An Analysis Between Two Catchments - Assignment Example

INTRODUCTION Water is vital in order to sustain life and based on this fact, it is important to acquire the knowledge in ways of its proper use and conservation. This is where hydrology comes in. Since hydrology is basically the science of water, its properties and laws, it is a handy tool in addressing the hydrologic cycle of the processes in nature. Such processes include surface runoff and infiltration, rainfall and snowfall, surface water evaporation, surface water exchange (between streams and lakes, etc), and groundwater storage. In this study, I have chosen to analyze, compare and contrast the recorded flow data from two catchments. The first catchment is Thrushel at Tinhay found in west Cornwall. In operation since 1969, shales and sandstones of Carboniferous Culm Measures predominate the catchment and is affected by the Roadford Reservoir (mainly for storage, hydro-electric power and pumped water transfers). The other is Wellington at North Fareham, situated in the southeast of Southampton. The catchment is predominantly mainly of chalk with clays (in Reading Beds) and sands (Bracklesham Beds). It has been in operation since 1951 and has been changed by the careful but increasing urbanization of the area in the headwaters. After analyzing the gathered data, we will be able to develop an understanding of the water resources of the catchments, evaluate flood risks, and to provide further information regarding construction of impoundments. METHODOLOGY Most of the data obtained for this analysis are described in the National River Flow Archives UK. To compare and contrast the two given Catchment Stations, the analysis uses the frequency flow histograms of both catchments. Evaluate the bar charts of the monthly flow characteristics and plot the flow duration curves of the data from both catchments. Using the Weibull and Gringorten formulae, and the Gumbel method to plot the data, we contrast the recorded flow data of both catchment stations. RESULTS Table 1: RANKED ANNUAL PEAK STREAMFLOW OF THRUSHEL AT TINHAY STAION WITH ESTIMATED QUANTILES, EXCEEDENCE PROBABILITIES AND RECURRENCE INTERVALS (1993-2003) YEAR FLOW, m3/s RANK, i ESTIMATED QUANTILE, q = i/ (N+1) EXCEEDENCE PROBABILITIES, 1 - q RECURENCE INTERVAL, 1 / (1 - q) 1994-95 31.29 1 0.091 0.909 1.1 2001-02 32.37 2 0.182 0.818 1.2 1996-97 33.43 3 0.273 0.727 1.4 1995-96 34.59 4 0.364 0.636 1.6 2002-03 35.11 5 0.455 0.545 1.8 1997-98 35.42 6 0.545 0.455 2.2 1993-94 41.28 7 0.636 0.364 2.8 1998-99 62.40 8 0.727 0.273 3.7 1999-00 65.35 9 0.818 0.182 5.5 2000-01 80.58 10 0.909 0.091 11.0 Table 1: RANKED ANNUAL PEAK STREAMFLOW OF WALLINGTON AT NORTH FAREHAM STATION WITH ESTIMATED QUANTILES, EXCEEDENCE PROBABILITIES AND RECURRENCE INTERVALS (1993-2003) YEAR FLOW, m3/s RANK, i ESTIMATED QUANTILE, q = i/ (N+1) EXCEEDENCE PROBABILITIES, 1 - q RECURENCE INTERVAL, 1 / (1 - q) 1996-97 5.77 1 0.091 0.909 1.1 1997-98 10.40 2 0.182 0.818 1.2 2001-02 18.06 3 0.273 0.727 1.4 1995-96 19.03 4 0.364 0.636 1.6 1994-95 20.51 5 0.455 0.545 1.8 2002-03 20.51 6 0.545 0.455 2.2 1998-99 22.31 7 0.636 0.364 2.8 1999-00 24.68 8 0.727 0.273 3.7 2000-01 32.17 9 0.818 0.182 5.5 1993-94 34.56 10 0.909 0.091 11.0 Figure 3: COMPARISON BETWEEN THE FLOWS OF BOTH CATCHMENTS (1993-2003) Figure 4: FLOOD-EXCEEDING PROBABILITY CURVE Figure 5: FLOOD-RECURRENCE INTERVAL CURVE DISCUSSION To estimate the quantiles (probability distribution), first rank each of the streamflows from lowest to highest using the "sort" command in Microsoft Excel; i indicates the rank. Then use the Weibull plotting-position formula to estimate the quantiles (q): q = i / (N + 1), where, i is the rank and N is the total number of observations (# of years in the record). From Tables 1 and 2 we can see that streamflow value of 80.57 and 34.56 cubic meters per second (cms or m3 /s) has a quantile of 0.909, or in other words, 90.9% of the time, streamflows are less than 80.57 and 34.56 m3 /s. Alternatively, we can think of the estimated quantiles in terms of exceedence probabilities. For example, looking again at Table 1 we see that 81.8% of the time streamflows exceed values of 10.2 cfs. Remember that these quantiles provide estimates only for values between i and N and