Objectives

Urban flooding studies are typically undertaken to analyze flooding problems in developed watersheds. Characteristics of these watersheds include:

  • Engineered drainage systems throughout.
  • Relatively short response times.
  • Localized flood damage of properties adjacent to drainage channels.
  • The objectives of urban flooding studies are to:
    • Characterize existing flood impacts.
    • Predict impact of future development.
    • Identify solutions to current and future flooding, including controls on land use.

Authority and Procedural Guidance

USACE activities in urban flooding studies are authorized by:

  • The Flood Control Act of 1936. This is the general authority under which the Corps is involved in control of floods (and associated damage reduction) on navigable waters or their tributaries. The 1936 Act and the Water Resources Development Act (WRDA) of 1986 stipulate details of Federal participation, including the requirement for benefits that exceed project costs.
  • Section 206 of the Flood Control Act of 1960. This authorizes the Corps to provide information, technical planning assistance, and guidance in describing flood hazards and in planning for wise use of floodplains.
  • Executive Order 11988. This directed USAVE to take action to reduce the hazards and risk associated with floods.
  • Section 73 of Public Law 93-251. This endorses USACE's consideration, selection, and implementation of nonstructural flood damage reduction measures.

The following USACE guidance on urban flooding studies includes:

  • ER 1105-2-100 (USACE, 2000) provides guidance and describes procedures for all civil works planning studies.
  • ER 1165-2-21 (USACE, 1980) defines USACE involvement in urban flood studies. A Federal interest exists for the portion of the watershed where the channel flow exceeds 800 cfs for the ten percent chance flood (0.10 AEP). However, if this criterion is not met, a Federal interest can exist for the portion of the watershed where the 1 percent chance flood exceeds 1,800 cfs.
  • EM 1110-2-1413 (USACE, 1987) describes general considerations when evaluating interior areas, commonly found in urban watersheds protected by levees from large bodies of water.
  • EM 1110-2-1417 (USACE, 1994a) describes methods, procedures, and general guidance for hydrologic analysis including rainfall, snowmelt, infiltration, transformation, baseflow, and stream routing.
  • EP 1110-2-9 (USACE, 1994b) describes the components needed to develop the HEMP for the different phases of a study.

Study Procedures

To meet the objectives of an urban flood study, typically peak flow, total runoff volume, hydrograph timing, peak stage, and floodplain delineations are required. These values are calculated for current development and future development conditions. In general, the procedure to develop a watershed model and calculate these values includes steps such as:

  1. Select appropriate methods to represent watershed.
  2. Collect watershed data and characteristics.
  3. Utilize regional studies and equations to estimate parameter values.
  4. Calibrate the model if historical data are available.
  5. Exercise the model with various precipitation events, using either historical or hypothetical frequency based events as needed.
  6. Analyze results to determine required values such as the peak flow or total runoff volume.
  7. Modify the watershed model to reflect changes in the watershed.
  8. Re-exercise the model with the same precipitation events.
  9. Compare the results to quantify the impact of the watershed changes.
  10. The development and modification of a watershed model to analyze the impacts of development is described herein.