Comparing urban sub-catchment delineation approaches for dynamic hydrological modelling

<p>In the face of rapid urban and population growth and with climate change altering precipitation patterns, urban water management is becoming increasingly demanding. Numerous software, tools and approaches to study urban water flow behaviour and model hydrological processes exist. However, the understanding of water movement in urban areas, especially during extreme events, and the physical principles behind them, as well as the interaction between the natural and the urban hydrological cycle is still incomplete. For decades, models suited for urban hydrological analysis greatly impacted the improvement of flood protection, public health and environmental protection, changing the way we look at urban water and stormwater management. In order to calculate accurate quantities of runoff in any rainfall/runoff model, information about urban sub-catchment characteristics plays an important role. Size, shape, topography, as well as land use influencing infiltration rates and evapotranspiration, are of great importance to calculate accurate runoff quantities on the urban scale. New implementations to reduce runoff towards the sewer system, such as decentralised stormwater techniques, increase the urgent need for accurate and high-resolution local/neighbourhood-scale information. Spatial and temporal developments require water management models to be connected with GIS (Geographical Information Systems). Initially not being developed to interact with each other, multiple approaches exist to combine GIS with water management models. Nevertheless, defining urban sub-catchments for rainfall-runoff modelling is often still performed manually using specific maps or using simple surface partitioning algorithms such as the Thiessen polygons. A significant disadvantage in generating urban sub-catchments manually is the fact that natural surface inclination is usually not considered, influencing the size and shape of the delineated sub-catchments. So far, only a few studies have devoted attention to improving the way urban sub-catchments are delineated and the information about their surface characteristics is generated. This study evaluates a GIS-based approach to automatically delineate urban sub-catchments accounting for the location of nodes (actual manholes or drain inlets) as sub-catchment outlets. In order to compare the influence of the sub-catchment delineation methods (1 to 3), we use (1) a digital surface model (DSM) and (2) a digital elevation model (DEM) to automatically delineate the urban sub-catchments and compare these two methods with each other as well as with (3) already manually derived sub-catchments of a specific case study. Furthermore, we compare hydraulic simulation results from the software SWMM with measured flow data to infer the most accurate sub-catchment delineation method.</p>

Stratigraphical influence on chalk cave development in Upper Normandy, France: implications for chalk hydrogeology

Classically, the Upper Cretaceous Chalk Group aquifer of northwest Europe is conceptualized as a homogenous dual-porosity aquifer, with high porosity related to its fine-grained porous matrix, and intermediate hydraulic conductivity associated with fractures. However, an increasing number of hydrological studies visualize the Chalk as a heterogeneous karst aquifer due to the localised presence of dissolutionally enlarged conduits. Field investigation suggests that cave development is guided by distinct stratigraphical and tectonic discontinuities within the rock mass. Identifying which potential inception horizons within the Chalk aquifer are favoured, and why, is important for developing future robust conceptual models of groundwater behaviour. This study focusses on the Chalk of the Upper Normandy region in France where karstic conduits are common and are linked to major sources of groundwater for public water supply. We analyse the geometry and geomorphology of six chalk caves exposed in the Seine Valley with an aggregated length of over 5.7 km, along with other caves in southern England, and identify the key inception horizons associated with their development. The data shows that prominent Turonian, Coniacian and Santonian hardgrounds have influenced the development of 68% of the studied caves length, with sheet-flints and marl seams also playing a prominent role. Caves developed on or between hardgrounds typically display a complex interlinked anastomotic passage network, whereas passages subjected to paragenetic conditions caused by a high sediment flux tend to be concentrated into fewer, larger conduits. The new evidence from Normandy and Southern England demonstrates the role of lithostratigraphy, and in particular stratigraphical discontinuities on conduit development. The data reinforces the idea that the Chalk aquifer should be viewed as a heterogeneous triple porosity karstic aquifer, in which conduit development is influenced by key stratigraphical discontinuities. This improved conceptual model can be used to develop better groundwater flow models and improved catchment delineation.

Automatic calibration of SWMM using NSGA-III and the effects of delineation scale on an urban catchment

The study aims at calibration of the storm water management model (SWMM) with non-dominated sorting genetic algorithm-III (NSGA-III) for urban catchment in Hyderabad, India. The SWMM parameters calibrated were Manning's roughness coefficient (N), depression storage for pervious and impervious areas (DP and Di), sub-catchment width (W), curve number (CN), drying time (dry) of soil and percentage of imperviousness (I). The efficacy of calibration was evaluated by comparing the observed and simulated peak flows and runoff using goodness-of-fit indices. The calibration takes into consideration eight event rainfalls resulting in eight calibrated sets. Weights of goodness-of-fit indices were estimated and the best calibrated set was further validated for five continuous rainfalls/runoffs. Simulated runoff volume and peak runoff over the five continuous rainfalls deviated by 7–22% and 2–20% with respect to observed data. Results indicated that parameters calibrated for an event rainfall could be used for continuous rainfall-runoff modelling. The effect of catchment delineation scale on runoff was also studied. The study indicated that output of the model was sensitive to variation in parameter values of infiltration and imperviousness.

Estimation of Hydrological Outputs using HEC-HMS and GIS

Estimating runoff and understanding of the relationship between rainfall and runoff are of great importance in the management of flood. Several computer based hydrological models have been developed and used in simulating runoff in various watersheds in different parts of the world and in water resource studies. This study focuses on the combination of Geographic Information System (GIS) with Hydrologic Engineering Center –Hydrologic Modelling System (HEC-HMS) hydrological model to simulate runoff process of the adjoining areas of the Lagos Island and Eti-Osa Local Government Areas (LGAs). The study makes use of LIDAR Digital Elevation Model (DEM), drainage data and land use map for catchment delineation and hydrological modelling, using HECGeoHMS and ArcGIS 10.2. In HEC-HMS 4.2.1, the delineated catchment with all hydrological parameters and average daily rainfall data, are used to simulate and compute rainfall runoff volume, peak discharges for 10 months (between Jan to October) and a total of three years (2012, 2015 and 2017) were considered. Direct runoff volume and depth estimation for the years under review were determined. Results show that the peak discharge occurred on the 2nd of July 2012 at a rate of 14m3/s with an estimated runoff volume at the basin outlet of 39,669.70 x 103m3 (this date tallies with the severe flood events that occurred in that year). The study shows that estimating hydrological outputs is possible with the use of HEC-HMS and GIS. It recommends the application of such technologies in the prediction and development of basic flood warning systems for the area.

Empirical quantification of lacustrine groundwater discharge – different methods and their limitations

Groundwater discharge into lakes (lacustrine groundwater discharge, LGD) can be an important driver of lake eutrophication. Its quantification is difficult for several reasons, and thus often neglected in water and nutrient budgets of lakes. In the present case several methods were applied to determine the expansion of the subsurface catchment, to reveal areas of main LGD and to identify the variability of LGD intensity. Size and shape of the subsurface catchment served as a prerequisite in order to calculate long-term groundwater recharge and thus the overall amount of LGD. Isotopic composition of near-shore groundwater was investigated to validate the quality of catchment delineation in near-shore areas. Heat as a natural tracer for groundwater–surface water interactions was used to find spatial variations of LGD intensity. Via an analytical solution of the heat transport equation, LGD rates were calculated from temperature profiles of the lake bed. The method has some uncertainties, as can be found from the results of two measurement campaigns in different years. The present study reveals that a combination of several different methods is required for a reliable identification and quantification of LGD and groundwater-borne nutrient loads.