Common use of Runoff Clause in Contracts

Runoff. Runoff enters the Barataria Bay estuarine system through a complex series of coastal swamps and wetlands, mostly from local precipitation. On a long-term (1961-1990) annual basis, precipitation over coastal Louisiana exceeds evaporation, thus, resulting in a net runoff into the Barataria Basin. Annual mean precipitation for the 30 years was reported as 160 cm (▇▇▇▇▇▇▇, 1987). In 1999, however, total precipitation was recorded as 114 cm due to the prolonged impact of the 1997-1998 El-Nino Southern Oscillation event, while the total amount of evaporation estimated using the GDIL1 data was about 124 cm. Hence, net freshwater input from the hydrological cycle must dictate the salinity distribution within the bay. Estuarine salinity decreases during periods of high runoff as the freshwater-saltwater interface moves down the estuary toward the sea, and it reverses when runoff decreases. There are several previous studies of runoff from land in Barataria Basin. Light et al. (1973) developed a hydrologic model to analyze freshwater flow in the Barataria area using the watershed management unit method. This model used precipitation, evapotranspiration, and physiographic data to calculate annual discharge from Bayous Chevreuil, Boeuf, and des Allemands. The investigators also developed a mean annual precipitation map based on a long- term record (1945-1970), and found mean annual rainfall excess values of more than 50.8 cm (20 inches) in the upper-basin watershed. Similarly, ▇▇▇▇▇▇▇▇ et al. (1973) modeled runoff from land and freshwater inputs to water bodies using the cell method and assuming water losses from the water surface, both open and vegetation-covered. They computed the mean geographical distribution of freshwater flow over the basin. Wax et al. (1978) produced a water budget based on climatic conditions to estimate periods of freshwater surplus and deficit for the Barataria Basin system. ▇▇▇▇▇▇ (1975) studied the characteristics of freshwater discharge and the drainage area near Lac des Allemands. He indicated that the freshwater inflow into Lac des Allemands was 42~54 m3 sec −1 under average flows and ~80 m3 sec −1 under peak flow conditions. ▇▇▇▇▇▇▇ and ▇▇▇▇▇▇▇ (1989) pro-rated this number to give a total runoff into the basin ~150 m3 sec −1 . ▇▇▇▇▇▇ (1975) estimated that the freshwater input to the Barataria Basin was 12 × 106 m3 per tidal cycle or 266 m3 sec −1 . ▇▇▇▇▇▇ (1982) estimated that the total precipitation over Barataria Basin was 21 × 106 m3 per tidal cycle. ▇▇▇▇▇ (1983) produced an annual water budget for the upper portions of the Barataria Basin system based upon data from 1914 through 1978 and estimated that 40 % of the precipitation was available for runoff. His results showed that most of the surplus of freshwater occurred in winter, with deficits of freshwater most likely to occur during the summer. He also noted that deficits should not be expected to occur regularly, because precipitation is usually greater than evaporation. Recently, ▇▇▇▇▇▇▇ and ▇▇▇▇▇▇ (1998) calculated a water budget using the 28-year average (1960~1988) of precipitation and found similar results to these of ▇▇▇▇▇. For this study, freshwater input by rainfall was estimated simply by multiplying total amount of rainfall by the total drainage area. The long-term average rainfall is known to be 160 cm , and the total drainage area is about 4,400 km2 . Therefore, the freshwater input into the basin is 4,400 km 2 × 1.6m year −1 = 1.15 × 106 m3 per tidal cycle or 223 m3 sec −1 , of which 25 percent flows to Lac des Allemands, which is comparable to some of the previous results. Runoff modeling depends on the records from single point rain gauges, some of which provide estimates of rainfall intensities in time steps of one hour or better, while others provide daily estimates. In large catchments, models using a daily time step may be adequate for applications. In small catchments such as the one under consideration here, a daily time step may be longer than the storm response time of the catchment and finer time resolution may be required (Beven, 2000). The water that contributes to streamflow may reach the stream channel by any of several paths from the point where it first reaches the ground as precipitation. The rainfall-produced runoff enters the system through a complex series of coastal swamps and wetlands, providing a mechanism for the slow release of fresh water over large wetland areas. The primary physical characteristics of the drainage basin are its area, shape, elevation, slope, orientation, soil type, drainage channel system, water storage capability and vegetal coverage (Raudkivi, 1979). Some water flows over the soil surface as surface runoff and reaches the stream soon after its occurrence as rainfall. Other water infiltrates through the soil surface and flows beneath the surface to the stream. The groundwater contribution to streamflow cannot fluctuate rapidly because of its very low flow velocity. Because the relation between precipitation and runoff is influenced by various storm and basin characteristics, usually many approximate formulas are used to relate rainfall and runoff. Since most of the land area for the Barataria Basin is less than 1.5 meters above mean sea level (▇▇▇▇▇▇▇▇ et al., 1973) and covered with wetland that is saturated by water, no groundwater flow is assumed. Only surface flow is considered significant and is incorporated into the model. In many environments, evapotranspiration is as significant a