Hi

In such area, the landslides with different sizes can produce huge volume of sediment, in addition to surface runoff. so to prevent these sediments reaching in dam reservoir the first step is to reconnaissance the catchment area using areal photo and walking over the area. This step leads to identifying the total  volume of landslide-generated sediments and finally this sediment volume should be added to the volume of sediment calculated by hydrological studies. The final volume shows the majority of sediment generation in service life of dam. the final decision should be addopted to this volume of sediments. as it can be a small or large real dam or just a small retarding gabion dams.

First suggestion would be to select a relatively stable forested watershed with suitable site for dam to fill the needs.  For various reasons, this may not be possible, and geology, soils and land uses not always mapped.  As suggested by Dr. Hosseini, consider actual or potential landforms that are landslide prone, and avoid them when you can in the location of site.  Gullied lands will also likely have high sediment loads, and good to avoid.  Braided streams are an indicator of too much sediment for stream to move, so avoid them too.  When I get to my computer, I will try to send publication on design of small dams.  It has much information to offer.

You might look over articles on BMPs or soil and water conservation measures ~2003 on my researchgate for some ideas dealing primarily with forest practices and associated land uses, practices.  

It is difficult to control sediments when they reach streams, so practices are often aimed primarily at keeping landforms stable, and with forest or other vegetation cover to limit erosion, and preserve low impact, vegetated streamside zones.  Roads and clearing should avoid unstable geology and steep slopes, and adequate cross drains or dips to remove road surface drainage into forest/vegetated filter zone before excessive erosion can occur.  

If high sediment loads cannot be avoided, there may be some ways to promote sediment flushing through dams, or regularly dredging the recent dam sediments for fill or possibly construction materials.

Large document on design of small dams.

I fully agree with the solutions provided by Mr. Hansen. First & for most is to avoid regions which are prone for heavy sediment yield like landslides; downstream of agricultural fields (wherein regular tilling and loosening of top soil takes place); barren lands exposed for direct soil erosion due to impact of rainfall; etc. Many times the regional geology; morphometric characters; soil texture; land slope; etc., are not considered seriously, which in turn reduce the age of the dam.  As already suggested where there is no alternatives, in such cases watershed conservation practices should be done in the upstream portion of the dam. When ever the dam is silted, then of course dredging of the sediments is the only solution. The sediments so extracted can be used in construction activities.

thanx to all by giving beneficial answer.