Riprap Revetments Research Articles

Property owner shoreline modification decisions vary based on their perceptions of shoreline change and interests in ecological benefits

Even under current sea level conditions, many communities are working to protect their coastlines against flooding and shoreline erosion. Coastal communities often protect their shorelines against excessive erosion by using armoring techniques (e.g., bulkheads, riprap). Yet hardened structures reduce many of the natural adaptive mechanisms present in coastal ecosystems and reduce the sustainability of the coastal system. In contrast, natural and nature-based features (e.g., living shorelines) can better protect coastal properties from storm damage and reduce erosion while also having the potential to adapt to new conditions. Since property owners are installing armoring structures more often than living shorelines, we sought to understand the factors motivating their shoreline modification decision. We surveyed property owners in Virginia, U.S. that applied for a shoreline modification permit. Most property owners, regardless of modification sought, perceive riprap revetment to be effective, able to withstand storm damage, and able to adapt to sea level rise. Interestingly, property owners that sought out living shorelines were not highly confident in living shorelines’ protection benefits. While most property owners perceived the ecological benefits of living shorelines, these benefits did not substantially impact the decision over what type of shoreline modification to implement. Our work highlights pathways that can improve coastal resilience given the important role that shoreline property owner decisions contribute to coastal community resiliency. Our results indicate there is a need to better engage property owners about the protection and adaptation benefits of living shorelines as their perceptions were not aligned with scientific assessments of living shorelines. Concurrently, coastal policies could be strengthened to support more natural approaches to shoreline management, as the more common armoring techniques are not resilient to sea level rise or storm damage.

Wave Run-Up on Mortar-Grouted Riprap Revetments

The wave run-up height is a crucial design parameter that determines the crest height of a sea dike and is used for estimating the number of overtopping waves. Therefore, a reduction of the wave run-up height is generally aspired in the design of dikes, which can be achieved by mortar-grouted riprap revetments (MGRR). Although MGRRs are widely utilized revetments along the German North Sea coast, no investigations into the wave run-up height on this revetment type are available to date. Full-scale hydraulic model tests were hence conducted to investigate wave run-up heights on partially grouted and fully grouted MGRRs. The wave run-up was determined using 2D-LIDAR measurements, which were validated by video data. Partially grouted MGRRs, due to their roughness, porosity, and permeability, reduce wave run-up heights from 21% to 28%, and fully grouted MGRRs due to their roughness reduce wave run-up heights from 12% to 14% compared to smooth impermeable revetments. Influence factors have been determined for four widely used revetment configurations, which can now be used for design purposes. A comparison and subsequent discussion about the representation of the physics of wave run-up by different parameters is carried out with the results presented.

Tensile Bending Stresses in Mortar-Grouted Riprap Revetments Due to Wave Loading

One of the most common revetment types in Germany is the mortar-grouted riprap revetment (MGRR), which is constructed by placing riprap on a filter or separation layer and subsequent grouting with mortar. Existing technical standards and guidelines for MGRRs do not consider the interaction between dynamic wave loading and structural response. To date, scientifically sound design approaches verified by model tests are missing. Therefore, the aim of this work is to establish a process-based model for the calculation of the acting bending tensile stresses during wave attack for MGRRs to asses crack formation. To this end, MGRRs were modelled as plates on an elastic foundation (PEF). Hydraulic boundary conditions were determined with full-scale hydraulic model tests. Model parameters of the PEF model were established by investigations into the mechanical parameters of the constituents of MGRRs. The results show that tensile bending stresses are particularly dependent on the pressure difference between the top and bottom edge of the top layer, which varies significantly for MGRRs as their porosity and permeability varies significantly depending on the amount of mortar used for grouting. Enveloping functions for maximum relative tensile bending stress σx,max/(ρwgHm0) are given for four configurations of MGRRs that are of great practical relevance.

