Tag Archive for: high water marks

Simple Way to Find Depth of Flooding Near You

Ever wonder how to find the depth of flooding on a bayou, river or stream near you? Here’s a simple way. But this only works for those in and around Harris County, Texas, and those who live on streams with gages.

Step-By-Step Guide

Go to HarrisCountyFWS.org.

FWS stands for Flood Warning System. The main function of this website is to alert you when streams are in danger of coming out of their banks. But the site also shows historical information for dozens of gages that blanket Harris and surrounding counties. That information includes flood peaks and bank elevations. By subtracting the bank elevation from the peak, you can easily determine the height of a flood and compare the height in your area to other areas.

For this exercise, start by selecting ALL gages in the left column. Gages across the region will pop up. See below.

By default, the map shows how much rainfall all those gages received in the last 24 hours. But there’s much more information behind them. Next…

Click on Any Gage

Another box pops up that is the gateway to historical information about that location. In this example, I clicked on the gage at the San Jacinto West Fork and US59.

Click on the hyperlink that says, “More information and alert signup.” Then, in the next screen…

Click on the Stream Elevation Tab
Scroll Down

You should see a box that looks like the one below.

The red “Flooding Likely” line represents the top of bank in that area. They say “Likely” because bank heights may vary slightly around a gage. But for most locations in a flat area like ours, that’s the point at which water starts to come out of the banks. Near this gage, the river starts coming out at 49.33 feet above sea level.

Below that box, three more boxes show:

  • Gage readings during the current time period (or any historical time period if you specify one).
  • Flood frequency for that gage location. For instance, the height of a 10-, 50-, 100- or 500-year flood.
  • Historical records for major storms.

This last box contains the information you want (if you’re looking for the Harvey peak). For instance, you can see that at this location, the West Fork reached 69.6 feet.

Subtract Flooding-Likely Elevation from the Flood Height

Subtracting the flooding-likely elevation from the high-water mark tells you the depth of flooding, i.e., how high the water got above the banks. At this location, that was more than 20 feet! (69.6 minus 49.33)

Step and Repeat

To compare the depth of flooding at other locations, repeat the same process. To visualize the differences, it helps to develop a spreadsheet with four columns: Location, Flooding Likely, Peak, Difference. Then you can then easily create a graph that looks like the one below.

worst first
Chart showing feet above flood stage at 33 gages during Harvey.

In this case, you can see that the San Jacinto, Spring and Cypress Creeks had the deepest floodwaters in the northern part of the county during Harvey. Some gages at other locations show that water didn’t even come out of banks.

What About Minor Floods?

The Harris County Flood Warning System contains information about stream levels going back 20+ years. If you’re looking for information about a flood not shown in the Historical Record box, you can search for it by specifying a time period and date range above the stream elevation and rainfall tabs.

Caution

Usually, Harris County Flood Control District personnel manually verify the historical records. So, you can trust the information. But if you’re researching smaller floods by inputting your own dates and time periods in the search fields, you may run into a problem.

Before 2010, sometimes gages recorded faulty readings. Gages during that period used pressure transducers, which could clog with floating debris and report false information.

So, if you see a hundred-foot flood that lasted 15 minutes, you’re looking at error. Cross check such readings against rainfall at the same gage. Also, check the readings immediately up- and downstream to rule out spurious readings.

The graph above shows wide ranges in the depth of flooding on the same bayous. Don’t assume that because a flood was 20 feet deep at one location that it will be the same along the entire stream. The topography could narrow, widen, deepen or flatten. All could affect the depth of flooding. So could other factors, such as the amount of surrounding development or previous flood mitigation efforts in an area.

Identifying Causes of Flooding

Using information from the Flood Warning System, you can help narrow down the source of flooding. If a neighborhood flooded, but the channel didn’t come out of its banks, you’re looking at a street-flooding issue. Most storm sewers and roadside ditches in Harris County and Houston are sized to handle a two-year rain. But older ones may have only a one-year level of service. And many become clogged over time. See below.

street flooding

For More Information

To learn how to find and verify other flood-related information, make sure you check out this post.

Posted by Bob Rehak on 10/29/22 with thanks to the Harris County Flood Control District

1887 Days since Hurricane Harvey

How High Water Marks Help Fill Gaps in Flood Knowledge

Note: the post below was condensed and adapted from a longer USGS article. The U.S. Geological Survey (USGS) records continuous stream stage (height of a river) and streamflow (how much water is flowing) with thousands of gages throughout the nation. But how can they determine flood height where no gages exist?

