One of the most thought-provoking articles I have read lately is “The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality.” This brief, well-written article brings many flood-related issues into sharp focus. The Rhode Island Department of Environmental Management produced it. And the National Park Service helped fund it. Even though many of the recommendations would not fly politically in Texas, one might. The report is worth reading just to understand the factors that contribute watershed degradation and their relationship to each other.
Central Role of Impervious Cover In Variety of Issues
One main premise: As impervious cover rises above 10% there is almost always a measurable loss in water quality.
Between 10% and 25% impervious cover, these impacts increase, and pollution and flooding become evident.
25%-plus impervious cover creates water quality impacts so severe that it may not be possible to restore water quality to pre-existing conditions.
The report claims that by keeping overall impervious cover below 10%, towns can ensure that land will be able to absorb and filter runoff from developed areas. This, they say, will also prevent excessive flooding, ecosystem impairment and contamination of water supplies.
A second major premise: Because water spends less time on site, infiltration declines dramatically. This can reduce groundwater in urban and suburban areas because there is not enough rainfall soaking into the ground.
The increased runoff that occurs during this process reduces groundwater recharge AND dramatically increases erosion.
Relationship Between Cover, Runoff, Other Measures
According to the EPA, undernatural forested conditions, only about 10% of precipitation runs off the surface of a site. Another 50% soaks into the ground. And trees and other vegetation take up a surprising 40% and send it back into the atmosphere through the process of evapotranspiration. This protects the watershed and water quality.
But higher rates of runoff can impact and degrade them. With increasing development, both the rates of infiltration and evapotranspiration decrease as runoff increases. On average, runoff increases more than 5X between natural and fully developed conditions. But extremes can be much higher. See below.
Increases in Runoff Rates
Total runoff for a one-acre parking lot is about 16 times that produced by an undeveloped one-acre meadow.
Higher rates of runoff create several types of impacts:
Hydrological
Biological
Chemical
Physical
Health
Hydrological Impacts
The report sums up the hydrological impacts by saying, “Because the water is spending less time on site, infiltration declines dramatically. This is a particular concern in many urban and suburban regions, where groundwater has been reduced because there is not enough rainfall soaking into the ground. The increase in runoff that occurs during this process, combined with the loss of recharge to groundwater, has dramatic impacts on streams.”
Biological Impacts
Biotic integrity is the most sensitive indicator of impervious cover according to the report. “The decline of biological indicators is the first sign of stream degradation, and has been the most commonly studied result of increased impervious cover. As a result of a high percentage of impervious cover, naturally occurring aquatic insects, wetland plants, and amphibians decline and are gradually replaced by species that are adapted to pollution and flooding. … Impacts on overall biotic measurements were seen within a range of 3.6% to 15% impervious cover; the threshold for fish population health ranged from 3.6% to 12%, and macroinvertebrate health declined between a range of 8% to 15%.”
Chemical Impacts
“Impervious surfaces such as roads, driveways and parking lots collect a variety of chemical pollutants and hydrocarbons and discharge them to aquatic systems with every heavy rain.”
“The study found a strong correlation between water quality and percent impervious cover across a range of contaminants, including organic residue, nitrogen, phosphorus, dissolved chloride, and fecal coliform. In each case, as impervious surface increases so does the contaminant of concern.”
Physical Impacts
“Development of impervious cover in a watershed can happen so quickly that stream systems can’t adjust, resulting in erosion of stream banks and alteration of the stream bed, which tends to become straighter, deeper, and more U-shaped.”
“This…sends silt downstream, creating further damage.”
“There is a strong correlation between increased impervious cover and increased risk to human health. A variety of chronic and acute illnesses are caused by microorganisms that either are swept into water bodies by increased runoff, or flourish because of increased nutrient pollution.”
“People can contract these illnesses through direct contact or through the consumption of tainted seafood.”
Smart-Growth Strategy Reduces Impervious Cover
So what are people to do. We need places to live. As population grows, so must developments.
The next sections of the report deal with strategies to control the growth of impervious cover. Most amount to fighting words in Texas, i.e., regulation and zoning. So here, I will only cover one strategy that the report discussed; it’s market based.
“Generally speaking, as density increases,” says the report, “the amount of impervious cover also increases. However, the overall pattern of development is also important.”
The next part of the report is counter-intuitive. It quotes the EPA, “…the large-lot zoning currently used to accommodate growth requires houses to be far apart, creating unnecessary impervious cover and encouraging more off-site impervious infrastructure, such as roads and parking lots.”
“Moreover, many of the surfaces remaining after large-lot development that are believed to be pervious actually behave like impervious surfaces. Research indicates that the volume of runoff from highly compacted lawns is almost as high as from paved surfaces.”