Past hurricane damage and flood zone outweigh shoreline hardening for predicting residential-scale impacts of Hurricane Matthew

Hurricanes and tropical storms are among the most frequent and costly natural disasters in the United States, and their impacts are expected to intensify in the future. Understanding the best predictors of hurricane damage in coastal areas is of paramount importance for reducing recovery costs and protecting coastal infrastructure and human lives. This project used surveys of 295 homeowners in coastal North Carolina to evaluate damage to estuarine shorelines and homes caused by Hurricane Matthew in 2016. Specifically, we were interested in the following questions: 1) did homes with hardened shorelines (e.g. bulkheads and riprap revetments) experience more or less damage than homes with natural shorelines during the storm; and, 2) what were the strongest predictors of hurricane damage to shorelines and homes. Overall, we found that past hurricane damage to shorelines was the strongest predictor of shoreline damage during Hurricane Matthew and that flood zone and past hurricane damage to homes were the strongest predictors of home damage. However, homes with bulkheads also sustained more hurricane damage than homes with natural shorelines, perhaps because homes with bulkheads were on average 2 times closer to the shoreline than homes with natural shorelines. Our results show patterns of repeated shoreline and home damage during hurricanes and indicate that environmental context and vulnerability can outweigh individual residents’ shoreline management decisions.

An Investigation on Performance and Structure of Ecological Revetment in a Sub-Tropical Area: A Case Study on Cuatien River, Vinh City, Vietnam

The current study was performed with an aim to investigate the performance of ecological revetments implemented on the bank of the Cuatien River in Vinh city, Vietnam. Based on the ecological, topographical, and hydrological conditions of the Cuatien River, the gabion and riprap models were introduced to investigate the effect of ecological revetment on the slope stability and ecological restoration characteristics. The effect of prevailing climatic indicators, such as temperature, precipitation, sunlight hours, and humidity were investigated to ascertain the characteristics of weather conditions on the subtropical area. On the surface soil layer of the gabion and riprap, the nutrient indicators of soil organic matter (SOM) and available nitrogen (AN) increased in the spring, summer, and winter, but decreased in autumn, and available phosphorus (AP) did not show an obvious change in the four seasons. The biomass growth rate of Vetiver grass on the gabion and riprap revetments was found to be the highest during the summer, at 15.11 and 17.32 g/month, respectively. The root system of Vetiver and other native plants could increase the cohesion of soil. After 6 and 12 months, the shear strength of the soil behind the gabion revetment increased by 59.6% and 162.9%, while the shear strength of the soil under the riprap also increased by 115.6% and 239.1%, respectively. The results also indicated that the gabion and riprap revetments could improve the river water purification effect and increase the ecological diversity in the region. In the current study, 26 floral and 9 faunal species were detected in the riprap revetment, whereas 14 floral and 5 faunal species were detected in the gabion revetment, respectively. Through high sequencing technology, the number of bacterial species in the present study was found to be 198, 332, and 351 in the water, gabion, and riprap samples, respectively.

In Situ Effects of Shoreline Type and Watershed Land Use on Submerged Aquatic Vegetation Habitat Quality in the Chesapeake and Mid-Atlantic Coastal Bays

Submerged aquatic vegetation (SAV) is an ecologically and economically valuable component of coastal estuaries that acts as an early indicator of both degrading and improving water quality. This study aimed to determine if shoreline hardening, which is associated with increased population pressure and climate change, acts to degrade SAV habitat quality at the local scale. In situ comparisons of SAV beds adjacent to both natural and hardened shorelines in 24 subestuaries throughout the Chesapeake and Mid-Atlantic Coastal Bays indicated that shoreline hardening does impact adjacent SAV beds. Species diversity, evenness, and percent cover were significantly reduced in the presence of riprap revetment. A post hoc analysis also confirmed that SAV is locally affected by watershed land use associated with increased population pressure, though to a lesser degree than impacts observed from shoreline armoring. When observed over time, SAV recovery at the local level took approximately 3 to 4 years following storm impacts, and SAV adjacent to natural shorelines showed more resilience to storms than SAV adjacent to armored shorelines. The negative impacts of shoreline hardening and watershed development on SAV shown here will inform coastal zone management decisions as increasing coastal populations and sea level rise drive these practices.