Simple, USGS correlates high water marks after floods with peak gage heights during floods.

“If, for example, records show that stream stage reached 17 feet during a storm, a high-water mark will show the hydrologist what a stage of 17 feet means in terms of how high the water was on the riverbanks and surrounding land.”

USGS Water School Website

Finding high water marks on or near buildings is easy. You look for the mud line or the edge of debris fields. The same principle applies in nature. Sometimes it’s easy.

Think of high water marks like a bathtub ring around a flooded area. Shown here: East End Park after Imelda.

But they’re not always that obvious.

Can You Spot High Water Marks In Pictures Below?

Below are two pictures used by USGS for demonstration purposes. They took the pictures a few days after a record storm. High-water marks show in both pictures, although a hydrologist would only regard one of the marks as being reliable.

Photos showing high water marks on vegetation after a storm.
Spot the high water marks.

The pictures below are close-ups of the high-water indicators in the top pictures. Did you spot them?

Pictures showing high water marks on vegetation.

The left picture shows a poison ivy vine with the bottom leaves covered in dried mud. Where the mud stops shows how high the floodwaters reached.

The right-side picture shows a limb that hangs over the same creek. During a flood, rapidly-moving water carries leaves and pine needles, etc.! They stick on limbs that are partially submerged. When the stream recedes the signs remain. The top of the leaves and pine straw indicate how high the creek was during the storm.

The mud on the vine is a much better high-water mark than the tree limb, though. During high water, the fast-moving water will cause partially submerged limbs to move up and down. Therefore, hydrologists would not use the limb to estimate high water.

How High Water Marks Are Used

Planning Development

Documenting high water marks helps plan development near floodplains. If you know that water reaches a certain mark on the bank every few years, you certainly don’t want people building homes and businesses there.

Determining Extent and Severity of Flooding

Gages can determine the height of a flood. But high water marks can also show the width and extent relative to topography.

After most major flood events, USGS partners with FEMA and other state and federal agencies to flag and survey high water marks in areas that flooded. USGS did so here after Harvey to determine the extent and severity of the flooding.

Prediction of Future Floods

Forecasters can use the data associated with high-water marks to predict the severity of future floods, delineate flood zones, and update current maps that may account for changes in upstream conditions.

Flood Frequency Calculations

High-water mark data is also part of the flood-frequency (or recurrence interval) calculations that FEMA uses to identify areas that are likely to experience a 1 percent chance of happening in any given year. These floods, known as 100-year floods, serve as the foundation for flood management planning.

Inundation Mapping

Another significant use: Flood-Inundation Mapping. A flood-inundation map shows the extent and depth of flooding that occurred in various communities as a result of a major storm or flood event. Inundation maps help determine things like:

  • Changes needed in building codes
  • Evacuation routes
  • Heights of bridges and roads.

Once inundation maps are complete, USGS documents them and makes them publicly available online.

Recreating Data from Damaged Gages

If a flood knocks out a gage, as it did with the one at US59 bridge and the Kingwood Country Club on the San Jacinto West Fork, high-water marks can provide maximum height of a flood after the fact. If the cross section of the river is known (or surveyed), hydrologists can even back-calculate the flow rate.

Posted by Bob Rehak on 7/16/2020 with thanks to USGS and Diane Cooper

1052 Days after Hurricane Harvey and 301 since Imelda

USGS Report on Peak Streamflows During Harvey Significantly Revises Flood Probabilities

The U.S. Geological Survey (USGS) in cooperation with the Federal Emergency Management Agency (FEMA) released a report this week that shows inundation maps, peak streamflows, detailed flood information, and new flood probabilities from Hurricane Harvey. Hurricane Harvey, it says, was the most significant multi-day rainfall event in U.S. history, both in scope and peak rainfall amounts, since records began in the 1880s.

Flood during Harvey looking east from the south side of the West Fork of the San Jacinto. Photo courtesy of Harris County Flood Control District.

Hurricane Harvey’s widespread 8-day rainfall, which started on August 25, 2017, exceeded 60 inches in some locations. That’s about 15 inches more than average annual amounts of rainfall for eastern Texas and the Texas coast. The area affected was also much larger than previous events.

New High Water Marks and Record Streamflows

USGS field crews collected 2,123 high-water marks in 22 counties in southeast Texas and three parishes across southwest Louisiana.

