Small Basin Runoff
H-Flume for peak flow measurement
State of Virginia has excellent potential for economic growth and development.
However, with economic growth, there is the responsibility of planning for construction of new or modifying the current
statewide transportation infrastructure. Extensive hydraulic analysis and design
are needed to reduce the impact of highways and bridges on the drainage basins
which they cross. With any modification to existing basin drainage, there is
potential for stormwater runoff to create flood and water-quality problems. Many
government agencies are trying to mitigate the increased runoff and diminished
water-quality associated with economic development through better drainage
structure design. Detention structures and channel improvements have often
proved to be good solutions for managing runoff volume and improving water
On average, Virginia's highways contain one culvert or flow structure for every
half mile of road constructed. A large percentage of these structures drain
small basins with areas less than 200 acres.
The Virginia Department of Transportation (VDOT) follows several well documented, standard engineering methods to
estimate runoff volumes and peak flows from small drainage basins. However,
significantly different results are calculated for the same basin when using the
separate methods for estimating flood hydrographs.
objectives of the study are to evaluate existing methods used by VDOT to
estimate flood hydrographs from small drainage basins, and evaluate the use of
dimensionless hydrographs to estimate runoff volumes.
Relevance and Benefits
collection will enhance the knowledge of runoff from precipitation events in
small drainage basins. Current
methods are inadequate for accurately determining runoff volumes and peak flows
from small drainage basins. Because of the inaccuracies of the available methods
to estimate runoff volumes and peak flows from small drainage basins, drainage
structures are usually over designed. A better understanding of the runoff
process and more accurate flow estimates will contribute to the protection of
life and property.
will be selected that are less than 200 acres in size and which have previous
hydraulic design work by VDOT. The basins will be instrumented for precipitation
and stage and rated for discharge. Weirs or flumes will be installed to assist
in discharge determinations as appropriate. Rainfall data and stage data will be
collected to obtain 6-8 single-peak runoff events per site (estimated 1.5 to 2
years per site). Runoff volumes and peak flows calculated in the initial
hydraulic design by VDOT will be compared to single-peak rainfall-runoff data
collected at each site. If at any time during the project, it is determined that
the proposed analysis will not supply an adequate solution to the problem, two
alternate methods should be addressed: the rainfall-runoff model, and regression equations.