Linking the Abundance of Estuarine Fish and Crustaceans in Nearshore Waters to Shoreline Hardening and Land Cover

Human alteration of land cover (e.g., urban and agricultural land use) and shoreline hardening (e.g., bulkheading and rip rap revetment) are intensifying due to increasing human populations and sea level rise. Fishes and crustaceans that are ecologically and economically valuable to coastal systems may be affected by these changes, but direct links between these stressors and faunal populations have been elusive at large spatial scales. We examined nearshore abundance patterns of 15 common taxa across gradients of urban and agricultural land cover as well as wetland and hardened shoreline in tributary subestuaries of the Chesapeake Bay and Delaware Coastal Bays. We used a comprehensive landscape-scale study design that included 587 sites in 39 subestuaries. Our analyses indicate shoreline hardening has predominantly negative effects on estuarine fauna in water directly adjacent to the hardened shoreline and at the larger system-scale as cumulative hardened shoreline increased in the subestuary. In contrast, abundances of 12 of 15 species increased with the proportion of shoreline comprised of wetlands. Abundances of several species were also significantly related to watershed cropland cover, submerged aquatic vegetation, and total nitrogen, suggesting land-use-mediated effects on prey and refuge habitat. Specifically, abundances of four bottom-oriented species were negatively related to cropland cover, which is correlated with elevated nitrogen and reduced submerged and wetland vegetation in the receiving subestuary. These empirical relationships raise important considerations for conservation and management strategies in coastal environments.

Wave Exposure Structures Oyster Distribution on Natural Intertidal Reefs, But Not on Hardened Shorelines

Intertidal oyster reefs can protect estuarine shorelines from wave erosion and sea-level rise, and recognition of these ecosystem services has fueled global efforts to conserve and restore these reefs. Although intertidal oyster reefs are valued for attenuating wave erosion, little attention has been paid to the effects of wave exposure on their distribution. The present study characterized the role of wave exposure in determining the distribution of natural intertidal oyster reefs and of oysters on hardened shorelines (bulkhead and riprap revetments). Wave exposure was determined using the National Oceanic and Atmospheric Administration (NOAA)-developed Wave Exposure Model (WEMo), which integrates adjacent water depth, fetch, and processed wind information, among other variables. Field mapping of oyster reefs, defined as ≥10 oysters m−2, in Pamlico and Core sounds, North Carolina, USA, was conducted in summer 2014. Hardened shorelines and associated oyster densities were mapped for Pamlico Sound only. A narrow wave exposure threshold (~500 J m−1) was identified above which natural intertidal reefs did not occur and below which reef presence was apparently dependent on other structuring variables, such as salinity at the time of sampling and the grain size of surrounding sediments. Wave exposure was not correlated with the presence of oysters on hardened shorelines. The application of WEMo in the present study should be useful for selecting locations and materials for intertidal oyster reef restoration.

Design of Minimum Cost Earthen Channels Having Side Slopes Riveted With Different Types of Riprap Stones and Unlined Bed by Using Particle Swarm Optimization

AbstractSeveral design alternatives for a trapezoidal earthen channel whose side slopes are riveted with riprap stones and bottom is unlined were analysed to compare the performance of different types of stones (round, sub‐round and sub‐angular, and angular) for minimum cost and performance effectiveness. Particle swarm optimization (PSO) algorithm was implemented to determine the minimum cost channel in contrast with earlier alternatives relating to the most hydraulically efficient sections. Both clear water and sediment‐laden flow conditions were applied to investigate the effect of flowing water characteristics on optimality, which is not available in the literature. Optimal design of channels incorporating land acquisition cost is scanty; therefore, land cost was also included in the objective function. In addition to the design of a channel without freeboard, different freeboard scenarios, including depth‐dependent equations, were applied to assess the impact of freeboard provision methodologies on optimal channel dimensions and cost. It was found that the channels having angular riprap revetment for sediment‐laden flow conditions offered the minimum cost of construction, with sub‐round and sub‐angular, and round stones coming next in sequence, and the freeboard provisions also impacted the channel shape, dimensions and cost. Copyright © 2016 John Wiley & Sons, Ltd.

Comparison of fish assemblages in restored and natural mangrove habitats along an urban shoreline

