I. Theoretical Background
Flooding in urban areas is an unavoidable problem in many cities, and it causes huge costs to the society in structural and non-structural damage. The impact of urban flooding is often very difficult to evaluation precisely. The damages in urban flooding may be divided into three groups; direct damage, indirect damage, and social consequences.
A highly developed urban place results to an increase of runoff volumes and flow rates, which may cause more frequent flooding. To abate the adverse downstream effects of urban stormwater runoff most communities use flow retardation structures. Detention basins, retention basins, infiltration basins, infiltration basins, and roof top storage are some examples of flow retardation structures. A detention basin provides protection in floodplains by containing floodwater for a brief period of time. It can be constructed by damming a channel or by excavating a pond into the existing ground. The excavation of a detention basin is constructed by a combination of cut and fill.
A detention basin is a non-natural flow control structure that is used to hold flood water for a limited period of time. Detention basins are more widely used for stormwater management than any other type of control. However, at present, its primary application has been for drainage control, i.e. peak-flow attenuation, rather than the water quality control. Detention basins designed for peak flow attenuation can be given an effective water quality control function at little added cost. (The Urban Water Resources Research Council of the American Society of Civil Engineers and the Water Environment Federation)
In designing a multipurpose detention pond there are certain guidelines that must be followed to ensure that the pond is properly maintained and do not become hazardous. 1.) Side slopes should not be greater than 3m horizontal to 1m vertical. 2.) An underdrain system should be constructed to minimize the wetness of the pond bottom. 3.) An alternative to the underdrain is the sloping of the pond bottom with at least 2% grade form inlet and outlet. Another means of elimination the wetness is to slope the pond to a gutter that flows through the pond. 4.) the pond bottom and the side slope should be finished with at least four inches of topsoil and seeded. This seed or sod should be capable of withstanding periodic flooding conditions. 5.) Outlets should be provided with rip-rap protection to prohibit erosion if soil is weak. 6.) An overflow spillway or weir should be placed at the high-water elevations and a minimum of 8m (3in) of freeboard should be provided. (NIPC 1986)
According to Debo (1982), poorly sited detention ponds may cause increased flood risked due to concentrated discharge. It fails in flood control for smaller that design of the storm. A detention pond can reduce flooding if the location is properly selected A proposed detention pond must be evaluated on a site-to-site basis.
Pre-development conditions are the most accurate estimated assumptions that a certain area hasn’t undergone any adjustments since before settlement. Under this scenario assumptions for run-off should match the pre-construction conditions with the post construction conditions. Improvement of the run-off conditions can be done if the run-off management within the area is poor.
Rational Method is an approach in designing for water shed or any small structures limited to maximum drainage of 200 acres. The rational method is applied by using a simple formula that relates runoff coefficient of a certain water shed, the rainfall intensity for a period of time and the watershed drainage area. The formula is
Q = CiA
Q = design discharge (L3/T),
C = runoff coefficient (dimensionless),
i = design rainfall intensity (L/T), and
A = watershed drainage area (L2).
TR-55 Tabular Hydrograph Method is another approach that can be used in the study. As summarized by Akan (1993), the tabular method develops multiple partial flood hydrographs at any point of the watershed. This is done by sectioning the watershed into homogeneous subareas. This method is only limited to the SCS 24-hr type I, IA, II, and III rainfall distributions. For this method the size of the drainage area, the time of concentration, the runoff curve number, and the travel time from the subarea outlet to the watershed outlet of each subarea should be known. The flow contributions of each sub area is calculated by the formula:
q = qtAmR
q = discharge coming from a sub-basin at time t (cfs),
qt = tabular unit discharge at time t (cfs/mi2/in.),
Am = drainage area of subarea (mi2), and
Am = drainage area of subarea (mi2), and
According to Akan (1993), the characteristics of a detention basin can be depicted by as a stage-storage and stage-discharge (outflow) relationships. For a regular-shaped basin, the stage storage relationship can be solved geometrically. For trapezoidal detention basins that have a rectangular base of W by L and a side slope of z, the relationship between the volume (or storage) S and the flow depth d is: S = L W d + (L + W)z d2 + (4/3)z2 d3.
The detention basin outflow is dictated by the structures’ outlet type and size. The stage-discharge relationship can be obtained by the governing hydraulic equation of the structure. There are 3 common categories for the outlets for the detention basin: orifice-type, weir-type, and riser-pipe type.
According to Iowa Stormwater Management Manual an orifice by the definition is a circular or rectangular opening of a prescribed shape and size. The discharge of an orifice is geometrically controlled by the opening and the depth of submergence and is determined by the formula:
Q = ko ao (2gh)0.5
Q = the orifice flow discharge (cfs)
ko = dimensionless coefficient of discharge
ao = cross-sectional area of orifice or pipe (ft2)
g = acceleration due to gravity (32.2 ft/sec2)
h = effective head on the orifice, from the center of orifice to the water surface
According to Brater and King (1976) a weir by definition is a hydraulic control structure that is commonly used in detention basin. The structure’s discharge is calculated by the general formula:
Q = Cw L (2gh)3/2
Cw = dimensionless weir discharge coefficient
L = effective weir length, ft
h = water depth above the crest
According to Iowa Stormwater Management Manual a perforated riser is a special kind of orifice flow. The riser used is a vertical pipe punctured with evenly spaced holes. According to McEnroe (1988), the discharge of a perforated riser without orifice plate at the bottom is formulated as:
Q = Cs (2As/3hs) (2gh)3/2
Cs = dimensionless discharge coefficient of the side holes
As = total area of the side holes, ft2
hs = length of the perforated segment of the riser pipe, ft