The Role of Hydrology in Urban Drainage Management

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

Flood Control

Primary System: major waterways
Secondary System: stormsewers/small conveyances (e.g., regional detention, channel modifications, property acquisition, flood warning)

Water Quality: source controls, structural treatment controls
Streambank Erosion (quantification and management): source controls, structural restoration

Major Program Focus:

New Land Development:

Allowable development intensity (hydrologic and hydraulic models are used to determine allowable floodway encroachment)
Adequacy of drainage infrastructure (streets, stormsewers, water quality, detention basins)

Corrective Improvements in Older Urbanized Areas: retrofits for flood control, erosion, and water quality management

Austin Hydrologic Issues

Flood control: large, infrequent storm events: 10-12 inches/24 hours (25-100 yr storm)
Water quality and erosion management: small, frequent storm events: <2 inches/24 hours (90% annual total volume)
Application of simple hydrology for small areas (<100 acres) using the rational method
Application of the unit hydrograph and channel routing for larger watershed areas using HEC-1/HMS with SCS Unit Hydrograph

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:

direct observations
statistical analysis
computational methods for precipitation transforms and routing

Hydrology provides estimates of:

peak flow
timing (important for larger drainage areas)
total runoff hydrographs
statistical probability of events (used for risk/cost analysis in drainage structure design)

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:

Prevent creation of flood hazards

regulate land development
review plans
propose, enact, & enforce local ordinances and regulations

Mitigate existing flood hazards

structure buyouts
attempt to alter basin hydrology (detention)
attempt to alter conveyance (channel modifications, maintenance, bridge enlargements)

Provide flood insurance: administer National Flood Insurance Program
Provide flood warnings: model conditions based on synthetic storms and make on the fly judgement calls

road closures
evacuations
reduce contents loss

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
G_s = sediment load
d_i = 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:

Reinforce the channel with new material (e.g., concrete). But this tends to adversely affect both the aquatic habitat and aesthetic value of the stream.
Excavate the channel to form larger cross-sections, while stabilizing the slopes.

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