contribution to the water balance as stream discharge. Thus, for extended periods of runoff simulation, it will be necessary to estimate actual evapotranspiration losses from a catchment area. However, due to the relatively small drainage areas considered here, resulting in relatively quick runoff and short periods of simulation, transpiration was ignored for this study. As a result, the following simple model was adapted to relate rainfall to runoff for any drainage basin, Runoff ( m3 hour −1 ) = (Rainfall – Evaporation)(m hour −1 ) × Area ( m 2 ). (4) Delineation of runoff catchment areas is one of the real challenges in modeling estuarine hydrology in South Louisiana. The availability of discharge data is important for the model calibration process. Streamflow rates may be determined from stream stage data calibrated using measurements of velocity and cross-sectional area. ▇▇▇▇▇▇ (1975) estimated discharge rate, using this method, for two main streams, Bayou Chevreuil and Bayou Boeuf, draining into Lac des Allemands. Discharge data are, however, generally available at only a small number of sites in any region. Runoff modeling for sites where there are no available discharge data is a difficult task. Barataria Basin has numerous known or recognizable, as well as unknown, streams (Table 1-1) that vary considerably in size and length. It is impossible to install gauges to measure all their discharges. Even though most small streams are not immediately discernible on a map, one can assume that there should be runoff from all subaerial land. In order to estimate the discharge rate from unknown and ungauged streams, the basin was divided into twenty-two watershed management units using a pre-existing watershed chart (A Digital Map of the State). The area of each watershed management unit was estimated (Figure 1-8). Each area segment contains land and water surface. Since the hydrodynamic model that was previously developed for shallow water accounts for freshwater input due to direct rainfall on the water surface separately, runoff estimates should only account for precipitation over land. Therefore, the area of water surface was subtracted from each watershed unit area. Based on a minimum two square kilometers of land area, at least 1 stream was arbitrarily specified for unknown streams. Known streams were also associated with drainage areas (Figure 1-9). Most unknown drainages are smaller than nine square kilometers. The length of unknown streams was estimated by assuming that all drainages are semi-circular in 2 × area / pi shape, i. e., length= . 18 727 458 178 87 235 103 303 116 90 233 498 172 93 367 243 174 296 152 428 514 670 97 Figure 1-8. Twenty-two watershed management units of Barataria Basin. The numbers on the map are the area of each subbasin in km2 . Figure 1-9. Freshwater sources defined in the hydrology model of the Barataria Basin. most remote point in a subbasin or subwatershed to the outlet (▇▇▇▇▇▇, 1989). All drainages were categorized based on their size and estimated persistence time (Table 1-2). The longest drainage system, Bayou Chevreuil (26 km), was known to have a 3-day (72 hours) persistence time. Other streams’ persistence times were estimated by a linear extrapolation relative to the longest stream’s persistence time. Streamflow, at a given location on a watercourse, is represented by a hydrograph. The hydrograph produced in a stream is the result of various hydrologic processes that occur during and after any precipitation event. This continuous graph displays the properties of streamflow with respect to time, normally obtained by means of a continuous recorder that shows stage versus time, and then transformed into a discharge hydrograph by applying a rating curve. The shape of a hydrograph depends on precipitation pattern and characteristics and basin properties (▇▇▇▇▇▇▇▇ et al., 1989). A stream’s behavior is greatly affected by the characteristics of its watershed. For instance, the steadiness of the stream’s flow at a given point is controlled by the area of the watershed upstream of the point. Typically, during a rainfall event, the hydrograph of an undisturbed stream rises fairly rapidly, and after reaching a peak value, falls off rather gradually. Streams differ from one another in many features besides area, though. A complete analysis of the relation between rainfall and runoff, determining the characteristic shape of hydrographs for a basin, involves knowledge of the basin’s physical, vegetative, and climatic characteristics, all of which affect the quantity of streamflow generated in a drainage basin. These parameters are not well known for the Barataria Basin. Another, simpler methodology must be employed for estimating the relationship between rainfall and runoff for the various sub-basins discharging into the Barataria Bay estuary. The unit hydrograph, expected in response to a unit input of rainfall per unit area of ▇▇▇▇▇, may be considered to consist of three general parts: 1) the rising limb or concentration curve, 2) the crest segments, and 3) the recession curve or falling limb. Generally, the falling limb lasts longer than the rising limb, skewing the curve to the right. In order to estimate the total volume runoff entering the estuarine system, such a hydrograph should be multiplied by the amount of precipitation and the area of the sub-basin. Using an one-sided filter, one can relate rainfall per unit time per unit area, x , to the resultant discharge from the sub-basin, y, as follows, y(ti ) = a1x(ti−1 )+ a2 x(ti−2 )+ + a j x(ti− j )