FLOODPLAIN STORAGE AND LAND USE ANALYSIS AT THE WOODLANDS,TEXAS1

ABSTRACT: The Woodlands, a totally planned community north of Houston, Texas, was studied in detail in an effort to develop a master drainage plan which minimizes adverse impact on the 100-year floodplain. A total of 40 HEC-1 and HEC-2 computer runs were made to evaluate various development, channelization, and reservoir storage options.     

THE HYDROLOGIC ENGINEERING CENTER EXPERIENCE IN NONSTRUCTURAL PLANNING1

ABSTRACT: The Hydrologic Engineering Center, Corps of Engineers, has been engaged in research, training, and project assistance in non-structural flood control planning for Corps offices across the United States since 1975. Lessons learned from this experience deal with the role of nonstructural measures in flood plain management, the role of creativity in analysis, the role of analysis, and tools for analysis. The role of nonstructural measures in flood control planning depends upon the scale of the problem, the nature of the measure, the degree of protection desired, and whether damage is to existing or future property. An earnest seeking for nonstructural opportunities, a field presence for their formulation, and compatbility with local infrastructure plans are prerequisite to creative use of nonstructural measures. Analysis is a necessary complement of creativity. Several tools for nonstructural analysis have been developed and applied to flood problems involving several hundred and several thousand structures.     

USE OF HISTORIC FLOOD INFORMATION IN ESTIMATING FLOOD PEAKS ON UNGAGED WATERSHEDS1

Regional procedures to estimate flood magnitudes for ungaged watersheds typically ignore available site-specific historic flood information such as high water marks and the corresponding flow estimates, otherwise referred to as limited site-specific historic (LSSH) flood data. A procedure to construct flood frequency curves on the basis of LSSH flood observations is presented. Simple inverse variance weighting is employed to systematically combine flood estimates obtained from the LSSH data base with those from a regional procedure to obtain improved estimtes of flood peaks on the ungaged watershed. For the region studied, the variance weighted estimates of flow had a lower logarithmic standard error than either the regional or the LSSH flow estimates, when compared to the estimates determined by three standard distributions for gaged watersheds investigated in the development of the methodology. Use of the simple inverse variance weighting procedure is recommended when “reliable” estimates of LSSH floods for the ungaged site are available.     

A FLOOD DAMAGE MODEL FOR FLOOD PLAIN STUDIES1

ABSTRACT: Hydrologic and economic information must be integrated in flood plain management. This study describes an integrated approach which includes consideration of the hydrologic, hydrodynamic, physical, and economic components of the total system. On the basis of these components, a theoretical model is proposed which provides a rational procedure for estimating flood damages from projections of economic development within an area. The utility of the model is demonstrated by applying it to a flood-prone region in Southern Quebec, Canada.     

ESTIMATING RUNOFF VOLUMES FROM URBAN AREAS1

ABSTRACT: Estimations of runoff volumes from urban areas can be made by the equation Q = a A σ(P_e– b), where Q is the runoff volume, a is the part of the total area A Contributing to runoff, P_e is the rainfall amount for a single event, and b is the initial rainfall losses. For the evaluation of a and b, rainfall/runoff measurements were made in five areas of sizes between 0.035 km² and 1.450 km². By linear regression analysis of rainfall volumes versus runoff volumes, the initial rainfall losses were found to vary from 0.38 mm to 0.70 mm for the different areas. The parts of the areas contributing to runoff were found to be proportional to the impermeable parts of the mas. The equation is applicable in urban areas with well defined paved surfaces and roofs and with a negligible amount of runoff from permeable areas.     

APPLICATION OF SCS INFILTRATION MODEL1

ABSTRACT: The SCS infiltration model was applied to the Ralston Creek watershed in eastern Iowa. The criteria to determine the various model parameters were revised to obtain a better agreement between the observed and computed total runoffs. A procedure to calibrate the infiltration model is presented. The infiltration model was used in conjunction with an overland flow model to develop flood hydrographs. The results indicate that SCS infiltration model adequately describe the distribution of losses.     

Peak Runoff Model Comparison on Central Illinois Watersheds1

ABSTRACT: Four peak runoff rate models were tested with 183 gage years of record to determine the model most applicable to small watersheds of mild topography in east-central Illinois. The Cypress Creek, Rational, Chow, and SCS peak runoff models were evaluated for their performance. Statistical analyses indicated the Soil Conservation Service model was most appropriate for the watersheds tested.     

FIRST-YEAR EFFECTS OF CLEARCUTTING AN OAK-HICKORY WATERSHED ON WATER YIELD1

ABSTRACT: Two intermittent streams on oak-hickory watersheds in southern Illinois were gaged with a V-notch weir and sampled with an automatic water sampler. Baseline data was collected for a period of three years. Flow volume showed large variations between years and watersheds. Water samples were analyzed for Na, K, Ca, Mg, ortho-P, and NO₃-N. Water quality was consistently high, but there were significant differences between the watersheds during the calibration period. One watershed was clearcut in November 1979. One year of postharvest data has been analyzed. Flow volume increased 95 percent, but there was no evidence of increased sedimentation. There were significant increases in the stream water concentrations of K, Mg, and NO₃-N of 18 percent, 8 percent, and 274 percent, respectively. Nutrient budgets for the site were not adversely affected by the harvest. The clearcutting operation appears to have had a small impact on the watershed due to minimal disturbance during the logging and below normal precipitation the first year following the harvest.     

THE VRANA LAKE HYDROLOGY (ISLAND OF CRES - CROATIA)1

ABSTRACT: The Vrana Lake on the island of Cres in the Adriatic Sea represents a specific phenomenon of karst hydrology. The island of Cres covers an area of 404.3 km² with an average volume of 220 × 10⁶ m³ of fresh water in the lake. The island has an average rainfall of 1,063 mm, with a Mediterranean climate. The lake has a bottom reaching a depth of 62 m below mean sea level. The average water level is 14 m above mean sea level. The most probable theories on the origin of the lake and its hycirologic-hydrogeologic functioning state that it is a flooded poije in karst. The water budget method was used to define the lake catchments area at approximately 25 km². During the last six years, there has been drastic decrease of about 3 m in the lake's water level. This phenomenon was analyzed and it was calculated that 53 percent of the water-level decline was caused by water discharges from the lake to satisfy water supply demands, and 47 percent was due to a period of low precipitation during the analyzed period.