What’s a watershed and why should I care?

In a word… …..       “Flood damage reduction”.  

In a sentence…     “A Watershed is where the water in your bayou comes from, so it’s important to manage the watershed to control the amount of stormwater flowing into the bayous,  to improve the quality of the water and to elevate the visual and ecological qualities of the bayou corridors.” 

In a little more detail………….

Rainfall:

When it rains, whether drizzling or pouring, each drop of water is pulled earthward by gravity and after hitting the earth, gravity tugs it and all its companions downhill. They collect in each fold of land and concentrate into rivulets and each rivulet is then pulled downward and concentrated into a stream, and the water in each stream is pulled into a river and each river is pulled into the sea. There’s a balance of energy at work all the way, with gravity pulling and tugging on the water to move it downwards and the earth resisting with friction, slowing the passage of the water. The story of stormwater is really a story about energy and movement- fast or slow.

When it rains along the Gulf Coast it can come down in buckets; we can get thunderstorms that drop more than two inches per hour. But understanding the effect of rainfall on our landscape requires you to know three variables: 1) the intensity or how hard it’s raining at any given moment in time; 2) the duration or how long it rains at that intensity, and 3) the area or how wide a swath does the rain cover during the rainfall event. A really heavy thunderstorm for 15 minutes over a several block area isn’t likely to cause any flood damages, but a moderate rainfall for several days over half the city can overcome our drainage systems and cause our bayous to flow over their banks.

We care about water when we’re thirsty, when we want to wash the car or water the lawn or when we want water for a hundred other necessities and conveniences; we also care about water when it rains and we get wet because we forgot the umbrella, when our feet get soaked, when the water rises and blocks the street or when it rises further and enters our homes and businesses, destroying our property and disrupting our lives.

Watersheds:

So what’s a watershed? It’s the fold in the landscape that directs fallen rain into a rivulet, and it may be a larger fold that collect rivulets into a stream, or it may be the concentration of streams that turn into a river. A watershed might be as small as your backyard or as large as half the North American continent. If none of your neighbors drain their property onto your yard, your yard could be described as a very small watershed. On the other hand, the watershed of the Mississippi River reaches from the Rockies to the Appalachian mountains and from the Canadian border to the Gulf of Mexico. Watersheds are like the branching veins of a maple leaf, with the fine lines joining to become the largest stem at the base.

Most of Harris County is part of a large watershed that drains into Galveston Bay and out to the Gulf of Mexico. There are 22 significant secondary watersheds in Harris County and they are usually named for the bayous and creeks that drain them, such as Buffalo, White Oak and Brays Bayous and Spring, Cypress and Clear Creeks. The 640 square miles of the City of Houston spread out across many of these watersheds and, of course, most political boundaries don’t necessarily follow watershed boundaries.

Floodplains and Better Drainage:

In Harris county we don’t have much in the way of mountains. Our landscape is relatively flat and our streams are shaped by the flat topography and soft soils of the coastal plains. Natural stream channels are shaped and sized by average annual rainfalls, year after year, millennia after millennia, and the runoff from an average annual rainfall in a watershed will just about fill up a natural channel. Rainfalls that are greater than the annual average will overflow the natural channel and spill out into what’s called the floodplain. Almost every river has its own floodplain, that is just part of the natural balance of energy in the river system, and in our flat coastal region, the floodplains tend to be wide since a foot or two of rise above the natural channel can stretch the water out as much as several thousand feet from the stream.

Because our natural floodplains were frequently inundated, they were often filled with thick forests and their deep fertile soils made them attractive to early settlers for farming and homesteading. Farmers and ranchers soon dug drainage ditches to dry out the wet land and to speed the drainage after frequent summer storms. Small natural streams were cleared of trees and straightened and deepened to better carry away the drainage.

As rural communities grew into urban populations, subdivisions sprang up along the streams in the pleasant shade of the trees, deep in what were actually historic floodplains. The builders of the subdivisions insisted on fast and effective drainage, so wider and deeper ditches were dug, and underground storm drainage systems were installed to move rainwater quickly from the streets out to the nearest stream.

But soon even the largest streams were overcharged with all the additional water flowing into them. Neighborhoods were flooding, either because they had been built in existing floodplains or because the stream channels could not keep up with all the drainage improvements further upstream in the watershed.

So governments responded to the cries of the citizens and, at the cost of millions, created huge public works to shrink the floodplains and to move ever more storm water, ever more quickly, through the city and out to Galveston Bay. Parts of Buffalo Bayou, Brays Bayou, White Oak Bayou and others were turned from natural, slow moving streams into large, efficient, high speed drainage channels to protect the growing city from flood damage during large tropical storms.

These large infrastructure projects were often initially large enough to reduce broad floodplains completely into the newly constructed channel or at least very nearly so. But few anticipated the intensity of Houston’s growth or its impact on the drainage systems, and within a couple of decades, the cumulative effect of new construction and improved drainage over hundreds of square miles overwhelmed even these ‘hydraulic highway’ channels.

