Tuesday, at the open house in Kingwood to review work to date on the San Jacinto River Basin Master Drainage Plan, the Plan task force members showed two very interesting posters. Together, they show where the water came from during Harvey and the extent of inundation. They also show the amount of rainfall in different areas throughout the watershed.

When you put these two maps together, one high-level message screams through:

1.5 to 3.5 feet of rain fell over 2,885 square miles. That’s an area bigger than Delaware. And it all drained toward Lake Houston.

Watersheds Within River Basin

Looking at these posters gives you an appreciation for how complex flood forecasting can be, especially for areas like Kingwood where so many watersheds converge. The river basin map below shows the number of square miles drained by each of the major tributaries. The upper right corner inset map shows the same tributaries mapped over the major roads, counties and cities in the region to help you place the streams.

Rainfall and Extent of Inundation

The second poster shows the extent of inundation along each of those major tributaries during Harvey. The upper right inset map shows rainfall across the region. Note how the rainfall was heavier toward the lower and eastern parts of the river basin. As water came downstream and the rain kept falling in those areas, the floodwater just kept building higher and higher.

How Local Factors and Channel Hydraulics Come Into Play

The maps also reveal much about smaller areas within the watershed.

Matt Zeve, Deputy Executive Director of Harris County Flood Control, has studied channel hydraulics for more than 20 years. He emphasized flooding in The Woodlands and Cypress did NOT flood because of water backing all the way up from Lake Houston. Lowering water in Lake Houston faster will not prevent flooding that far upstream, he says. A wide variety of local conditions govern upstream flooding, such as:

• Rainfall rate, volume and location
• Time of accumulation
• Channel width/depth
• Gradient
• Flatness of terrain
• Blockages
• Rate and timing of runoff
• Time of year
• Amount of vegetation vs. impermeable cover
• Soil type
• Ground saturation and more.

This rainfall and inundation map clearly shows the effect of some of these factors. Notice, for instance, the three pockets of heavy flooding at the west end of Cypress Creek on the left. Also notice how the flooding narrows downstream toward the right. There are a several things going on here, according to Zeve.

• The area that flooded so badly was extremely flat. The area used to contain rice paddies. Farmers made the land even flatter.
• That area also received more rainfall. Note the small pocket of orange on the rainfall inset map over the area that flooded so badly on Cypress Creek.
• As you move east on Cypress Creek, the flooded area gets less wide. That’s because the channel gradient increases. The creek therefore creates a deeper channel and the floodplain narrows.

As you look at these maps, apply your knowledge of local conditions to see if you can explain similar anomalies.

For Future Reference

These maps still exist in draft form. The river basin survey is only half complete. The maps may change before completion of the study.

For easy reference in the future, I will post the high-res PDFs under the Hurricane Harvey tab in the Reports page.

Posted by Bob Rehak on 12/20/2019

843 Days since Hurricane Harvey