of river migration raises serious questions about the lack of setback requirements for Texas sand mines. As rivers migrate toward mines, they can breach dikes and increase the risk of future breaches. Sediment then sent downstream can block rivers and streams, and contribute to worse flooding.

In some cases, mining companies may still be around to repair breaches. But what happens when the mine is played out and no one is there to repair the dike? Hundreds of acres of silt could suddenly be exposed to river currents and washed downstream. As more and more mines on the West Fork approach the end of their lives, this is becoming a huge concern.

#### Before Sand Mines

This series of satellite images from Google Earth starts in 1995, before there were any sand mines on either side of the river at this location. I created the red line in a separate layer above the satellite images. As we move forward in time, the location of the line will NOT change; but the location of the river WILL.

1/18/1995 before sand mining in this area of the West Fork

By 12/31/2001, the river had shifted slightly. We now have a sand mine on the east side of the river. Note the width of the dike and the road on top of it.

By 1/25/2004, the river had eaten away at the dike and threatened the road.

1/14/2006: The river has almost completely shifted from its original bed and wiped out a large part of the road

1/8/2008: The dike has become dangerously thin, and the road has completely disappeared.

3/14/2014: The mining company has shored up the road by adding fill to both sides of the dike, increasing sedimentation in the river.

On 5/31/2015, the Memorial Day Flood inundated the mine and wiped out the road again. Note the large body of water at the far left. This was a new pit started on the west side of the river that year. Notice how the dike on the left has been breached and silt from the mine is flowing directly into the river.

7/31/2015: The dike on the left remains open and erosion from the Memorial Day flood has eaten the road on the right dike. Twenty years after the start of this sequence, the river has now completely migrated from its original path.

Then along came the Tax Day Flood of 2016.

By 1/23/17, we see sediment building up at the south end of the both pits from the storm during the previous year. This shows that the current was strong enough to move sand within the pits, something the miners say is impossible.

By 8/30/17, the entire area was inundated. Peak flow during Harvey actually happened the day before this photo was taken.  It was four times greater than what you see above.

On 10/28/17, two months after Harvey, the dike on the right has almost disappeared. It is now a mere 38 feet wide. The red line, which represents the original riverbed, no longer overlaps the current river bed. The pond next to the G in Google has almost completely filled in, more evidence of sediment migration within the pit.

#### Reckless Endangerment?

This series of river migration images shows the relentless forces of erosion. Mining in the floodway so close to the river increases sedimentation, and as a consequence, the risk of flooding.

We’re already spending tens of millions of public tax dollars to dredge the San Jacinto and restore its carrying capacity. Sediment clogged it, in large part, because sand mine dikes have failed repeatedly to protect the mines from floods.

At what point does the honorable pursuit of profit become reckless endangerment? At what point does hope that the dikes will hold become willful blindness? Since when does one man’s unfettered right to mine sand give him the right to damage others and imperil public safety? Why do legislators allow business practices that endanger neighboring communities? When will regulators see the partial truths spread by TACA for what they are – an deceptive attempt to escape liability for egregious business practices? And above all, what happens when miners walk away from the property but floods continue as they always have.

#### Property Rights Vs. Public Safety

Miners claim they have the right to do what they want on their property. But not at the expense of public safety. Should the owners of commercial buildings be allowed to operate without fire alarms, sprinkler systems and safety exits just because it’s their property?

Miners have choices. They don’t need to compromise safety. The meander belt of the San Jacinto stretches for miles. There’s plenty of sand out of the floodway to mine.

At the current rate, without human intervention, river migration should capture the mine on the right side of these photos in about three years. It won’t be the first time something like this has happened.

To prevent such disasters in the making and protect public water sources, other states and countries have established setback regulations from rivers. Texas should do the same.

Posted on August 29, 2018 by Bob Rehak

365 Days since Hurricane Harvey flooded the Lake Houston Area

As always this is my opinion on a matter of public policy and is protected by the First Amendment of the United States Constitution and the Anti-SLAPP statutes of the Great State of Texas.