Ming-Han Li

Biotechnical engineering as an alternative to traditional engineering methods A biotechnical streambank stabilization design approach

Focus on ecologically fragile streams in the US has resulted in heightened recognition and popularity of biotechnical streambank stabilization methods. This ancient technique re-emerges in the US in response to the link between traditional protection measures and numerous occurrences of streambank failures. The purpose of this study was to investigate biotechnical engineering as a viable alternative to traditional channelization and hard-armoring methods. Primarily by literature review, this study analyzed and organized various streambank stabilization approaches in traditional engineering, fluvial geomorphological, ecological and biotechnical engineering perspectives. Strengths and weaknesses in these four perspectives are discussed, suitable biotechnical alternatives are presented, and a cost-strength matrix of biotechnical techniques is introduced.

Overland Flow Time of Concentration on Very Flat Terrains

Two types of laboratory experiments were conducted to measure overland flow times on surfaces with very low slopes. One was a rainfall test using a mobile artificial rainfall simulator; the other was an impulse runoff test. Test plots were 6 ft (1.83 m) wide by 30 ft (9.14 m) long with slopes ranging from 0.24% to 0.48%. Surface types tested include bare clay, lawn (short grass), pasture (tall grass), asphalt, and concrete. A regression analysis was conducted to construct models for predicting flow times. Results predicted with regressed models were compared with those from empirical models in the literature. It was found that the slope variable in the regressed model from rainfall test data is less influential than that in existing models. Furthermore, the exponent for the slope variable in the regressed model for the impulse runoff condition is only 1/10th of those in existing models. Overall, most empirical models underestimate overland flow time for laboratory plots with very low slopes. The slope variable becomes insignificant in governing overland flow time when the slope is small. Antecedent soil moisture, not included in most empirical models, significantly affects time of concentration, which is included in the regressed models.

Design-with-Nature for multifunctional landscapes: Environmental Benefits and Social Barriers in Community Development

Since the early 1970s, Ian McHarg’s design-with-nature concept has been inspiring landscape architects, community and regional planners, and liked-minded professionals to create designs that take advantage of ecosystem services and promote environmental and public health. This study bridges the gap in the literature that has resulted from a lack of empirical examinations on the multiple performance benefits derived through design-with-nature and the under-investigated social aspect emanated from McHarg’s Ecological Determinism design approach. The Woodlands, TX, USA, an ecologically designed community development under McHarg’s approach, is compared with two adjacent communities that follow the conventional design approach. Using national environmental databases and multiple-year residents’ survey information, this study assesses three landscape performance metrics of McHarg’s approach: stormwater runoff, urban heat island effect, and social acceptance. Geographic Information Systems (GIS) was used to assess the development extent and land surface temperature distribution. Results show that McHarg’s approach demonstrates benefits in reducing runoff and urban heat island effect, whereas it confronts challenges with the general acceptance of manicured landscapes and thus results in a low safety perception level when residents interact with naturally designed landscapes. The authors argue that design-with-nature warrants multifunctionality because of its intrinsic interdisciplinary approach. Moreover, education and dissemination of successful examples can achieve a greater level of awareness among the public and further promote multifunctional design for landscape sustainability.

Improved Time of Concentration Estimation on Overland Flow Surfaces Including Low-Sloped Planes

AbstractTime of concentration (Tc) is one of the most used time parameters in hydrologic analyses. As topographic slope (So) approaches zero, traditional Tc estimation formulas predict large Tc. Based on numerical modeling and a review of relevant literature, a lower bound for slope (Slb) of 0.1% was identified as a threshold below which traditional Tc estimation formulas become unreliable and alternate methods should be considered. In this study, slopes less than Slb are defined as low slopes. Slopes equal to or exceeding Slb are defined as standard slopes where traditional Tc estimation formulas are appropriate. A field study was conducted on a concrete plot with a topographic slope of 0.25% to collect rainfall and runoff data between April 2009 and March 2010 to support numerical modeling of overland flows on low-sloped planes. A quasi-two-dimensional dynamic wave model (Q2DWM) was developed for overland flow simulation and validated using published and observed data. The validated Q2DWM was used in a ...

Estimating Time of Concentration of Overland Flow on Very Flat Terrains

Time of concentration is a primary basin parameter which represents response time of a rainfall runoff system. The accuracy of estimation of peak discharge or flood hydrograph is sensitive to the accuracy of the estimated time of concentration. Existing commonly-used empirical models for estimating time of concentration all include surface slope in the denominator, that is, the greater the surface slope, the less the time of concentration and vice versa. The problem occurs when such empirical models are to be used for estimating time of concentration on very flat terrains because as the surface slope approaches zero, the time of concentration approaches infinity. Based on engineering judgment such long times of concentration are not representative of reality. In this study, two types of laboratory experiments were developed to measure overland flow times on flat surfaces. One is the rainfall test using a mobile artificial rainfall simulator, and the other the impulse runoff test. Test plots were 6 feet wide by 30 feet long with slopes ranging from 0.24 to 0.48 percent. Surface types tested include bare clay, lawn (short grass), pasture (tall grass), asphalt and concrete. The experiment results were compared with those predicted by other empirical models. It was found that most of the empirical models under-estimate the time of concentration parameter. The antecedent soil moisture not included in most empirical models played an important role in affecting the time of appearance of runoff at the outlet, which affected the time of concentration.