The installation of living shorelines is one strategy used to ameliorate habitat degradation along developed coastlines. In this process, existing hard structures, such as sea walls and riprap revetments, are supplemented with habitat forming species, e.g., oysters and mangrove trees, to improve habitat quality and function. Shoreline restorations in Biscayne Bay, Florida, USA, often utilize red mangroves, Rhizophora mangle (Linneaus, 1753), in addition to riprap revetments, to help stabilize the shoreline. This riprap-mangrove habitat provides structure for marine organisms to utilize and is believed to improve shoreline habitats in areas previously cleared of mangroves. We examined whether habitat provisioning was similar between restored mangrove habitat with the inclusion of riprap boulders and natural mangrove shorelines. We compared fish assemblages between natural mangrove and riprap-mangrove habitats within two areas of northern Biscayne Bay. Fish community structure and certain benthic cover types varied between mangroves and riprap-mangrove habitats. Total fish abundance was greater in mangrove habitat, while taxonomic richness was highest in riprap-mangrove sites in the northern part of the bay. Our findings suggest that fish assemblages and community structure are different between these habitat types, although the geographic context may mediate the effect of habitat type. Therefore, it is likely that these restored mangroves provide different ecological services than unaltered mangrove shorelines.

하천만곡을 고려한 사석호안공 설계기법 제안

In Korea, most levee breaks had been occurred at steep or meandering region in river. To mitigate inland flood by protecting levee, riprap design formula should be modified by adopting channel meandering. In this study, experimental study had been performed to suggest modified riprap design formula. Unstability conditions of riprap revetments were regenerated in a straight and two meandering experimental channels. After experimental observation of unstability of riprap, the modified riprap design formula had been suggested. Furthermore, the design formula adopts correction coefficients which can consider cross section averaged velocity and channel meandering.

Marshes with and without sills protect estuarine shorelines from erosion better than bulkheads during a Category 1 hurricane

Acting on the perception that they perform better for longer, most property owners in the United States choose hard engineered structures, such as bulkheads or riprap revetments, to protect estuarine shorelines from erosion. Less intrusive alternatives, specifically marsh plantings with and without sills, have the potential to better sustain marsh habitat and support its ecosystem services, yet their shoreline protection capabilities during storms have not been evaluated. In this study, the performances of alternative shoreline protection approaches during Hurricane Irene (Category 1 storm) were compared by 1) classifying resultant damage to shorelines with different types of shoreline protection in three NC coastal regions after Irene; and 2) quantifying shoreline erosion at marshes with and without sills in one NC region by using repeated measurements of marsh surface elevation and marsh vegetation stem density before and after Irene. In the central Outer Banks, NC, where the strongest sustained winds blew across the longest fetch; Irene damaged 76% of bulkheads surveyed, while no damage to other shoreline protection options was detected. Across marsh sites within 25 km of its landfall, Hurricane Irene had no effect on marsh surface elevations behind sills or along marsh shorelines without sills. Although Irene temporarily reduced marsh vegetation density at sites with and without sills, vegetation recovered to pre-hurricane levels within a year. Storm responses suggest that marshes with and without sills are more durable and may protect shorelines from erosion better than the bulkheads in a Category 1 storm. This study is the first to provide data on the shoreline protection capabilities of marshes with and without sills relative to bulkheads during a substantial storm event, and to articulate a research framework to assist in the development of comprehensive policies for climate change adaptation and sustainable management of estuarine shorelines and resources in U.S. and globally.

Ecological tradeoffs of stabilized salt marshes as a shoreline protection strategy: Effects of artificial structures on macrobenthic assemblages

Armoring shorelines to prevent erosion is a long-standing global practice that has well-documented adverse effects on coastal habitats and organisms. A relatively new form of shoreline protection, referred to as hybrid stabilization, incorporates created marsh in combination with a stabilizing structure such as a low-profile stone sill and is being implemented in many US coastal states as a means to not only control erosion but also to restore coastal habitat. However, there has been limited scientific investigation of ecological benefits and impacts associated with implementation of hybrid stabilization. We evaluated relative habitat capacity of marsh-sills by comparing plant, sediment, and benthic macroinvertebrate attributes in intertidal and subtidal zones of existing marsh-sills, natural marshes, tidal flats, and riprap revetment within two subestuaries of Chesapeake Bay, USA. Low and high marsh plant characteristics (stem count and height) of marsh-sills were similar to or greater than natural marshes. However, sediment was coarser, total organic carbon and total nitrogen concentrations were lower, and benthic macrofaunal community structure differed in marsh-sills compared to natural marshes. Marsh-sills supported lower deposit-feeding infaunal biomass than marshes in the intertidal. Epifaunal suspension-feeders were most prevalent at sites with artificial structure (riprap and marsh-sill), but highly variable among subestuaries. Infaunal abundance, biomass, diversity, and proportion of suspension/interface and deposit feeding animals were greater in shallow subtidal than in intertidal environments. Conversion of existing habitat to marsh-sills may cause localized loss of benthic productivity and sediment bioturbation and nutrient-cycling functions, with the opportunity to enhance filtration capacity by epifaunal recruitment to structures. When creating marshes that require structural support, there should be a balance of minimizing loss of existing habitats while encouraging use of suitable structural habitat for suspension-feeders. If properly implemented, the addition of structural habitat could subsidize secondary productivity particularly in areas where loss of complex biogenic habitat (e.g., oyster reefs) has occurred.

