Detention basins as best management practices for water quality control in an arid region

Flood control detention basins （DBs） can act as water quality control structures or best management practices （BMPs）. A key pollutant that DBs serve to settle out is particulate phosphorus, which adsorbs onto sediment. This study examines the sediment phosphorus concentration and its relationship with the particle size of sediment microcosms from pre- and post-rain event samples obtained from six DBs located in Clark County, Nevada. DBs were allotted a land use classification to determine if there was a correlation between the sediment phosphorus concentration and surrounding land use. The curve number method was used to calculate the runoff and subsequent phosphorus carried into the DB by the runoff. Our data show sediment phosphorus concentrations to he highest in soils from undeveloped areas. Runoff amount also plays a substantial role in determining the amount of phosphorus brought into the DB by sediment. This research has implications for improvement of water quality in arid regions.

Chinese Satellite-based Flood Mapping and Damage Assessment in Dongdian Flood Detention Basin,China

Against the background of rapid climate warming,frequent and severe flooding disasters significantly impact socio-economic development and human life.In 2023,due to the northward movement of Typhoon Doksuri,extreme precipitation occurred in northern China,which resulted in a massive flood event in the Haihe River basin.Seven flood detention basins(FDBs)in North China were successfully implemented to effectively alleviate the downstream flood pressure.Leveraging all available Chinese satellite data,we monitored the flooding process daily,focusing on reviewing the flooding in the Dongdian FDB.The results indicate that since the activation of Dongdian FDB on August 1,the flood reached the urban area of Tianjin in just nine days and inundated the entire detention basin.Flooding persisted in the detention basin for about a week before gradually receding.The total maximum inundated area in the whole region was 307.5 km^(2),including 240.5 km^(2)of arable land,7.0 km^(2)of greenhouse land,and 9.7 km^(2)of built-up land,with an average inundation duration of 19 days.The total cumulative inundated arable land area was 240.5 km^(2),with an average inundation time of 21 days.This study shows that multi-source Chinese satellite data can provide comprehensive information and adequate references for post-disaster assessments.

Stormwater runoff pollution loads from an urban catchment with rainy climate in China

An older urban district in Wuhan, China, is transitioning from discharging sewage and stormwater directly into lakes, to directing the sewage to wastewater treatment plants （WWTPs）. Dealing with polluted storm- water discharge is a great challenge. Stormwater runoff from an urban catchment with a combined sewer system was sampled and analyzed over a three-year period. Results indicate that wet weather flows account for 66%, 31%, 17%, and 13 % of the total load of suspended solids （SS）, chemical oxygen demand （COD）, total nitrogen, and total phosphorus, respectively. The first flush of COD and SS was significant in all runoff events. More than 50% of the SS and COD loads were transported by the first 30% of runoff volume. Storage and treatment of the first 10 mm from each combined sewer overflow event could reduce more than 70% of the annual COD overflow load. An integrated solution is recommended, consisting of a tank connected to the WWTP and a detention pond, to store and treat the combined sewer overflow （CSO）. These results may be helpful in mitigating CSO pollution for many other urban areas in China and other developing countries.

Zero increase in peak discharge for sustainable development

For urban land development, some or all natural land uses （primarily pervious） are converted into impervious areas which lead to increases of runoff volume and peak discharge. Most of the developed countries require a zero increase in peak discharge for any land development, and the policy has been implemented for several decades. The policy of zero increase in peak discharge can be considered as historical and early stage for the low impact development （LID） and sustainable development, which is to maintain natural hydrological conditions by storing a part or all of additional runoff due to the development on site. The paper will discuss the policy, the policy implementation for individual projects and their impact on regional hydrology. The design rainfalls for sizing LID facilities that are determined in 206 weather stations in USA are smaller than design rainfalls for sizing detention basins.The zero-increase policy links to financial responsibility and sustainability for construction of urban stormwater infrastructures and for reducing urban flooding. The policy was compared with current practices of urban development in China to shine the light for solving urban stormwater problems. The connections and differences among LID practices, the zero-increase policy, and the flood control infrastructure were discussed. We promote and advocate the zero-increase policy on peak discharge for comprehensive stormwater management in China in addition to LID.

Stormwater treatment： examples of computational fluid dynamics modeling

Control of rainfall-runoff particulate matter （PM） and PM-bound chemical loads is challenging; in part due to the wide gradation of PM complex geometries of many unit operations and variable flow rates. Such challenges and the expense associated with resolving such challenges have led to the relatively common examination of a spectrum of unit operations and processes. This study applies the principles of computa- tional fluid dynamics （CFD） to predict the particle and pollutant clarification behavior of these systems subject to dilute multiphase flows, typical of rainfall-runoff, within computationally reasonable limits, to a scientifically acceptable degree of accuracy. The Navier-Stokes （NS） system of nonlinear partial differential equations for multi- phase hydrodynamics and separation of entrained particles are solved numerically over the unit operation control volume with the boundary and initial conditions defined and then solved numerically until the desired convergence criteria are met. Flow rates examined are scaled based on sizing of common unit operations such as hydrodynamic separators （HS）, wet basins, or filters, and are examined from 1 to 100 percent of the system maximum hydraulic operating flow rate. A standard turbulence model is used to resolve flow, and a discrete phase model （DPM） is utilized to examine the particle clarification response. CFD results closely follow physical model results across the entire range of flow rates. Post-processing the CFD predictions provides an in-depth insight into the mechanistic behavior of unit operations by means of three dimensional （3-D） hydraulic profiles and particle trajectories. Results demon- strate the role of scour in the rapid degradation of unit operations that are not maintained. Comparisons are provided between measured and CFD modeled results and a mass balance error is identified. CFD is arguably the most powerful tool available for our profession since continuous simulation modeling.