Record streamflows were measured at 40 USGS streamgages in Texas that have been in operation at least 15 years. At two streamgage locations, scientists determined that the percent chance for flooding of this magnitude to happen in any given year was 0.2 percent. This probability is also referred to as a 500-year flood. Thirty other USGS streamgages experienced flooding at levels with a 1 percent chance of occurring each year, also known as a 100-year flood.

Check out the “event viewer” noted in the report, especially if you are interested in high water marks in your neighborhood.

How Data Will Be Used

The USGS conducts research on the physical and statistical characteristics of flooding, estimating the probability of flooding at locations around the United States.

The purpose of the study was to check the probability of future occurrences and map the extent of flooding in Texas.

These records will assist officials in updating building codes, planning evacuation routes, creating floodplain management ordinances, providing environmental assessments and planning other community efforts to become more flood-resilient. FEMA will also use this information to revise their Flood Insurance Rate Maps. These maps help identify areas most likely to experience flooding in any given year.

Gages Closest to Lake Houston

The section on the San Jacinto Watershed starts on page 33. The maps for the San Jacinto watershed appear on pages 35 and 36. Use the maps to see the new high water marks in the area and to find the USGS gages nearest you. For most people in the upper Lake Houston Area, it will be one of these gages:

  • 08068090 – Grand Parkway and West Fork near Porter
  • 08069500 – West Fork and I-69 near Humble/Kingwood
  • 08070500 – Caney Creek near Splendora
  • 08069000 – Cypress Creek near Westfield
  • 08068500 – Spring Creek near Spring
  • 08071000 – Peach Creek near Splendora
  • 08070200 – East Fork near New Caney
  • 08071280 – Luce Bayou above Lake Houston near Huffman

After you locate the gages nearest you, cross reference the numbers of those gages with data at the front of the report. It helps to use the search function in Adobe Acrobat because much of the information is in tables with very small type.

Examples of What You Will Find

Here’s an example of what you can find. For the gage nearest many of the sand mines on the West Fork (08068090), peak streamflow was estimated at 131,000 cubic feet per second. That was the highest of the 33 peaks previously observed at that location (from Table 3 on Page 9).

Now here’s the big news: From the same table, we can see that the Annual Exceedance Probability (AEP) is 2.4.  That’s the likelihood of occurrence of a flood of given size or larger occurring in any one year, expressed as a percentage.

AEP is often expressed as the reciprocal of ARI (Average Recurrence Interval). For instance, A 10-year flood has a 10 percent probability of occurring in any given year, a 50-year event a 2% probabaility, a 100-year event a 1% probability, and a 500-year event a .2% probability.

In this case, a 2.4% AEP would have a likely recurrence interval of 42 years, given the new realities of upstream development, any changes in climate, and pocket calculators with more computing power. This means that the West Fork Gage ((08068090) at the Grand Parkway DID NOT even experience a 100-year flood! Yes, we can expect to see worse in the future.

That’s a far cry from the 1,000-year flood that some talked about earlier and raises real public policy questions about locating sand mines in floodways.

Despite the fact that Harvey was the largest rainfall event in recorded U.S. history, USGS now predicts that it would take even bigger floods to reach the reconfigured 100-year, 200-year and 500-year recurrence intervals: 196,000, 263,000 and 374,000 cfs respectively for West Fork Gage at Grand Parkway (Gage #08068090 from Table 5, Page 14). So the new 500-year flood would have almost triple the volume of Harvey.

Humble Gage Data Missing From Report

Unfortunately, the Humble Gage at I-69 does not show up in the tables even though it is on the map and the cover of the report. This is likely in part due to the fact that the gage stopped reporting during the event due to the excessive streamflow

They may also have not reported the exceedance probability due to the shorter recent record.

For all the other gages, the Annual Exceedance Probabilities translate to new recurrence intervals ranging from 35 to 250 years. The gages at the low end of that range tend to be in the fastest developing neighborhoods.

Implications of New Findings

The report will stimulate public policy debate about development near rivers and the most effective methods of flood mitigation.

After reading this, I believe more than ever that we need more detention, dredging and gates (DDG). We need all three to help us handle the volumes of floodwater that USGS expects at more frequent intervals. Prayer, while advisable, is a less certain option in my mind than including DDG in the flood bond and passing it.

BTW, there was some confusion Tuesday night at the flood bond meeting. A small number of flood control employees incorrectly told residents that dredging would not be possible under the bond. It will be according to Matt Zeve, whom I contacted today.

Posted July 12, 2018 by Bob Rehak

317 Days since Hurricane Harvey