“The solution is to maintain the overall density [by] encouraging the use of more compact growth techniques that can reduce impervious cover on a per unit basis.”
“…by greatly reducing roads, utilities and other infrastructure costs, this approach can be profitable for developers while reducing house prices for consumers.”
Visually, the strategy looks something like this.
Food for thought as we turn over a new year. As land prices escalate, the market is driving new development in this direction anyway. Land now comprises 40% of the cost of a new home. A homebuilder told me it’s the single largest component of the cost of a new home.
But when I look at Scenario C, it raises a question. What’s the incentive to preserve the open space around the development?
Posted by Bob Rehak on 12/31/2020
1220 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.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/12/Changes-in-Hydrology-w-Increasing-Impervious-Cover.jpg?fit=1152%2C804&ssl=18041152adminadmin2020-12-31 11:40:552020-12-31 12:02:40Need to Reduce Impervious Cover to Prevent Flooding, Protect Water Quality
2 Zip Archives of Existing Conditions Models in HEC-RAS, an engineering program (Appendix L)
That’s more than 3,600 pages, EXCLUDING the zipped materials.
Executive Summary Summarized
The executive summary includes a heat map of historically flooded areas, estimated structural damage costs in the next 50 years, and projected population growth during the same period. Not surprisingly, the three fasted growing areas (West Fork, Spring Creek and Cypress Creek) also show the most projected damage.
The summary then proceeds to flood-damage-reduction strategies. They include:
Detention Basins
Channel Improvements
Floodplain Preservation
Buyouts
Flood Warning Improvements
Floodplain Re-mapping
Policy updates
Formation of a vision group
Flood Response Improvements (Evacuation planning, protection of critical facilities such as hospitals, etc.)
The exec summary also lists the top sixteen project priorities, estimates their costs, outlines possible sources of funding, and lays out next steps.
Project Location Map
Project rankings.Note: Rankings do not necessarily coincide with numbers on map above.
The proposed projects will provide tangible benefits, including reduction in the number of at-risk structures for a range of storms as shown in Figure 1.10 below.
With these projects in place, the level of a 100-year flood at I-69 and the West Fork could be reduced by 5.94 feet, Likewise, where Caney Creek meets the East Fork, the 100-year flood would be reduced by 2.82 feet.
Next Steps
That’s good news indeed for everyone who lives in the San Jacinto Watershed. With this information now in hand, we now can quantify the benefits of projects, priorities them, and get on with the hard work of actual mitigation.
More news to follow.
Posted by Bob Rehak on 12/30/2020
1219 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/12/Screen-Shot-2020-12-30-at-2.06.17-PM.png?fit=1024%2C356&ssl=13561024adminadmin2020-12-30 14:51:422021-01-01 07:17:03Final San Jacinto River Basin Master Drainage Study Released Today, Recommendations Revealed
The National Weather Service has issued a flash flood watch from 6PM tonight through Thursday afternoon. In the north Houston Area, it includes:
Houston
The Woodlands
Coldspring
Trinity
Shepherd
College Station
Huntsville
Conroe
Livingston
Madisonville
The watch extends much farther north and west. It includes the Dallas/Fort Worth Area, Oklahoma, Arkansas where NWS predicts up to six inches of rain.
Where heaviest rains will fall.
Jeff Lindner, Harris County Meteorologist reports increasing risk of excessive rainfall along and west of US 59 and west of SH 288. A strong upper level storm system will combine with increasing moisture and a slow/stalling surface frontal boundary over SE TX tonight. The stalling boundary will produce training.
Lindner adds, “Heavy rainfall of 1-2 inches per hour looks possible and this could be maintained for several hours along/near the stalling front. Where the heaviest rains fall will be determine where the front slows and stalls and at this time areas from Wharton and Fort Bend into western Harris and then northward into Montgomery County have the highest chances.”
“Widespread rainfall amounts of 2-4 inches with isolated totals of 5-6 inches will be possible,” says Lindner. “We need to watch for flooding.”
Areas At Highest Risk of Flooding
While grounds west of I-45 are especially dry, Lindner predicts that the entire San Jacinto basin (west, east, and mainstem) and Trinity basins will be affected – especially in the longer term as water works its way downstream.
Northwest and western Harris County will likely see the highest totals. Responses on the creeks in those areas are likely early Thursday.
Lindner believes most watersheds will be able to handle incoming rainfall up to 4.0 inches. Should western Harris County get more, minor flooding would be possible. Especially along the lower end of South Mayde Creek, the upper end of Little Cypress Creek, the upper end of Spring Creek, and the lower end of Keegans Bayou.
