Rucker Pond. Background

Transcription

1 Rucker Pond Background The Rucker Basin consists of two subbasins (East and West) that drain to a single area known as Rucker Pond. Both subbasins have the same hydraulic parameters, but have different areal extents. The basin is usually dry, however when heavy rainfall occurs the basin floods due to blockage by a natural rock embankment at the end of the pond. The top of the embankment sits roughly 1.5 feet above the ground elevation, and behaves as a rectangular weir when flood levels exceed the height of the embankment. The Simon Construction Company has plans to develop a new gated community at Rucker Basin. Before beginning construction, the company needs to verify the necessary slab height for each house. In order to keep the residences from flooding, the slab height must be six inches above the 500- year flood level of Rucker Pond. Using HEC- HMS, develop a model that accurately depicts the basin, the embankment, and the flood level of Rucker Pond for a 500- year rainfall by using two subbasins connected downstream to one reservoir. Be sure to use US customary units. Subbasin Parameters The West Subbasin has an area of.0075 square miles. The East Subbasin has an area of.005 square miles. The rest of the basin parameters are the same for both subbasins. Both subbasins connect downstream to Rucker Pond and do not connect to one another. Both use a Simple Canopy method, no surface method, Green and Ampt loss method, Clark Unit Hydrograph transform method, and no baseflow method. The canopy has an initial storage of 0% and a max storage

2 of 0.8 inches. The loss has an initial content of 0.03, a saturated content of 0.46, suction of 10.3 inches, conductivity of 0.4 inches, and is 58% impervious. Both subbasins also have a hour time of concentration and a hour storage coefficient. Paired Data The paired data manager is needed to create two functions (curves): an elevation- storage function and a storage- discharge function. The elevation- storage curve represents the volume of water stored in the pond (reservoir) when the water level is at a certain elevation, while the storage- discharge function represents the relationship between the volume of water in the reservoir and the discharge rate. With the two curves, the equation for flow over a weir, and the elevation of the embankment, one can accurately model the natural weir at the end of the pond. The following figure contains the data points for the elevation- storage function. After the values for the elevation- storage function are determined, one can create a set of values for the storage- discharge function based on the corresponding elevations and the equation for a rectangular weir flow derived from the Bernoulli s equation:! = 2 3!!! 2!!! h!! Where,! =!"#$!!!!, h = h!"#!"!h!!"#$ (!"),

3 ! =!"#$h!"!h!!"#$!",!! =!"#$h!"#$!"#$%&#%!"#!h!!"#$, and! =!"#$%&' 32!"!! In this case, the discharge constant for the weir is 0.62 and the width of the weir is 30.0 ft. Ground elevation is 55.0 feet, and as stated earlier, the embankment rises 1.5 feet above the ground elevation. Therefore, it is safe to assume that there is no discharge from the pond when the water elevation is 56.5 feet or below. Using the above equation and the elevations that correspond to the storage capacity, one is able to develop the following relationship.

4 Rainfall Data In order to determine the 500- year flood level, one must first input the rainfall data for a 500- year storm. The level of rainfall in inches that produces a 500- year event varies with the duration of the event. The model storm to be used in this study occurred in December 11, 2011 from 8:00 AM to 3:00 PM with measurable rainfall from 9:00 AM to 2:00 PM. This information should be input in the Control Specifications module and the time series data manager. The following data should be entered as a precipitation gage in the time- series data manager and would result in a specified hyetograph that should be selected in the meteorological model. Results At what elevation should the new property be constructed in order to keep the houses from flooding? Keep in mind that the slabs must be built 6 inches above the 500- year peak flood level of the pond.

5 Further results should include the following: Further Investigations The Simon Construction company found that building foundation slabs, driveways, and sidewalks above the flood level costs a large amount per inch of slab height. In order to cut down on costs, the company has decided to install a concrete culvert in the natural rock embankment to increase outflow from the ponding area. If the bottom of the culvert is at ground elevation, how large must the culvert be in order to bring the flood level below the height of the embankment? After extensive surveying and price negotiation, the Simon Construction company has found that the optimum slab height is 1 foot above ground elevation. With the houses being built at 56.5 feet

6 msl, how large would the culvert have to be in order to bring the maximum 500- year flood level down to an acceptable level for the community? Is this sized culvert feasible to install in the embankment?