The expansion of cities into rural watershed areas without adequate grading ordinances, building codes and management guidelines has been responsible for many predictable disasters and property losses. Woodland fires, mudslides, landslides, etc., are often the result of a lack of insight by the public agencies dealing with urban growth and development. These agencies make changes in their public safety codes primarily in response to disasters and often are not geared to avoiding them through effective land management of which effective erosion control is a key objective.

Effective Watershed Management

A watershed can be defined as all the land and water within the confines of a certain drainage area. Vertically, it extends from the top of the vegitation to the underlying rock layers that confines water movement. An effective watershed manager may, therefore, also be required to be a vegitation manager as well as a wildlife fire manager.

A development, or building site watershed is the area of land whose drainage directly affects the safety of the individual property. In many cases this area will include neighboring properties over which a owner will have little control. While effectively written tract easements, conditions, covenants, restrictions, and reservations (CC&R’s) governing the management and maintenance of a tract, or unit of development, are key items to cooperation among neighbors, Landscape Architects and designers must understand the erosional processes affecting a watershed area. Landscape Architects and land use planners must understand changes brought about by vegitation, fires and their own actions or inactions.

Controlling Drainage

As an example, a typical grading ordinance and guideline may state that a 2 percent overall gradient must be maintained from the rear of the property to the curb or the drainage structure. Designers or investors often defeat this requirement by leveling off the property for lawn or planting. With time the property will settle and dip towards the slope. The surface may then channel water onto an already saturated slope and cause slips and massive slope failure.

The above example is just one of many considerations that must be taken into account when designing and controlling drainage.

Maintaining Slope Stability

Landslides are often the major erosional processes on steeper’ slopes ranging from about 25 to 45 degrees. In addition to the basic. requirements of controlling drainage, one of the most effective ways to retain slope stability and reduce the probability of landslides is to maintain deep rooted woody vegitation on steep slopes. Deep roots serve the dual functions of anchoring the soil to the bedrock and pumping the water out of the deeper soil layers. This leaves the soil mantle dry to absorb heavy winter rains like a sponge and permits percolation to the deeper layers.

When removing or thinning out vegitation on steeper slopes one must be aware of the importance of the angle of repose in effective watershed management. This angle is defined as the angle between the horizontal and the maximum slope that a particular soil or geological material assume though natural processes. For loosely heaped dry sand and mixed earth, the angle of repose is about 21 to 25 degrees but can be increased through such factors as compaction, an increase in soil moisture up to the saturation point and effective plant materials. For standard soils on natural slopes the angle of repose is about 34 degrees (67 percent). On slopes that exceed the angle of repose, gravity causes soil and rocks to slide or fall downhill unless anchored by plants.

Effective Landscape Design

The aim of landscape design is to make a specific property more attractive. Many investors and owners equate attractiveness with colorful, water demanding plants and soon discover that they have created a landscape setting that requires costly maintenance and reduces property safety by requiring too much watering on unstable watershed areas.

The mixture of native plant species found in any particular area is determined by such factors as aspect and steepness of slope, soils, elevation, fire frequency and local climate. Landscape design in watershed areas should therefore aim towards making the property more functionally attractive within the confines of the natural environment.

Xeriscape Design

Water-conserving landscape design should include low-waterusing plants, effective irrigation systems and proper soil preparation. Drought tolerance, resprouting ability, root depth and strength and disease resistance are some of the most important plant characteristics for long-term watershed landscape design. After fire, roots of resproutring plants provide slope stability, and helped along by additional irrigation, can greatly reduce surface erosion by forming an effective surface cover before heavy rains return.

Plant Compatibility

Plant compatibility becomes a critical concept when mixing native and exotic species in a landscape setting because of varying watering and maintenance requirements.

Among individual species, those with higher moisture requirements also seem to have greater tolerance to landscape irrigation than most drought-tolerant species. The time and amount of watering is also critical. Occasional deep watering at night during the summer months is less damaging than intermittent, more shallow watering in the daytime. Heat and moisture encourage root rot, especially in heavy or darker soils.

Effective Watering of Landscaped Slopes

Excessive watering creates serious problems even among exotic species. It leads to slopes already saturated through landscape irrigation. It is estimated that about 80 percent of the near-surface slope failures involve the upper one to four feet of soil and are caused by over-saturation from sprinklers and heavy rains.

Sprinkler systems on slopes should only be operated manually and then sparingly. Most drought-tolerant landscape shrubs can survive on 1/2 inch of rainfall during the hot summer months, but may require greater watering where aesthetics are important. Most other landscape plants can survive on less than 30 inches of additional annual rainfall.

During dry years, fairly complete soil moisture recharge is often necessary through irrigation; however, only one deep watering may be required by the most drought tolerant species. If effective rains do not occur during the winter, occasional shallow watering may be required to keep the more water-demanding plants alive. Keep in mind that complete moisture recharge may initiate slope failures and major slides if heavy rainstorms fall on saturated soils. On the other hand, when late season rains have recharged the soil moisture, one watering during the summer months may be sufficient for plant survival.

Today, public agencies, planners and designers are using more effective land use in response to regularly occuring flood and landslide disasters. Factual management information is more readily available to anyone, landowners and Landscape Architects are increasingly becoming the key to adequate long-term management of watershed resources.

The information contained in this article was obtained from an article written by Klaus Radtke and published in The IECA Handbook, ?EUR??,,????'??Erosion Control...You?EUR??,,????'???re Gambling Without It,?EUR??,,????'?? proceedings of Conference XVIII, International Erosion Control Association, February, 1987, Reno, Nevada, with permission from Carol L. Forrest, P.E., President, IECA.

Klaus Radke is president of GEO Safety, Inc., an environmental research, resource management and consulting firm. He is also president of the National Foundation for Environmental Safety.