Urbanization has taken its toll on a Houston Treasure
Buffalo Bayou has eroded substantially in the vicinity of Memorial Park and the River Oaks Country Club. Historic aerial photography sets show clearly that acres of riparian forest have been lost, and riparian forest canopy that once shaded the channel has largely disappeared, creating a wider channel and vertical slopes. While erosion of the bayou’s channel and banks is a natural process, what is not natural is the volume of water being sent through the bayou, combined with the accelerated timeframe during which stormwater is delivered to it resulting in higher peak flows.
Causes and effects
The bayou channel evolved when the watershed consisted of natural prairie and forest at a time when the delivery of stormwater was much slower – with longer lag times between storms and peak flows in the channel – than what we see today.
However, with increased urbanization, the amount of water-shedding surfaces – such as buildings, streets, parking lots, and sidewalks – not only adjacent to the bayou but also well beyond the project reach, has produced more rapid, higher flows that exceed channel capacity. Additionally, two reservoirs totaling 26,000 acres, which were completed in the mid 1940s to protect Houston’s downtown district, have dramatically changed the natural characteristics of the drainage sent to Buffalo Bayou from the western portion of the watershed.
River science (“fluvial geomorphology”) recognizes that a stable river channel depends on the connection of a primary channel with an adequate floodplain (the low, vegetated area adjacent to the channel) to allow dissipation of erosive forces during high water events before stresses reach a critical point at which the failure of the bank occurs. In a well-functioning natural stream, the floodplain serves to dissipate erosive energy, slow the flow of water, trap sediment, and store part of the stormwater within the soil, from which it can be gradually returned to the channel over time.
The erosion continues
Buffalo Bayou, both upstream and within Memorial Park, has lost the connection between the channel and its adjacent floodplain, which is essential to a stable channel configuration. Urbanization has unleashed a process that river scientists call “downcutting” (deepening of the channel, often at the foot of a vulnerable bank) and “aggradation” (deposition of sediment in areas of slower flow). Even if there were to be no further development in the watershed, these processes would take a very long time (decades to centuries) to alter the banks and streambed until the channel forms a new, adequate floodplain at a lower elevation in the landscape. The effect – visible in the time series of aerial photographs beginning in 1930 and available on Google Earth and the Harris County Flood Control District website – is erosion that has already modified much of the channel and the riparian forest and other vegetation beside it, and continues to do so. Left to its own devices, the bayou would eventually find its stable form, but in doing so it would destroy homes and bridges and other important infrastructure.
Individual property owners and public agencies have been applying stopgap measures to stop the destruction of individual parcels of property, and the bayou is slowly being straight-jacketed with walls of concrete bags, rip-rap, steel sheet piling and timber embankments. These piecemeal projects often just shift the erosive energy of the stream to the opposite bank or to downstream properties. Without a better, more holistic alternative based on the science of natural channel design, these Band-Aid solutions will continue to harden and line the banks of the bayou.
Water Quality Degradation
Erosion continues to cause excessive sediment loads within the stream. Excessive sediment loads are not merely a cosmetic problem of muddy water. The clay particles prevalent in our soils are made up of microscopic plates that can shelter and harbor bacteria and each speck of dirt suspended in the current is, or can be a host to, undesirable bacteria that degrade water quality. This is a key factor contributing to the consistently poor water quality in Buffalo Bayou.Proposed Solution