Evaluating erosion control performance of crimped straw mulch in comparison with RECPs on simulated highway roadsides

Abstract The objective of this study is to compare the crimped straw mulch with commercial, manufactured products on erosion control performance of (1) sediment loss and (2) vegetation establishment. Artificial rainfall was used to evaluate erosion control materials on two different slopes (2H:1V and 3H:1V). Two soil types (clay and sand) were tested. Four weight applications (2⋅24, 4⋅49, 6⋅73, and 8⋅98 metric tons ha−1) of crimped straw mulch were tested; and the vegetation establishment was tested on small greenhouse trays. The results show that sediment loss decreased with increasing weight of crimped straw mulch. On 3H:1V slopes, all rates of the crimped straw mulch could protect the soil surface either equal to or better than the proprietary products. For 2H:1V slopes, heavier applications of crimped straw mulch were able to reach the protection level of a manufactured product. Vegetation density testing yielded inconsistent results with no clear trend, which is likely because of the small test trays, the lack of replications, and the insufficient time for vegetation establishment.

Estimating Time of Concentration on Low-Slope Planes using Diffusion Hydrodynamic Model

The time of concentration (Tc) is an important parameter for hydrologic design, analysis, and modeling. Error in the estimation of Tc will cause error in prediction of peak discharge (Qp), resulting in an incorrect design. Overland flow Tc depends on several factors, such as rainfall intensity (i), length (L), topographic slope (S0), and flow resistance (n). Generally, Tc has an inverse relation to S0. For the areas with very small or zero S0, use of such small values for S0 in traditional methods (empirical equations) for estimating Tc can produce very large values of Tc that seem to be unrealistic and incorrect. This study was conducted to identify a lower slope bound (SLB) for the use of traditional equations in determination of Tc and to propose the use of an alternate method for the determination of Tc on low-slope planes. The diffusion hydrodynamic model (DHM) was validated using data collected from published studies and field data collected in this study, including observed rainfall hyetographs and runoff hydrographs for 27 rainfall events for watersheds with relatively low-slope. After model validation, DHM was used to generate parametric relations between Tc and its explanatory variables (rainfall intensity, length, slope and resistance to flow) and extend the regression model to low-slope planes.

Comparison of Field and Laboratory Experiment Test Results for Erosion Control Products

The purpose of this study was to compare soil loss results from field and laboratory experiments on five rolled erosion control products (RECPs) and one spray-on bonded fiber matrix (BFM). Beginning in 1990, erosion control products to be used on Texas highway embankments, such as rolled blankets and hydromulches required evaluation of their soil erosion control effectiveness in a field laboratory developed by Texas Department of Transportation (TxDOT) and Texas Transportation Institute (TTI). Products were tested on 6.1-meter wide test plots on an embankment of either a 33% (21.3 meters long) or 50% (15.2 meters long) slope during vegetation growing seasons, using simulated rainfalls. Products that performed well received the approval for Texas highway application. This field testing program was revised in the end of 2001 and transformed to be conducted in an indoor laboratory. The standard test plot size was reduced to 1.8 by 9.1 meters. Tested slopes maintained at either 33% or 50%. The comparison results of field and indoor laboratory data indicate that regardless of the slope and the soil type, the soil loss ratio between field and indoor data maintains almost the same. The effects of rainfall magnitude, raindrop size, and test plot size are discussed.

Watershed Slope Lower Bounds for Hydrologic Methods

Engineers design a substantial fraction portion of infrastructure that accommodates storm water drainage and conveyance. Estimation models of the response for a watershed typically contain some form of watershed slope as a principal parameter, and the response is usually inversely proportional to that slope. Therefore, as topographic slope decreases, the watershed timing parameter increases. The consequences at low enough slope, is that the timing parameters approach infinite time and the precipitation intensity that is associated with a long averaging time is so small as to be meaningless -- yet low slope environments exist, are populated, and precipitation does generate runoff. BACKGROUND The parameters important for understanding overland flow are topography, surface roughness, soil infiltration characteristics (abstractions), and the distribution, duration, and intensity of precipitation. Of these components, the focus of this research was the impact of topographic slope on hydrologic estimates of runoff.

Proposed Test Protocol for Evaluating Performance of Sediment Control Devices for Roadside Storm Water Runoff

This paper details an official testing program developed by researchers at the Texas Transportation Institute (TTI) to classify sediment retention devices on the basis of their sediment removal performance. The testing facility designed as part of this study was constructed at the Texas Department of Transportation–TTI Hydraulics, Sedimentation, and Erosion Control Laboratory (HSECL) located at the Texas A&M University Riverside Campus. The facility consists of a parabola-shaped testing channel 4.6 m (15 ft) wide and 5.5 m (18 ft) long with a maximum depth of 0.8 m (2.5 ft) at the center and a slope of 3%. Two commercially available artificial sediments were mixed in equal proportions to create the artificial storm water for testing. The flow rate and turbidity of the water entering and leaving the channel were monitored for the duration of the test. The turbidity measurements were converted to suspended solid concentration by using a relationship developed in the HSECL laboratory. A simple mass balance calculation was used to determine the amount of sediment trapped in the channel.