31 Mar 95
a. Agricultural areas. Hydrologic engineering analyses
for agricultural areas generally involve a single subbasin
adjacent to the levee. Volume and duration of flooding are
usually more important than peak inflow to the line-of-
(8) Regulatory policies affecting development off and on
protection. Seasonal effects are often important due to crop
growing patterns and changing damage potential throughout the
year. A continuous record analysis is normally used in the
(9) Identification of environmentally and culturally
b. Urban areas. Urban area analyses are usually more
(10) Secondary water effects such as water quality,
complex than agricultural areas. Rainfall-runoff analysis may
sediment, debris, and ice, which may affect study procedures
include multiple subbasins. If natural or detention storage is
and analysis costs.
limited, peak flow may be as important as volume. Layout,
design, and operation of existing and potential future storm
b. Information sources. The following are common
sewer systems must be considered. Investigations involving
sources of information:
trade-offs between pumping capacity and nonstructural
measures, such as relocation to gain more ponding area, may be
Corps files of previous studies.
required. The feasibility of flood-warning-preparedness
components should be investigated.
(2) Local agencies such as drainage and levee districts,
planning commissions, public works departments.
Hydrologic engineering requires coordination early on with the
Federal agencies such as USGS, SCS, USBR, FEMA,
study manager and other study team members to clarify the type
of study, study objectives, and general scope of the requirements
and constraints. Known problems and issues that affect the
(4) State agencies such as Department of Water
detail, cost, and conduct of the study should be described.
Resources, Natural Resources or Conservation.
Communication with counterparts are established and
Railroads, highway departments.
information and insights about the study. The use of previous
Relationship Between Interior and Exterior
study data and information should be scrutinized and used to the
a. Information needed. The following information
A detailed description of the relationship between interior and
exterior stages is found in EM 1110-2-1413. The following
typically is needed to develop hydrologic engineering analyses.
paragraphs summarize that material.
Previous study data and reports.
a. Fluctuating water levels both exterior and interior to
the line-of-protection make interior area analysis unique. If the
(2) Maps, including USGS quadrangle sheets, topographic
exterior and interior occurrences display a consistent
maps, aerial photographs, ortho-photographs, zoning plans,
relationship with each other, then, to a certain degree, one can
storm sewer layouts, etc.
be predicted from the other. The interior and exterior events are
said to be correlated. If the physical and meteorologic
(3) Historic flood events information including storm
processes of the interior and exterior events are related to one
intensity and distribution patterns, high-water marks, frequency
another, they are said to be dependent. If the interior and
of overtopping, flow patterns, debris and sediment, and response
times and actions.
exterior events produce stages that coincide, e.g., the interior is
high when an exterior event occurs, they are said to be
coincidental. Coincidence can exist whether or not the interior
(4) Existing and potential future flood control facilities
including design capacities and operation procedures of gravity
and exterior occurrences are correlated or dependent.
outlets and pumping stations.
b. It is possible, though not likely, that there is complete
(5) Survey cross-sectional
noncoincidence in a study area, e.g., the interior and exterior
water levels will never be high or low at the same time. The