As of noon on Wednesday, the SJRA shows Lake Conroe at 199.41, slightly below its normal level.
The City of Houston began lowering Lake Houston at noon Wednesday as a precautionary measure. Property owners should secure property along the shoreline. Lake levels can be monitored in real time by visiting the Coastal Water Authority website.
Heavy rainfall will end Thursday afternoon.
PRECAUTIONARY ACTIONS
A Flash Flood Watch means that conditions may develop that lead to Flash Flooding. Flash Flooding is a very dangerous situation. You should monitor forecasts and be prepared to take action should Flash Flood Warnings be issued. See:
Need to Reduce Impervious Cover to Prevent Flooding, Protect Water Quality
One of the most thought-provoking articles I have read lately is “The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality.” This brief, well-written article brings many flood-related issues into sharp focus. The Rhode Island Department of Environmental Management produced it. And the National Park Service helped fund it. Even though many of the recommendations would not fly politically in Texas, one might. The report is worth reading just to understand the factors that contribute watershed degradation and their relationship to each other.
Central Role of Impervious Cover In Variety of Issues
One main premise: As impervious cover rises above 10% there is almost always a measurable loss in water quality.
The report claims that by keeping overall impervious cover below 10%, towns can ensure that land will be able to absorb and filter runoff from developed areas. This, they say, will also prevent excessive flooding, ecosystem impairment and contamination of water supplies.
A second major premise: Because water spends less time on site, infiltration declines dramatically. This can reduce groundwater in urban and suburban areas because there is not enough rainfall soaking into the ground.
The increased runoff that occurs during this process reduces groundwater recharge AND dramatically increases erosion.
Relationship Between Cover, Runoff, Other Measures
According to the EPA, under natural forested conditions, only about 10% of precipitation runs off the surface of a site. Another 50% soaks into the ground. And trees and other vegetation take up a surprising 40% and send it back into the atmosphere through the process of evapotranspiration. This protects the watershed and water quality.
But higher rates of runoff can impact and degrade them. With increasing development, both the rates of infiltration and evapotranspiration decrease as runoff increases. On average, runoff increases more than 5X between natural and fully developed conditions. But extremes can be much higher. See below.
Increases in Runoff Rates
Higher rates of runoff create several types of impacts:
Hydrological Impacts
The report sums up the hydrological impacts by saying, “Because the water is spending less time on site, infiltration declines dramatically. This is a particular concern in many urban and suburban regions, where groundwater has been reduced because there is not enough rainfall soaking into the ground. The increase in runoff that occurs during this process, combined with the loss of recharge to groundwater, has dramatic impacts on streams.”
Biological Impacts
Biotic integrity is the most sensitive indicator of impervious cover according to the report. “The decline of biological indicators is the first sign of stream degradation, and has been the most commonly studied result of increased impervious cover. As a result of a high percentage of impervious cover, naturally occurring aquatic insects, wetland plants, and amphibians decline and are gradually replaced by species that are adapted to pollution and flooding. … Impacts on overall biotic measurements were seen within a range of 3.6% to 15% impervious cover; the threshold for fish population health ranged from 3.6% to 12%, and macroinvertebrate health declined between a range of 8% to 15%.”
Chemical Impacts
“Impervious surfaces such as roads, driveways and parking lots collect a variety of chemical pollutants and hydrocarbons and discharge them to aquatic systems with every heavy rain.”
“The study found a strong correlation between water quality and percent impervious cover across a range of contaminants, including organic residue, nitrogen, phosphorus, dissolved chloride, and fecal coliform. In each case, as impervious surface increases so does the contaminant of concern.”
Physical Impacts
“Development of impervious cover in a watershed can happen so quickly that stream systems can’t adjust, resulting in erosion of stream banks and alteration of the stream bed, which tends to become straighter, deeper, and more U-shaped.”
“This…sends silt downstream, creating further damage.”
Health Impacts
“There is a strong correlation between increased impervious cover and increased risk to human health. A variety of chronic and acute illnesses are caused by microorganisms that either are swept into water bodies by increased runoff, or flourish because of increased nutrient pollution.”
“People can contract these illnesses through direct contact or through the consumption of tainted seafood.”
Smart-Growth Strategy Reduces Impervious Cover
So what are people to do. We need places to live. As population grows, so must developments.
The next sections of the report deal with strategies to control the growth of impervious cover. Most amount to fighting words in Texas, i.e., regulation and zoning. So here, I will only cover one strategy that the report discussed; it’s market based.
“Generally speaking, as density increases,” says the report, “the amount of impervious cover also increases. However, the overall pattern of development is also important.”
