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 USACE 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 1105-2-103 (USACE, 2023) provides the policy for conducting civil works planning studies; Chapter 4 addresses flood risk management. It superseded Chapters 1 through 3 of ER 1105-2-100 (USACE, 2000), the Planning Guidance Notebook.
  • 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), under conditions expected during the period of analysis. Drainage areas that lie entirely within the urban area and are smaller than 1.5 square miles are assumed not to meet this criterion. An exception can be granted where the one percent chance flood (0.01 AEP) exceeds 1,800 cfs and the 0.10 AEP flow is below 800 cfs because of a hydrologic disparity, such as highly pervious soils, natural storage, or detention.
  • EM 1110-2-1413 (USACE, 2018) describes the hydrologic analysis of interior areas, which are common in urban watersheds protected by levees from large bodies of water. It covers existing and future conditions and the performance of interior drainage facilities such as ponds, gravity outlets, and pumps.
  • EM 1110-2-1417 (USACE, 1994) describes methods, procedures, and general guidance for hydrologic analysis including rainfall, snowmelt, infiltration, transformation, baseflow, and stream routing.
  • Appendix C of EM 1110-2-1619 (USACE, 2025), which superseded EP 1110-2-9 (USACE, 1994), describes the components needed to develop the hydrologic engineering management plan (HEMP) for the different phases of a study.
  • Paragraph 6-5 of EM 1110-2-1619 (USACE, 2025) addresses interior drainage facility performance. Where an area behind a levee can flood from both interior runoff and the exterior channel, the two hazards must be combined in a single expected annual damage (EAD) computation. Computing EAD separately for each source and adding the results double-counts some damage.
  • ECB 2026-1 (USACE, 2026) requires an assessment of long-term hydrometeorological conditions for studies that rely on flow-frequency or stage-frequency relationships. It lists urbanization, through increased impervious surfaces and stormwater and interior drainage infrastructure, among the drivers of changing runoff response. The bulletin expires 9 January 2028.

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.

The case study in this chapter illustrates developing and modifying a watershed model to analyze the impacts of development.

Current HEC-HMS Capabilities

Several HEC-HMS features support these steps. The GIS tools use terrain data to delineate subbasins and reaches and to estimate physical parameters such as flow length and slope (Steps 2 and 3). Percent impervious area is an input to the loss method of each subbasin, so a with-development condition can be represented by copying the basin model and changing only the parameters that development affects (Step 7). Optimization trials estimate parameters from observed events (Step 4). The Uncertainty Analysis compute type samples parameter values to show how their uncertainty affects computed flows, and the Frequency Analysis compute type assembles a flow-frequency curve from a set of frequency events (Steps 5, 6, and 9). Reservoir elements can represent detention facilities proposed to offset increased runoff.