are not very reliable near either limiting value. Nevertheless, this method will suffice for your analysis, but be aware of the limitations. The definition of a 100-year flood is the annual peak discharge with an exceedence probability of 0.01, or 1%. A 10-yearflood has an exceedence probability of 0.1, or 10%. From Table 1and 2 we see that in 2001 and 1993 the daily peak discharge was 80.57 and 34.56 m3 /s with an exceedence probability of 0.091, or 9.1%, which is a value very close to the 10-year flood definition. In fact, we see that the recurrence interval for the annual peak discharge of 80.57 and 34.56 m3 /s is 11 years. In other words, our statistical model (Weibull plotting-position formula) suggests that this event has a return period, T (or recurrence interval), of 11 years. The return period can be estimated using the following relationship: T = 1 / 1 - q To add to the contrasting elements would be their physical characteristics. The Tinhay station, of the CC type, has a three-bay compound Crump profile weir with crests of 3.66m and 10.97m total length with a catchment area totaling to 112.7 km2 and a POT threshold of 27.37m3/s. Its flood banks could contain flow up to 0.96m so there is no problem with floods. With sandstone and shale, it was a natural catchment until the Roadford Reservoir began to fill and affected flow post in 1988. Its upper end needs confirmation with more gaugings but rating development is considered acceptable. It has an indicative suitability for pooling but rating is not confirmed for the highest flows (since flow is contained in the bank, the rating would still be probably acceptable). On the other hand, the North Fareham station, of the FV type, has a flat-V weir which was installed in the year 1991 with a 250m upstream of the M27 river crossing with a total catchment area of 112km2 and a POT threshold of 9.44 m3/s. Current meter measurement is difficult in flood flows due to the bridge across the weir and it also aggravates problems in flooding. It also has a series of interconnected pools with a considerable bank erosion and debris in the channel. There is a high confidence in rating to the 1.24m stage but has a lower confidence because of bypassing. CONCLUSION There are data used are subjected to limitations due to certain factors. The Weibull plotting-position formula to estimate the quantiles are only estimates of the value, thus not very reliable near either limiting value. However, the estimates used in the study are the best available estimate of flows of the present time. In the North Fareham station, there was a loss of flow records in November 1953 due to vandalism and digital data only started in August 1981, in contrast to Tinhay station which started operation already with digital data. Also, the bypassing beyond the structure limit of North Fareham via the upstream eastern bank leads to uncertainty in flows greater than 9.8 m3/s. It could be said through this study there are factors that cause flood risks in catchments beyond it s physical structure. Such might be the intervention of bridges or the present predomination of such sandstones, sand, clay or shale. Finally, this study has shown that attention must be given to the catchments since they have proved that they have a variety of uses; not only provide water but also for hydro-electric power, pumped water transfers, water storage and also preventing flood. REFERENCES Napa County Resource Conservation District. (2006). Hydrology. Retrieved July 22, 2006 from http://www.naparcd.org/hydrotext.htm#hydrointro _________. (2005). Hydrology. Retrieved July 22, 2006 from http://www.everythingbio.com/glos/definition.phpID=1404 Taylor, R. (2006, June 27). Recurrence interval. Retrieved July 22, 2006 from http://www.geog.ucl.ac.uk/rtaylor/data_disk/1002/1002_flood_time_files/fram e.htm#slide0022.htm Croarkin, C. & Tobias, P. (2006, July 18). Engineering statistics handbook. Retrieved July 22, 2006 from http://www.itl.nist.gov/div898/handbook/eda/section3/eda3668.htm Centre For Ecology and Hydrology. (2006). National river flow archive. Retrieved July 22, 2006 from www.nwl.ac.uk/ih/nrfa/index.htm The Environment Agency. (2006). HiFlows-UK. Retrieved July 22, 2006 from www.environment -agency.gov.uk/hiflowsuk Read More