“Moreover, many of the surfaces remaining after large-lot development that are believed to be pervious actually behave like impervious surfaces. Research indicates that the volume of runoff from highly compacted lawns is almost as high as from paved surfaces.”
“The solution is to maintain the overall density [by] encouraging the use of more compact growth techniques that can reduce impervious cover on a per unit basis.”
“…by greatly reducing roads, utilities and other infrastructure costs, this approach can be profitable for developers while reducing house prices for consumers.”
Visually, the strategy looks something like this.
Food for thought as we turn over a new year. As land prices escalate, the market is driving new development in this direction anyway. Land now comprises 40% of the cost of a new home. A homebuilder told me it’s the single largest component of the cost of a new home.
But when I look at Scenario C, it raises a question. What’s the incentive to preserve the open space around the development?
Posted by Bob Rehak on 12/31/2020
1220 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.
Final San Jacinto River Basin Master Drainage Study Released Today, Recommendations Revealed
This afternoon, Harris County Flood Control District (HCFCD), San Jacinto River Authority (SJRA), Montgomery County, and City of Houston released final results of the massive San Jacinto River Master Drainage Plan (SJMDP) study which began in April 2019.
It will take weeks to digest all this information. It consists of:
That’s more than 3,600 pages, EXCLUDING the zipped materials.
Executive Summary Summarized
The executive summary includes a heat map of historically flooded areas, estimated structural damage costs in the next 50 years, and projected population growth during the same period. Not surprisingly, the three fasted growing areas (West Fork, Spring Creek and Cypress Creek) also show the most projected damage.
The summary then proceeds to flood-damage-reduction strategies. They include:
The exec summary also lists the top sixteen project priorities, estimates their costs, outlines possible sources of funding, and lays out next steps.
“Benching” in reference to the Kingwood Area involves lowering the floodplain near the West Fork to increase flood capacity. This link shows how a similar project in California worked.
The proposed projects will provide tangible benefits, including reduction in the number of at-risk structures for a range of storms as shown in Figure 1.10 below.
Next Steps
That’s good news indeed for everyone who lives in the San Jacinto Watershed. With this information now in hand, we now can quantify the benefits of projects, priorities them, and get on with the hard work of actual mitigation.
More news to follow.
Posted by Bob Rehak on 12/30/2020
1219 Days since Hurricane Harvey
Flash Flood Watch
The National Weather Service has issued a flash flood watch from 6PM tonight through Thursday afternoon. In the north Houston Area, it includes:
The watch extends much farther north and west. It includes the Dallas/Fort Worth Area, Oklahoma, Arkansas where NWS predicts up to six inches of rain.
Jeff Lindner, Harris County Meteorologist reports increasing risk of excessive rainfall along and west of US 59 and west of SH 288. A strong upper level storm system will combine with increasing moisture and a slow/stalling surface frontal boundary over SE TX tonight. The stalling boundary will produce training.
Lindner adds, “Heavy rainfall of 1-2 inches per hour looks possible and this could be maintained for several hours along/near the stalling front. Where the heaviest rains fall will be determine where the front slows and stalls and at this time areas from Wharton and Fort Bend into western Harris and then northward into Montgomery County have the highest chances.”
“Widespread rainfall amounts of 2-4 inches with isolated totals of 5-6 inches will be possible,” says Lindner. “We need to watch for flooding.”
Areas At Highest Risk of Flooding
While grounds west of I-45 are especially dry, Lindner predicts that the entire San Jacinto basin (west, east, and mainstem) and Trinity basins will be affected – especially in the longer term as water works its way downstream.
Northwest and western Harris County will likely see the highest totals. Responses on the creeks in those areas are likely early Thursday.
Lindner believes most watersheds will be able to handle incoming rainfall up to 4.0 inches. Should western Harris County get more, minor flooding would be possible. Especially along the lower end of South Mayde Creek, the upper end of Little Cypress Creek, the upper end of Spring Creek, and the lower end of Keegans Bayou.
As of noon on Wednesday, the SJRA shows Lake Conroe at 199.41, slightly below its normal level.
The City of Houston began lowering Lake Houston at noon Wednesday as a precautionary measure. Property owners should secure property along the shoreline. Lake levels can be monitored in real time by visiting the Coastal Water Authority website.
Heavy rainfall will end Thursday afternoon.
PRECAUTIONARY ACTIONS
A Flash Flood Watch means that conditions may develop that lead to Flash Flooding. Flash Flooding is a very dangerous situation. You should monitor forecasts and be prepared to take action should Flash Flood Warnings be issued. See:
Do not enter or cross flowing water or water of unknown depth.
Posted by Bob Rehak on 12/30/2020 based on information from the National Weather Service and HCFCD
1219 Days since Hurricane Harvey