CE 394K.2 Hydrology 
The Role of Hydrology in Urban Drainage Management
Speakers from the City of Austin Watershed Protection Department

Overview:  George Oswald, Watershed Engineering Division Manager

The jurisdiction of the City of Austin Watershed Protection Department includes the city proper (~250 square miles), the extra-territorial jurisdiction (ETJ) covering an additional radius of 5 miles, and parts of Hays county. Together, this area covers approximately 500 square miles. Major sources of revenue are ~$20 million from landowner fees, and bonds such as the recent $25 million approved by voters for construction of the Waller Creek tunnel.

Service Missions

Major Program Focus: Austin Hydrologic Issues

Role of Hydrology in Flood Control Mission:  David Walker, Floodplain Manager

The Austin area is extremely flood-prone. The cause is a combination of terrain characteristics (Edwards aquifer uplift zone), and climate (low pressure zones from the Pacific sucking up moisture from the Gulf of Mexico). Therefore, hydrologic analysis is very important. The primary tools of hydrology are: Hydrology provides estimates of: These are the basic inputs for hydraulic analyses/floodplain delineations and design of hydraulic structures.  They are needed to analyze existing conditions and project the impacts of future/proposed conditions. Typical applications include the following:

Channel Instability and Stream Bank Erosion:  Mike Kelly, Erosion Control Engineer

High stream flows can cause a lot of damage, but the frequency is low. As such, the overall effect on stream shape is low. We're more interested in storms with sufficient frequency and magnitude to shape channels through sediment redeposition. Channel equilibrium is described by the following relationship:

,  where:

Q = flow
S = bed slope
Gs = sediment load
di = sediment size

In the typical scenario, Q increases due to a storm. The result is generally a combination of decreased bed slope, increased sediment load, and increased sediment. Some applications are as follows:

1. Land development. The law requires that the channel should withstand changes from increased flows resulting from land development. Hence, detention ponds are sometimes used as a tool to reduce flows.

2. Remediation. Channel geometry changes in response to changes in shear stresses:

  and   where:

t = shear stress
w = stream power
g = specific weight
R = hydraulic radius
S = bed slope

Channel degradation typically occurs by the following process. High flows and frequency cause an increase in water power, which in turn leads to higher sediment loads, which results in increased cross-sectional area (deeper channel). The channel can now convey more flow. But this comes at a cost of steep stream banks devoid of vegetation and decreased slope stability. There are two common solutions:

Return to Class Home Page