8:1 DESIGN CONSIDERATIONS
Hydrology is the science concerning precipitation and the resulting storm water runoff, interception, infiltration, and stream flows. These natural phenomena are modeled to predict the potential for flooding, as well as the impact of development and land use on our waterresources.
Hydraulics is that branch of engineering that calculates how fluid flows through channels, pipes, or over the landscape. Velocity, as well as depth and width of flow, can be calculated. Hydraulic design uses the design flow and volume determined by hydrologic calculations to design and evaluate proposed design alternatives. A design storm is chosen as the basis of a design.Consideration is given to the land use being enhanced or protected when picking a design storm. Economics, longevity, risk, environmental considerations (e.g., habitat protection or restoration), and aesthetics are also used to evaluate proposed alternatives. Secondary design flows (low and high) are also considered when evaluating proposed alternatives.
(1) Design StormEngineers refer to design storms based on the statistical probability of their occurrence. For example, the 100-year storm has a 1 in 100 or 1% probability of being equaled or exceeded each year. The total rainfall amount varies based on the frequency (probability) of the design storm.
A hydrograph is a plot of the rate of runoff that occurs in a drainage area overtime. There is a unique hydrograph for every rainfall event. Different storm water management techniques can alter the shape of a hydrograph in a drainage area for the same design storm. Detention reduces the peak rate of runoff while extending the time over which the runoff occurs.
(3) Time of Concentration
The periods of time that it takes storm water to reach the outlet of adrainage area is referred to as the time of concentration. Small subcatchments with impervious surfaces have a short time of concentration and will realize their highest flow rates during brief, intense thunderstorms. A large undeveloped watershed will realize peak runoff after a long continuous rain over many hours or days.
The ability of the soil in a watershed toinfiltrate storm water influences the volume of surface water runoff and the peak rate of flow. Soil that is already saturated from snowmelt or a previous rain will have less infiltration and a higher rate of runoff. Frozen ground will impede infiltration and result in runoff from normally permeable soil at a rate similar to impervious surfaces.
Another water loss thatreduces the rate and volume of runoff is interception. Trees, shrubs, and plants retain precipitation, resulting in evaporation or evapotransporation instead of surface water runoff. Small depressions in uneven ground store water, which then infiltrates or evaporates. These water losses reduce the peak rate and total volume of runoff from a drainage area.
(6) Land Use and Topography
Landuse has a large effect on the type of drainage system and the choice of the design storm. Land use affects the peak flow rate as well as the total volume of flow. The economic impact of flood damage to the land and improvements are considered. Generally speaking, protecting a farmland and increasing yield will have less total value per acre than protecting homes, commercial, or industrialproperties. The risk of the loss of life, disruption of transportation, or threat of pollution may support a larger expenditure per acre and a larger design storm. The topography of the area will also influence the choice of the design storm. Rolling terrain will usually result in more depression storage and localized flooding whereas flat regions such as the Saginaw Bay area would have extensive...