Upper Cahaba

watershed – vital to the health of the entire

Cahaba systemMany rivers have their big city at their mouth, but the Cahaba is particularly challenged because it has its big city

at its headwaters. The Cahaba starts in what are now the suburbs of Birmingham. It begins in Springville and Clay and flows through Trussville, Leeds, Irondale, Vestavia Hills, Hoover and Helena – almost a third of its

length -- before finally entering more rural lands in Bibb, Perry and Dallas counties and merging with the Alabama River just south of Selma. We, the people of the Birmingham region, thus have a very special responsibility to protect the Cahaba, because everything we do here in the headwaters affects the entire watershed. We are the

guardians of this special river.

I-459

I-59

I-20

Cahaba River

Lake Purdy

Drinking Water Intakes

Highway 280

Trussville

Leeds

Liberty Park

Irondale

Proposed Northern Beltline

Beltlineextensio

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Growth Challenges in our Drinking Water Source

The way we have been growing in the upper Cahaba watershed has a severe impact on the river. The problem is not with the growth itself, but with the way we have been growing.

Growth Challenges

Sediment

Hunter Nichols

Sediment is the primary pollutant in the Cahaba and many of its tributaries.

Here is the Cahaba on a clear day seen from the bridge at Grant’s Mill Road in Irondale. Notice the rocks visible in the clear water and the deer crossing the stream. The second picture shows this same view of the river after

a typical rain. It’s full of mud. Not a fun place to fish, not a good place to reproduce if you are a fish, and a terrible sight for Water Works Board officials charged with removing all that mud from our drinking water. Not

fun for ratepayers either whose utility fees must be spent cleaning the water and building infrastructure for new water sources.

Sediment from Land Clearing

This mud has two major sources–both related to growth and construction. The first is easy to understand: it’s run-off from poorly managed and maintained construction sites that allow mud to run into our waterways when it

rains.

This first source of mud is construction sites that look likea this.

And this.

Poorly maintained construction sites can fill a tributary with mud – this is not muddy water, it is the mud that has settled out and filled the stream. This will make its way into the river.

Forest cover

ShallowInfiltration

DeepInfiltration

SurfaceRunoff

The second source of sediment takes a little more explanation. When rain hits undeveloped forested land, most of it soaks into the ground where it recharges groundwater. Relatively little of that rain becomes surface

run-off.

After typical development

ShallowInfiltration

DeepInfiltration

SurfaceRunoff

Residential – 2-3x more runoff

Commercial & industrial – 5x more runoff

But after a typical development, most of the rain can no longer soak into the ground because of the hard, impervious surfaces that we build – roofs, driveways, parking lots, and roads. Most of the rain hitting the site now

becomes surface run-off. For typical residential developments, a site may generate 2-3 times more volume of run-off when it rains. A typical commercial or industrial development may generate 5 times more volume of run-

off.

Storm Water Impacts of Large

Commercial Development – 2005

Here is an example from our region of how stormwater run-off has been dealt with in the past. This culvert was designed to funnel rainwater from a large commercial development into a tributary of the Cahaba River. Rainwater from acres of asphalt and roofs, as well as run-off from above the development is channeled through this culvert and other outlets. The large boulders in front were intended to slow the water’s velocity as it enters a Cahaba tributary. In case you missed the size of this culvert, note the person standing next to it. It doesn’t take a lot of imagination to understand what the volumes of water coming through this culvert may do to the stream below.

Here is the stream below the culvert before the development was built. Note the flat rocks at water level on the

left side of the picture. The stream is only about 6 inches deeper than these rocks.

Here is that same stream two years after the development was built and the volume of storm runoff from large areas of paving and roofs increased. Now the flat rocks are 3+ feet up in the air and the stream is much wider.

The huge volumes of water pouring off the development site and through the culvert every time it rains have scoured out the stream bed. This in-stream erosion is the second source of sediment pollution in the Cahaba

and its tributaries, and a growing problem.

Now, there is a semi-permanent island of sediment where that tributary runs into the Cahaba River. It will take

many decades for that sediment pollution to make its way down the river. Multiply this impact by many developments on many small creeks throughout the watershed.

River channels widen and deepen when

flow increases

The cumulative impact of development on the tributaries and streams in the upper Cahaba watershed is also having a major impact on the volumes of water in the main stem of the river. When it rains in the Birmingham

area, enormous volumes of water pouring off impervious developments are shunted into tributaries of the Cahaba River. This huge increase in water volume causes the river to widen and deepen in order to accommodate the

flow. This picture shows one impact of the widening river: a sewer line that used to be many feet back from the edge of the river was broken as the river expanded, and dumped raw sewage into the Cahaba (a situation CRS

investigated and helped to get fixed).

These enormous volumes of water hitting the river’s tributaries every time it rains in the Birmingham area are causing bank collapses down the main stem of the Cahaba River. This bank collapse came within twelve feet of this building. These bank collapses further increase the sediment load in the river (and prove one reason why

buildings should be set back further from the river).

Growth Challenges

Flooding

Another effect of this conventional way of handling stormwater is increased severity of flooding. When rain cannot soak into the ground and instead is funneled into the river tributaries during the storm, the increased

volumes of water from many developments mean that flooding is more frequent and more severe.

Loss of water supply

Growth Challenges

The flip side of this increased flooding is a diminishment of our drinking water supply. Groundwater is what keeps a river flowing when it doesn’t rain. If we don’t allow rain to soak into the ground, we reduce our overall

amount of groundwater. As a consequence, when it doesn’t rain, our rivers and streams may dry up. One assumption underlying the Birmingham Water Works Board’s future water supply scenarios is that there will be less drinking water available from the Cahaba River because of the way we handle stormwater in developments in the upper watershed. Less groundwater means less drinking water supply from the river, which means that ratepayers will have to pay for development of other drinking water – this contributes to the need for the $358

million intakes recently proposed for the Black Warrior River. Conventional stormwater management doesn’t just negatively impact our drinking water supply – it also negatively impacts our wallets.