River Training and Ecological Enhancement Potential Using In-Stream Structures

The use of in-stream flow training structures for channel stabilization has become increasingly popular due to its potential cost effectiveness and ecological benefits. When properly designed and maintained, these structures help to protect the channel from erosion and lateral migration and may also provide grade control. Additionally, in-stream structures may improve fish and macroinvertebrate habitat and increase hyporheic exchange. However, a large number of these projects fail due to inadequate design guidelines. In this study, various types of in-stream sill and deflector structures are described. A literature review including case studies, journal articles, and standards developed by various agencies suggests design guidelines which are currently available but do not meet the rigorous engineering-based hydraulic design criteria necessary for success. A practitioner opinion survey of personnel from state agencies, federal agencies, and private firms indicates that these structures are being used extensively in at least 76% of the USGS physiographic provinces. In general, respondents indicated that in the areas of cost, performance, maintenance, and environmental enhancement in-stream structures are preferable to the most common alternative: riprap revetment. Information from 39 case studies suggests that successful projects involve multiple structures and are located in rivers with relatively high aspect ratios.

Seasonal persistence of a small southern California beach fill

Torrey Pines State Beach, a site with large seasonal fluctuations in sand level, received a small shoreface beach fill (about 160,000 m 3) in April 2001. The 600 m-long, flat-topped nourishment pad extended from a highway riprap revetment seaward about 60 m, terminating in a 2 m-tall vertical scarp. A 2.7 km alongshore span, centered on the nourishment region, was monitored prior to the nourishment and biweekly to monthly for the following 2 years. For the first 7 months after the nourishment, through fall 2001, significant wave heights were small, and the elevated beach fill remained in place, with little change near and above Mean Sea Level (MSL). In contrast, the shoreline accreted on nearby control beaches following a seasonal pattern common in southern California, reducing the elevation difference between the nourished and adjacent beaches. During the first winter storm (3 m significant wave height), the shoreline retreated rapidly over the entire 2.7 km survey reach, forming an alongshore-oriented sandbar in 3 to 4 m water depth [Seymour, R.J., Guza, R.T., O'Reilly, W., Elgar, S., 2004. Rapid erosion of a Southern California beach fill. Coastal Engineering 52 (2), 151–158.]. We show that the winter sandbar, most pronounced offshore of the nourishment, moved back onto the beach face during summer 2002 (following the usual seasonal pattern) and formed a wider beach above MSL at the site of the original nourishment than on the control beaches. Thus, the April 2001 shoreline nourishment was detectable until late fall 2002, persisting locally over a full seasonal cycle. In an extended 7-year time series, total sand volumes (summed between the back beach and 8 m water depth, over the entire 2.7 km reach) exhibit multi-year fluctuations of unknown origin that are twice as large as the nourishment volume.

Effects of coastal development on nearshore estuarine nekton communities

Coastal development affects estuarine resources by severing terrestrial-aquatic link- ages, reducing shallow water habitats, and degrading ecosystem services, which is predicted to result in measurable declines in nekton community integrity. We assessed the effects of landscape features on nearshore habitats and biological communities, relating subtidal habitat, shoreline condition, upland land use and nearshore fish communities in a Chesapeake Bay tributary, the James River, Virginia. Both upland development and the placement of erosion control structures on the shoreline were associated with reduced fish community integrity, and shoreline alterations were linked with the amount of subtidal structural habitat in the nearshore. Ecological thresholds in nek- ton community integrity were evident at ≥23% developed land use within 200 and 1000 m buffer increments. Nekton assemblages at sites with low development (<23%) and natural or riprap shorelines were different from all other combinations of altered conditions (low development with bulkhead, and high development with riprap or bulkhead). Species composition along natural or riprap revetment shorelines with low upland development tended to be diverse and inclusive of tidal marsh species, while highly developed sites or bulkhead shorelines were dominated by a few gener- alist species. The complex interaction between watershed (both nearshore and inland) and shoreline development presents a unique challenge for coastal planning. Alternate moderating approaches for coastal development may include preservation of riparian buffers, the placement of living shorelines for erosion control where appropriate, and development of targeting tools to identify landscapes near an ecological threshold.

