Applying Classical Geomorphology in Disturbed Urban Basins: A Design Approach

摘要

This paper describes an approach to urban stream intervention designs in loess or cherty Ozark Plateau soils. The approach incorporates the large body of accepted knowledge, procedures and methodologies already available to design professionals. There is a misconception among some engineers that fluvial geomorphology is neither applicable nor useful in highly disturbed urban streams. In these situations, the streams are used for stormwater conveyance alone and managed accordingly. Decades of development under this charge have left urban streams severely altered. Budget constraints and community demands have lead municipalities to approach stream interventions as an endless cycle of independent site-specific repairs. In many cases, the municipalities have no patience for anything that smacks of a study; they want the problem "solved". The substantial amount of data necessary for a fluvial geomorphic analysis seems incompatible with the urgent demands of these cities. While it may seem that there is no role for the philosophies and values of natural channel design in this situation, the contrary is true. Not only is it possible to apply fluvial geomorphology to urban stream interventions, it is the only way to avoid perpetuating the problem. Most of the classical analysis methods are relevant even in urban conditions. The peculiar features of applying classical geomorphology in these disturbed urban basins are: 1) The rapidly changing watershed makes assessment of dynamic equilibrium challenging. In most cases, the temporal scale has been severely compressed. 2) Infrastructure such as culverts and utility crossings cause the stream to "start over" downstream of each crossing, functionally creating a series of mini-watersheds. 3) Wild card variables abound. Dumped rip rap, yard waste, failed grout, failing gabions, washing machines, shopping carts and other anthropogenic insults causes the stream to continuously re-adjust to new conditions. 4) Stream classification is problematic under these conditions. The stream may enter a new classification after every infrastructure imposition and urban insult. 5) Reference reaches are rare at best. It is difficult to match morphology, sediment characteristics and hydrology for any meaningful length. So what to do? Rely heavily on lots of data. Classical analysis methods apply, but lengths of similar and somewhat predictable geomorphology through the generation, transport and depositional reaches are replaced with dominant fluvial processes changing dramatically and frequently from one urban interference to the next. It is common to find processes that are unexpected based on their location within the watershed. The design team must identify these nested processes for each reach as they move through the basin. When the data has been gathered and reduced, the design team must consider both the reach function based on its location and role in the overall basin as a well as the sometimes conflicting processes nested within the reach.