Time of Concentration:

As forests, prairie, farmland and ranchland were converted to streets, homes, schools and businesses, the speed with which water collected into drains, ditches and streams was radically reduced. When rainfall lands on undeveloped areas it might take hours or days for all the water to drain off the land: witness the water that dribbles for days out of a vacant lot after a heavy rain.

In contrast, when rainfall lands on rooftops or streets and parking lots, it takes just seconds or minutes to flow into drain pipes or ditches. Multiply this across entire watersheds and you have a lot of water, all at once, converging on a channel only capable of carrying a small fraction of that quantity. Right away, neighborhoods, businesses, schools and transportation systems were subject to significant and serious flood damage during the Gulf Coast’s recurrent tropical storms.

Remember that a “flood” is just made up of too many raindrops at one time, and in one place. Remember also that a flood is only bad when all those raindrops just happen to congregate where we have built our homes and businesses.

The speed with which rainfall collects into natural or man-made drainage channels is called the “Time of Concentration”. A shorter time of concentration means quicker drainage, but that’s not necessarily better since it can also lead to flash floods and out-of-channel flows downstream. A longer time of concentration means the water is held back in the watershed for long periods of time, only flowing into the stream slowly and within the capacity of the stream to accept the flows. “Time of Concentration” is key to understanding urban stormwater management: you want to concentrate the runoff when and where it will do least harm!

In some parts of the world, where soils are made up of highly permeable sands and gravels and where the earth can absorb water quickly, a significant amount of rainfall can be absorbed by the ground during a rainfall event. But our city is built on a the bed of an ancient seafloor, and we sit on heavy clays, thousands of feet thick. These clays absorb water only very slowly, and once they are just a little bit wet, they can be almost completely waterproof. During a rainfall a fraction of an inch might soak into a shallow aquifer, but most of the rain water will run off without soaking in. Remember that a good thunderstorm or tropical event can easily drop several inches of rain in just a few hours.

So along the Gulf Coast, it’s not so much whether or not a surface is permeable that governs how fast rainwater accumulates in the bayous, it is the overall roughness of the surface that controls the speed of runoff or the time of concentration. The roughness can be measured at a small scale, for instance in the difference between the roughness of a parking lot and the roughness of a pasture or the roughness can be measured at a larger, system scale, as in the difference between the “smoothness” of a neighborhood street system (and its drainage piping) and the “roughness” of an equivalent area of wooded acreage where the land has dips and bends and the surface is covered with tree trunks and brush.

This takes us back to the balance between energy and movement: gravity working on water to move it downhill and obstructions in the overall landscape working to hold it back. This is one of the critical challenges of managing storm water flows: finding ways to hold the flow back in the watershed, and storing it without causing flood damage, until the main receiving streams have the capacity to accept the water. So, when it rains hard, how do we store all the water in our city’s landscape?

Storage:

Detention basins for storage:

If the land is available, the most effective way to temporarily store storm water is to construct large detention basins on major bayous. Detention basins are usually empty, until they are needed during a big storm, when they can quickly take a lot of water out of the stream to lessen the risk of flooding. They often have a large spillway to let water in really fast, but just a small pipe to let the water slowly back out to the stream. After filling up during a storm, they usually drain back out after a day or two, depending on their size. Well designed large detention basins can be planted with trees and grass and can function very well as community parks and as significant open space with trails and other features that won’t be damaged by being occasionally submerged.

Smaller detention basins can be created in tributary watersheds, where small streams or ditches come together to join the larger bayous. In urban areas it can be hard to find large land areas but smaller 5-10 acres tracts can provide an important amount of stormwater storage in a smaller watershed. Small scale detention basins can also provide valuable open space for a city that is fast increasing in density and becoming more urban. If the detention basin is properly designed, the basin can also clean the water that enters it before passing it back into the bayou.

Commercial and residential developments larger than 5 acres are required to provide their own detention storage and these are often created as open basins. When properly designed, even these small basins can add valuable, green open space and habitat to the cityscape and can also cleanse the water that passes through them.

Channel enlargement for storage

Many of our region’s man-made drainage channels could be enlarged, not to move more water, but to store more water. Most channels and ditches are constructed with maintenance roads along each side; these maintenance roads could be lowered so that during big rainfall events, they could also hold water (since channels are not usually maintained during a storm event). This might require a short wall at the edge of the right of way, but it would allow the planting of trees in many of these barren corridors, since the added capacity in the channel is for storage and not conveyance.

Some of the bayous, such as Brays and White Oak that have been channelized, also have large flat areas at street level that could be excavated down to some intermediate level to store large amounts of water within the channel cross section. It would be important to replant these areas with groves and forests of trees to slow the flow of water downstream so that it would not increase and worsen flooding downstream areas. (The Brays channel is currently being proposed to be widened, but in that case to move more water, not to store water.)

Although many people find it hard to accept, our city streets absolutely have to play an important part of moving and storing water during heavy rainfall events. Because streets take up a large percentage of the land area in urban areas, the street becomes the primary relief system for the underground storm drain pipes, which can only be economically designed to carry the rainfall from a relatively small rainfall event. This protects the valuable property (houses and businesses) at the expense of the street being inaccessible for a few hours at a time. It requires that the street be at least a couple of feet below the elevation of the properties and in older neighborhoods it may mean having to reconstruct the street at a lower elevation to provide this protection. And although it is not appropriate to store rainwater in the streets for long periods of time, they can act as a good buffer to prevent the overloading of the nearby bayou and potentially damaging flooding downstream.