Bank stabilization, riparian land use and the distribution of large woody debris in a regulated reach of the upper Missouri River, North Dakota, USA

AbstractLarge woody debris (LWD) is an important component of ecosystem structure and function in large floodplain rivers. We examined associations between LWD distribution and riparian land use, bank stabilization (e.g. riprap revetment), local channel geomorphology, and distance downriver from the dam in the Garrison Reach, a regulated reach of the upper Missouri River in North Dakota, USA. We conducted a survey of shoreline‐associated LWD in the reach during typical summer flow conditions. Reach‐wide LWD density was 21.3 pieces km−1 of shoreline, of which most pieces (39% ) were ‘beached’ between the waterline and the bankfull level, 31% of pieces had evidence of originating at their current location (anchored), 18% of pieces were in deep water (&gt;1 m), and 13% were in shallow water. LWD density along unstabilized alluvial (sand/silt) shorelines (27.3 pieces km−1) was much higher than along stabilized shorelines (7.2 pieces km−1). LWD density along forested shorelines (40.1 pieces km−1) was higher than along open (e.g. rangeland, crop land; 9.2 pieces km−1) or developed (e.g. residential, industrial; 7.8 pieces km−1) shorelines. LWD density was highest overall along unstabilized, forested shorelines (45 pieces km−1) and lowest along open or developed shorelines stabilized with a blanket‐rock revetment (5.5 pieces km−1). Bank stabilization nearly eliminated the positive effect of riparian forest on LWD density. A predicted longitudinal increase in LWD density with distance from the dam was detected only for deep LWD (including snags) along unstabilized alluvial shorelines. Partial resurvey in the summer following the initial survey revealed a reduction in total LWD density in the reach that we attribute to an increase in summer flow between years. Changes in riparian management and land use could slow the loss of LWD‐related ecosystem services. However, restoration of a natural LWD regime in the Missouri River would require naturalization of the hydrograph and modification of existing bank stabilization and channel engineering structures. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Comparison of Subyearling Fall Chinook Salmon's Use of Riprap Revetments and Unaltered Habitats in Lake Wallula of the Columbia River

Subyearling fall chinook salmon's Oncorhynchus tshawytscha use of unaltered and riprap habitats in Lake Wallula of the Columbia River was determined with point abundance data collected by electrofishing in May 1994 and 1995. We documented the presence or absence of subyearlings at 277 sample sites and collected physical habitat information at each site. Based on logistic regression, we found that the probability of fish presence was greater in unaltered shoreline habitats than in riprap habitats. Substrate size was the most important factor in determining fish presence, with dominant substrates larger than 256 mm having the lowest probability of fish presence. Water velocity, also included in our model due to its biological importance, was not a significant factor affecting presence or absence (P = 0.1102). The correct prediction rate of fish presence or absence in our sample sites using cross validation was 67%. Our model showed that substrate was the most important factor determining subyearling habitat use, but the model did not include other habitat variables known to be important to subyearlings in more diverse systems. We suggest that resource managers consider alternative methods of bank stabilization that are compatible with the habitat requirements of the fish that use them.

Seepage Effects in Some Riprap Revetments

Seepage can be an important cause of failure in riprap revetments, especially on streambanks consisting of layered alluvium, and on stage-controlled rivers. Revetments can be protected against seepage by filters designed according to widely accepted principles. Nevertheless, numerous widespread failures caused by emergent seepage have been observed in riprap revetments. The factors that most significantly affected the apparent severity and extent of seepage effects on revetments were alluvial layers of protected soil in which gradations varied from layer to layer and the persistence of a source of seepage water. Design and construction of these failed revetments did not prevent or accommodate the effects of seepage through the multilayered soils behind the revetments.

Construction of the profile of a dynamically stable riprap revetment of a sloping protecting structure

Construction of the profile of a dynamically stable riprap revetment of a sloping protecting structure