Many of our older neighborhoods have roadside swales or ditches that store water effectively during rainfall events. These swales and ditches could be improved by a gentle widening wherever possible. The current maintenance practice of cutting deep trenches by the side of the road needs to be changed to a practice of widening the swale to have gentle slopes to store more water, to allow the sides of the swale to be maintained with good vegetative cover and to create a safer street condition. Gentle street swales may require a drainage easement along the front line of adjacent properties and revisions to the current standards for driveway culverts. Well-vegetated street-side swales can also filter rainwater runoff and result in cleaner water in our streams and bayous.

Underground conduits for storage

In urban areas of the city where land is scarce and expensive, it may be appropriate to store rainwater underground. And while it might be appropriate to move and store water temporarily in neighborhood streets, it is, for public safety reasons, important that collector streets be able to function during major storm events. In these instances the high cost of underground storage is justified to store the water that would otherwise be on the surface obstructing emergency and other essential traffic flow.

Public institutions and large commercial sites may also elect to store their run-off underground if the cost of their land justifies the high cost of underground storage. They might store the water under their roads in oversize drainage pipes or they might construct vaults under their parking lots. Because stormwater storage is now being required in many parts of the country, there are many techniques and systems being used and marketed to capture the run-off, store it and cleanse it before sending it into the city drainage system.

Building systems (architectural) storage:

In our region, most retail, warehouse and industrial buildings have flat roofs. Many of these buildings are huge, with rooftops that can measure in acres. Rooftops in many parts of the country are already being used for rainwater detention by simply being designed to temporarily store the rainwater that falls on them before releasing it to the drainage system. A roof designed to detain just six inches of water can store more than half of the water falling during a 100year storm event. In areas of town with lots of big retail outlet stores or in industrial warehouse districts, this could make a very big difference in the rate of stormwater leaving a site during a storm.

“Green” rooftops are a similar way to store limited amounts of stormwater on top of the building structure before sending it on its way. A green roof has a thin, very lightweight layer of soil (or a soil substitute) that allows special grasses or other plant materials to grow on the rooftop. Green roofs cut down significantly on the urban heat build up that large expanses of roof can create; green roofs insulate and cool the underlying building; green roofs increase the life of roofing materials by stabilizing the temperature of the roof and by protecting the roofing from the sun’s rays and, importantly for this discussion, green roofs detain, store and cleanse rainwater. Green roofs can also be applied to gently sloping roofs, where roof ponding is not possible.

Smaller structures, such as residential structures, can detain or store rainwater from their rooftops by collecting the water coming in the downspouts in cisterns. The cistern might be a plastic or concrete box underground next to the house. If only used to mitigate stormwater flows, the cistern would release or pump the water slowly out to the street collection system after the storm has passed. In many parts of the country people are practicing what is called “rainwater harvesting” to offset the high cost of municipal water, especially for non-potable uses such as watering the garden or washing the car. Since many new houses are being built as pier and beam there is room under the house for a gravity drain system from the rainwater collection cistern.

Non-structural approaches for storage:

Studies have shown that trees in the city can play an important part of helping to reduce peak stormwater flows off urban land during rainfall events. A full canopy of large trees will collect and detain a small amount of water in the foliage itself; the soil under a tree canopy will be higher in organic content and will absorb more rainwater than soils without tree cover; and the trees will transpire or consume a certain amount of water, opening the soil up to receive water from the next rainfall event. Studies are under way to evaluate and quantify the actual benefits that can be derived from urban forest cover so they can be compared on a cost/benefit basis to alternative structural peak flow reduction measures.

Although our clay soils generally do not lend themselves to high levels of percolation or absorption during large storm events,  it is possible to significantly enhance the permeability of the top one to two feet of soil, which is the active root zone for most of our Gulf Coast plants. If soil can be improved to raise the amount of water it will absorb from a small fraction of an inch to a couple of inches, that can accumulate to be a significant amount of stormwater runoff when spread out over large enough areas.

The ability of this shallow zone to absorb rainwater can be enhanced mechanically by adding sand and organic materials to the soil (which every local gardener knows about).  In addition, the permeability of this active root zone can be enhanced organically by biological means. Healthy soil has a myriad of small and microscopic organisms that change the structure of the soil by binding together clay particles and creating openings for water to move through the earth. Drastically reducing the amount of pesticides, herbicides and fertilizers, and carefully re-introducing the correct balance of mineral nutrients, will allow these natural soil builders to do their job.

Summary:

There is no one simple solution to properly manage an urban watershed to reduce flood induced damages to life and property.

The successful solution will include many different interlocking parts, each part appropriate to a different scale and to different conditions. Not all the parts need to be implemented simultaneously, nor do they need to be implemented immediately.

But in a fast growing urban area like the Houston metroplex, it is critical that we begin to understand our watersheds: what they are, how they behave, and how we can live comfortably and graciously within them with an ever increasing urban density.